What do you want from a GPSDO? - Page 1
Forum section: electronics / general

author: rahandford
author_rank: Regular Contributor
timestamp: March 12, 2019, 06:41:08 am
content: Hello everyone,I was looking on Ebay and Aliexpress for a GPSDO. Most of the units available (and information obtained on here) are all the same and many are based on references that are second hand and of an unknown age with regard to ageing characteristics.So I decided to try to make my own based on the uBlox LEA-M8F time and Frequency Module.What I would like to know is if I make this design open source and make it available as a product to others, what features do you want from a GPSDO for your bench?So far I have 1PPS and 10MHz(x2) also a 30.72MHz outputs. Do people use the RS232 outputs and if so what data do you extract? DO you want TTL or true RS232 levels?Do you need an LCD and if so what information do you find most useful? Should I include a fractional PLL so you can generate a range of outputs?All input would be most appreciated and the results will be shared here.Thanks to all for your help.Rupert

author: rfspezi
author_rank: Regular Contributor
timestamp: March 12, 2019, 07:00:20 am
content: I would appreciate at least six (better 10) 10 MHz outputs to keep all my measurement equipment in sync.A LCD display that shows the accuracy that can be expected at the moment would also be important to me.Generation of different output frequencies is less important to me since i generate them with a better suited function generator anyway.USB Interface would be nice.

author: rahandford
author_rank: Regular Contributor
timestamp: March 12, 2019, 07:41:46 am
content: Thanks for the feedback

author: timgiles
author_rank: Regular Contributor
timestamp: March 12, 2019, 07:52:00 am
content: I dont know how much the 6 or 10 outputs would add to the cost - but - prehaps consider the ability to add a daughter board that splits / filters.... the signal. Certainly 2 outputs (10Mhz) is useful to many, not sure 6 or 10 outputs is but I agree, an option would be excellent.

author: rfspezi
author_rank: Regular Contributor
timestamp: March 12, 2019, 08:01:55 am
content: Do you already have an idea of the system design?Which VCOCXO, DAC, uC...?A block diagram to discuss would be great.

author: ramon
author_rank: Regular Contributor
timestamp: March 12, 2019, 03:29:31 pm
content: I would want a good 'holdover' (that module seems to have a vctcxo, so maybe it is good).True RS232 better than TTL. But never USB.Ability to reprogram/reconfigure/hack the Ublox module as much as possible.I do not need LCD if we can check status via RS232. One 'power on' LED and another 'locked LED' (or TTL signal) is enough. But I don't mind if LCD option is included.Fixed 10 MHz output + 1 PPS, and also configurable output (Because I need a good 2.048MHz reference output too, others might need different frequency).The last one : a good case with universal power supply (100V/110V/220V), and good antenna kit.

author: tkamiya
author_rank: Super Contributor
timestamp: March 12, 2019, 07:10:31 pm
content: No external power brick for power source. I hate those things. Built in power supply with built-in short term battery backup would be nice. It would only need to keep it running for few hours.Other than that, I'd like to have it simple. 2 PPS, 3 10Mhz, Power indicator, 1 PPS indicator, RS232C/USB, and big UNLOCK alarm LED, and a reset button. I really don't need an LCD.I'm actually on a fence on RS232C or USB. Most PCs today no longer come with RS232C. It is also starting to lose compatibility. DTE/DCE was bad enough. Now 3.3V, 5V, and TRUE RS232C involving +/- 24V. USB may be a reality, and since we don't use that for timing, I'm not sure if it's a bad thing. If it was for NTP feeding pin9, I would be against it.

author: David Hess
author_rank: Super Contributor
timestamp: March 12, 2019, 10:38:17 pm
content: I am not so hot these days on an internal off-line power supply if it is custom, undocumented, unrepairable, and unreliable which is often the case. So an external power brick is fine as long as external 12 volt DC from any source is also supported.At this point I think configuration via Ethernet is more practical than RS-232 but both would be useful.Must have:Power IndicatorLock IndicatorSupport for some type of redundant power or battery backup.Should have:Self charging of external battery if supported.Some type of Ethernet connected real time monitoring. This could be as simple as Telnet.Would be nice:NTP if an Ethernet port is included.Built in distribution amplifier.

author: Theboel
author_rank: Frequent Contributor
timestamp: March 13, 2019, 12:28:28 am
content: 1. ethernet2. backup power conection3. +12v DC input4. Parameter can be adjusted with external correction (PPP etc)5. Dual freq receiver

author: tkamiya
author_rank: Super Contributor
timestamp: March 13, 2019, 12:42:27 am
content: Ethernet to serial converters are already on market - except most are bulky and expensive. Maybe something *like that* can be created and placed in where serial port would be?

author: tkamiya
author_rank: Super Contributor
timestamp: March 13, 2019, 12:48:51 am
content: I'm wrong. Some quite small tcp/ip to serial modules are available.Try doing some searches on Amazon. They start around 20 dollars.

author: beanflying
author_rank: Super Contributor
timestamp: March 13, 2019, 01:06:38 am
content: Going down the path of battery backed GPSDO at present. Base unit I chose some time agoeBay auction: #253375324294Multiple output options is a must IMO this many gives plenty for my usage. There is some shots of the internals here.https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/msg1440787/#msg1440787I can't see to many shortfalls with it as far as inputs and outputs go, sync's to Lady Heather, PPS, Square and Sine out.8-10 hours GPSDO battery life from 10 D sized NiMh in the HP shell or circa 3 hours with the counter on as well using the GPSDO as the reference. Very worthwhile addition to the project or even a sub project. Reason for the NiMh over Lithium was ease of retrofitting the box and also less fussy for charging and discharge management.

author: rfspezi
author_rank: Regular Contributor
timestamp: March 14, 2019, 07:13:17 am
content: I just thought about what happens if you don't have/want it powered on 24/7 and don't wanna use a battery.To achieve lock and good stability as fast as possible after heating up of the VCOCXO, i guess the following would be useful.* Every update of the DAC value needs to be stored in NVRAM* Use frequency lock instead of phase lock (maybe optional in software?)

author: rahandford
author_rank: Regular Contributor
timestamp: March 15, 2019, 05:54:29 am
content: Thanks for all the feedback so far - I will try to make all this into one deviceDatasheets for parts chosen so far:LEA-M8F u-blox M8 time & frequency reference GNSS module:https://www.u-blox.com/sites/default/files/products/documents/LEA-M8F_DataSheet_%28UBX-14001772%29.pdfDAC8571:http://www.ti.com/lit/ds/symlink/dac8571.pdfBliley TCVCXO:http://www.farnell.com/datasheets/2674668.pdf?_ga=2.168041506.2031284271.1552628280-648269207.1446653940&_gac=1.15460994.1552288765.EAIaIQobChMI6cXx7MX54AIVTZPtCh1MbgSdEAAYASAAEgL4pvD_BwEPlease let me know where I have gone wrong already!

author: beanflying
author_rank: Super Contributor
timestamp: March 15, 2019, 06:06:55 am
content: I haven't looked at the M8F variant or specs but I have done some playing with the M8N. Depending on what frequencies you are going to try and generate other than a PPS the 8N suffers badly from Jitter. While my HP Counters measure 10MHz correctly against my GPSDO the Waveform is shocking at that point.I just re did some testing here and it only works reasonably at integer divided frequencies of 24MHz (Max spec is 10Mhz but 12 is still reasonable)https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg2246022/#msg2246022

author: dkozel
author_rank: Regular Contributor
timestamp: March 15, 2019, 03:05:46 pm
content: I haven't seen the distribution of the 1PPS signal mentioned. Ideally there'd be several 1PPS and 10 MHz outputs all of which are length matched to keep the signals in alignment. The hardware for buffering and splitting the signals isn't that expensive, I'd say a minimum of four ports each makes for a useful system.You'll find that square waves vs sine waves for the 10 MHz is a bit of a topic, I'd tend towards square waves by default, possibly with non-populated pads for a high order low pass filter to convert it to a sinewave.For inspiration, the Octoclock has an open schematic of control and distribution. It uses a separate GPSDO, but maybe it will be of some use as a reference.https://kb.ettus.com/OctoClock_CDA-2990

author: texaspyro
author_rank: Super Contributor
timestamp: March 15, 2019, 04:57:17 pm
content: I think that you will find out that a M8F+TCXO will wind up having rather poor performance as a GPSDO.

author: rahandford
author_rank: Regular Contributor
timestamp: March 15, 2019, 09:48:22 pm
content: Why is that? What would you want to change? All advise much appreciated

author: texaspyro
author_rank: Super Contributor
timestamp: March 16, 2019, 03:32:06 am
content: Quote from: rahandford on March 15, 2019, 09:48:22 pmTo start with, [eople that use GPSDOs want a stable, accurate output... something a TCXO is not known for. Use a good OCXO.

author: DimitriP
author_rank: Super Contributor
timestamp: March 16, 2019, 04:14:15 am
content: The only features I need in my next GPSDO are:A). FreeB) 4 months of timeC) room for one more bench to fill up with gear and projectsD) spare cash for mouser, digikey, and arrow orders

author: rahandford
author_rank: Regular Contributor
timestamp: March 16, 2019, 04:29:22 am
content: Quote from: texaspyro on March 16, 2019, 03:32:06 amYes I appreciate that a OCXO is better but also (as far as I have found) much more expensiveDid you look at the specification for the Bliley TCVCXO? - its quoted at 2ppm.I also believed that this is only really used in a holdover situation as during normal operation the pll will be locked to the GPS?Please feel free to educate me as I am no expert!!Thanks for your feedback it's the only way to learn

author: DimitriP
author_rank: Super Contributor
timestamp: March 16, 2019, 05:35:45 am
content: Since you asked ....QuoteThat's the first wrong turn you took right out of the gate.There is no such things as a "cheap" GPSDO. Perhaps "good enough for the price".OXCOs are either expensive, small and consume very little power or are cheaper (by almost an order of magnitude) , old, bulky and a little on the power hungry (comparatively speaking ) side .There are plenty (more than 2 ) published designs out there that can be built using 1PPS or 10KHz from a cheap GPS module ($10-$25), a surplus OCXO (like a 10811 for about $100) and a control board for about $20-$30 in parts.A search for GPSDO will returnalmostas many results as searching for kardashian

author: texaspyro
author_rank: Super Contributor
timestamp: March 16, 2019, 06:01:31 am
content: Quote from: rahandford on March 16, 2019, 04:29:22 amNo correction mechanism is perfect or instantaneous. You want to minimize the amount and number of corrections that needs to be done. An OCXO might be in the 2 ppb range, a far cry from 2 ppm.

author: texaspyro
author_rank: Super Contributor
timestamp: March 16, 2019, 06:19:32 am
content: Quote from: DimitriP on March 16, 2019, 05:35:45 amThe Trueposition GPSDO was around $50 and the Samsung/Symmetricom/Trimble "UCCM" GPSDOs can be had for less than $100. The Samsung, in particular, is surprisingly good.

author: awallin
author_rank: Frequent Contributor
timestamp: March 16, 2019, 08:20:04 am
content: Quote from: dkozel on March 15, 2019, 03:05:46 pmThis wiki page has a list of open designs for pulse/frequency distribution:https://www.ohwr.org/project/pda-8ch-fda-8ch/wikis/Similar-ProjectsMy own and John Ackermanns phase-noise measurements are here:https://www.febo.com/pages/amplifier_phase_noise/http://www.anderswallin.net/2016/09/ettus-octoclock-distribution-amplifier/fda_2016-09-12_pn/If you go with an OCXO and care about phase-noise and/or short-term ADEV then the distribution-circuit should be chosen so it doesn't degrade the phase-noise of the oscillator.

author: awallin
author_rank: Frequent Contributor
timestamp: March 16, 2019, 08:27:09 am
content: I guess it's hard to please everyone...uBlox is constantly releasing new products, is there a common interface so the GNSS-chip/module can be pluggable? F9P, F9T, etc. etc. just outCan you make a board which takes many different local oscillators? There should be an option to go from ~5 eur VCXO (ABLNO?) to ~50eur (e.g. connor-winfield) OCXO's, to high-end 500eur OCXO (Morion etc)Ethernet connectivity and monitoring over SNMP is nice, for permanent lab deployment. DAC-voltage etc. should be something that can be logged.If the future GNSS-modules are dual-frequency and spit out raw measurement data then a suitable data-bus to the GPSDO-uC/brains and ability to push that out over Ethernet (RTCM?) starts to be interesting for prosumer/pro tinkering.

author: Towger
author_rank: Super Contributor
timestamp: March 16, 2019, 08:33:08 am
content: These are probably the cheapest on ebay at the moment:http://www.ebay.com/itm/163064988493And, come with all the cables and additional ewaste goodness :-)

author: awallin
author_rank: Frequent Contributor
timestamp: March 16, 2019, 08:45:26 am
content: Quote from: Towger on March 16, 2019, 08:33:08 amCan you get even two of the fans at the back new from farnell/digikey for $29

author: DimitriP
author_rank: Super Contributor
timestamp: March 16, 2019, 08:46:49 am
content: Quote from: texaspyro on March 16, 2019, 06:19:32 amThat is indeed cheap in dollars for qty 1 ...but who ends up with just one ?

author: Theboel
author_rank: Frequent Contributor
timestamp: March 16, 2019, 10:09:28 am
content: are some one know anything about this offer :http://www.ebay.com/itm/163064988493beside use the GPS module ?

author: Towger
author_rank: Super Contributor
timestamp: March 16, 2019, 10:37:56 am
content: Last photos are of the inside. The module is top left. Right beside one of the fans, which answers a number of questions on should they be insulated to keep the oven at a constant temperature.

author: nctnico
author_rank: Super Contributor
timestamp: March 17, 2019, 06:10:17 pm
content: Derive the 1PPS output from the 10MHz and not directly from the GPS. I have a BG7TBL GPSDO and the 1PPS output wanders around for 100ns.

author: texaspyro
author_rank: Super Contributor
timestamp: March 17, 2019, 06:14:59 pm
content: Quote from: nctnico on March 17, 2019, 06:10:17 pmIf you do that, pay attention to syncing the PPS output to UTC time... not an easy thing to do. PPS needs to be stable AND on time. Just dividing the 10 MHz down will yield a PPS with some random offset to UTC.

author: David Hess
author_rank: Super Contributor
timestamp: March 17, 2019, 07:48:03 pm
content: Jamming the 1pps divider of the 10MHz output to phase lock with UTC will be a fun challenge but I agree this is a good idea. It is a small step from this to allowing a programmed positive or negative delay to compensate for any cable propagation times although a timing GPS should support this already.

author: texaspyro
author_rank: Super Contributor
timestamp: March 17, 2019, 08:25:40 pm
content: A PPS output that is not synced to UTC is a pretty useless signal. There are not many use cases for an unsynced PPS output. I don't know of any DIY GPSDOs that implement a proper PPS output.

author: David Hess
author_rank: Super Contributor
timestamp: March 18, 2019, 01:03:42 am
content: Quote from: texaspyro on March 17, 2019, 08:25:40 pmThere not not many use cases but the problem identified by nctnico is that practically all GPS receivers provide a time synced PPS output which has considerable jitter and it would be nice to instead regenerate this PPS output based on the jitter free 10MHz disciplined oscillator which will require some care to maintain sync with UTC.

author: texaspyro
author_rank: Super Contributor
timestamp: March 18, 2019, 04:31:15 am
content: Quote from: David Hess on March 18, 2019, 01:03:42 amHere is a histogram log of the PPS output of a Thunderbolt GPSDO. PPS jitter is around +/- 1.2 nsec (+/- 800 psec 99% of the time).

author: rfspezi
author_rank: Regular Contributor
timestamp: March 18, 2019, 08:12:00 am
content: Quote from: beanflying on March 15, 2019, 06:06:55 amWhere exactly did you get the information about the 24MHz from?A link would be helpful.Thanks

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2019, 08:54:04 am
content: Quote from: rfspezi on March 18, 2019, 08:12:00 amI have forgotten the original source, there is a youtube video on it too. Certainly backed up by testing, the spec is listed as 10MHz max but 12 is clean as is 8. I have had a breadboard play with a PLL but as to be expected not usable and I haven't had a play with a soldered option.Quick three videos/captures just for reference. Terminated into 5010Meg followed by 8 and 12. Full Bandwidth so no filtering.https://youtu.be/77B80MOPc8whttps://youtu.be/TOKRDG19PxUhttps://youtu.be/ozflNHosEQs

author: rfspezi
author_rank: Regular Contributor
timestamp: March 18, 2019, 09:21:24 am
content: @beanflying:Is it correct, that when i set the output-frequency of the NEO-M8N to 10kHz (=24MHz/2400) i will get a clean signal aswell or does this only apply to binary divisors (2/4/8...) ?

author: nctnico
author_rank: Super Contributor
timestamp: March 18, 2019, 09:33:57 am
content: Quote from: texaspyro on March 17, 2019, 06:14:59 pmResetting the divider when the 1PPS is more than 2 10MHz cycles off fixes that. You could get fancy and do some averaging but such a circuit is likely to introduce extra jitter.

author: beanflying
author_rank: Super Contributor
timestamp: March 18, 2019, 09:40:41 am
content: Quote from: rfspezi on March 18, 2019, 09:21:24 am10kHz capture. Just grab one and have a play, cheap GPS fun. add an FTDI or similar USB 3.3V board and enjoy.

author: rfspezi
author_rank: Regular Contributor
timestamp: March 18, 2019, 06:53:09 pm
content: I just set my NEO-M8N to output 8 MHz and checked the signal on the scope.What i observe are a bunch of 20ns longer high-pulses every approx. 1 second.Setting the frequency somewhere between 10 and 8000 Hz higher lets these pulses completely disappear.Does this mean, that instead of adjusting the 24MHz oscillator, only the output is digitally adjusted?

author: beanflying
author_rank: Super Contributor
timestamp: March 20, 2019, 02:23:59 am
content: A bit OT but as a result of a PM I tried upgrading my year old M8N to the newer V3 firmware. While it may or may not have the Flash under the cover it seems there may also be a need for a Hardware upgrade to allow it to work. Not sure if this stops the upgrade from being allowed by U-Center but mine certainly wasn't happening. One of the reasons to maybe want it is to use the Galileo Satellites but mine on the Bench in the Shack is getting data from 8-10+ on the stock antenna as is so at least for me a non issue.Quote

author: Towger
author_rank: Super Contributor
timestamp: March 20, 2019, 07:15:19 pm
content: Quote from: texaspyro on March 16, 2019, 06:19:32 amThere are a couple of different Samsungs. The STP2945LF which has a smaller oscillator than the STP2878LF (Double Oven???). In the examples for sale, both appear touse the UBlox LEA6T-1-001 xx.Do you know that the difference between them is and which is better?Their age appears to over lap, I assume using the xx in the UBlox part number is the revision, so higher = newer/better?

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: March 21, 2019, 03:56:49 am
content: Quote from: rfspezi on March 18, 2019, 06:53:09 pmI've been experimenting with a NEO-M8N module over the past 3 or 4 weeks since I bought one designed for RPi/Arduino projects and I can confirm that the module simply adds or drops one cycle of its 48MHz TCXO clock signal every half to five seconds or so, depending on where the temperature compensation has managed to steer it back towards its nominal frequency (retrace effect limits the desired effect of temperature compensation in this case).The pulses are actually some 20.8333ns long (one period of the nominal 48MHz TCXO used in the M8N based modules). Like you, until I started monitoring the 2MHz jitter free output on the PPS line (I plan to multiply this up to a jitter free 10MHz) by triggering my scope with a 2MHz Sinc pulse waveform from my upgraded FY6600 AWG (adjusted by the 10 to 20mHz or so required to minimise the drift between the two traces), I used to naively think the M8N kept it synchronised to GPS time by adjusting the TCXO frequency. It turns out that it's not using quite as subtle a technique as that.Keeping a 1Hz PPS pulse in sync with GPS time for hundreds of years can be readily achieved using this simple adding or dropping of a clock cycle to the frequency divider input whenever it drifts as much as one full cycle from sync with the GPS time datum, whatever the actual XO or TCXO frequency happens to be. This is exactly the same principle as using leap seconds adjustments to maintain synchronisation with UTC.The rate at which these corrective phase shifts occur depends on how close to the exact (in this case, 48MHz) frequency the XO or TCXO happens to have settled on. I've seen the rate at which these corrections are applied vary from around two per second to one in more than five seconds with my own GPS module.I've experimented with temperature changes by blowing cooling air over the module and using the warmth of my hands to warm it up to try and bring the background drift to a halt, slowing the frequency of the corrective jumps but the temperature compensation does seem to rather limit the effects in spite of the retrace issue. I suspect such experimentation with modules relying on just an XO rather than a TCXO will produce more extreme responses.BTW, when you program PPS frequencies as low as 8 or 10KHz, the only reason you can no longer detect these phase shifts is simply because you've scaled them down by some three orders of magnitude from the 8MHz case where these phase jumps are a mere, yet still problematic, 15 deg. There's no way you're going to see 15 millidegree phase shift jumps on any 'scope you're likely to own.If you care to go to the other extreme, you can program a jitter free PPS frequency of 12MHz and observe 90 degree jumps in phase. Belay that! My bad, I managed to misinterpret your statement. I'll get back to this once I've repeated your tests with my own module. It looks like all you've done is introduce a non-integer divisor induced jitter component that's masking or smearing out the corrective pad/skip pulse corrections. Methinks the "cure" is worse than the "disease" in this case.I was using a signal generator to trigger the scope on one channel with a Sinc waveform matched to the PPS frequency, adjusted to minimise the drift between them whilst displaying the PPS waveform on the other channel. When I triggered from the PPS signal set to 8010000Hz, I could see faint ghosts of jitter behind the more persistent trace. The best way to observe these corrective phase shifts is by triggering from a basic TCXO or OCXO clocked DDS AWG which can be adjusted to within milliHertz of the desired frequency. I'm guessing this was what you were doing. I'm only mentioning here and now it for the record to benefit any newbies reading this topic.Using a long time constant PLL to multiply a 10KHz PPS frequency might be a neat way to filter out this phase noise without the expense of a VCOCXO module if you don't mind the lack of a one ppb or better 'hold over' feature, typical of a full blown GPSDO. The NEO M8N can be programmed to output a matching frequency from its TCXO in the event of loss of lock on the GPS and/or Glonass satellite constellations but frequency accuracy will be degraded to that of the TCXO which may be anywhere from +/- 100ppb in error with a 10 to 30 ppb variation with temperature. Better than nothing but nowhere near the normal locked frequency output. However, this may be a cost effective starting option, there's nothing to prevent you adding a VCOCXO module at a later date while you're in charge of the design and build of your very own DIY "GPSDO".If you're after a cheap solution, a clever way to get a lock/unlock indicator without the minor expense of an Arduino nano is simply to program the free running output to a duty cycle of say 10 to 25 percent and monitor the average DC voltage with a comparator to drive the indicator LED (assuming you use a 50% ratio for the locked frequency output). Since this signal is going to be processed via a PLL and/or divide by two flip flop stage, the final output will remain a perfect square wave regardless.In this (navigation centric) application, there's simply no call for anything better (ie production of an ultra low phase noise polluted reference frequency) hence the need to add a separate VCOCXO disciplined with a long time constant PLL to filter out these phase jumps to obtain an ultra low phase noise polluted reference clock output fit to feed into the EXT 10MHz reference sockets on the rear panels of T&M and communications gear.Also, the benefit of using a disciplined VCOCXO module is that it can offer a hold over accuracy and stability matching the locked state whenever the GPSDO unit loses lock with the satellites for any reason, keeping it to within nanoseconds of GPS time over periods of hours or even days provided power to the GPSDO isn't interrupted in the meantime.However, I'm investigating a possible way to eliminate the expense of a separate VCOCXO module via the use of a 3N502 ultra low jitter PLL multiplier chip and an effective filter to turn the square wave output into a pure sine wave signal. The only fly in the ointment with the 3N502 clock multiplier chip is that, buried deep in the data sheet, it's only specified to a lowest output frequency of 14MHz in spite of the lowest clock input frequency being 2MHz and it having a lowest multiply option of two leading me to infer otherwise. I may need to multiply a jitter free 4MHz PPS signal up to 20MHz and divide it back down to 10MHz with a divide by two flip flop to hopefully get a more acceptable quality of 10MHz sine wave output.If this idea of mine fails to produce the goods, then at least I'll know why there's no cheaper alternative to investing in a VCOCXO module which won't leave me quite so distressed by the expense of such an "essential component", especially when this is tempered by the fact that it will provide my GPSDO with a much superior hold over performance than that of the module's own built in TCXO. With this in mind, there's no reason why you'd need to stick with a 50% duty cycle on the locked frequency output, you could use a 67% setting for the locked output and a 33% setting for the unlocked output, making the difference between the average DC voltage levels even easier to distinguish by the lock/unlock comparator circuit.JBG

author: rfspezi
author_rank: Regular Contributor
timestamp: March 21, 2019, 08:32:57 am
content: Quote from: Johnny B Good on March 21, 2019, 03:56:49 amThank you for the detailled experience report.Concerning the lock-indication - the dutycycle method is exactly what i am already using.I got a test setup running now which goes exactly the other way as "usual".I set the GPS module to ouput a frequency that produces as much as possible jitter and got an odd division factor. (140625 Hz is certainly not the best setting - maybe 19.2 MHz?, but for testing it's ok)That way i hope to get adjustment of the output signal in a faster, more fluent and finer manner.Since i have no frequency counter to measure it's performance, i can only compare it to my HP high stability OCXO.A block diagram is attached.I wonder what you think about it?[EDIT:]Attached the voltage curve of the Vfc input to the Trimble VCOCXO.The voltage variations were worse before i put the setup under a cardboard box, however it doesn't allow me to make a real statement about the frequency stability/accuracy.The frequency/voltage relation of Vfc is about 1.5 Hz/V.Power-on/cold-start of the setup was about 20 minutes before the recording started.[EDIT:]The 470uF should be a 100uF.

author: JBeale
author_rank: Frequent Contributor
timestamp: March 21, 2019, 10:54:02 pm
content: What I want for a GPSDO is to have a decent MTBF. I got a Trimble UCCM 57964-80 GPSDO board rebuilt into a little case with connectors for $120 from ebay and it works OK (OSC peak error within +/- 25 ppt) but looking at the Lady Heather screen I see that the DAC currently shows 11.03% and it has been dropping steadily 0.1% every day for two weeks. So in less than 4 months it will be dead and now it does not seem to be the bargain I was hoping for. Unless there is some way to retune the oscillator range.

author: texaspyro
author_rank: Super Contributor
timestamp: March 21, 2019, 11:12:54 pm
content: Quote from: JBeale on March 21, 2019, 10:54:02 pmThere is no documentation on what the UCCM DAC range is, but a lot of receivers use -100% to +100% with 0% being the center of the DAC range.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: March 22, 2019, 02:15:41 am
content: Quote from: rfspezi on March 21, 2019, 08:32:57 amWhilst considering your reply, I happened to notice out of the corner of my eye (when I'm sat at the computer, my workbench is behind me so I'm using a shaving mirror as a rear view mirror to keep an eye on the 'scope display without having to swivel round) that the GPS module had lost lock by displaying the 10% duty cycle waveform I'd chosen as an unlocked indicator just over a day ago.This the very first time I've seen it lose lock since then so it was quite a coincidence. I swiftly switched to the winXP VM where I've got the u-Blox u-centre software running to check what was happening. Strangely it was still showing 6 locked satellites at reasonable strengths considering the module is only using the patch antenna rather than the external antenna which doesn't stretch to the workbench right now. Even odder, I noticed a couple more unlock events whilst the u-centre software was still blithely claiming good locked signals from all 6 satellites.Clearly, such indications of satellite signal strengths and quality are no reliable indicator of "Lock". The rather cunning idea of using different duty cycle ratios to indicate the locked/unlocked state looks to be an effective and reliable way to indicate the true state of 'Lock'. I'd have the Arduino nano monitor this hardware signal at a high priority to issue any 'unlock alarms' rather than relying on it to interpret this event from any status messages issued by the module.140625 certainly is an odd division factor on the 48MHz TCXO. That calculates to 341.33333 by my reckoning!I think 19.2MHz may be too much of a stretch for the M8N. The jitter on a scope trace of the 10MHz is more a cosmetic issue than any particular detriment to disciplining a VCOCXO via a long time constant PLL. The once every second or so corrections need to be filtered out just as much as any non-integer divisor induced jitter would need to be filtered out.Such jitter might help mask or dilute these corrective phase shifts. I still haven't figured whether I was witnessing a 'scope trace display artefact or an actual smearing of these phase shifts into a lower level of broad band phase noise analogous to the use of dither noise in CDDA processing. The M8N module is compensating by using the smallest width pulses (the 48MHz clock pulses) to achieve the finest granularity of control so I can't quite see how the choice of output frequency, whether it's jitter free or not, would effect this process.However, this is yet another configuration option I can test with the 3N502 clock multiplier chip in my search for an alternative to incorporating a VCOCXO to generate an acceptably clean 10MHz waveform. Normally, I wouldn't expect problems with satellite reception sufficient to cause the module to unlock so, for me, reversion to the unlocked frequency output would be an acceptable temporary downgrade by virtue of its sheer rarity (at least for now - I would normally be using an external active antenna to feed it with strong reliable signals making such events virtually non-existent).As for your block diagram, the only unusual feature is your choice of PPS frequency but other than the fact that it's obviously working quite well (the Vfc plot suggests a p-p variation of around half a ppb), there was no obvious reason to think why it wouldn't. However, I've no actual expertise in laying out PLL circuits (yet) so you can take that for what it's worth.An OCXO should be more than adequate to let you observe what's actually happening on the PPS line at jitter free integer division ratios such as 2, 3, 4, 6, 8, 12 and 16MHz. If that HP OCXO isn't part of a signal generator but just the OCXO module by itself, assuming it's a 10MHz module, you can still use the raw, jittery 10MHz PPS signal to re-calibrate it if necessary. before using it as a trigger reference to observe other harmonically related frequencies such as 2 and 4 MHzHarking back to my opening remarks about seeing the GPS module losing lock on the satellites despite u-centre's claim to having a lock on 6 of the blighters, I now have the following observation to add.At 19:22 UTC, the GPS module dropped out of 'Lock' and, when I brought the u-centre program up a couple of seconds later, I'd caught it with its knickers down. That is to say, no satellite signals other than two or three showing blue bars. A few seconds later, five green bars lit back up followed shortly thereafter by the GPS module reverting back to the locked state. This is something I've never ever seen before. I think I may have witnessed a system-wide event rather than a local problem with my GPS module, hence the time reference for anyone prepared to check their GPSDO logs.JBG

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: March 22, 2019, 02:57:41 am
content: Quote from: texaspyro on March 21, 2019, 11:12:54 pmThat's an interesting conundrum. Hopefully, you're right about JBeale's situation but, even so, it does look an alarmingly high drift rate notwithstanding that it will have reached the halfway point rather than the end in about 4 months time. The ageing effect slows down with, well, age so hopefully it won't have reached the end of its tether in a mere 44 months time. I guess JBeale is going to find out whether it's got another 40 months or longer when it hits the zero percent mark in four months time. Fingers crossed that it starts showing negative values as it crosses that point.I find this interesting simply because one virtue of a used VCOCXO module is the fact of its pre-ageing having already slowed the pace down somewhat. However, this does assume it has been handled with TLC during the recovery process and subsequent shipping to you the end user and still has a useful ageing margin left to give many more years of service.Being the cheapskate that I am, this is an important question to me. Do I 'cheap out' on a used module or do I blow a wad of serious (to me) cash on a brand new pristine item that's had lavish TLC in both its manufacture and subsequent delivery into my hands. Choices, choices, choices. I think you can guess which option I'll be going for, after all the clue is in the above statement.Still, any advice on this subject will be most welcome.JBG

author: texaspyro
author_rank: Super Contributor
timestamp: March 22, 2019, 03:25:14 am
content: If your GPSDO has not been run in a while, it can take a couple of months to settle down. A few months ago I fired up a NTBW that came off the boat from China a few years ago. The DAC was falling at over 5-10 mV per day for three months, then it settled in at +/- 1 mV/day.If your GPSDO EFC had been stable for a long time before, then just starting dropping rapidly... you might have a problem.

author: FransW
author_rank: Frequent Contributor
timestamp: March 22, 2019, 10:43:40 am
content: Quote from: Towger on March 16, 2019, 08:33:08 amAnyone with info about this LMU300 or documentation?I could not find anything.Thanks in advance, Frans

author: Towger
author_rank: Super Contributor
timestamp: March 22, 2019, 11:11:08 am
content: Quote from: FransW on March 22, 2019, 10:43:40 amIt contains the newer version of the 'Trueposition GPSDO', the rest of the unit probably cannot be used for much except parts. There are multiple threads about it here and other sites.Beware the Packrat PDF document, it is useful but contains multiple mistakes. e.g.:The GPSDO is 3.3v RS232, but they connect it directly to a 5v Arduino.They state the old version of the GPSDO needs 13.5 to 15v, while it is actually 12v. They probably needed the extra voltage to get it working with the 5V ArduinoThe pin out for the new version says 'Pins 5,6 TX', but 7 and 8 are TX.

author: FransW
author_rank: Frequent Contributor
timestamp: March 22, 2019, 01:12:55 pm
content: Hi Towger,Thanks. I am aware of Packratt and the True Position GPSDO. No problems there.However, the question was about the LMU300 19" module.The only info was in the Time-Nuts archeve which I could not directly get.So, the question remains, what does the unit contain and what exactly is the functionality of the various units on the board?FransThis the person that was offering info: Paul - K9MRHowever, I do not know how to contact him.Anyone?

author: JBeale
author_rank: Frequent Contributor
timestamp: March 23, 2019, 06:11:18 pm
content: Sorry about further hijacking this thread, but there is some TruePosition LMU300 info here:https://www.mail-archive.com/time-nuts@febo.com/msg86139.htmlQuoteSounds like significant work is required for it to be of much use, I guess that's why it's relatively cheap.In other news, after reading the replies here about my Trimble UCCM GPSDO (which I've only had 2 weeks so far) I plotted the DAC % values from the log, and yes it's not quite a straight line, there is a hint of levelling off or at least a slight improvement over the past week in the downward drift, so maybe it won't crater after 4 months after all. I'll keep my fingers crossed.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: March 27, 2019, 04:27:15 pm
content: Quote from: rahandford on March 15, 2019, 05:54:29 amThat tiny 3.2 by 2.5 mm smd TCVCXO chip for starters! Did you not realise just how tiny this thing was?For a home hobbyist project, the smd XO chip does seem rather out of place imo. Are you trying to limit the appeal of this project to just those with regular access to high grade SMD reflow workstations?I'd suggest using a VCOCXO in a metal canned DIP package since not only is it easier to work with, it's also more readily handled both as something rather more tangible for the 50 to 100 dollars' worth of investment than an itsy bitsy smd component that could disappear down a crack in the floor should you sneeze it off the bench plus the fact that it's a lot easier to retrieve from the circuit board should you have a rethink of the board design in favour of a completely new PCB layout. A tiny smd part designed for a mobile phone would be my last choice for a GPSDO project like this.BTW, apropos of OCVCXOs, I'm currently looking at Digikey UK's offerings right now and seriously thinking of blowing some 40 to 60 quid's worth of my hard earned. However, I'm a little dismayed at the seemingly low stability performance of even these ovenised VCXOs (5 to 50ppb figures). I'm hoping this is the basic 'stand alone' at a fixed Vfc setting with no external assistance to hold them to within 50ppt and that such, seemingly unimpressive ppb figures are normal for OCVCXO modules typically used in a GPSDO. Really, I'm just after some clarification and assurance on this point before I splash the cash.I've finally come to realise just why a VCOCXO (or at the very least, a VCTCXO) is an absolute requirement to make a GPSDO based low phase noise 10MHz frequency reference suitable to feed the external 10MHz reference input sockets on the back of communications and high end test and measurement kit, hence my new found interest in VCOCXO modules (I have a u-Blox NEO M8N based module as the basis of my very first GPSDO project).An interesting fact about this u-blox module is that, whilst it can output 18MHz clock signals (and even 19.2MHz - checked it just now) on its PPS line, the 16MHz option produces a rather curious toggling on each phase correction event between a 33 and 67 percent duty cycle as a consequence of this division by 3 ratio of its 48MHz TCXO.Trying to avoid the clock jitter effect such as typically observed on the 10MHz frequency setting by choosing only integer based division ratios for output frequencies such as 1, 2, 3, 4, 6, 8 and 12MHz will be doomed to failure if the integer is devoid of any even number factors such as that factor of 3 on its own in the example above. Configuring a frequency of 9.6 and 6.857143 MHz ( factors of 5 and 7) for example, produces the same toggling effect as witnessed in the 16MHz case (Oh! The new found joys of 'scope ownership!). I've no doubt this will also apply to other stand alone odd division ratios such as 9, 11, 13 etc. but ICBA to do any more tests. I tried tracking down used VCOCXO modules on Ebay but there are pitifully few to be found and of those, the Chinese sellers don't seem to understand the concept of providing meaningful technical data on what appears to be obsolete kit that the manufacturer no longer offers any data sheets for. Unlike Victor Kyam, I wassounimpressed by these sellers, Ididn'tbuy the product, hence my trawling sites like Farnell and Digikey with a preparedness to spend somereal(by my standards) money.JBG

author: jpb
author_rank: Super Contributor
timestamp: March 27, 2019, 07:21:34 pm
content: Quote from: Johnny B Good on March 27, 2019, 04:27:15 pmThe Ublox timing application note discusses it a bit but their example is 8MHz so the extra divide by 2 probably evens things out.https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_%28GPS.G6-X-11007%29.pdfA couple of sources of VCOCXOs on ebay that are worth looking at are :(The first I haven't bought from but they look interesting and have data)https://www.ebay.com/itm/1-x-CTS-OCXO-Crystal-Oscillator-Model-1190027-3-3V-20MHz/273776000063?hash=item3fbe523c3f:g:saMAAOSwMDhcZtYH&autorefresh=trueandhttps://www.ebay.com/itm/UCT-OSA-MY-108663-01-10MHz-12V-Double-Oven-OCXO-Crystal-Oscillator/264230742882?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649I've bought three of the 108663 which are equivalent of the Oscilloquartz 8663 I think. I'm currently measuring them at the moment.They are new old stock and are taking a little while to settle down.

author: rfspezi
author_rank: Regular Contributor
timestamp: March 27, 2019, 08:32:14 pm
content: By the way... the NEO-M8N maximum clock output is 24 MHz.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: March 29, 2019, 12:53:54 am
content: Quote from: rfspezi on March 27, 2019, 08:32:14 pmI've just tested that. Any frequency selection in 1Hz steps, from 1Hz, all the way up to 24000000Hz, will produce a valid result. The jitter will vary depending on the programmed frequency but the averaged frequency output will match the programmed output with a precision that improves with ever longer integration times. Trying to set a frequency of 24000001Hz or higher via the u-centre application will be rejected as invalid.The jitter free output frequency options include this 24MHz maximum which will now exhibit corrective phase jumps of 180 deg. The next lower set of jitterless frequencies in descending order, all derived from even numbered division ratios of the 48MHz TCXO, are:- 12, 8, 6, 4, 3, 2, 1.5, and 1 MHz before dropping into the KHz range with corrective phase jumps corresponding to:- 90, 60, 45, 30, 22.5, 15, 11.25 and 7.5 degrees, getting ever smaller with each reduction in frequency below the 1MHz mark. By the time you get down to 15625Hz, these corrective phase jumps (correctionally applied jitter) will have shrunk to a mere 117.1875 millidegrees.If we take the idea of using an even integer value of divisor to the limiting value of 48000000 to generate a 1Hz output, these corrective phase jumps will be reduced to just 7.5 microdegrees. Since the rate at which these corrective phase jumps seems to vary from around 5 per second down to one every 5 to 10 seconds (and possibly much longer, given the right conditions) with the particular example of M8N module I've been testing, it would seem the key thing in phase locking a VCXO (OC or TC type) to the GPS reference is that of using a PLL with relatively long time constants in the range of tens of seconds or longer, regardless of which frequency has been chosen to lock the PLL onto.If you're going to forego the microcontroller with 16 bit DAC option to drive the Vfc line and just use a PLL directly, the best you can do for the 'hold over' function is to have a separate manually calibrated 'hold over' voltage reference for the lock/unlock detection circuit to switch the VCXO module to, requiring that you regularly adjust this 'hold over' voltage source on a routine basis to match the control voltage output being generated by the PLL in the locked state. This might mean daily calibration/adjustments during the initial weeks of the commissioning period until the VCXO has settled down to the point where once a week calibration checks/adjustments will suffice.For hobby use, this will probably be a satisfactory solution in a home built GPSDO, especially when the microcontroller with DAC option can be added as a later upgrade to complete the GPSDO project, otherwise, the alternative would be to allow the 48MHz TCXO of the module to provide a degraded reference during any such loss of lock events as a "Better than nothing" choice to maintain the supply of 10MHz pulses, hopefully for only seconds to minutes per once in a blue moon event that may never happen barring having the GPS antenna knocked out of commission by storm damage or sabotage, whether by neglect or malicious intent.For a home built GPSDO, you can be as pragmatic as you like when it comes to projects like this. There'll be ample opportunity to upgrade it later on after you've learn't a few valuable lessons in the art of designing and building your very own GPSDO.JBG

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: March 29, 2019, 01:37:57 am
content: Quote from: jpb on March 27, 2019, 07:21:34 pmI had a read of that u-blox app note. I noticed it didn't fully explain the issues but it said enough to confirm that my own conclusions on how the M8N module keeps the PPS synchronised to GPS (or UTC) time had been right on the money.That extra divide by two to generate the 8MHz doesn'tprobablyeven things out, it simplydoeseven out the 16MHz waveform irregularity created by dividing the 48MHz TCXO by 3 which the drop/add corrections so nicely displays as an alternation of the resulting 2:1 mark/space ratio on a 'scope trace. Probability doesn't come into it.I took a look at those oscillator modules you linked to but neither were of the VC type, both being simply OCXOs with, presumably some sort of trimmer to calibrate them manually. Both had been described as OCXOs and neither mentioned tuning voltage rates in their descriptions.OCXOs just aren't suited for use as a disciplined oscillator, there's simply no means by which a GPS referenced PLL can electrically fine tune the frequency of such modules. I wasted a fair bit of time looking at such Ebay traders' OCXO offerings before finally coming to the conclusion that there were only two or three far eastern traders offering VCOCXOs, typically used units, hence my turning to Digikey in desperation.As yet, I haven't placed any orders with Digikey, I'm still mulling over my options.JBG

author: texaspyro
author_rank: Super Contributor
timestamp: March 29, 2019, 01:44:00 am
content: The 8663 has an EFC input. All mine are on freq with am EFC in the 4 .. 5V range. The 8663 also has a very stable Vref ouput around 7V.

author: rfspezi
author_rank: Regular Contributor
timestamp: March 29, 2019, 09:07:33 am
content: Does anybody know, what best case ppb-range can be expected from a PLL based implementation?What i need from a GPSDO would be an "absolutely" clean, or at least cleaner than my best measurement equipments internal 10MHz clock.Otherwise i would only use the GPSDO to calibrate one instrument from time to time and use the output of that particulare one to sync all my other measurement equipment.For me it does not make sense to have a 10 Mhz reference with 1 ppt accuracy, but poisoned with phase noise etc. that ruins my spectrum analyzer measurements.I also wonder why the designs i saw so far don't care about apprupt changes in DAC value instead of smoothly adapting the EFC voltage to the new level.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: March 29, 2019, 01:53:53 pm
content: I have not seen an OCXO without an EFC input. (that is how you control the freq. externally, such as GPS) Can anyone show one? Seems no EFC would be a waste of an OCXO.The NOS MY 108663 on ebay seem quite full of phase noise. I have been burning one in for months.Maybe years of burn-in required? Can't touch the old used 8663-XS.

author: jpb
author_rank: Super Contributor
timestamp: March 29, 2019, 05:52:05 pm
content: Quote from: Johnny B Good on March 29, 2019, 01:37:57 amThey are both of the VC type. I know the 108663 are because I have three of them - they come with a Vref output that can be used to set the control voltage via a pot but you don't have to use it - you can just apply voltage.On the CTS ones, if you look at the Frequency Control section you can see that they are tunable over +/- 0.8ppm with a control voltage range of 0 to 3.3V. They even state that there is sufficient range to tune back to nominal for up to 10 years.

author: jpb
author_rank: Super Contributor
timestamp: March 29, 2019, 05:59:00 pm
content: Quote from: SoundTech-LG on March 29, 2019, 01:53:53 pmQuite a few OCXOs are non-tunable, you have to be a little careful when buying say from Digikey as often the same type have a V variant (tunable) and an F variant which isn't. I've also found quite a few on ebay are non-tunable or just physically tuned.Here is one example :https://abracon.com/Precisiontiming/AOCJYR-10.000MHz-M5625LF.pdfInteresting what you say on the phase noise. I'm measuring short-term stability and they are definitely worse than the old 8663-XS in my GPSDO but they have been getting better over the few weeks that I've been measuring them.Perhaps they take a long time to settle. Or perhaps these were all ones that were deemed failures which is why they are now being sold cheap.What power supply are you using? I found this made a huge difference on my measurements. I got very wavy results when using my Hameg supply and much better (smoother results) when using a Keithley supply or a purely linear TTi supply.

author: kj7e
author_rank: Frequent Contributor
timestamp: March 29, 2019, 07:40:48 pm
content: I'm currently testing 2 of the NOS UCT 108663's and two older 8663's. The 8663's short term stability drifted rapidly over the first few days, then leveled off quickly. The 108663's are slowly settling down, the drift is slowing more everyday. I've been taking EFC voltage readings twice a day for the last month. I suspect it will take months for the 108663's to really settle down. I don't have a way to measure the phase noise unfortunately.

author: texaspyro
author_rank: Super Contributor
timestamp: March 29, 2019, 07:49:19 pm
content: Here is some drift testing I did on a UTC8663 that I put into a 53132A counter:UTC 8663 mounted on Sweeny DOCXO card in a HP 53132A counter. The 8663 hadnot been powered up for over 10 years. Below is the frequency drift per dayas the 8663 aged in. THe 8663 is spec'd at 1E-10 (1 mHz) per day aging. Frompast experience, they are capable of much better than that (1E-11 to 1E-12 perday). They seem to be limited in this application by the EFC DAC / VREFperformance.The 8663 was mounted on the Gerry Sweeny aftermarket OCXO board for the 531xxcounters. For the first 30 days, the board was configured for the 0 .. 10VEFC DAC range. The Sweeny board powers the DAC with +/- 12V which is a bitout of the spec'd minimum +/- 13.5V required for optimum performance onthe 0 .. 10V range.The Sweeny OCXO board is based upon the HP design with a couple of nice mods.First it has footprints for several common OCXO modules. Second, it allowsthe DAC range to be configured for 0 .. 5V, 0 .. 10V, and +/- 5V.The circuit feeds the OCXO output through a differential line driver. The linedriver sends the OCXO to the HP counter mother board through a ribbon cable.The motherboard makes the OCXO output available on a BNC on the counterback panel. Checking the ADEVs of the raw OCXO outout and the counter outputshows that the 53132A degrades the DOCXO output by around 1.5E-12 (raw DOCXOvalue around 5.1E-12 at 200 seconds, counter 10 MHz output BNC around6.5E-12)For the first 30 days the EFC DAC reading was recorded and the HP53132A wasthen auto-cal'd and the new EFC DAC output was recorded. (The DAC readingsfor the first 10 days were lost when a momentary power failue occured and thesystem was not shut down cleanly).The initial 30 day run was done with the Sweeny board DAC configuredfor the 0 .. 10V range. Given the DOCXO EFC sensitivity (around 0.75 V / Hz)and the 12-bit DAC, the minimum auto-cal resolution is 0.002 Hz. Severalauto-cal cycles were performed each morning until one was within 0.001 Hz.DAC readings were made by a Tektronix DMM914 4.5 digit DMM, not the best toolfor the task, but it was available.drift (Hz/day) DAC (after cal -> 24 hrs later)day 1: .298000 Hzday 2: .057000 Hzday 3: .033000 Hzday 4: .022000 Hzday 5: .017000 Hzday 6: .011800 Hzday 7: .009640 Hz 4.471Vday 8: .008400 Hz 4.458Vday 9: .006380 Hz 4.451Vday 10: .003880 Hz 4.443Vday 11: .002840 Hz 4.446V -> 4.438V // power glitch caused resetday 12: .003470 Hz 4.437V -> 4.437Vday 13: .002100 Hz 4.432V -> 4.433Vday 14: .001590 Hz 4.430V -> 4.430Vday 15: .001380 Hz 4.426V -> 4.428Vday 16: .001660 Hz 4.424V -> 4.426Vday 17: .000526 Hz 4.422V -> 4.423Vday 19: .000284 Hz 4.421V -> 4.423Vday 20: .000038 Hz 4.423V -> 4.424Vday 21: .001019 Hz 4.420V -> 4.420Vday 22: .001350 Hz 4.417V -> 4.418Vday 23: .000402 Hz 4.417V -> 4.417Vday 24: .000953 Hz 4.418V -> 4.419Vday 25: .000304 Hz 4.417V -> 4.416Vday 26: -.000441 Hz 4.417V -> 4.418Vday 27: .000214 Hz 4.415V -> 4.415Vday 28: .000517 Hz 4.415V -> 4.416Vday 29: -.000998 Hz 4.415V -> 4.415Vday 30: .001300 Hz 4.415V -> 4.414Vday 31: .000423 HzPowered down, replaced 53132A fan, changed DAC range to 0 .. 5V, andthe 53132A was put back into case (hence no more DAC readings). The 0 .. 5VDAC range reduced the auto-cal minimum step size to 0.001 Hz. Hopefullyit should also improve the DAC stability.Warmup frequency error after power up:1 Hz - 3m0.1 Hz - 3m 25s0.01 Hz - 5m 45s0.001 Hz - 6m 15sAfter 24 hours the unit was auto-cal'd once and left alone. The frequencyerror for the next week:day 32: .000152 Hz (0.00009 Hz/day for last 18 hours)day 33: -.000825 Hzday 34: -.000730 Hzday 35: -.000287 Hzday 36: -.000144 Hzday 37: -.000086 Hzday 38: -.000083 Hz

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 05, 2019, 07:07:40 pm
content: Quote from: jpb on March 29, 2019, 05:52:05 pmWhen you're dealing with Chinese traders on Ebay, the possible lack of an electrical tuning pin is something to watch out for when you consider the cost of return postage involved in returning such items for a full refund. Although the electrically tunable OCXOs are the more common type, it's not something one should ASS-U-ME, hence my concern over the use of "OCXO" as a shorthand for "VCOCXO".Anyway, I stumbled across a 13MHz CQE OCXO at last Sunday's NARSA rally which I snapped up for a mere 4 quid. There was a disturbing lack of traders selling OCXOs, ICs and other electronic components at an event where I'd have expected the traders to be falling over themselves to maximise such a golden opportunity to sell their wares to a discerning market demographic.One trader said he had OCXOs in stock but didn't think there was going to be sufficient demand to warrant bringing them to this major rally event (). Worse still, he wasn't the only trader with a similar excuse for not bothering with selling the many ICs he had in stock, using the excuse that it wasn't worth the effort to sort the many hundreds of types that he had into organised packaging (again). Talk about a self fulfilling prophecy!Regardless of imposing such a self fulfilling prophesy over the past decade or so since I was last disappointed by this attitude amongst traders, if the market demographic has actually shifted so far to the consumerist extreme as to suggest that no one in the amateur radio and hobby electronics camp today now even so much as sully their hands with soldering irons and side cutters et al is actually true, then I am truly dismayed by this state of affairs.All that aside, it seems my punt on that OXCO has paid off. After searching for any info on my acquisition, I eventually discovered an image of a single A4 page alongside of the image for a very similar looking 13MHz Vectron OCXO on an Ebay trader's page which implied the use of 12 volts supply along with the accurate pin out and dimensions details.Knowing that there was a real possibility that my unit could be a 5 volt part which would be instantly fried by a connection to a 12 volt supply, I tried it initially with 5 volts which, surprisingly, after a 7 or 8 minute warm up at 280mA, suddenly started to output a clean 13MHz 3Vpp square wave (after outputing what I suspect may have been a 4.3333MHz signal during the warm up phase - the use of 3rd overtone SC cut crystals being fairly common in OCXOs). This transition was accompanied by a relatively swift plummet in heater current down to 200mA (20 seconds or so period) where it remained more or less the same.The Vref pin voltage was just over 3 volts which isn't enough to allow it to be used as a stabilised tuning voltage (trimming the Vfc pin voltage to get it within +/- 0.5ppb of frequency results in a reading of 3.31v on my 3 1/2 digit DVM. That plus the rather low ratio between warming up and on temperature heater current readings (280:200mA) left me wondering whether it actually was a 5 volt part or a 12 volt one so I tested it with a 6.9 volt supply (6.5v after losing voltage in my moving coil multimeter's shunt circuit on the 600mA scale).I noticed that the heat up current was limited to 280mA and the output voltage rose to around the 4.5Vpp mark rather than remain confined to a TTL level from a 5v internal regulator circuit. I temporarily held a 1.3v NiMH cell in series with the 6.9v (6.5v) noting the same 280mA current limit and an even higher output voltage on the 'scope. I didn't dawdle at this voltage (7.8v) for fear of burning out a 5 volt part with an absolute maximum overvolting stress figure of only 7 volts. I guestimated a trend towards a 10Vpp at 12v which seemed a rather improbable specification for even a 12v part.This and subsequent testing strongly suggests that, contrary to my initial expectations, this must be a 5 volt part after all, rather than a 12 volt part that has a freakish ability to generate stable on frequency output off a 4.88v supply. The key aspect that led me to even consider the 12 volt possibility being that of the rather low 1.4:1 ratio of warm up to warmed up heater current levels. Most of the OCXO data sheets I've looked at generally show a ratio of 2:1 or even 3:1, hence my "Dicing with death" supply voltage experiment.I've already sought advice on this OXCO in my "Ublox-NEO-M8N GPS navigation signal amplify module for arduino Rasppery PI" thread but, after two days, not had any takers. Since there is rather more interest being shown in OCXOs in this thread, I've attached the same set of images I'd posted in the other thread in the hope that someone may recognise the particular OCXO module that I've currently got lashed up to a couple of NB3N502 clock multiplier chips with a 74193 slung between them as a divide by 13 circuit to generate 10MHz from this 13MHz OCXO.The unit appears to be unused NOS (the pin out markings were done by me in accordance with the pin out data in the last image showing the Vectron data sheet page).JBG

author: jpb
author_rank: Super Contributor
timestamp: May 05, 2019, 09:07:17 pm
content: There are quite a few 13MHz OCXOs around - I guess because they were used in GSM (?). I bought several MTI ones (new old stock) a while ago simply because they were good OCXOs even though the frequency is a little awkward.I also bought a couple off this seller (Aspen electronics) who is UK based:https://www.ebay.co.uk/itm/OCXO-Vectron-13MHz-High-Stability-Oven-Oscillator-New-old-Stock/272806056479I didn't pay the full £9 as (at the time at least) they were open to offers.I've not extensively tested it but it seemed to work ok (the frequency varied about the right value when the control voltage was varied.Looking at your datasheet it seems the same as the Aspen one (on their ebay listing) and does correspond to what they are selling I think but rather different from the OCXO you show in your photo.Incidentally I also bought a N-type connector Surge supressor off Aspen electronics which they are still selling and are good value - especially as there was a muddle up and they ended up sending me two, I offered to send it back if they would pay the postage but they just told me to keep it.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 06, 2019, 12:18:47 am
content: Quote from: jpb on May 05, 2019, 09:07:17 pmYeah, the data sheet is identical, right down to the incorrectly stated 36.1mm width (I guess it should have read 26.1 afaicr from my own measurements).If you look past the label, I think you'll see a very strong resemblance to mine.Just out of interest, were your units 12 or 5 volt ones? Mine, afaict, seems to be a 5 volt unit. The warm up period seems very protracted. Almost eight minutes when I thought to time it with a stopwatch this morning after leaving it off overnight followed by accidentally stopping the watch prematurely after just two or three minutes when I overreacted to what I thought was the appearance of the stable 10MHz waveform on my 'scope which resulted in my switching it off for a ten or fifteen minute cool down before starting another warm up timing test.I don't suppose the mere 4.8v that manages to reach the OCXO via the solderless breadboard is helping this situation in any way. However, the fact that even at this low a voltage it's managing to warm up and start producing a stable 13MHz 3Vpp square wave bodes well for it being an actual 5 volt part, rather than just a freak 12v part which can manage to produce the goods even at such a low voltage, which can prove very useful in eliminating the need to have an additional 12v supply accompanying the existing 5 volt supply needed to power my planned GPSDO project.Powering from a 5.25v rail should reduce the warm up to something nearer to the more acceptable 5 minute mark and probably reduce the warmed up heater demand current from 200 to 170mA. I've tried the effect of packing foam rubber pieces top and sides as insulation which knocks around 30mA off its 4.8v current consumption so a reduction to 150mA on the higher voltage supply looks low enough that I might be able to use a 10MHz variant with a NB3N502 multiplier to substitute the 50MHz smd XO chip (currently substituted with a 0.1ppm TCXO module) in my cheap as chips FY6600 signal generator.I'd only considered using a TCXO as an upgrade to the execrable 50ppm smd XO chip Feeltech had planted just one centimetre from a bunch of three LDO regulator chips generating 70 deg C temperatures which kept it running at 50 deg C . I'd bought the TCXO as a module simply because it was the cheapest way to get my hands on a 0.1ppm 50MHz TCXO but once I realised how bad an idea it was to transplant the DIP14 oscillator into the original XO location, I simply mounted the TCXO module well away and wired its output via a short length of co-ax to where the smd XO had been.The use of an OCXO had originally been discounted on the grounds of its much higher energy consumption not only due to the additional heat load but also the demand for a separate PSU of its own to boot. Now, suitably thermally insulated, not only is the consumption looking low enough for the existing PSU to handle, the extra heat (circa 1 watt or less) is not going to be a problem either.Of course, this all assumes that I can get hold of a 10MHz version of this OCXO since the additional ICs needed to generate 10MHz from a 13MHz OCXO draw around another 70 to 80mA which may prove to be the final straw for the much put upon smpsu board during the warm up phase when the heater current is at its maximum limit of 280mA. Still, it's fairly straight forward to test the limits of the FY6600's PSU before committing to such an upgrade should I be so lucky as to get hold of a 10MHz version to play with.In the meantime, I guess my next step is to rehome my rat's nest wired prototype concoction onto vero board so I can do more meaningful stability tests and have something that I can shoehorn into a petite case alongside of my u-blox GPS module and some more ICs and/or that Arduino Nano I speculatively purchased when I bought the GPS module some three months ago. At any rate, I'll be able to prototype some PLL designs once I've freed up my solderless prototyping board for further experiments.Oh, and just to reiterate, what voltage were those OCXOs of yours? 12 or 5 volts?JBG

author: jpb
author_rank: Super Contributor
timestamp: May 06, 2019, 10:33:46 am
content: It took me a little while to dig them out (it was about 2 years ago when I bought them).They are 12V ones (which is stated on the data sheet).I connected one up as can be seen in the attached pictures - I didn't allow much time for warming up and anyway my counter is in use currently as I'm trying to do some long term ADEV measurements.The counter on my scope normally reads a little low (it reads 9.99998 MHz at 10MHz) so the frequency is a bit high (reading exactly 13MHz) when it should be a bit low normally with VC not set but as I say, I didn't allow any warm up.I did check them when I got them.The control should be between +/-1000 to +/-3000 ppb with the control voltage being between 0 and 10V so it should be changing between 13 and 39 Hz and unfortunately my scope counter is only six digits so doesn't quite extend far enough.EDIT: I should have added that the voltage is only 2.2V because I'm loading it with 50 ohms - it is above 6V on 10Mohm setting but the waveform is rather ugly. The best load I guess is 600 ohms or so but this was just a quick demo to give you some info.

author: jpb
author_rank: Super Contributor
timestamp: May 06, 2019, 10:47:07 am
content: By the way - if you're after low power these are only 120mW !! :https://www.ebay.com/itm/CTS-16-384MHz-OCXO-3-3V-Oven-Controlled-VFControl-50ppb-120mW-VFOV405-TCEDH/113700631221I bought one to play with. Unfortunately they are 16.384MHz which is good for a clock but not so handy for a 10MHz reference.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 06, 2019, 08:50:29 pm
content: Quote from: jpb on May 06, 2019, 10:33:46 amThanks JPB.The data sheet obviously matches those OCXOs of yours but I'm not so sure it's an exact match to mine. It looks like the one I got must be a later 5 volt version - the rest of the info, pin out and dimensions (width typo excepted) matches but not the tuning voltage spec unless my unit is indeed a 12 volt unit that freakishly also manages to work from as low a voltage as 4.79 volts (perilously close to the lower 5% tolerance limit for a 5 volt part).I rather think the reason for the 8 minutes and 21 seconds warm up before mine springs into life (retested it again this morning) is probably on account the 430mA warm up current drags the voltage down below the -5% tolerance limit. I didn't think to check the voltage but since just a few seconds disconnection from supply leads to tens of seconds of another warm up phase, it's easy enough to run a quick voltage check... which reveals a further one tenth of a volt drop but there are so many iffy connections, the slightest disturbance to the USB connectors can vary this by +/- 0.1v making it rather difficult to verify whether an out of tolerance voltage condition is really the cause of this delayed output phenomena.The fact that a control voltage of 3.31 tunes it exactly to 13MHz, and that it can produce a 3Vpp square wave from a 4.8v supply and the output voltage is raised by the same voltage boost over and above this 4.8 volts (i.e. 6.5v supply produces a 4.5Vpp signal) leads me to the conclusion that it's a 5 volt rather than a 12 volt OXCO in spite of the quite protracted warm up period before it even produces any sensible output at all.It's not merely a case of a 3Vpp at slightly the wrong frequency during the warm up, it's a complete lack of output other than a low amplitude unstable high frequency riding on top of a much lower frequency square wave, possibly the fundamental of a 3rd overtone SC cut crystal. If I'm correct in my assessment, then applying 12v is guaranteed to destroy my 4 quid bargain in an instant. Since, aside from the protracted warm up period, it seems to be working just fine off a 4.8v supply, it seems my best bet is to assume that it is a 5 volt part that is best not subjected to a 12v supply.The best I can hope for is for someone who has been running a seemingly identical OCXO off a 12v supply for months or even years, to decide, purely out of curiosity you understand, to test them with a 5 volt supply to see whether they'll display the same or similar behaviour as my particular (peculiar?) example.Apropos of those ultra low power (by the conventional standard of OCXOs) 16.384MHz clock OCXOs, nice as that is, ultra low power is at the bottom of my list of priorities in my current GPSDO project. I only mentioned the relatively modest power requirement of that 13MHz bargain buy on account of the possibility of now being able to use a 10MHz example in my Feeltech FY6600 without having to add a second dedicated PSU board as would have been required to power a 12v 3W (warmup demand current rating) OCXO module which complication led me to choose that 0.1ppm 50MHz clock power module as a low power requirement compromise between the existing low quality smd XO chip and the ultimate in OCXO accuracy and stability. ... Several hours later:I decided to power the YwRobot Breadboard Power Supply from a 9v wallwart instead of via its USB port, only to discover that the AMS1117-5 was only producing 4.81vI was no better off so I perused the 1117's data sheet and lifted its ground/ref pin, bunged a 33R in series and, after an initial test to determine that I needed to add a 370R between the output and the Gnd/Ref pin to raise the voltage by almost another half volt, got the soldering iron warmed up again and completed the modification.Now I'm getting 5.31 to 5.39 volts depending on how hot the regulator chip gets - much better to be on the high side than on the low side - less waste heat in the LDO regulator for a start and I now see a 13MHz output (initially slightly off frequency of course!) within seconds of powering it up. This is more like it!Almost instant, though slightly off frequency, output followed by the warm up phase onto frequency more typical of normal OCXO startup behaviour. I guess my NOS example must be far less ancient than that 1997 data sheet would imply after all.JBG

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 19, 2019, 04:15:45 am
content: Quote from: Johnny B Good on May 06, 2019, 08:50:29 pmFurther developments in my cheap OCXO saga:I've more or less determined that my bargain of the century 13MHz OCXO acquisition last month is in fact a 5 volt part after all. Disappointingly, those 10MHz versions that I was alluding to turned out to be 12 volt parts (they were identical in appearance to my 13MHz example but as everyone knows, looks can be deceiving).One of the results of my search for a definitive data sheet for that 13MHz OCXO was the discovery of a very cheap source of 10MHz versions of this CQE branded OCXO from a UK supplier who, unbelievably, CBA to test them or track down a data sheet so was selling them off for a mere £4.99 each (plus £2.00 P&P one off or £1.0 per additional unit on a multiple order), claiming them to be unused NOS.<https://www.ebay.co.uk/itm/CQE-CRYSTAL-OSCILLATOR-10MHz-REDUCED-TO-CLEAR/253081992039?hash=item3aecdcbb67:g:x1gAAOSw~qNZh2rl><https://tinyurl.com/y6pl2xvk>At the time, I'd already ordered a set of three and was waiting on their delivery. I didn't want to mention this source for fear of him being inundated with orders and the stock drying up. Those first three cost me 4 quid in postage but I didn't want to order a "Lifetimes's Supply" in one go just to save a couple of quid in case they turned out to be not such a wise investment after all. As it turned out, all three performed just fine although I was a little suspicious about the claim for them being unused NOS (the pins didn't seem to be quite as pristine looking as my initial 13MHz rally purchase).Since I now had a 10MHz OCXO plus a couple of spares, after testing them out on a 5.33v supply and discovering after several minutes delay a 10MHz 600mVpp sine wave output finally putting in an appearance, I decided it would be well worth the risk of applying 12 volts to the Vdd pin to prove my, by then, very strong suspicion that these were the more usual 12 volt parts. The risk taken paid off handsomely. I've also been able to ascertain that, apart from an ac coupled 4Vpp sine wave output, the 13MHz Vectron data sheet I posted pretty much applies to these 10MHz CQE branded clones (including the 0 to 10 volt tuning range with positive sense).Once I had satisfied myself as to the worth of ordering more of these OCXOs, I placed an order for another four to secure myself a bargain basement lifetime supply. I've since used one of them (with a NB3N502 clock multiplier chip) to upgrade the TCXO upgrade to my FY6600's original 50MHz SMD XO chip. Of course, this involved adding a 12v 300mA smpsu (extracted innards from a wallwart) to power the OCXO board - the existing +12v rail has insufficient reserve to handle such abuse.However, the nice thing about this extra psu complication that I'd initially hoped to avoid is that I can shut the generator off rather than just put it into standby to keep the OCXO up to temperature since its smpsu is wired directly to the IEC C6 mains inlet socket to stay powered up whilst it remains plugged into a mains supply (it only takes 1.3W once up to temperature). Placing the generator into standby only drops the consumption from circa 7 to 9.7 watts down to 5 watts (plus the 1.3W of the OCXO), so I gain an advantage from my pain of adding a second smpsu to the box as well as a very slight reduction in the loading on the original 3 rail smpsu board. What's not to like about a mod like this?Incidentally, my suspicions about these 10MHz CQE OCXOs being unused NOS were confirmed when I received the last four units. A couple of them had the remnants of a through plated hole on a pin each disproving the 'unused NOS' claim. However, since they tested good with tuning voltage settings for exactly their 10MHz frequency ranging from a low of 2.32 to a high of 4.51 volts out of a 0 to 10 volt tuning range, at a penny shy of a fiver each, I could hardly complain at this modest bit of mendacity on the part of the seller.However, I can certainly question the need to mislead. The question in this case being why he felt the need to disguise the fact of them being used items at the price he was asking in the first place when such readily evident mendacity could only undermine his credibility. Still, at least you can be on your guard over this aspect of their condition if you decide to take a chance on such a cheap and local (for UK citizens at least) source of 10MHz OCXOs.I still have to lash my GPS up to one of these OCXOs to create a GPSDO better suited to calibrating the FY6600's OCXO upgrade but I thought it best to apply the OCXO upgrade to the FY6600 first before sorting the GPSDO project out since its enhanced frequency stability over the initial TCXO upgrade makes it an even more useful test tool for the next stages in this project.Another contributor to the FY6600 thread, 'Arthur Dent', had added a 10MHz OCXO with a NB3N502 clock multiplier chip to his FY6600 over a year ago and included an external 10MHz reference socket with a change over switch. I plan to do almost the same modification but without the complication of a change over switch.I recently spotted a reference to the use of injection locking a Wien bridge oscillator (very low harmonic distortion and spurious free sine wave oscillator) to a high stability XO and discovered literature on the use of such injection locking being used to synchronise XOs although I couldn't find any examples of the technique being applied to real world OCXOs.However, as far as I can see, it shouldn't be too difficult to injection lock a free running OCXO to an external reference source that's within 1ppb of its frequency. In my case, I'd expect my OCXO to be well within half a part per billion so quite amenable to this technique of getting it to lock into sync with an external 10MHz GPSDO reference without the need for a change over switch or the risk of any 'hand over glitches' that may crash the FY6600's FPGA logic during such transitions back and forth between its internal and external 10MHz reference sources (and fall back gracefully onto its internal source should the external reference fail unexpectedly for any reason whatsoever).I started a new thread yesterday called " Injection locking the 10Mhz OCXO to external reference (upgrading a FY6600)" but so far haven't had any takers despite my two attached photos having been viewed 14 and 13 times. It rather looks like I'll have to run my own experiments to find the best method of injection locking the internal TCXO to the 10MHz external source.Mention of those photos reminds me that it would be worth attaching them here for a bit of light entertainment. The first will be obvious but the second needs an explanation. Basically, out of frustration with having to make do with just the raw 10MHz PPS output as a calibration reference with all the non integer divisor induced jitter as well as the distracting effect of the sawtooth corrections applied by the GPS module to keep synced with GPS time, I made up a crude crystal and resistor filter using a couple of 10MHz crystals I'd bought at that radio ham rally last month to try and filter out the worst of the jitter on the raw PPS output.Rather surprisingly, it worked better than I'd had any right to expect. The picture shows the setup with just sufficient resolution to reveal the details and the effect on the scope traces. However, the still image can't convey the ever present though very toned down sawtooth effect which is now less of a jitter and more a swaying effect. It makes it just that little bit easier to adjust the OCXO to within 100ppt.However, since it is a direct output from the GPS's PPS line, it does suffer from variations in phase as reflected by the deviation map plot variations of +/-3 or 4 metres in the u-centre app which equates to some +/- 10 to 12ns phase shifts which makes trying to calibrate an OCXO to within 50ppt rather problematical (also, the sawtooth doesn't help either). As things stand right now, it's largely a matter of luck getting the OCXO tighter than 50ppt. I need a GPSDO source before I can do any better and get a more accurate idea of the ageing drift rate of these OCXOs. Hopefully, it won't be much longer before I finally get my GPSDO project completed sufficiently enough to properly calibrate and check these OCXOs but for now, I'm having to make do with a filtered 10MHz PPS signal.JBG

author: jpb
author_rank: Super Contributor
timestamp: May 19, 2019, 01:18:32 pm
content: Thanks for the link.Actually I'd come across them before (he also is selling a much more expensive one for £25 and has been doing so for at least a year) but I'd not bought as there was no info and I failed to link it to the datasheet that you've linked it to.I feel tempted to buy some but I've already purchased lots of OCXOs and now mainly try to seek out things like the HP10811 or perhaps a Wenzel but these are ridiculously expensive on ebay.Perhaps getting a few more to play with would be fun though.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 19, 2019, 06:53:14 pm
content: Quote from: jpb on May 19, 2019, 01:18:32 pmAt the price they were, I rather thought they'd make very nice toys for anyone with a cheap GPS module to hand. I mean, just how bad do they have to be before they're considered a total waste of money?I'd say "totally broken is just how bad imo." which isn't so bad in this case when, for me dealing with a local UK based trader, with Ebay's refund policy as a backup option, I don't have the hassle of dealing with an inscrutable Chinese operator almost half a world away to contend with.As far as I can tell, all seven examples are fully functional without even any signs of gross (or, indeed any) physical abuse so often evident in the photos offered by many Chinese purveyors of used recovered stock. I've no means of measuring phase noise performance but none have exhibited what I suppose might be considered alarmingly high rates of ageing related frequency drift and, imv, seem to have several years worth of life left in them, quite possibly another decade or more's worth.Having managed to secure my own personal 'Lifetime's Supply', I felt it only fair to disclose my 'secret find' for all to take advantage of. There must be a quite a few impoverished hobbyists out there who would be only too happy to invest a mere five quid (plus the two quid P&P charge) in something that would typically be three or more times as pricey and, like me, still leave serious doubts about their condition as well as the (tr)additional 3 to 6 week journey on a slow boat from China to cope with.Although the original 13MHz unit requires another three ICs to utilise it as a 10MHz OCXO source, I'm still considering it for use in the GPSDO project since it eliminates the need for a 12v rail (besides which, it would otherwise just go to waste in the spare parts drawer if I didn't make the modicum of effort required to put it to good use). The additional half watt from the three extra ICs on the power budget in this case, unlike the situation with the FY6600, isn't going to be an issue and I can substitute one of the two expensive 3N502s with an XOR gate and RC delay for the first times two multiplier stage to save it for a more deserving and productive fate.However, it looks like I've found another little side project to distract me from proceeding with that plan in the form of experiments to work out how best to implement my 'injection locking' these 10MHz OCXOs to a GPSDO reference using, for the time being the filtered raw PPS 10MHz output from the GPS module which may prove to be a halfway house between a full blown GPSDO implementation and that basic GPS based 10MHz calibration reference of Scully's <https://www.youtube.com/watch?v=lbns-FvpzK4> which had inspired me to buy the u-blox M8N module in the first place.It might be another week or three before I manage to finally concoct something that could actually be described as a GPSDO (of sorts), assuming I don't find yet another distraction to play around with in the meantime.JBG

author: jpb
author_rank: Super Contributor
timestamp: May 19, 2019, 09:18:57 pm
content: I'm sure, from what you say, they are fully functioning and at that price good value. It is just in my case I probably have around a dozen similarly specced ocxos I've acquired over the years so they would just add to the pile and make me feel guilty that I'd not got around to doing anything with them yet!

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 20, 2019, 02:00:08 am
content: Quote from: jpb on May 19, 2019, 09:18:57 pmI know exactly where you're coming from... now!I've just totalled up my spend on these OCXOs and it comes to 7 pence shy of 44 quid. However, to put that into perspective, I just have to remind myself that I'd been poised on the brink of shelling out some 45 quid or more plus 20% VAT to Digikey for a cheap brand new Abrams(?) OCXO just six weeks back. I'm glad I stepped back from that particular brink. For almost the same expenditure, I've got a set of seven from which to choose when I can eventually run the required tests to pick out the best of the bunch (assuming there is much to choose between the 'best' and the 'worst').Brand new, as Texaspyro pointed out, doesn't always guarantee a better quality of OCXO so having a few spares to play with for less expenditure gives me options I just wouldn't have otherwise had with a single brand new unit costing more than I ultimately spent. I might never use more than just the first two I've already earmarked for the sig genny and a GPSDO project but I've at least got the assurance of plenty of spares on hand to let me experiment at my leisure.Building ever better GPSDOs could become a bit of a hobby in itself so having spares will save me having to rob the earlier builds of their OCXOs. If I'm going to go down that road, I'd like to have at least three generations of GPSDO running to allow meaningful comparisons. I might land up using a lot more of my 'Lifetime's Supply' than just two or three.One can never really predict future usage so it just seemed a good idea to take advantage of such low pricing to amass an ample contingency reserve without breaking the bank. That 13MHz sample I'd bagged at the rally just three weeks ago turned out to be a much better investment of the 4 quid I'd paid for it since my searching for a datasheet led me to discover a source of very cheap 10MHz versions which I may otherwise have turned my nose up at for the lack of experience of this particular brand of OCXO. My collection is now large enough that I think I'll be able to resist further temptation to expand it any further (at least for the foreseeable future).JBG

author: jpb
author_rank: Super Contributor
timestamp: May 21, 2019, 08:33:44 pm
content: In a fit of extravagance I just ordered 4 to add to my collection. He was down to the last 7 and I worried that I might regret not getting some.I'm working on improving my ADEV measurements, and these will provide more things to measure and, as you say, it gives flexibility for designing GPSDOs and other things.

author: ignilux
author_rank: Unknown
timestamp: May 21, 2019, 09:13:56 pm
content: Just out of sheer curiosity I went to check how much it would be to ship to the States...I now have two on the way

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 21, 2019, 11:38:21 pm
content: Quote from: ignilux on May 21, 2019, 09:13:56 pmIt'll be interesting to see what you and jpb think of them once you've had a chance to check them out. In the meantime, I'm stuck without a working GPS module on account of my foolishly hitting the PPS line with a dose of 12 volts in the wee small hours yesterday. It's a long story which I won't bore you with here. You can find my tale of woe in my GPS navigation module thread here if you wish to satisfy your curiosity:-https://www.eevblog.com/forum/projects/ublox-neo-m8n-gps-navigation-signal-amplify-module-for-arduino-rasppery-pi/msg2426871/#msg2426871https://tinyurl.com/y6mvst5g(I've just figured out that the standard < and > hyperlink delimiters have been confusing the "insert hyperlink" button function on this web site's messaging system.)Suffice to say I've now got a cheap NEO-6M on order, along with a couple of USB to serial adapters (PL and CH but no pricey FTD ones) and SMA flylead adapters to tide me over. They should arrive by the end of the week (all bar one Chinese sourced order of flylead adapters at any rate).In hindsight, the worst of that fiasco was that I could have experimented with just a couple of those OCXOs without going anywhere near my precious GPS module.Never mind, treat yourselves to a bit of a laugh at my expense. At least the 21 quid I blew will be going to a good cause. I can laugh about it now - there's no point in crying and, besides, it's only a tiny fraction of "The Kid's Inheritance" I'm slowly burning my way through.JBGPS Oh BTW, does anyone have a GPS module to recommend? Perhaps a timing one. I might as well try and take advantage of the enforced GPS module replacement as an upgrade opportunity.

author: texaspyro
author_rank: Super Contributor
timestamp: May 21, 2019, 11:56:54 pm
content: Quote from: Johnny B Good on May 21, 2019, 11:38:21 pmSomething Ublox F9T based (around $200) or Furuno GT-8736 (around $60) ... but, I suspect you are a bit wimpy, and can't handle the performance

author: Theboel
author_rank: Frequent Contributor
timestamp: May 22, 2019, 12:38:44 am
content: Quote from: texaspyro on May 21, 2019, 11:56:54 pmHi Mark,Have You Try F9T ?

author: texaspyro
author_rank: Super Contributor
timestamp: May 22, 2019, 01:48:32 am
content: Quote from: Theboel on May 22, 2019, 12:38:44 amYes! I now Heather working with the F9T and F9P.Supposedly Digikey now has F9T modules and should soon have eval boards.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 22, 2019, 04:29:55 pm
content: Quote from: texaspyro on May 21, 2019, 11:56:54 pmIf "performance" is a euphemism for "expense", then yeah, you got that right!I'll have a search for "u-blox F9T". Something to drool over should offer a bit of distraction therapy.That was a short 'drool'. I'm afraid that's just a bit too avant-garde for my wallet.I was thinking of something a little less avant-garde than my original M8N module such as this item:https://www.ebay.co.uk/itm/Ublox-M8N-GPS-With-Compass-for-OMNIBUS-F4-Pro-V2-V3-Flight-Controller/323433150529?hash=item4b4e1e3841:g:aBAAAOSwDTNatpObThe only thing being that it'll lack the convenience of built in USB port and possibly flash ram (the PPS on pin three can be accessed easily enough). However, since I've now got three usb to TTL serial adapters ordered (the last being an FTD based Arduino adapter), the lack of a built in usb port shouldn't be a problem. I'm assuming this can be perverted to discipline an OCXO easily enough but the ASS-U-ME aspect worries me a little. Does anyone think this might not be such a good idea?I've already ordered one of these, yesterday:-https://www.ebay.co.uk/itm/GY-NEO6MV2-NEO-6M-GPS-Module-NEO6MV2-with-Flight-Control-Antenna-for-Arduino/273851748531?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908103841%26meid%3D2efd0832b1314d6c9cee050ae8b03de8%26pid%3D100227%26rk%3D2%26rkt%3D14%26mehot%3Dlo%26sd%3D153446586450%26itm%3D273851748531&_trksid=p2054502.c100227.m3827I've already spent just over three times as much on a variety of USB to TTL serial adapters (PL CH and FTD flavours) and an SMA pigtail or three:-https://www.ebay.co.uk/itm/PL2303HX-USB-to-TTL-RS232-COM-UART-Module-Serial-Cable-Adapter-for-Arduino/263864494229?epid=18022329686&hash=item3d6f8ca895:g:bYkAAOSwnKdbaaNrhttps://www.ebay.co.uk/itm/USB-to-RS232-Cable-CH340-DB9-Male-9Pin-Serial-Port-Adapter-Converter-Windows-7-8/162903533365?epid=506855824&hash=item25edcea735:g:lqcAAOSw44BYMHfXhttps://www.ebay.co.uk/itm/USB-To-TTL-FT232RL-FTDI-Serial-Adapter-Converter-Module-For-Arduino-3-3V-5V-Mini/222727769676?epid=8010050952&hash=item33db9c0e4c:g:5bcAAOSwZW5aFB08I might not be making extravagant purchases but I'm still running the risk of "Nickel and Dime-ing my way into bankruptcy". However, as surplus to my immediate needs as these 'chump change' purchases appear, they're still useful items to have to hand in the workshop.When you're starting all over again, there's no easy way to avoid this sort of 'expense' - it's just part of the process of bringing a neglected late 20th century electronics parts stock back up to date. Also, whilst I understand that the FTDI USB adapters are the ones to go for, I thought I'd sample the other, more problematical types to bring my experiences into line with everyone else's (it's only a bit of 'chump change' and I ain't going to be living forever - other than dental and medical implant costs, I won't be taking any of it to my grave).JBG

author: texaspyro
author_rank: Super Contributor
timestamp: May 23, 2019, 04:02:01 pm
content: This one is quite good... and cheaper than the F9T:https://www.buerklin.com/en/Products/Active-Devices/Multi-Chip-Module/GNSS--%26-GPS-Module/MULTI-GNSS-RECEIVER-GT-8736C/p/64S3196

author: jpb
author_rank: Super Contributor
timestamp: May 23, 2019, 04:55:49 pm
content: Quote from: texaspyro on May 21, 2019, 11:56:54 pmHave you had a chance to look at the newer improved Furuno the GT88 (I don't know if it has a GT-8836 board yet)?https://www.eevblog.com/forum/metrology/furuno-gt88-timing-gnss-receiver/I've not heard of the F9T, I will investigate.

author: jpb
author_rank: Super Contributor
timestamp: May 23, 2019, 05:01:47 pm
content: Quote from: texaspyro on May 23, 2019, 04:02:01 pmIn the UK it is also sold by these people (though I've never used them so I don't know if they are any good):https://ecommerce.linkwave.co.uk/gt-8736-gnss-timing-module/

author: texaspyro
author_rank: Super Contributor
timestamp: May 23, 2019, 05:15:03 pm
content: I don't know of any GT-88xx stuff that is available...

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 24, 2019, 03:29:00 am
content: Quote from: texaspyro on May 23, 2019, 04:02:01 pmIt's still over twice the price I'm prepared to pay at this stage of the game though.I was getting used to the quirkyness of the u-centre interface with my M8N example which, whilst not completely dead, is regretfully no longer able to discipline anything on account of the needless risk I took in my 'injection locking' experiment (I could have simply used one of the many spare OCXOs to act as a master in these experiments and kept the M8N out of harm's way.).If I wanted an exact replacement as per the item linked to below, not only would it cost me £6.64 more than I paid for the original I'd bought from a trader shipping from a UK address in less than a week but I'd have to suffer the typical 10 to 30 days shipping delay from Hong Kong which adds insult to injury, hence my choosing a cheap and cheerful NEO-6M module from a UK based supplier.https://www.ebay.co.uk/itm/Ublox-NEO-M8N-GPS-navigation-signal-amplify-module-for-arduino-Rasppery-PI/113231381729?hash=item1a5d1dc4e1:g:YfkAAOSwDiBZN502The ETA on the 6M module I've ordered had been given as the 23rd or 24th of May but it didn't turn up in this morning's post. Hopefully, it'll be here later this morning, possibly along a few other bits I'd ordered to overcome the absence of on-board usb and SMA sockets.The lack of an on-board usb port will provide me an opportunity to slum it with the serial port crowd and expand my experience. The most difficult problem I'd had to overcome with the now broken module had been in setting up the USB port sharing in my winXP VM (host os is Linux Mint 17.1). After that, it was plain sailing.I've never used a USB to serial adapter ever, so it'll be a new experience. The only thing I know of these USB adapters is what I've read and what I've read has generally not been good, seemingly due to incompetent chip implementations by Prolific and others, hence my ordering PL, CH340 and FTDI chipped adapters to try and sidestep the compatibility issues that seem to be a common theme in end users' tales of woe with these adapters. AFAICR, the FTDI chipped adapters seemed to be the least problematical.With a bit of luck, I should be able to set up a USB link to the NEO-6M module without too many tears of frustration. If all else fails, I suppose I could fit a DB9 backplate and use the COM port header connection on the MoBo to access a real serial port (I'm sure I've still got a few serial cables knocking around somewhere on the premises).These backplate adapters were wired in two different configurations so I'll have identify the correct backplate to install before I can even begin sorting out a connecting cable and deal with the voltage level/polarity issues that may be involved.The last time I had any dealings with RS232 was 5 or 6 years ago when I lashed up a basic three wire link to my ancient APC SmartUPSes via their bastardised DB9 'serial ports'. My main concern in this case is in dealing with a potentially fatal incompatibility between a TTL level interface and an RS232 bi-polar 5 or 12 volt level interface on the PC side of the link. USB may have its driver software issues but at least there's no such danger over the wired connection itself. Hopefully, I'll be able to get it working with at least one of the USB adapters I ordered and avoid the hassle of setting up an RS232 interface.JBG

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 26, 2019, 04:17:56 am
content: That NEO-6M module turned up a day late on the Saturday, along with the Prolific and FTDI USB serial adapters (the CH340 and the SMA to IPEC pigtail had arrived on Friday). I spent most of Saturday afternoon and evening sorting out the USB adapters, mostly the same old same old business of getting the VBox USB settings sorted out before I could install the windows drivers.I'd prepped up the GPS module (it had needed a 5 pin header soldering to it - one wasn't supplied but I have a small stock to cover such parsimony) and applied 5v power and plugged the SMA pigtail into its IPEC socket (when I first saw that pigtail on the Friday morning, it had seemed far too small for the job) so I could test it with my external active patch antenna to see if it would indicate satellite lock on its PPS LED as an initial check. , It obliged with the default setting of 1 Hz 10% duty cycle inverted output by the usual winking excessively bright LED. So far, so good.I tried the Prolific adapter first on account the serial cable was handily terminated with four individual header connectors which allowed me to connect to the header pins directly. Other than the usual faff with VBox's USB settings, I'd had no problems other than a small suspicion that my not being able to reprogram the Timepulse 5 setting to give me 10MHz output might have been due to a weird compatibility issue despite being able to monitor its activity. My suspicion was swiftly dispelled once I'd gone through the VBox USB faff all over again to set up the FTDI adapter which made no difference to what I was seeing.After spending several fruitless hours trying to get something more useful than a 1Hz 10% inverted duty cycle out of the PPS line, I eventually proved it was the module itself simply ignoring commands it didn't recognise by discovering I could filter satellites in and out of the picture at will by changing the elevation angle mask value over the range of 5 to 55 degrees.It seems that 6M module is either a fake or else an old enough version that doesn't support the TP5 commands. The best I could do was to reduce the pulse period to its 1ms minimum in the timepulse menu, along with a reduction of the pulse length to 0.5ms to give me a 1KHz square wave. I've yet to dig out u-blox's data sheet on this ancient module to figure out whether I got my money's worth or was simply cheated with a fake. Its sensitivity isn't all that far behind that of the M8N so, who knows?I'm glad I took the precaution of ordering an SMA to IPEC pigtail because the supplied active ceramic antenna proved to be remarkably deaf. Indeed, deafer than that passive patch antenna I'd removed from the M8N module a month or so back. To put not too fine a point on it, it was even deafer than the SMA to IPEC pigtail when disconnected from my external antenna. I'd had to cup my hands around the module to force it to lose lock so I could check the response to my attempts at trying to program the TP5 settings. If I didn't keep it shielded for long enough after losing lock, it would jolly well resurrect its lost satellites!Once I'd convinced myself that I wasn't going to get a PPS signal any higher than a mere 1KHz out of this module, I set it up alongside my 'scope and the FY6600 to see whether I could make any use of that 1KHz square wave. It turns out that it can still be used as the trigger reference to compare the 10MHz output from the signal generator. It's a little bit touchy but I was able to prove that the OCXO was only 85ppt adrift after the four or five days I'd been forced to leave it to stew shortly after adjusting it to within 100ppt just prior to my damaging the original M8N module.I'd revamped the foam rubber insulation around the OCXO module, including adding a layer to the underside of its PCB in order to stabilise against unwanted random 'wind chill' effects from the fan driven cooling air flow just prior to my last attempt at calibrating it. It seems to have worked quite well.I'll power everything down again (the OCXO stays powered up whilst the sig genny is plugged into the mains though) and see if the NEO-6M loses its settings by the time I rise from my slumbers. The mini supercap (60 to 80 milli-Farads) drops below the 1.4v data retention threshold of the BBRAM after only an hour or so at most. If it has hung onto its settings after 6 or more hours, it's a safe bet that it does possess flash ram after all. I'll know for sure one way or the other soon enough.JBG

author: texaspyro
author_rank: Super Contributor
timestamp: May 26, 2019, 06:00:48 am
content: Lady Heather talks to all Ublox GPS modules (start with the /rxu command line option to make sure the module is speaking Ublox Binary... uninitalized modules power up in NMEA mode). Heather's "P" menu lets you set the timepulse outputs.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: May 26, 2019, 09:34:00 pm
content: Quote from: texaspyro on May 26, 2019, 06:00:48 amThank you very much! I just voted a thank you to be sure that your reply had been appreciated even if I were to drop dead suddenly before issuing this little missive.I don't know why I didn't do that sooner, like two or three hours back after D/L and installing LH into my winXP VM. I guess I'd gotten rather too pre-occupied with searching out the command line switches and related key entered commands to remember to hit the 'thank you' button. Never mind, better late than never and better a more considered reply than a hasty note of thanks (though I suspect some folk here would rather I did the latter rather than the former).It's an interesting take on u-blox's idea of a GPS programming and monitoring interface in their u-centre application. Of course LH is rather more pre-occupied with the DO aspect of the GPSDO concoction.As far as I've been able to discern, you're the author of this rather nifty freeware for which I'm grateful. If I've managed to grab hold of the wrong end of the stick in reaching this conclusion, then please set me straight and accept my profuse apologies for any embarrassment I may have caused.I suppose I should have searched out this little gem before attempting to program non-existing features - it would have saved me several hours of pointless testing:-https://www.u-blox.com/sites/default/files/products/documents/NEO-6_ProductSummary_%28GPS.G6-HW-09003%29.pdfBelieve it or not, I only just thought to track down this nugget of information just after I'd written the previous paragraph!Never mind, at least it confirms my eventual conclusions over the lack of any TP5 support which is implied in the configuration menu in the u-centre application even when you've chosen the specific receiver generation which could have been used to trim out the 'dead features' from the all too inclusive list that covers every possible u-blox GPS module model and variant.At a penny shy of just four quid, I can hardly complain since it's doing exactly what u-blox says it does 'on the tin'. The seller wasn't entirely to blame (only partially) since I could've so easily discovered that pdf before hitting the 'pay button'. In any case, the 1KHz limit isn't really a problem against its use in a DIY GPSDO where the end result hardly depends on whether you're limited to a 1Hz reference or have the freedom to choose a 10 or 100 KHz or a 1MHz reference where you need a long time constant filter to iron out the sawtooth corrections anyway. Being able to program that M8N module to output 10MHz was just a convenient way to access a quick 'n' dirty calibration signal and I'm way past that stage now.I'm pretty certain that I can't push the PPS line beyond the 1KHz limit I'd discovered using the u-centre app by using Heather's P command - it's simply a hard limit that's been baked into the non-upgradable firmware. It's interesting to see it correctly displayed as a 1KHz PPS though.Your hints on command line switches helped considerably in seeing results almost straight away. I just found myself adding a bunch of other command switches to the desktop shortcut piece by piece until I got it in tune with my timezone and desired screen size.I've taken a photo of my test setup and a screen-shot (after finally rediscovering the GLP command to anonymize my location data) which I've attached for added 'entertainment value'.JBGPS btw, I forgot to explain the breadboard setup which I was merely using as a convenient way to link the FTDI USB to serial adapter on the left, to the GPS module. The other bits 'n' bobs are just the remains of my earlier experiments, left in place for convenient 'storage' - I might need them for more experiments.

author: jpb
author_rank: Super Contributor
timestamp: June 01, 2019, 05:14:57 pm
content: Quote from: Johnny B Good on May 21, 2019, 11:38:21 pmThey arrived a little while ago but I've only now had a chance to look at them.Measuring the first one:With 12V applied they produce a nice sine wave though the magnitude is a little low.It is 1Vrms into 10Mohms but only 1Vpp into 50 ohms.The reference voltage is 5.1V approximately.The frequency varies ok with the control voltage, it varies enough to show up on my Oscilloscope counter (my main counter is tied up with my ongoing ADEV measurements on my GPSDOs).The current draw is 70 mA after warming up.I'll update this if the other three are different.The second one is similar except the current dropped to 11mA then went back up beyond 130mA before settling a bit in the range 73 to 80 mA and finally settling at 75mA.The frequency is < 10MHz at 0V and > 10MHz at 5V so a 5.1V reference level is probably right.The third one is the same.BUT the fourth one is duff! It draws the same sort of current (340mA to start and then goes down to below 100mA but it produces no RF output and the reference voltage is approximately zero (a few mV).I suppose 3 out of 4 isn't bad but it does push up the average cost - I'll contact the seller on ebay.Edit: the seller has very promptly offered to send me a replacement.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 02, 2019, 04:46:07 pm
content: Thanks jpb,Other than for your receiving a broken unit (kudos to the seller btw), that pretty well echoes my findings too. Strangely, the warm up current closely matches what I see with the original 13MHz unit I'd bought at the radioham rally just because it was an OCXO (the only one I managed to track down) and it happened to be so cheap (at a mere 4 quid, even if I'd been wrong about how easy/difficult it would be to generate an OCXO sourced 10MHz, just the mere ownership alone of an OCXO I could experiment with would have justified the purchase).I hark back to that chance purchase because it basically kicked off my modest collection of these CQE 12v 10MHz OCXOs as well as giving me some additional insight into the characteristics of these units. I rather doubt that the oven heater is anything more than a TO220 power transistor (heater resistor elements would just add another point of failure and the 65 to 85 deg C temperatures used are well within the limits of a silicon transistor's thermal ratings), so the constant current draw I noticed when trying different voltages on the 13MHz unit (4.8 to a brief peak of 7.8 volts or so) was revealing if no great surprise. Unfortunately, this characteristic just muddied the waters when it came to determining whether that first unit was a 5 or 12 volt part.Once I realised just how piss-poor the performance of those LDO regulators used by the breadboard PSU adapter was and applied the necessary mod to mitigate the resulting low voltage (a mere 4.8v after the OCXO finally came up to temperature some eight minutes after power up), I was more or less convinced that it could only be a 5 volt part after all (at a starting voltage less than the 4.75v minimum, it was no wonder it took a good 3 or 4 minutes before I could even see any oscillator activity). Once it was being hit with 5.13v from the start, it generated output within milliseconds of my applying power although it still took almost a full eight minutes to warm up.Although I tested the first three 10MHz units using my now improved 5v breadboard psu adapter (it does both 5 and 3.3 voltage levels), I jumped straight to the 9 volts of the wallwart I'd chosen to power the breadboard psu adapter after seeing a very low sine wave output gradually emerge from each of those first three 10MHz units I'd initially purchased. I was a little less cautious in risking an over-voltage induced burn out on these parts since I had an extra two to spare from the consequences of such a rash action (no such luxury with the original 13MHz unit).I've since worked out that once all of these OCXOs (including that 13MHz unit) have reached their operating temperature, they settle down to around the 800mW mark give or take 50mW (around a 100mW less when cocooned in a layer of foam rubber to minimise random wind chill effects and thermal loading in my much modified fan cooled FY6600). The 12v units current limit around the 300mA mark versus the 280mA of the 5 volt one which incidentally draws around 190mA off a 4.8v supply when up to temperature and about 170 when powered with 5.13v and insulated with a foam rubber jacket. My 12v 10MHz OCXOs, like yours, also dropped down to around the 70 to 75mA markThe only reason I'm pretty sure the original is a 5 volt one, in spite of the penalty of a very protracted 8 minutes warm up (versus a 2 1/2 minute warm up for the 10MHz 12v units) to suggest otherwise, is the fact of its 3Vpp sq wave which looked like it was increasing in voltage in direct proportion to the applied supply voltage (ie. a 10Vpp for a 12v supply... in theory - a theory I wasn't going to put to the test) whilst the 10MHz units were producing a 4Vpp sine wave at 12v into a Hi Z load.I don't think I bothered testing the last four OCXOs at 5 volt since the first three had all shown (eventually) a low level sub 2Vpp on-frequency sine wave output (which had neatly answered the question I'd posed in my earlier posting, "Can you still get an output from a 12v OCXO even when powered from just 5 volts?").One noteworthy thing about these CQE labelled OCXOs is their remarkably avant garde low power consumption once warmed up. less than 900mW versus the 3 or 4 watts typically mentioned in regard of OCXOs in earlier threads on this subject here and elsewhere during the past five years. This reduction in power consumption doesn't seem to have compromised their performance since I've seen less than 200ppt drift with the one I fitted to the FY6600 over a week ago, confirmed by similar stability, despite the al fresco conditions with a breadboard lash up for a couple of the other 10MHz OCXOs I've been testing with these past two or three days.Another thing worth mentioning is that the tuning voltage range isn't limited by the Ref voltage. The actual limit seems to be diode clamping to the ground and the supply rails rather than to the ground and ref voltage rails. I found this out when testing with a PP3 9v supply to the tuning pot on the 13MHz 5 volt unit where I saw the voltage clamping to 5 volt, rather than the 3.3v of the Ref pin voltage and subsequently being able to tune using the 12v rail to supply the tuning pot to go beyond the limit imposed by the circa 5.1v Ref pin voltage.I happen to be using the 5.09v Vref pin voltage to supply the trimpot on the one I'm using in my FY6600 but you're not limited by this, you can use the full 12v supply voltage for tuning these OCXOs (presumably, the official 0 to 10v tuning range given in the data sheet for the Vectron units).This is just as well considering that the on-frequency tuning voltages on the 7 units in my possession ranged from a low of 2.32 to a high of 4.51 volts!As for the 5v 13MHz unit, its ref voltage falls short of the required 3.41 tuning voltage by around 10mV the last time I checked (I suspect the Vref in this case is meant to be 3.30v versus the 5.0v of the 12v units).TBH, despite its convenience in the case of the one being used in the FY6600, I'm not quite sure how these Vref pin voltages are intended to be used. I suspect they're more intended as a reference for a 16 or 32 volt rated RRO opamp to drive a stabilised supply to a tuning pot or other RRO opamp driven from a PLL phase detector or DAC. You're not obliged to use the Vref as a tuning supply voltage directly (or even at all). They just seem to be there as a courtesy 'convenience feature'.As it happens, I'm now poised to install a couple of the three 74HCT390 dual decade divider chips I ordered last week which turned up yesterday (rather than this Monday/Tuesday) into my breadboard lash up. I want to test my u-blox NEO-6M GPS module (limited to a maximum of 1KHz on its PPS output pin) with a PLL and my 13MHz OCXO (chosen to avoid having to mix 12 and 5 volt supplies on my breadboard setup).That's a choice I'm beginning to regret since it took me all of the wee small hours to reassemble the times two to divide by 13 to times five conversion required to generate the 10MHz with this 13MHz OCXO (effectively a divide by 1.3 - I've done it once before - how hard can it be? - a lot harder when you forget to apply a logic zero to the 'clear' pin of the '193 which is an active high input rather than an active low as per most of the other inputs,).The only saving grace in this case being that I'd managed by chance to have already programmed the correct preload value to count down in a divide by 13 ratio.. However, by that time (05:55 BST), I thought it best I get an "Early Night"(tm) and put in a good six hours of Zeds before resuming this milestone project (it'll be the very first time that'll I'll be attempting to actually phase lock an OCXO to a GPS module!).However, as I've mentioned, I'm regretting doing this and think I now don't have enough space to add the extra 3 or 4 chips required to complete this project. I'm now considering how to make it a "13MHz reference" to avoid the 12/5 volt mixed supplies issue or else put up with the inconvenience and small risk involved (it's only the OCXO that needs a 12v feed - the 5 volt derived tuning voltage will do the job just fine with my hand picked OCXO and the output is capacitor coupled anyway).In a way, it's a little unfortunate that these 10MHz CQE OCXOs are able to produce a usable, if a little on the low side, frequency output off just a 5 volt supply since this doesn't offer such a defining distinction between the 12 and 5 volt versions, leaving me only with a bunch of other circumstantial evidence as a guide to my current determination as to the 13MHz OCXO's "Voltageness".Since it seems to work fine, once the rather protracted warm up period has expired (the price of a low peak power 5 volted version of a CQE OCXO it would seem), I'll let discretion remain the best part of valour in this case (I've subsequently seen the destructive effect of 12 volts misapplied to a 3.3v part already, thank you very much, dear Lord Murphy).I've attached some photos of the current project. Please note the loose white wire bottom right in the first picture. This is the tuning connection to the TCXO, poised for connection to the filtered output from a phase comparator when I finally get to that stage. This means the TCXO hasn't been trimmed to frequency, hence the last picture. Also, the NEO-6M is simply parked on the board awaiting connection into the final circuit which will probably not feature the two 3N502 multipliers or even a 13MHz TCXO if I do finally decide to risk mixing 12 and 5 volt supplies.JBG

author: jpb
author_rank: Super Contributor
timestamp: June 02, 2019, 06:39:35 pm
content: Regarding the reference Voltage, I've noticed with a number of OCXOs it is set to rather less than the total tunining range (e.g. around 7V or so when the range is up to 10V). In this case the 5.1V (or 5.09V to be more precise) seems to be roughly twice the initial tuning voltage for 10MHz (based on what I measured on my not very precise counter on my oscilloscope).I would have thought that dual power supplies (12V and say 5V or 3.3V) wouldn't be too much of a problem as the 5V parts are presumably very low current compared with the OCXO so a simple linear regulator chip with a couple of capacitors would be ok and doesn't add much to the number of components.I admire your tenacity in working with the 13MHz part. I have a number of 13MHz OCXOs some of which are quite good but if I was to use them in a GPSDO I'd probably make it a 1MHz one (divide by 13) or perhaps have a second PLL with a 10MHz crystal.The 13MHz parts are all from GSM systems I think where it is an exact multiple of the bit rate (according to Wikipedia). I wondered if it was possible to try locking onto a GSM signal but I suspect that idea is a non-starter (having got these 13MHz OCXOs because they were cheap I am now hunting around for applications!)

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 03, 2019, 04:58:59 pm
content: Quote from: jpb on June 02, 2019, 06:39:35 pm'Scope based frequency counters leave a lot to be desired, even the 6 digit hardware ones as used in the SDS1202X-E I bought just over six months ago (that read 30Hz high to begin with but now seems to be within 10Hz of actual, presumably due to the XO reference having now aged itself closer to its design frequency).Since I've upgraded the cheap 'Ten a Penny' smd xo chip used by Feeltech in their otherwise excellent, penny pinching cost cutting deficiencies aside, FY6600 to a 10MHz OCXO with 3N502 clock multiplier to improve the original 50MHz reference by a good five orders of magnitude of accuracy and six orders of magnitude of stability, the 'scope counter is now merely a useful sanity check that the signal it's locked onto is in therightball park (never mind 'theball park' - I've been chasing down measurements in the ppt range for the past month now - ppb accuracies are now just a memory of my earlier ambitions).It's true that providing stable regulated DC voltages in a multi-rail setup is now routine thanks to LDOs of various types, the big problem when it comes to frequency stability measured in ppt, is that the once insignificant voltage changes on these rails due to variations in loading can no longer be tolerated when it comes to OCXO tuning voltages. The classic way to obviate this is simply to use a separate regulator dedicated to only this function, preferably one that is itself powered from another regulator at a voltage between the relatively unstable supply (12v to 9v LDO feeding a 5 or 3.3 volt LDO or even a separate mains to 12vdc smpsu feeding a 9v then 5v with possibly even a 3.3v LDO hung off the end of that chain.The OCXO manufacturers are obviously aware of this demanding requirement so will typically offer the internal LDO's output on a Vref pin (it only has to supply some 10 to 15mA at most to the constant load from the XO itself - I've noticed the undershoot of the heater current during initial warm up dropping to a steady 14mA minimum before the temperature control turns the heater transistor current back on in both the 5 and 12 volt CQE OCXOs - as I mentioned earlier, this is one of the insights I gained into the inner workings of these CQE OCXOs).In most cases, this can be used directly to feed a 50K trimming pot or a cmos RRO opamp/dac arrangement, free of the millivolt variations of the general 5 or 3.3 volt supply rail. Failing that (the voltage isn't high enough for example) it can be used as an amplified reference voltage with another suitable cmos RRO opamp to create a higher tuning voltage supply with a stability matching that of the OCXO's internal oscillator supply voltage. As you said, it's only a minor complication to provide a separate dedicated tuning voltage supply, free of the effects from the rest of the GPSDO's loading on the main power supply rail.Thanks for noting my 'tenacity' with recreating afunctioningdivide by 13 circuit out of a '193 IC.Putting aside my oversight of the need to ground the clear input pin, it's not difficult to convert the 13MHz into a 1 or 10 MHz sq wave frequency reference (the 10MHz option requires at least one 3N502 multiply by five chip and a frequency doubler to make sure it gets a 2MHz input (the 3N502 isn't specced for input clocks below 2MHz - I used another '502 to double up the 13MHz before feeding the divide by 13 circuit to meet this 2MHz requirement but a simple RC delayed input XOR gate frequency doubler would have done the job, saving that extra '502 for more productive use elsewhere). I wouldn't be quite so quick to discount those cheap 13MHz OCXOs as a substitute for a 10MHz reference oscillator if I were in your position, unless you've got a surfeit of 10MHz OCXOs to hand of course.In the end, I stripped the '502s out of the circuit leaving just the '193 to convert the 13MHz into a 1MHz signal which I then fed into one and a half '390s to feed a simple '86 based PD with the required 1KHz to lock to the NEO-6M's 1KHz output from its PPS line. At this stage of the game, any "reference frequency output" will do for my current purpose - I'm just trying to get a functioning PLL circuit proven. Even with this chip count reduction, it was still a tight fit on my prototyping breadboard but I was able to get results of a sort. I rather suspect I might be needing an inverting buffer amplifier between the output from the RC filtered PD output and the tuning control pin on the OCXO.As it happens, the ten 5v cmos RRO opamps I'd ordered from a Chinaman about a month back did finally turn up last week but I need to mount one on a SOT to DIP adapter board before I can add it to my breadboard lashup. In the meantime, I did try cheating a solution to this suspected requirement by using a spare XOR gate as an instant phase reverser for one of the PD's inputs (clutching at straws rearguard action on my part) by feeding one of the input signals into an input pin of a spare gate in the '86 IC and grounding the other input to change it from a logic high (internal pullup) to a logic low as an easy and convenient means to effect instant phase reversals at will. Needless to say, it didn't really make a useful difference in spite of it producing an immediate (and initially promising) effect.When it had seemingly 'locked up', I could see that the GPS was doing something useful by disconnecting the antenna which then caused the frequency to drift rapidly from the target frequency when satellite lock had been lost, followed by a rapid shift back onto frequency when lock was regained after reconnecting the antenna. The PD circuit was obviously doingsomethingbut it had a tendency to slip inexorably out of lock, first to the high side then to the low side over periods of 10 to 15 minutes.I'm not entirely sure what the problem is. I have many theories and contenders by way of poor component choice(s) to play around with. It's basically a negative feedback setup so one might think there'd be a lot of latitude in component value selection. However, I'm aware of the issue of 'phase stability' in such negative feedback loops turning them into positive feedback loops at critical frequencies when the gain can equal or exceed unity. Creating stable negative feedback networks can prove more difficult to achieve than it appears on the face of it and I obviously need to study this problem some more.One possibility is that the rather ancient, original TTL 7486 XOR gate I'm using isn't really suited to this PD circuit but it was the only '86 I could find in my four decades old collection to test with. I've got a five piece set of the more modern 74HC86s headed towards these shores on the slow boat from China at just 99p the lot (as opposed to the silly money being asked per single IC by the greedy, profiteering UK dealers - I'd hoped my single 7486 chip would save me the indignity of going to one of these UK dealers, begging bowl in hand - it may do so even yet, given enough lateral thinking).Of course, there's also the effect of the jumper styled wiring used on such prototyping breadboards, along with the less than reliable contact performance and the problems of arranging for effective high frequency bypassing of the supply voltage feeds to the ICs used to contend with. Then, in this case, comes the issue of the quality of the 220, 22 and 1 microfarad Al electrolytics I used (taken from a PD circuit from off the web), any one of which could be contributing excessive leakage effects.Reassembling with the original components, Manhatten 'dead bug' style on a piece of groundplane PCB might instantly cure the prototyping breadboard issues but, since the high frequency digital processing circuitry seems to have survived such indiscretions of construction and I'm now dealing with a mere 1KHz (admittedly sq wave) signal, I'm not so sure such an improvement in the reduction of high resistance connections and unwanted lead inductances would do much for the PD circuit in this case.However, I do recall the time when I was 'breadboarding' my design of a 200W RMS per 4 ohm loaded channel bridged output PA design some 50 years back onto tag board, soldered connections al fresco layout (better than the current prototyping breadboard setup) and experiencing strange instability issues which had me convinced that the only way to properly test it out would be to commit it to a PCB (hand drawn and etched with ferric chloride in my workshop) which conclusion turned out to be correct, so there may yet be some mileage in going this route after all, as unlikely as that would appear.BTW, my 'phase lock loop' appears to have stabilised (I powered it all back up about an hour ago) but at a 100mHz offset from the FY6600's 13MHz setting. There's a slow but quite discernible drift with the control voltage sat at 3.26v at the moment (it can swing, for no apparent reason, between some 2 and 4 volts). I know that the drift should be a barely discernible 10mHz or so when 'locked' which makes the resulting "locked" state, quite frankly, rather inexplicable.I know this isn't the locked state because if I trigger directly from the PPS output, the 13MHz Sinc pulse waveform takes some 5 to 10 minutes to slip one full cycle rather than the ten seconds or so that it's doing right now.Never mind, it's a result of sorts... I've just noticed it was starting another trip to the high frequency end of its range (I've now got a 50K pot wired to the tuning control line to limit the extreme excursions in the hope it would lend some stability - it helps a little but it doesn't cure this 'locked to not exactly the correct frequency' syndrome). I disconnected the antenna to force loss of satellite lock and, sure enough, when this happens it suddenly jumps up in frequency and then drops back onto its 100mHz above frequency 'lock' state when satellite lock is regained.To my mind this "almost works properly" is a really weird effect, I would have expected it to either work or else keep dancing from one extreme to the other without any pause close to the desired frequency. I wasn't expecting this sort of behaviour from this circuit so it's quite a conundrum.JBG

author: jpb
author_rank: Super Contributor
timestamp: June 03, 2019, 05:14:37 pm
content: Quote from: Johnny B Good on June 03, 2019, 04:58:59 pmThe one on my WaveJet is pretty good (within its limitations). It has consistently read 9.99998MHz for 10MHz over the time I've had it (about 5 years) so when the tuning had it go from 9.99997 to 9.99999 I was pretty sure it was going through 10MHz. Obviously it would be better to use my proper counter but for a quick look it works well. It is certainly a lot better than the frequency read out on the scope itself (based on the waveforms).

author: Electro Fan
author_rank: Super Contributor
timestamp: June 03, 2019, 10:24:35 pm
content: Not sure if this fits in this thread but since there are a lot of experienced GPSDOers here.... (and maybe the power supply should be a consideration in "what do you want from a GPSDO?")….Has anyone measured the impact of simple switching power supplies vs. linear supplies on the performance of GPSDOs? Does anyone have any insight regarding to what extent ripple impacts GPSDO performance?

author: texaspyro
author_rank: Super Contributor
timestamp: June 04, 2019, 01:36:55 am
content: It all depends upon the GPSDO. A lot of them have on-board switching supplies of who-knows-what quality. The ones from reputable / knowledgeable makers usually use rather good ones.

author: Electro Fan
author_rank: Super Contributor
timestamp: June 04, 2019, 03:37:27 am
content: What would a typical spec be for ripple on the power supply for a reputable GPSDO?On the order of 50 mV peak-to-peak, 15 mVrms, or something better? What is the impact if the PS's ripple is higher?

author: texaspyro
author_rank: Super Contributor
timestamp: June 04, 2019, 04:43:53 am
content: Quote from: Electro Fan on June 04, 2019, 03:37:27 amI dunno... they are all different... check the GPSDO manual... you do have proper manuals for all your undocumented GPSDOs?

author: jpb
author_rank: Super Contributor
timestamp: June 04, 2019, 05:19:44 pm
content: I was surprised by how much difference the type of supply I used had on ADEV measurements.See this thread for a discussion on the subject:https://www.eevblog.com/forum/metrology/rubidium-gpsdo-ocxo-use-of-switching-regulator/msg2291295/

author: Electro Fan
author_rank: Super Contributor
timestamp: June 04, 2019, 11:45:14 pm
content: Quote from: texaspyro on June 04, 2019, 04:43:53 amNot sure what that was all about but fwiw I'm kind of new to GPSDO's. I have a 2018-06-05 BG7TBL and the 1.1-D version of the manual, and I have a Star 4 that is on order that has not yet been delivered. I've found some documentation on it from Oscilloquartz but I think that is at the oscillator and board level rather than the full GPSDO level. Anyway, I appreciate all your good info and help on this forum.

author: texaspyro
author_rank: Super Contributor
timestamp: June 05, 2019, 12:12:59 am
content: Quote from: Electro Fan on June 04, 2019, 11:45:14 pmBaically a jab at the lack of decent documentation on all but a very few GPSDOs... most of the ones you find all are pulls from telecom equipment or rather amatureish devices. The telecom stuff is usually pretty good, but were custom built for paying customers that want to keep their goodies and specs proprietary and secret.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 05, 2019, 03:06:45 pm
content: I've just posted a half marathon contribution to that other thread on the subject of smpsu issues, relating my experience with the FY6600 function generator by way of example (along with mention of my current distress at PLLing the 1KHz PPS of a cheap NEO-6M to a test setup using a 13MHz OCXO).The fact that the Oscilloquartz unit actually mentions ripple limits for its external smpsu requirement strongly suggests that they've taken account of the need for proper and effective supply rail filtering (andactuallyapplied the appropriate measures). Other manufacturers that assume you use a perfect PSU, completely free of such ripple suggests they may have put no thought whatsoever into this aspect of their design (i.e. they've not tried hard enough to create a high quality product).When you think about this issue of PSU ripple, it's a sobering thought that this is something that's been on the minds of designers to varying degrees of concern for nigh on 80 years or more. It's not as though there's not been plenty of time for this issue to have been well and truly sorted out by now. Any shortfalls in this department are now largely down to pragmatic concerns over customer expectations and to what level of bean counteritus you allow your accountants to dictate to your design and production departments.Although manufacturing cost can't be allowed to run away unless you're making a very niche product, far too many manufacturers are taking this saving of pennies on product selling at hundreds and even thousands of pounds or dollars to a piss taking extreme which risks their credibility.Over the past 40 years or so, I've seen enough examples of rampant bean counteritus (and even one counter-example!) to convince me that most manufacturers of high tech electronic goods can be total and utter twats (I'm looking at you, Akai that once was in the business of high fidelity magnetic tape decks).If you can get hold of the service manual, complete with circuit diagrams, for your high tech electronic device (in this case, a GPSDO), you do at least have a chance of putting right the deficiencies of bean counteritus and sometimes even outright "skoolboy howlers" perpetrated in the design to turn your "Sow's Ear" into, if not a silk purse, then at at least a rayon purse facsimile of its intended function.Although more and more electronic products are hiding their inner workings from the gaze of the would be fettler by using proprietary designs within FPGA chips and the like, when it comes to sorting out PSU ripple issues, there are several approaches to solving such deficiencies of design including, if necessary, that of filtering out the ripple before it even reaches the device in question (either use of a better PSU or additional filtering between the existing PSU and the device's power rails). A quick 'n' dirty way to discover whether such an improvement to the PSU would be worth pursuing is simply to use a battery or batteries as a substitute for the original supply (removes the question of whether your hundreds of dollars worth of bench supply will introduce problems of its own or not).This all sounds like a lot of trouble but if you're motivated enough to be asking such a question, then it's not really that much trouble after all.JBG

author: Electro Fan
author_rank: Super Contributor
timestamp: June 05, 2019, 07:05:06 pm
content: Quote from: Johnny B Good on June 05, 2019, 03:06:45 pmOk, thanks for the good info and battery suggestion. It does seem that an answer to determining how much impact external power supply ripple makes on the ADEV or other aspects of GPSDO operations is “try a battery.” Of course, the results might be somewhat influenced by the internal/integrated power supply circuitry of the particular GPSDO design. As previously noted, if for example, the internal board level design is spec’d at 150mV peak to peak ripple and noise, maybe a battery power supply won’t be able to show off it’s full impact. But I think it’s worth a try.Results of 1 baseline test with respect to some simple startup observations: My BG7TBL seems to go from a cold start through a ~30 second period where it draws 1.2A and climbs to 1.3A; at about 30 seconds it starts drifting downward to less than .5A which it reaches in about 5 minutes and eventually (in about 60 minutes) it will settle at about 352mA. I don’t know for sure which comes first, the Lock symbol on the display or 352mA but I think they occur about the same time. At about 25 minutes the GPSDO reaches 360mA so the whole current descent process goes faster at the start and slower at the end. By about 28 minutes the LCD shows 10,000,000.0000 for the first time, then the 10 MHz signal bounces up and down a bit and eventually settles out at or near 10,000,000.0000 (at least that’s what the display says – who knows?).(I have developed a new theory that the GPSDO warm up cycle takes somewhat longer to reach 352mA and Lock when I’m measuring it with my handheld DMM than when I just observe it without the DMM measuring current. Burden voltage? I was pretty sure it only took about a half hour to reach Lock without the DMM inline. With the DMM inline the last 8-10mA takes a relatively long time (~30 minutes?) before it reaches 352mA. Update: I now think the Lock occurs when the current settles to about 354mA, and then the GPSDO will bottom out at 352mA.)If you plug and replug the BG7TBL while it is warm it will come back to 352mA much faster (a few minutes?). It consistently runs rock solid at 12.0V from startup to full warmup. This is all using the seller supplied wall wart that is spec’d at 12V, 2.0A.So it would seem that with a 12V battery rated at 9Ah it might be possible to run a 12+ hour test. Maybe not long enough to learn long term impacts but maybe enough to learn something about the impact of external power supply ripple (or the lack thereof in the case of a linear supply) on short term GPSDO performance.As a side note, it’s funny how a mild time nut addiction can lead to purchasing a $200 GPSDO followed by a second GPSDO and maybe a better SMPS and maybe a 12V battery and then maybe lead to justifying why it would be useful to purchase a bench DMM capable of recording and displaying trends. Stuff you previously didn’t know existed leads to the purchase of stuff you don’t really have a use for other than to observe and learn which leads to a need for more test equipment to measure stuff with more detail. I guess it's a cycle.

author: Electro Fan
author_rank: Super Contributor
timestamp: June 05, 2019, 08:30:11 pm
content: While deciding on how to come up with a suitable battery I replaced the 12V, 2A wall wart with a linear PS set for 12V, 2A. Below you can see the impact on the graph in LH.Latitude is light blueLongitude is orangishAltitude is purpleDOP is maroon

author: Electro Fan
author_rank: Super Contributor
timestamp: June 05, 2019, 08:43:42 pm
content: Other than less stair step effects on the lines in the LH graph I'm not sure what effects the linear supply has on GPSDO performance, but I can say that at a constant 12V the wall wart has less fan noise.

author: jpb
author_rank: Super Contributor
timestamp: June 05, 2019, 08:46:32 pm
content: Quote from: Electro Fan on June 05, 2019, 07:05:06 pmI justify similar expenses as being educational, though expensive for a hobby they are a lot less than say paying for a course at university. I also tell myself that I can sell off the kit when I'm finally cured.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 06, 2019, 12:59:31 am
content: Quote from: Electro Fan on June 05, 2019, 07:05:06 pmWow! I can certainly relate to that final side note (as I'm sure most here would).I believe it's something referred to as TEA addiction.For myself, it took the form of my purchasing a cheap Siglent DSO last November by way of returning from a 30 year sabbatical in my hobby interest in electronics enforced by marital and family responsibilities when the availability of modern test gear much cheaper and better specced than the Tektronix kit I couldn't afford second hand examples of three decades back when two or three hundred quid used to be 'real money', was pointed out in the SED news group just over six months ago, citing the FY6600 thread in this forum by way of example of just how much you could now get for your money if you set your sights below the more well known profit... manufacturers such as Tektronix and Keysight.Although this inspired me to buy a Siglent SDS1202X-E as a 'best bang for my buck' purchase (and that after doing without for thirty years, I really could now afford to treat myself to a piece of kit that could outdo the 2nd hand Tektronix offerings of three decades back for only slightly more than their then asking price), it took less than a week of ownership to realise that some sort of signal generator would now be a useful item to complement my 'scope purchase.Initially, I did briefly consider a cheap Siglent 2 channel 30MHz AWG until I started comparing it against the FY6600 at just a quarter of the price. Despite all the negatives against the FY6600 in an almost two years old thread, it still appealed to me, especially as most of the criticism was due to the readily fixable penny pinching induced shortcomings, most of which I've now fixed.One of the glaring deficiencies being the use of a "ten a penny" smd xo chip which would be of no detriment at audio frequencies but a severe annoyance for its lack of both accuracy and, even worse, stability for radio frequency work beyond the 100KHz mark, let alone, up in the Gods of the HF bands (3 to 30MHz).At the time when I read Arthur Dent's OCXO modification contribution, I thought he was going just a little over the top and so settled for just a 3 orders of magnitude improvement in the form of a neat little 50MHz 0.1ppm TCXO oscillator power board I'd initially planned on extracting the TCXO from to drop in place of the original and rather execrable smd XO chip until my IR thermometer revealed that it was running at 50 deg C due to being just 10mm away from the three LDO regulators which were showing a 70 deg C temperature on the main board. I landed up mounting the whole module above the cooling fan to act as a deflector plate to keep it within just two or three deg C above room ambient, connecting it to the vacated XO smd chip location via a short length of small bore co-ax.That gave me ample stability and accuracy to let me observe in sufficient detail what was happening with the NEO M8N module I'd bought as a starter item for my GPSDO project - I've since upgraded from the TCXO to a 10MHz OCXO with, as per Arthur Dent's OCXO upgrade, a 3N502 low jitter clock multiplier to generate the required 50MHz clock drive. That's now resulted in a free running OCXO calibrated within 300ppt with a drift rate over the past week or so of less than 200ppt (although I haven't been able to compare against a GPS reference for almost a week now). Suffice to say, it's been accurate and stable enough for me for use as a secondary reference in developing my DIY GPSDO project.I mention this on account this new found interest in test and measurement kit has set me off on a seemingly never ending quest for better and more accurate gear, aka a GPSDO I can rely on and be proud of. Lately, in earlier postings here, I've been bemoaning my problems with getting a 5 volt 13MHz OCXO to phase lock against my cheap NEO-6M's 1kHz PPS (it's a make do for now substitute for the original M8N module I damaged with an injection of 12 volts into its PPS line a couple of weeks back).All this time I'd been blaming first the XOR gate phase detector circuit and then the 4046 phase detectors for not doing 'their job' and, only now between this and the previous post, discovered a rather strange fact about the PPS signal from a NEO-6M (or at least this NEO-6M) when it's programmed to output a 1ms second pulse with a 50% duty cycle (1KHz sq wave).Up until just prior to this post, I'd only ever quickly checked out the PPS and divider chain 1KHz output waveforms, preferring to monitor the 13MHz OCXO and FY6600 outputs when testing my crude PLL setup. It's only since I decided to take a much closer look in detail at the 1KHz signals going into the 4046's phase detectors, whilst monitoring the output voltage, that I've finally discovered the root cause of my crudely lashed up on prototyping bread board GPSDO test setup's failure to lock onto the GPS reference. It turns out to have been all to do with the NEO-6M's strange habit of slowly wittling away at the positive going pulse, shrinking its width until it disappears up its own backside to instantly reincarnate itself as a full fat sq wave, remaining so for the following five or so minutes before starting another cycle of shrinkage over the next 5 or so minutes.If it hadn't been for the fact that I'd upgraded the FY6600 to an OCXO clock, I doubt I'd have been able to hand adjust the tuning pot on the OCXO to keep it close enough to the leading edge of the 1KHz PPS pulse to observe such weirdness (I'm not sure the original 0.1ppm 50MHz TCXO upgrade would have been enough of an improvement to have pulled off such a stunt).It's no wonder the XOR gate failed to produce any meaningful failure behaviour let alone actually function as intended other than for relatively brief moments when it seemed to be locking in an offset fashion. The type 2 phase detector in the 4046 should have gotten round the non-square waveform issue but I'm guessing it was triggering on the trailing falling edge which would have been slowly happening at earlier and earlier times in the cycle for half of this 10 or so minute cycle before jumping back to a sq waveform for another 5 or so minute round of fun and games. No wonder it was slowly driving me round the bend these past few evenings (including midnight to 5 or 6 in the am if truth be known).Anyway, that's just a taster of the sort of process that sucks you ever deeper into the mad mad world of DIY test gear and electronic projects in general. BTW, I think, now I've got the use of the type 2 phase detector in that 4046 which doesn't rely on the waves being square, I'll have a go at reprogramming the NEO-6M to generate the narrowest pulse possible (10 microsecond, afaicr) and see how it behaves at that setting.Obviously, I could try an inverting gate on the PPS line but I don't have any room on my prototyping breadboard for another TTL chip. Paraphrasing the famous line out of the movie, "Jaws", I think I'm going to need a bigger board. I think I might have one in the attic somewhere so I might have a look for it later on.I've just taken a look and found it straight away! It's actually a "Microprocessor Trainer model ET-3400" made by Heathkit and it's complete with original manual. Taking another look at it now, I see the only item of value to me is the older style breadboard block that's simply stuck down onto the main circuit board to allow additional analogue circuits to be assembled (the 50/60Hz 120/240v transformer supply, according to the circuit diagram has two centre tapped secondary windings, one biphase rectified to produce circa 8vdc for the 5v regulator IC and the other with bridge rectifier to, presumably, provide a +/- 18 to 20v dc for the 12v + and - regulators which, according to the manual, are only for use with the prototyping breadboard that had been stuck, centre stage on the mainboard/panel.They'd designed this 'Trainer' around a Motorola MC6800 cpu so it's no wonder I never even bothered to fit a plug onto its captive mains lead. If they'd more ambitiously based it around a Zilog Z80 or even its kakamaimee cousin, the Intel 8080, I might have deigned to fit the mains plug and have a play with it. With the association of the words "trainer" and "potty" in my mind and the fact that I was probably already writing Z80 assembler code to run on my S100 bus desktop machine at the time, it's no wonder it spent 30 years or so gathering dust in the attic.I think it was a novelty item even back when I picked it up for cheap (or possibly even as a freeby) at some hamfest in the early to mid 80s. I was probably attracted to the prototyping breadboard (and possibly the plugged in TTL ICs) more than anything else. I suppose the case might be repurposed to stick a bunch of prototyping breadboards down on a replacement panel to the original mainboard and the transformer supplies replaced with more modern smpsu modules extracted out of redundant wallwarts and so on but that seems it might consume too much of my time right now and it may well have some value as a "Collectable" for all I know (it seems to be pretty well complete, only lacking its "Original Packaging"(tm)). I might prise the breadboard off for my immediate needs since it's an easy enough 'restoration job' should it prove to have any value as a 'collectable'.As it happened, I did use the right tool for the job, a flat bladed screwdriver, just not the right way since it proved to be screwed down to the panel rather than, as I'd assumed, stuck down with double sided adhesive tape. Still, it did save me having to remove a whole bunch of screws just to separate the top and bottom halves of the case to gain the required access to use the screwdriver as Ghod had intended (there was enough flex in the ventslots to grab hold of the loose screws with long nose pliers and with careful jiggling to extract them for reuse to attach the breadboard onto a plastic baseplate to prevent the contact strips being pushed out the underside.I'm pretty certain it's a twin to another prototyping breadboard I already had but which I had managed to misplace at some time during the past 35 years or so. Making up a baseplate for my liberated breadboard will be worth the modicum of time and effort required since I can use it as an annexe to the existing board to support the luxuries of extra logic gates, hex Schmitt trigger inverters, CMOS RRO opamps and such like.I've just taken the time to time the pulse width cycling on the PPS line with a stopwatch and it cycled in 11 minutes and 12 seconds during which time it would remain at a 50% duty cycle for a few seconds longer than 5 minutes, just as I suspected was the case. I think the addition of a hex inverter gate in the PPS line should fix the phase locking issue that's been torturing me these past three days or so.I monitored the output voltage on the output of the LPF connected to the PD2 output line and it has been cycling from zero to 2.37 volts, shadowing the phase difference between the trailing edges of the OCXO divider and the PPS outputs. It would seem I have finally solved the mystery of this phase locking failure. As to what the best cure is, I've yet to investigate my options. An inverter should get me at least 99% of the way to a fix (possibly even 99.9% or better). I shall see.JBG

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 07, 2019, 10:37:37 pm
content: Quote from: Electro Fan on June 05, 2019, 07:05:06 pmThe question of "Burden voltage" might have some credence (ITYMTS, burden load from the DMM's 10 or 11 Mohm voltage sensing circuit impedance) since I noticed a definite detuning effect on the PLL when disconnecting the meter from the output of the LPF feeding the OCXO's tuning pin after it had finally established a rather fragile lock. Obviously, a better measuring point would have been on the output of a buffer amp between the LPF and the tuning control pin but, since I hadn't been able to add such a luxury item to my breadboard lashup for the want of space, that simply hadn't been an option.Quote from: Electro Fan on June 05, 2019, 07:05:06 pmAn alternative to that scheme, if you feel the need for protracted days long runs, would be to use a pair of 7AH alarm batteries (the cheapest SLA battery option if you can have a friendly chat with your local alarm shop's proprietor) and connect them via Schotky barrier rectifier diodes so you can attach a freshly charged alternate when the first starts to show signs of exhaustion, allowing a breakless changeover to put the changed out battery on charge. Thinking about it, you might want to swap the batteries between load and charger on a shorter cycle to avoid going beyond a 50% DoD and the risk of premature wear out.The "Try using batteries" suggestion works fine when, for example, testing to see whether weird noise pollution on a signal generator's output is a power supply artefact or not when such testing can be over and done with in less than half an hour but the cost of sufficient battery capacity to power a GPSDO for several day's worth of testing makes the building of an over specified high grade LPF to put between the cheap wallwart and your precious GPSDO rather more appealing.The big appeal of using a battery is the complete elimination of switching noise which trumps the obvious downsides of slowly drooping voltage during discharge (which usually isn't an issue in most cases) and the inconvenience of having to swap out or recharge exhausted battery packs.Quote from: Electro Fan on June 05, 2019, 07:05:06 pmI've already said more than enough on that subject.The latest report on my own travails with trying to get a 13MHz OCXO to lock to the "One KiloHertz PPS" signal from a cheap (and, as it turned out,NASTY) NEO-6M is that I did manage to get it to lock for a good ten to fifteen minutes yesterday before it suddenly threw a wobbly (the 1 microsecond wide pulse disappearing up its own backside to reincarnate itself as another cycle of 1 microsecond wide pulses at a 1KHz repetition rate. It seems that even setting a 1 microsecond pulse width doesn't stop it being wittled right down to nothing over the final half second or so of the 10 to 15 minute cycle (it varies - presumably a side effect of the sawtooth corrections to keep the original one pulse per second's leading edge to within half a cycle of the free running on board 48MHz XO synchronised to GPS time).I tried various concoctions of inverting buffer configurations on the PPS and divider chain output lines but since the process of getting the 1KHz pulse edges lined up takes ten thousand times longer than it would at 10MHz, it's not so much a case of waiting for paint to dry so much as waiting for it to peel.In the end, I just left it running to see what tricks it was going to get up to whilst I dealt with other stuff.By then, last night, I was rather resigned to waiting for my cheap NEO M8N to eventually arrive any time from the middle of next week onwards so I was rather pleasantly surprised when it, along with an order for ten 74HC390s, turned up in this morning's post.I set the new M8N up with a USB to serial adapter and hooked it up to the winXP VM to check it out with u-blox's u-centre application. I tried programming the TimePulse but that doesn't apply with the M8N as I had a deep suspicion would be the case.There's a shed load of programming options which seems to be a superset of all the options that have ever applied to the various GPS receivers the u-centre app can control. Unfortunately, this means you can never be certain which options work and which ones don't because the particular receiver you're using simply doesn't happen to support that function.This wouldn't be so bad if the configuration options not supported by your specific u-blox module were greyed out or you were offered some other means to indicate the futility of programming an unsupported feature. Anyway, with my suspicions confirmed over the slapdash programming effort that went into the u-centre UI, I turned to the TP5 item which I knew from experience was the correct menu option. Although I can program any frequency from 1 to 24,000,000Hz in 1 Hz steps, I decided to program a 1KHz 50% duty cycle pulse so I could both carry on testing my existing PLL setup and also look out for any of the symptoms of a shrinking pulse that had plagued the NEO-6M.After waiting for 15 minutes to confirm that, even after disturbing the divider chain to induce a better alignment of the pulses, the PLL was behaving in a rational manner (tuning voltage glacially dropping from 4.6v as the divider chain pulse edge approached the balance point, around a phase lag of 125 deg, I left it for an hour or two to its own devices whilst me and the missus nipped out for a bit of shopping, returning to find it had finally settled at said balance point with a tuning voltage of 3.22v verifying it had finally phase locked the OCXO.It seems stable in that any disturbance, which detunes the OCXO, once removed, causes the PLL to glacially restore the balance rather than, in the case of the NEO-6M, send it tumbling out of control (LPF voltage nose diving or climbing to the Vcc limit). Basically, I've just proved that all my efforts at getting an OCXO of any sort to phase lock to that NEO-6M's PPS had been doomed from the outset - who knew? Seriously, did anyone know?Anyway, "Honour being satisfied" as it were, I plan on programming the PPS to output a 100KHz 50% duty cycle pulse by which to phase lock one of the 10MHz OCXOs (although I may still ultimately decide to utilise the 13MHz one with its convenience of a 5 volt power requirement).Several points of interest I noticed when testing the replacement M8N module were that the SV signals were down by around 18dB or so on the original M8N module but, by eliminating the 5m antenna extension cable, I saw the expected 6 to 7dB boost, unlike the inexplicably low 1 to 2 dB loss it had displayed with the original module. I'd surmised at the time, that the incredibly low loss from 5 metres of RG174 at 1.6GHz was more to do with a fortuitous impedance mismatch at the RF input of the original whilst now, I'm possibly just witnessing the effect of a proper impedance match (or possibly a Lo-Z mismatch because a 270 ohm antenna feed resistor had been substituted in error by a 27 ohm resistor [Edit:- Belay that! a more likely possibility would be a shorted out inductor] - I'll take a closer look later, leave no stone unturned and all that - you never know what you might discover if you don't take a very close look).The other difference being that despite configuring to receive the SBAS geo-synchronous SVs over 6 hours ago, I have yet to see any signs of one. As for the Galileo constellation, I'd already given them up as a lost cause since, apart from a fleeting appearance during the first month since I acquired an external GPS antenna after buying the first module, there's been no further signs, presumably due to a protocol change in the Galileo system requiring a firmware update to get the M8N to recognise them again.The damaged M8N was still showing the SBAS SVs whenever I programmed the module to display their presence until quite recently so that's another test to run. It's possible I might have a 'fake' M8N but whatever its shortcomings, it still appears to be just as useful as the original module for making up a GPSDO (it's kept the OCXO locked to the 1KHz square wave output from its PPS line for the best part of the past 6 hours.).Now that I have the option once again, I'm going to reconfigure the PLL for 100KHz operation and shift my life into the 'fast lane' (FVSVO "fast"). I'll report any further progress once I've made some.JBG

author: Electro Fan
author_rank: Super Contributor
timestamp: June 09, 2019, 02:49:13 am
content: "An alternative to that scheme, if you feel the need for protracted days long runs, would be to use a pair of 7AH alarm batteries (the cheapest SLA battery option if you can have a friendly chat with your local alarm shop's proprietor) andconnect them via Schotky barrier rectifier diodes so you can attach a freshly charged alternate when the first starts to show signs of exhaustion,allowing a breakless changeoverto put the changed out battery on charge. Thinking about it, you might want to swap the batteries between load and charger on a shorter cycle to avoid going beyond a 50% DoD and the risk of premature wear out."That's a pretty neat idea.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 10, 2019, 03:33:34 am
content: Quote from: Electro Fan on June 09, 2019, 02:49:13 amThank you, it's not particularly original but at least it gives you a means of checking whether the switching ripple noise from a cheap wallwart is going to compromise the GPSDO's performance.The 7AH rating is based on the 20 hour discharge at 350mA which, in my case with those CQE OCXOs settling down to around the 80 to 90mA mark and my simple GPSDO (basically Gyro's design here:https://www.eevblog.com/forum/projects/my-u-blox-lea-6t-based-gpsdo-(very-scruffy-initial-breadboard-stage)/msg1493431/#msg1493431) taking maybe only another 100mA all in (fake NEO M8N - no flashand an FTD232 usb adapter, just one '390 and a '86 for the PD, not forgetting the obligatory 74HC14), a pair of those batteries would only reach a DoD of 34% on a 12 hour cycle turnaround so in my case, it would be entirely do-able.If your GPSDO draws rather more than the 200mA estimate I made for mine you might need to swap the batteries around on an eight hour cycle if you don't want to wear them out prematurely. With a nod to Peukert's law, a 400mA draw will get you to a 50% DoD in just about 8 hours. Any higher than that, to misquote "Jaws", and you're gonna need a bigger battery.Mind you, if you can prove the point in a 16 hour session, you can get away with a shorter turn around cycle if you have a suitable constant voltage charger to keep pace with the higher discharge and recharge cycles involved. Alternatively, if you can borrow the use of a car starting battery, you could save on that investment in a couple of cheap alarm batteries. After all, you're looking at the most pragmatic way to take the questionable quality of a 'cheap wallwart' out of the equation.Getting back to the latest development with my own little GPSDO project, after swapping from the 5v 13MHz 3Vpp sq wave out OCXO to one of the 12v 4Vpp sine wave output ones by using my liberated prototyping breadboard as a 12v annexe to mount it alongside of the original rat's (Vipers?) nest wiring lashup last night, I decided to live dangerously and rebuild it all onto the one breadboard.Before I could rebuild a now much simplified configuration (two less ICs to find room for), I had to figure out a safe way to provided a +12v feed to the OCXO. I soldered a short 2 wire female header plugged flylead (the sort used to connect a PC's front panel soft on/off switch to the MoBo header pins) onto the switched 6.5 to 12 volt feed from the wallwart socket of the YwRobot MB102 breadboard power supply using hot melt glue to fix it in place and provide strain relief. The ends of the jumper wires fit quite nicely into these female header pin connectors making it a neat way to safely provide a 9 or 12 volt feed directly from whatever wallwart I choose between the 9 or 12 volt ones I've kept aside for powering my breadboard concoctions.Whilst I'd not had any stability issues with the 13MHz OCXO, I couldn't say the same for the 12v 10MHz units. I found myself having to hold onto then slowly release the wire taking the oscillator output to the 74HC14 I was using as a buffer between the 74HCT390 dual divide by ten IC to get a rather fragile lock. In the end, it occurred to me that maybe I just needed to bias the HC14's input to the mid point since, unlike the 13MHz unit which outputs a DC coupled 3Vpp square wave, the 10MHz outputs a capacitively coupled 4Vpp sine wave.Once I'd managed to retrieve a couple of 2K2 resistors from my 'salvage' collection and fitted them, all was sweetness and light once more (although it did help improve phase locking performance a tad when I remembered to replace the earthing link wire to reconnect the ground side of the LPF).Anyway, the new setup with the 10MHz OCXOs now demonstrates the same remarkably stable behaviour that, considering the rat's nest breadboard wiring involved, I'd seen with the 13MHz unit. It even works just fine when using the 9v smpsu wallwart instead of the 12v one (it just takes a little longer to warm up, maybe five minutes versus three on the 12v supply). The cheap 'n' cheerful wallwarts don't seem to be cause for concern right now as far as I can tell. That, of course, may well all change once I've had time to do more thorough testing on the finalised boxed up version.TBH, apart from the effect on the warm up time, I don't think the 12 volt supply to these OCXOs is particularly critical. They all include a well filtered low noise LDO regulator to power the really critical oscillator and oven controller circuit which regulates the heating current drawn from the 12v rail in the power transistor that acts as the heating element. Its collector voltage doesn't matter so much as long as there's sufficient voltage to allow the maximum current limited flow to generate enough heat.I've had them get up to temperature when I was testing the first batch of three off a 5.15v supply. It took a good eight minutes or so and the output was a little on the weak side although at the correct frequency. Since all the indications were that I wasn't supplying enough voltage (that, and the fact that I now had another two spares), I went straight to a 9 volt supply (a five volt part would have been damaged almost straight away at another two volts beyond the 7 volt absolute maximum limit typically quoted for five volt parts) and then to the full twelve volts once my test unit had managed to survive this potential voltage abuse for an hour or so.I was treading very carefully since the vendor had absolutely no data on these OCXOs and my first 13MHz example had proved to be a 5 volt part once I'd realised that my 5 volt supply was being dragged down to less than 4.75v due to the warm up demand, just below the +/-5% tolerance limit I'd have expect to have been applied in this case. As soon as I fixed the low 5 volts issue, it would then spring immediately into life unlike before when it had taken several minutes before even any sign of activity became apparent on the sub-standard supply voltage which had eventually reached the dizzy heights of 4.82 volts.Whilst I'd expect the supply voltage to be of super-critical importance with a 5v OCXO, there's a lot more leaway in this regard for the 12v units as my current lashup is so clearly demonstrating with an 8.98v supply as I type this little missive.You could probably get away with using a low noise 10v LDO to clean up the ripple from a cheap 12v wallwart and save the expense of an ultra low ripple 12v supply which savings can be used to stock a couple of 'cheap spares' against the risk of your one and only suitable PSU expiring unexpectedly.I was observing the effects of positional deviations on the phase between the signal generator's "10MHz" (I have to tune down by some 7 or 8mHz to get a 'stationary' display of the two waveforms) which can turn any calibration attempts into a game of "Chase Wil o' the Whisp", if you overlook this phenomena. This is what must drive the need for extremely long time constants and the desire to get hold of Timing GPS Modules. TBH, since I was expecting to see this effect, I wasn't at all surprised and it rather confirms that my basic GPSDO test rig is functioning as intended, considering the relatively short time constant being used in the PD's output filter.I'm actually using 22 and 220μF aluminium electrolytics rather than the 33 and 330μF tantalum caps shown in Gyro's circuit. I'm thinking of substituting these with a 47 and a 470μF cap by way of an experiment. TBH, I'm surprised at how well this filter is working considering the issue of leakage in normal electrolytics. I'd normally only consider the use of non-electrolytic types in filters but needs must and electrolytics were the only type in these values I had to hand.Unlike Gyro's circuit, I've missed out the cmos rro opamp buffer and taken the filter's output directly to the OCXO's control pin. I might strap one of the ten 5v cmos rro opamps, that had finally arrived nearly three weeks ago, onto a SOT adapter board so I can test out a higher impedance version of the LPF (1MR with a 2.2μF and a 22μF with 100KR series resistor combination). TBH, I'm surprised Gyro didn't take advantage of the buffer amp himself to reduce the caps down to a more manageable size. Since he posted that circuit diagram over a year ago with no further follow ups in that thread, it's a little late in the day to query him on that. I might get an answer to that when I come to add the buffer amp and rescale the filter values.Anyway, that's the latest situation regarding what I want from a GPSDO which is simply a basic functioning frequency reference good enough to feed the external reference inputs of test gear and HF transceivers and such like (I'm not planning on working with GHz rated kit - at least not just yet). It makes a pleasant change for once, to be be able at long last, report some real progress. What this means is that I'll have to add a CR2032 coin cell holder to get round the limitations of the tiny 80 milli-Farad super-cap that provides less than an hour's worth of data backup (about an hour when only used to maintain the RTC which is the primary purpose of the BBRam). This should give me at least 250 days of data retention (220mAH and a 30μA drain current).Considering the fault with the NEO-6M's shrinking PPS, that did at least have the option to save the config data into either the SPI-Flash or the I2C-EEPROM as well as the Flash and the BBRam. I found that if I selected all four options, the settings would remain intact even overnight. I'm guessing it was either the SPI-Flash or the I2C-EEPROM which was saving the data but ICBA trying to figure which it was. I simply selected all four options whenever I wanted tp make sure the settings didn't evaporate overnight. Unfortunately, the same trick doesn't work with this fake M8N module, hence the additional complication of wiring up a lithium coin cell holder to complete the project.JBG

author: jimmc
author_rank: Frequent Contributor
timestamp: June 10, 2019, 04:02:09 pm
content: An alternative way of long term saving and setting the configuration is to program a small MCU to set-up the module via the serial port.This is only needed at start-up and the MCU is then free to be used as a divider (if clocked from the OCXO).I did this with a NEO-7M and an ATtiny85 to give a 10kHz comparison frequency, the ATtiny85 also turned on the NMEA output only when an external USB-RS232 converter was powered.This was done to reduce the possibility of ripple on the power rail at the1Hz rate of the data bursts when monitoring not required. (Simple lock LED used in normal operation.)As an aside a second ATtiny85 (again clocked from the OCXO @10MHz) was programmed to give an output frequency, switch selectable, from 5MHz, 2.5MHz, 1MHz down to 1Hz (with 1:1 M/S ratio).Jim

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 11, 2019, 04:13:49 am
content: Quote from: jimmc on June 10, 2019, 04:02:09 pmHi Jim,Thanks for the suggestion. I did consider making use of an Arduino nano I'd bought at the beginning of February just after I'd bought the first (genuine but now broken) NEO M8N module I'd purchased right at the end of January, but adding a CR2032 seemed the more pragmatic approach in view of my lack of Arduino programming expertise. I'll be incorporating the FD232 USB to serial adapter that's currently on the prototyping board lashup to let me monitor and configure the fake M8N module so even if I allow the CR2032 to go flat after some 250 to 300 days worth of not being powered up, it'll only be a minor inconvenience to replace the coin cell and reconfigure it.I've not even bothered unsealing the anti-static bag the Arduino nano had arrived in although I do intend to try my hand at programming it in the not too distant future. The last time I did anything like this was 35 years ago writing the control routines in Z80 assembler to adapt a Philips solenoid controlled data cassette drive as a floppy disk substitute (332KB formatted storage each side of a C60 with a worst case end to end search time of 14 seconds and 19200 bps read/write speed).This had allowed me to load the 8K TCL BASIC, which had been originally supplied on a set of four 2KB eproms which I'd relocated into ram space, in just 5 seconds flat - faster than that crappy waferdrive system of Clive's at a time when 3.5 inch SD floppy disk drives had just dropped in price to a mere hundred quid a pop (and S100bus FDC cards also happened to be around the 100 quid plus VAT mark).I'd had plenty of financial incentive at the time to put these surplus decks into use as a poor man's alternative to floppy disks - all the convenience of a floppy disk drive system but at a slower pace. Of course, this all changed over the next five years as the IBM PC created a mass market to drive competition which drove prices right down to mere 'chump change'.I've still got that old S100 bus kit but the original main board with its dozens of socketed chips appears to have succumbed to the damp basement conditions it had been kept in for some 30 years or so and I'm in the middle of swapping to a spare (but unmodified) board which had been stored in a much warmer and drier part of the basement.However, I've been in the middle of this swap out job for over a year now since I couldn't maintain my enthusiasm to bring it back to life. Still, at least it's now parked in a more benign environment for the time being so I may yet finish the repair project if the urge to get it going again ever returns.Anyhow, my GPSDO setup seems to be working just fine on the breadboard setup. TBH, it's a wonder it worked at all when I realised I'd actually forgotten not just one earthing strap wire in the LPF circuit but two! Even more remarkable was that the 74HC14 was able to perform at all after I'd misplaced the ground wire onto the adjacent output pin when I'd fitted a 100nF disk ceramic cap over the IC to improve the supply decoupling the day before when I'd been trying to get to the bottom of the weird instability on the oscillator drive signal which had proved to be the lack of dc biassing resistors on the input of the gate I'd selected to buffer the OCXO's ac coupled output.I wouldn't have spotted this Obe Won Kenobe error if I hadn't been looking to use one of the spare inverters to buffer the oscillator feed to the FY6600 counter's input/trigger sockets just to take the opportunity to take pictures of it showing an exact reading of 10,000,000.00Hz on its display at the 100 seconds gate time setting (maximum digits).The frequency counter function built into the FY6600 is not something I've ever bothered with before other than for a few brief tests to verify that it actually worked. It seemed a good opportunity, now that the internal reference is comfortably within 1ppb of exactitude, to test just how it would display a frequency more accurate than its resolution by at least one more order of magnitude.Funnily enough, it would display the correct frequency about half the time and under-read it by 0.01Hz the other half whilst the 1 and 10 second gate times resulted in either an exact reading about two thirds of the time or else show a "1" in the final digit position for the other third.Considering that I've seen much higher specced counters displaying even larger discrepancies on the least significant digit(s) when hooked up to a 10MHz GPSDO reference in various youtube vids, this doesn't seem so bad, especially considering the price difference involved.Still, I had rather hoped to be bored by an unremitting display of all zereos after the Leading "1". I guess it's all down to how well the gating circuitry has been implemented. Incidentally, I saw the same results using either the front panel input socket (AC coupled) or the rear panel 'trigger' input socket (DC coupled).I'd like to recalibrate the signal generator's OCXO reference to better than the +600ppt it currenly seems to be set at but the apparent short lived frequency drifts that arise out of the phase shifts on the GPS's PPS line, due to the wandering in the calculated position data as displayed in the deviation map, lends an element of confusion to the calibration process. I'll resist the urge to fiddle right now until I have a better setup in regard of a longer time constant filter to minimise this effect (and possibly a relocation of the GPS antenna to get a less obstructed view of the whole sky - it's currently blocked on a large part of the eastern sky).The fact I can now confirm my suspicions over the effect of a wandering fix as per the deviation map display is a sign that I've made some useful progress in my basic DIY GPSDO project. Now all I have to do is rebuild what I've got onto a PCB and fit it into a case. I haven't entirely nailed down the details (I'll add one of those 5v cmos rro opamps so I can make a longer time constant filter with higher resistor values rather than use larger value caps) but I think it's more or less there as a cheap DIY GPSDO. I've got another half dozen of those CQE OCXOs from which to develop and build more ambitious GPSDO projects.JBG

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 15, 2019, 11:24:06 pm
content: I've managed to sort the breadboard lashup out with a 5v cmos RRO opamp to buffer the Phase detector's LPF output so I could experiment with using a 1MR in place of the 100KR series element. This worked but it didn't really solve the issue with the plot error induced phase shifts (as correlated with the deviation map plot).Since all it seemed to do was extend the lockup settling time by an extra 5 minutes or so, I decided I'd be better off reverting to the original filter value and look to improving the external patch antenna location to reduce the excursions in the deviation map plot (at least there's less lag between what's shown in the deviation plot and the resulting phase shifts in the 'scope trace of my signal generator and the GPSDO reference).BTW, when I added the buffer amp, I was expecting to eliminate the phase shift effect of connecting and disconnecting the 10MR loading from the DMM I'm using to monitor the EFC line but, strangely, the loading effect hadn't gone away. After wiring up the second amp in the dual RRO opamp to provide a buffered DMM feed from the output of the opamp driving the EFC, largely to avoid leaving the second amp with floating terminals and the possibility of random oscillations, I was still experiencing the same effect.Eventually, I realised it wasn't the 10MR loading from the DMM but just the fact of adding extra wired connections to my lashed together GPSDO that was the culprit. In the end I realised that I only needed to put my hand near the OCXO's output link wire to induce this phase shifting effect. It seems my concoction is suffering from 'Theremin Syndrome'(tm)It's not a major issue with something lashed up on a prototyping breadboard but it had me scratching my head for a while after I thought I'd sorted the initial and expected effect of loading the unbuffered EFC with the DMM's 10MR voltmeter impedance by adding the opamp.Anyway, that was last night's good deeds (of work which didn't go unpunished) which included my having another go at calibrating the OCXO upgrade to the FY6600, seemingly with a lot more success than the last time I'd tried this with the rather jittery 10MHz output from the previous M8N module.However, it's no longer an issue of non-integer division of a 48MHz TCXO down to a 10MHz reference, nor is it the damned "Sawtooth" corrections to hold the PPS to within 20.8333ns of the nearest 48MHz clock edge. When it comes to averaging over periods of thousands of seconds (and rising), such minor deficiencies eventually dilute themselves into utter insignificance. What I find most troublesome now, are the nanoseconds (5 to 30 of them at a time) of phase shift due to position plotting errors as evidenced in U-blox's U-centre application's deviation map plots.When you're aiming for better than 30ppt accuracy, such variations swamp the hoped for very slow drift between the 'scope traces, making it difficult in the extreme to achieve the desired precision that is now a real possibility given a stable enough reference.Since I knew that my external active mag mount patch antenna location (on a ballasted biscuit tin on the flat roof of my office bay window with obstructing brickwork to the East) wasn't helping this situation, I decided to use an old steel drawer, ballasted with four used car batteries, to support the antenna on a 'jamjar' lid screwed into the wooden plug into the top of an 8 foot aluminium pole clamped to one corner of said ballasted drawer which now sits on the bay window roof in place of the biscuit tin arrangement.As per usual with these DIY projects, more time is spent in preparation which, in my case, is largely spent trying to track down the various items I knew should still be lying around somewhere in the basement than on the actual construction.I managed to sort out the major issues last night, leaving the fiddly details to be dealt with this afternoon. Suffice to say, after some effort hoiking the bits (including those four car batteries!) to the second floor bedroom above my office from where I could access the bay roof via the window, I finally managed to elevate the antenna into a much less obstructed view of the open sky.Mind you, the ridge of the roof is still a good foot higher but an extra foot on the length of the pole would have meant having the join between the end of the 5 metre antenna cable and the 5 metre extension outside which I wanted to avoid. In any case, the combined length of the antenna cabling is barely sufficient as it is and not very much of the Eastern sky is being blocked compared to how it had been previously. Indeed, I was seeing some 19 to 20 SVs (after adding the GLONAS constellation to my previously GPS exclusive setup) "In the Green" (right now at 22:55 BST, it's down to 15 'greens'). The sky plot, even after just six hours or so of data, is looking far better balanced all round with the obstructing effect only showing for SV elevations in the lower 10 deg section of the SE quadrant pair.I'm still seeing deviations of position but they seem to be spread over a smaller diameter, typically around 4 metres versus the previous 6 to 8 metre diameter 'dance floor'. I can still see variations in phase that correlate to the deviation map display which swamp the extremely slow drift due to the discrepancy in frequency between the free running OCXO in the signal generator and the long term precision of a GPS reference.It's difficult to be precise on account of these phase shifts but since it's taken some 5 hours for the signal generator's 10MHz to drift out of phase by a whole cycle, I reckon, more by pure dumb luck than anything else, that it's now only adrift by a mere 5.5ppt. I suspect it's passing through the 0ppt error point at possibly some 5 to 15 ppt of drift per day. I'll have a better idea when I check it tomorrow.This issue with phase shifts due to plotting errors with navigation only GPS units such as the M8N can be minimised by using timing GPS modules. However, I don't know whether the expense of such timing modules could be justified in a DIY GPSDO project. My question in this case being, "Just how much of a reduction in such phase errors can I expect to see with a timing GPS module?" If it's as much as a tenfold reduction, then it can allow me to continue relying solely upon a simple PLL arrangement without resorting to the extra complication of microprocessor control. Obviously, some means of tracking the EFC voltage to record the current trend and maintain it in the event of loss of lock would be desirable, implying the use of a micro-controller in any case.However, for a cheap DIY GPSDO, the Luddites amongst us can always record this voltage on a daily then weekly basis from which to manually trim the OCXO during such loss of lock events. Of course, this would only be putting off the inevitable upgrade to a micro-controller. In the meantime, I'll put up with these variations of phase until I get to grips with utilising that Arduino nano that's still sealed within its protective anti-static packaging.As for my signal generator's OCXO, it appears to have halted and reversed its frequency drift. However, I suspect it might be as much to do with room temperature changes as an ageing effect. When you're down to parts per trillion accuracy levels, ambient temperature changes as small as just one or two degrees, despite the moderately insulated ovenised XO, will start to show up. The phase variations from a basic GPSDO just make such changes look all the more dramatic, hence my new found annoyance. TEA anyone?JBG

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 16, 2019, 06:50:54 pm
content: A quick follow up to my previous post:A check late this morning, after timing three cycles showed the signal generator had drifted from its previous 5.5ppt error figure to an error of 155ppt. Some six hours later and it would appear to have dropped to 110ppt. It does look as though ambient temperature is influencing the drift rate as I suspected despite it only being a matter of a couple of degrees (Celsius, FTAOD in the minds of the Leftpondian community).The skyplot does indeed show a marked improvement after yesterday's antenna relocation exercise. No doubt, an extra 18 inches of height would have provided some additional improvement but not a lot more in view of the upward sloping terrain to the east, along with the ground clutter effect of the dwellings located thereon.I've increased the default 5 degree elevation mask setting to 10 degrees which seems to have helped reduce the size of the 'dance floor' as it appears in the deviation map plots. This has reduced the number of usable SVs from a typical count of twenty down to eighteen (GPS and GLONAS constellations). We've had the odd heavy shower with variations between sunny blue skies and heavy overcast which does appear to have an effect on SV reception (possibly as much as a 5 to 6dB variation) which no doubt makes some small contribution to the "navigational fix errors" which impose a phase shifting effect upon the GPSDO output.With regard to GPS patch antennas, I am a little bit puzzled over an oddball unit I picked up at the Blackpool radioham rally some seven weeks ago. It's slightly larger than the mag mount gps 'puck' antenna I'm currently using (andisn'tmag mount) and has two (not one!) co-ax cables exiting from a corner located strain relief grommet. One is about two metres in length, sporting the usual male SMA plug whilst the second 15cm one sports an in-line SMAsocket. I'm guessing this second one is for GSM use rather than to allow another GPS antenna to be daisy-chained on but search as I might in the 'images' category, I've not seen any other examples that look remotely like it.Perhaps someone here might recognise it from the attached images and verify what the short co-ax flylead connection's intended function is. For added value, I've also included the ridiculously tiny active patch antenna that came with the NEO-6M module, and the salvaged passive patch antenna I'd rescued from the original NEO-M8N module. All three antennas are fully working (for ever increasing values of "working" going from left to right).Incidentally, the groundplane of that rescued patch antenna appears to have been 'tinned' with that low melting point desoldering alloy since a momentary contact with the dried out solder blob on my soldering iron bit resulted in an instant accidental tinning event showing itself as that marking on the opposite side of the centre pin to where I'd soldered the braid of the co-ax flylead.JBG