What frequency standard is sufficient? - Page 1
Forum section: electronics / testgear

author: Housedad
author_rank: Frequent Contributor
timestamp: October 08, 2018, 01:15:33 am
content: Tcxo, Ocxo, rubidium, GPS disciplined and all combinations thereof.I can really see the use in having a frequency standard on the bench. The problem for me is what level and cost is right for me? I know it is whatever you are working on will dictate it, but I'd rather just buy what I need for future uses now.I'm studying now for ham, and am looking to start repairing various things for resale. Older radios, test equipment, the odd repairable tv, God knows what. Calibrating test equipment is a good reason, too.What is a good balance type standard that is good for the medium beginner and will still be useful a ways down the road and not break the bank?

author: Vgkid
author_rank: Super Contributor
timestamp: October 08, 2018, 01:25:12 am
content: What is your budget? Do you have access to the sky(not living in a heavily crowded city) gps has many options...

author: tsman
author_rank: Frequent Contributor
timestamp: October 08, 2018, 01:35:20 am
content: What test equipment do you already have or plan to get? A frequency standard depends on what you're trying to do and what other equipment you have. If you're just starting out then getting good quality test equipment is far more important than a high precision frequency standard. You can always buy a frequency standard later and hook it up to the external reference inputs anyway. You may find that the built-in oscillators are good enough for what you need to do.

author: retrolefty
author_rank: Super Contributor
timestamp: October 08, 2018, 01:38:39 am
content: A used 10MHZ OCXO can be easily found on E-bay and is a good start for a beginners test bench More complex/accurate can follow as the need is justified.

author: radioactive
author_rank: Regular Contributor
timestamp: October 08, 2018, 01:52:00 am
content: I ran across this open-source GPS disciplined oscillator product the other day...https://www.tindie.com/products/nsayer/gps-disciplined-xcxo/I am not affiliated with this product in any way, but I think it looks interesting for the price. Worth a look for the budget conscious in my opinion.

author: Housedad
author_rank: Frequent Contributor
timestamp: October 08, 2018, 01:55:04 am
content: I have plenty of sky. I live just outside the edge of the suburbs. I'd like to keep itwellbelow $200 US.I have a few pieces of test gear and want to aquire more. Repairing used equipment if needed to keep the costs down.Equipment:DS 1054Z Oscilloscope (hacked 100MHz)Instek 100 mhz analog oscilloscope2 bench DMM's InstekRigol dg1022 function generator(2)Gdp 3032S power suppliesBK precision DMM handheldDe5000 LCR meter on it's way.Currently trying to repair a Wavetek 2410 Rf generator.And most of the standard bench repair/test/play stuff and tools.https://www.eevblog.com/forum/chat/whats-your-work-benchlab-look-like-post-some-pictures-of-your-lab/msg1179703/#msg1179703

author: xrunner
author_rank: Super Contributor
timestamp: October 08, 2018, 02:23:23 am
content: Quote from: Housedad on October 08, 2018, 01:55:04 amI have a BG7TBL GPSDO version and it works great for $133 -BG7TBL

author: tkamiya
author_rank: Super Contributor
timestamp: October 08, 2018, 02:38:27 am
content: Well, at that price range, you can expect to get OCXO GPDO. Remember; though, you need that, antenna, and distribution amplifier at minimum. So if you are looking to keep the cost down, all of that needs to be looked at. Silly thing as coax and connectors can add to quite a bit.At this point, I don't think you need one as there is nothing you will slave to a master oscillator.I have two Nortel GPSTM, which is a GPS trained oven crystal oscillator. A telecom surplus. It's enough for me. I have frequency counters and RF generators slaved. You can get these within your budget. (but you will break your bank when you add distribution amplifier) I use it to establish standard on frequency counters and maintain consistency in my lab.Realistically, with it, all of my frequency counters will all zero and display the same number. And RF generator will agree with frequency counter. Without it though, last digit or two will flicker and drift as my air conditioner cycles. BUT, with those drifts and flickers, they are far more accurate than ham radio equipment demands. Ham gears have TCXO as high stability options. Unless I'm using two RF generators at the same time, I think of them as sanity check devices.

author: Housedad
author_rank: Frequent Contributor
timestamp: October 08, 2018, 02:52:35 am
content: Quote from: tkamiya on October 08, 2018, 02:38:27 amThat's what I'm thinking about the first uses too. My counter and the wavetek RF generator I recently bought are used and I have no assurance if they are even close to spec. The wavetek has a ocxo so I probably could use one at least as accurate to check it against.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 08, 2018, 03:27:44 am
content: There are sanity checks you can do with almost no equipment. Zero beat with a commercial station. Zero beat with WWV. Run the oscillator into a long counter (or better a medium counter that generates interrupts for a computerized counter) and then compare the accumulated time to a clock like your cell phone.The frequency reference is a great convenience and can speed up a sanity check, but it isn't necessary.

author: ZL1CVD
author_rank: Regular Contributor
timestamp: October 08, 2018, 04:33:24 am
content: I picked up a Symmetricom 089-03861-02 GPSDO module for around $80US a while back. Added a $15 Chinese GPS antenna which came with 5m lead. You can also get some magnetic mount antennas for under $10. Set it up and left it running for a week. Came back and checked it was all good then calibrated my R&S CRTU analyser to it. Packed away the GPSDO for six to 12 months or so before I do it all again. Most quality brand RF analyzers have a good OCXO inside so once set, your good for at least a year. But yep, its important to have a good time base. I looked at rubidium but you get as good from any GPSDO designed for cellular timing. Plus for us outside the US, rubidium will probably = customs nightmare.

author: tautech
author_rank: Super Contributor
timestamp: October 08, 2018, 04:36:31 am
content: I'd be tempted by one of member Leo Bodnar's offerings......no pissing around, just plug it together and load some settings:http://www.leobodnar.com/shop/index.php?main_page=index&cPath=107

author: nctnico
author_rank: Super Contributor
timestamp: October 08, 2018, 05:52:42 am
content: I'd get a GPDSO and be done with it.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: October 08, 2018, 08:57:31 am
content: I have Bodnar's GPSDO. It can serve as sort of 2ch signal gen also, quite useful.

author: tggzzz
author_rank: Super Contributor
timestamp: October 08, 2018, 12:52:37 pm
content: Quote from: Housedad on October 08, 2018, 01:15:33 amTo simply test the equipment you mention, almost anything is good enough. CatalinaWOW's reply is spot on.There's no point in making any statements/guesses until you know what you want to measure, and to what accuracy, resolution etc.If you are interested in RF stuff, then sooner or later you will run into phase noise, which is often more important than absolute accuracy and resolution.Once you start getting into the frequency/timenut domain, you will want to find out about Allen deviations.Until you have a positive requirement, my inclination would be to go for a decent secondhand spectrum analyser before a frequency source. After all, they contain a decent frequency source plus a frequency synthesiser to observe any frequency you want! (Plus you can often use the LO output).

author: cdev
author_rank: Super Contributor
timestamp: October 08, 2018, 01:59:08 pm
content: I paid $45 on ebay for a telecom surplus TruePosition GPSDO, implemented on a card that plugs into a rack-resident mother board, which I dont have. But the instructions to use it are on the weband also here, (a long and informative thread about them).The TruePosition is built far better than anything I could ever build myself. Its likely as accurate as one of the Trimble or Symmetricom units that cost more than twice as much, even used.It's gradually emerged that it has a lot of configurability, thanks to a number of people's contributions. I wish we had the manufacturers specifications but what we do have is still quite good.This post is way too long and pieces are redundant.Basically, your needs are totally dependent on what you are doing, but the availability of this specific unit makes that choice a pretty easy one for most users here who are seeking a lab quality 10 MHz time and frequency standard.Why not buy a standalone timing GPS?For some uses, that makes sense too. For example, for an NTP server, I think a timing GPS is a better choice. A plain GPS which gives you 25-40 ns accuracy is much more accurate than you need.All of the better GPSs end up giving the exact same results to me, all the time, with NTP once I figured out how to configure the offset correctly.(Anybody who has more info, please share it here!)Any good GPS thats successful in a mobile or UAV application likely has a good TCXO, already (or they wouldn't work in the wide range of environmental conditions they encounter, or at best they would work unreliably)Timing applications in telecom and especially space to earth applications like GPS (where there is a very low signal to noise ratio are much more demanding than other radio applications.An OCXO smooths out the frequency vs. time. Otherwise the GPSDO's frequency will vary in a sawtooth manner between two extremes, within a range of typically 50 nanoseconds or so. The best ones might do better, maybe half that.Experts all will tell us that the jitter on any GPSs (and all GPSs have this problem) is too high for some (RF) applications (for the GPS-resident oscillator to act as a frequency standard in the most demanding applications) all by itself without an oscillator with a long time constant- a means of smoothing this signal out.. A quartz or rubidium oscillator - 'free running' will emit a pure sine wave that changes in frequency very slowly, but it may be off a bit unless its aligned with a GPS both periodically (calibrated to cancel out the crystals natural aging) and also on an ongoing basis by the GPS's atomic clock.One feature one should look for in a timing GPS is "Timing Receiver Autonomous Integrity Monitoring" (abbreviated "TRAIM")Once the GPS is installed in its permanent home and you know its not going to move using TRAIM you can train it so that after that it can maintain its lock for timing purposes with only one satellite.The commercial GPSDOs that originated in cell phone equipment, although used, are likely the cheapest high quality signal source youre likely to find. Some of the GPSDOs on ebay, taobao, etc. are comparable in quality or better. But I really doubt if most of the homemade ones - with the possible exception of the very best designs - perhaps ones that were in essence crowd sourced, improved by a community of high knowledge users.- without that I doubt if most attempts are going to be as good as a telecom surplus unit thats not at the end of its useful life due to aging or broken in some way.Personally I would make sure that I got one with an oven if I planned on serious radio use in the future. There probably is a lot of variation between ebay sourced GPSDOs with some of the best being made by a Chinese ham I think his call is bg7tbl. See this thread here.https://www.eevblog.com/forum/testgear/bg7tbl-gpsdo-master-reference/But, here is the problem with his unit, if somebody wants to have the ability to tweak the settings, you really can't.Some of the functionality on most GPSs is accessed by sending commands to the unit via a serial port (well, more often than not on newer units it is a 3.3 or 5 volt- serial port, make sure you know which - lower voltage UART)As far as I know some of the homemade units don't have any way of sending commands to the units to do much in the way of configuration. Some may have a button to initiate training the unit for a new location, and thats it. Some don't even have that.OTOH, some of the GPS-only units are very flexible and configurable to emit square waves over a wide range of frequencies, basically you get a DDS. Considering that some Ublox 6T units that have that dual output capability, two configurable square wave outputs can be found for as little as $25, thats an amazing deal.IF you can live with the jitter. Or get both.

author: tggzzz
author_rank: Super Contributor
timestamp: October 08, 2018, 04:16:57 pm
content: Quote from: cdev on October 08, 2018, 01:59:08 pmMylimitedunderstanding is that the cheap units have a 48MHz internal clock, and the 1pps (etc) output signal is synchronous with that. Consequences:it is avisable to choose an output frequency that can be derived from 48MHz by simple division: 1MHz/4MHz/etc is good, but 10MHz isn'tthere will be a drift between the 48MHz clock and the GPS received signal, resulting in an occasional 21ns jumpand then there is all the behaviour associated with GPS signalsMore expensive units may or may not derive the 48MHz clock from the GPS signal.So, GPS is very good for long-term absolute accuracy, but is non-ideal for short-term stability. Hence the use of GPS receivers to "discipline" (a.k.a. correct) rubidium and crystal oscillators - which are stable in the short term but can drift in the long term.Hence the OP needs to figure out what he needs. GPS may be precisely the wrong technology.In the absence of other information, I suspect the OP should stick to a simple TCXO, and optionally use that as a DDS's clock source if he wants other frequencies.

author: TurboTom
author_rank: Super Contributor
timestamp: October 08, 2018, 04:59:37 pm
content: I tend to defend a Rb oscillator, simply for the convenience of it. It's portable, independant of unobstructed view to the sky and three minutes after powering up, it provides a fairly accurate reference signal that gets better over the next thirty some minutes. Jitter, accuracy and cleanlieness of the output signal very much depends on the particular oscillator used. I've got very good results with my Efratom LPRO101, much better than the Temex LPFRS. I didn't get my hands dirty on other atomic oscillators so I cannot tell anything about those.

author: nctnico
author_rank: Super Contributor
timestamp: October 08, 2018, 05:05:24 pm
content: Recently I've seen some measurements of a Rb clock from Ebay compared to a very good oscillator and the Rb clock showed a lot of short term instability. The measurement was repeated using a different Rb clock from Ebay and it showed the same results.

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2018, 05:19:18 pm
content: Was it superior to a TC Crystal though? (like a good one in good agilent equipment)I had some doubts about using my cheap RB one.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 08, 2018, 05:35:14 pm
content: Define superior. As noted in prior posts and threads on the subject, each has areas of superiority. Which is why they are combined in GPSDO in an attempt to get the best of both worlds. Rb alone is satisfactory for many uses. As is OCXO. Depends on your application and requirements.When I was a young engineer I used to laugh at the note our internal calibration department placed on counters. "Only calibrated to 1 part in 10^8". Since the work we were doing at the time wasn't particularly sensitive to frequency errors, we were more than satisfied with accuracy of 1 part in 10^3, or if we were going to get all anal about it 1 part in 10^4. The extra four orders of magnitude of accuracy really was a joke to us. Of course this was back when you still had to tune a radio, not just set the frequency you wanted so by today's standards no one cared about accuracy.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 08, 2018, 06:04:41 pm
content: Outside of very specific applications, an internal high stability reference from almost any piece of timing equipment is more than sufficient for testing and verification. Once you get into OCXO territory, with single digit ppm accuracy even with some aging, there are very few applications for faster absolute time, and typically, a good OCXO is going to give you some of the lowest phase noise and short term stability, so it's sufficient for any tight timing requirements that don't need an absolute reference. While a regular internal reference with an output is likely sufficient for most applications (the output is required to synchronize with another piece of gear), a high stability reference gets you another couple digits worth of absolute accuracy and lower phase noise and guilds the lily, so to speak.That being said, most people looking at an external reference like this don't necessarily require it for measurements they're making, they just want the certainty of a disciplined reference and the stability a high performance reference can offer. For value concerns, I'd say the peak right now is a GPSDO with an OCXO, either one of the units brewed up by hobbists (like the BG7TBL) or one of the surplus telecom units (like many of the Trimble options). Both should get you around 10^-9 stability/accuracy after warmed up and with a good GPS fix, which is equivalent to the mid to high tier frequency reference upgrade option on many bits of gear, except with the disciplining to keep the long term drift at basically zero.Rubidium sources cost more, but good ones can achieve another digit or two worth of accuracy and stability when disciplined. You have to be a bit careful because some rubidium sources are just directly driving the output and these won't be as stable in the short term or have as low phase noise, the best rubidium references have a Rb source disciplining an OCXO (and which in turn are disciplined to a GPS). The biggest benefit of Rubidium is long term stability, so by using one to discipline a crystal oscillator you effectively reduce the aging and drift effects of an OCXO by a couple digits, so a rubidium reference disciplined to GPS is not really inherently better performing than a GPSDO OCXO because the GPS signal is providing that same kind of long term stability, but the rubidium reference is a much better performer if the GPS signal is ever lost, since it's long term stability is much better. A GPS signal disciplining a really well put together rubidium reference which disciplines an OCXO is going to be somewhat more stable than most GPSDOs without rubidium, but it's basically going to be due to the long integration time used to discipline the rubidium source being able to better compensate for small fluctuations in GPS time, so it's a much smaller difference between OCXO vs. Rb when not locked to GPS.Since GPS is ubiquitous and good OCXOs are fairly cheap, that GPSDO with OCXO offers really good performance for money. Since it's generally easy to get a signal, the benefits of rubidium are much smaller than it would be without GPS.

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2018, 07:18:47 pm
content: So it sounds for general lab work, if you have a choice between a OSCO and a Rb stand alone unit, you should just go for the OSCO, because of phase noise concerns.Over what time period does an frequency specific but not phase sensitive experiment need to be run to benefit from a pure Rb clock (the small ~4x3 inch or less modules) rather then a OSCO?Are there any other situations where a pure Rb undisciplined clock can benefit?I am imagining something like, if you are testing a newly made filter to see the effects of burn in/aging, you can hook it up to a pure Rb source to categorize the distortion easier with less post processing and measurement requirements, vs a OSCO, say you wanna monitor it for a month (some kinda active filter etc).. this way you can monitor the distortion drift carefully... and you don't give a hoot about short term phase noise or drift since it will just look like a sineusoid riding some lower frequency, in the frequency domain (say doing extrapolations from a waterfall)...Am I correct in this?I like the term OSCO (oven stabilized crystal oscillator) better then something with a X in it, I keep thinking of the kool aid man and the cheetos cheetah.. my sunglasses are not dark enough and my ollies are too lame for that, man

author: usagi
author_rank: Frequent Contributor
timestamp: October 08, 2018, 07:33:19 pm
content: GPSDO OCXO is going to be far beyond the needs of most hobbyists. Few will need anything like a Rb or Cs unit.

author: MrW0lf
author_rank: Frequent Contributor
timestamp: October 08, 2018, 08:59:27 pm
content: If OCXO one is far beyond needs sounds just right to have one to check if TCXO one is working good... So have to buy 2! I have only TCXO one and already very nervous because cannot check it

author: tkamiya
author_rank: Super Contributor
timestamp: October 08, 2018, 09:17:31 pm
content: With price of GPSDO with OCXO being what it is, there is no need to even think - I *think*.Mine was more of a want than a need. It's a good thing to have. Better have it than not to have it. It was a fun thing to play with. I can't think of myself needing any more than what I have now. Couldn't go much cheaper either by going to TCXO.Rubidium, I understand, is worse for short term stability than OCXO? With GPS taking care of long term, I wonder why would I actually want Rubidium? I've seen a AT&T product that has GPS/Rubidium/TCXO. Can someone chime in and elaborate on relative merits?

author: nctnico
author_rank: Super Contributor
timestamp: October 08, 2018, 09:26:15 pm
content: Like others said: with a Rubidium reference you don't need to wait for a GPS lock (and have a place of an antenna). If you put a cleanup oscillator behind a Rubidium reference then you'll get a very stable frequency reference. But you still don't know if the frequency from the Rubidium clock is accurate while a GPSDO output is -sort of- synchronised with the clocks at (IIRC) the US Naval observatory.

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2018, 09:32:25 pm
content: I want to know if what I have is useful for anything in particular or just be certain that I have a 'incomplete device'. I know you can do it, but I am curious if anyone has academic answers.

author: nctnico
author_rank: Super Contributor
timestamp: October 08, 2018, 09:37:00 pm
content: Quote from: coppercone2 on October 08, 2018, 09:32:25 pmYou'd have to check the short and long term stability of the time reference in your instrument compared to those of a Rubidium clock and the GPDSO. I don't know the numbers from the top of my head but you can find the numbers for the BG7TBL GPDSO in the (long) thread about it. For the Rubidium clock they are in the datasheet.

author: coppercone2
author_rank: Super Contributor
timestamp: October 08, 2018, 09:59:26 pm
content: I glanced at specs like that before but there is not that much information given . I will try to determine some error related time constants with that information

author: ArthurDent
author_rank: Super Contributor
timestamp: October 08, 2018, 10:36:35 pm
content: A good GPS, even an older one like a Trimble Thunderbolt, steers the OCXO to exactly 10Mhz by using a large time constant of 500 seconds or more so any minor corrections are pretty smooth. For comparing 2 frequency standards, I trigger my scope off one GPS standard and connect the 10Mhz GPS, rubidium, or OCXO I’m testing to a vertical channel and check for drift at a sweep rate of 5ns/DIV. Almost all the GPS standards I’ve tested stay within +/-5ns over a long period of time. I’ve tested about 200 Thunderbolts plus several other telcom makes from HP, Lucent, and other companies and they are all pretty good. Any GPS standard that can be monitored on a PC with the ‘Lady Heather’ program is a plus because you can easily see how the GPS standard is doing over long periods of time. I also monitor the output of the unit I’m testing on a good counter. (see attached photo)Both rubidium and crystal oscillators have warm-up characteristics and after they have been on for several hours or days are pretty good but both have to be adjusted against a higher accuracy standard like a GPS. I have one OCXO frequency standard that has been running (other than battery replacements) for over 35 years. All crystal oscillators exhibit aging and generally speed up by about 2 parts in 10E10 every day and are quite predictable. If an OCXO is powered down then turned back on there is an error caused by ‘retrace’ where the oscillator won’t come back to exactly the same frequency. They also may take several weeks or longer to reach maximum stability. Rubidium warms up fast and comes back to almost the same frequency each time but some take a day or two to reach maximum stability. The Rb standard I use the most is about 25 years old and very stable. It uses a FRK-HLN standard and is built into a unit that has buffered sine wave outputs of 0.1Mhz, 10Mhz, plus 1Ghz, and is great for checking counters.Just buying a rubidium or crystal oscillator from Ebay may not be the best way to go because either type could have drifted so far out from a long ago calibration that they won’t be adequate for what you want them to do. In the past I sold LPRO-101 and other Rb units that I had carefully tested and adjusted over time so I knew they were reliable and adjusted to be on frequency. Buying one from China is kind of a crap shoot because a lot of these sold are pulled out of scrap and may be untested or near end of life. Attached is a graph showing an LPRO-101 being tested/adjusted.Even if you don’t have requirements for really high accuracy I’d still go with a Rb unit calibrated by the seller or for better long term stability, if you have a good spot for an antenna, I’d definitely go with a GPS. If you have no way to check calibration I’d avoid an OCXO even though it will probably be the cheapest way out. The GPS units probably range from about $100-$300 and the Rb oscillators like the LPRO-101 are probably up to $275 for a good one. If you don’t want to experience the ‘joy’ of trying to build a unit from collected parts, probably the small UCCM GPSDO boards from any of the various makers built into a small case would be the quickest (but maybe not the absolute best) way to get a GPS receiver up and running.

author: cdev
author_rank: Super Contributor
timestamp: October 08, 2018, 11:08:02 pm
content: I leave my GPSDO on all the time when I am using it. 24/7Thats what I've been told is the best way to have it operate both for accuracy and longevity.Quote from: nctnico on October 08, 2018, 09:26:15 pm

author: Wolfgang
author_rank: Super Contributor
timestamp: October 08, 2018, 11:30:07 pm
content: Quote from: Housedad on October 08, 2018, 01:15:33 amHi,I do it this way:- a homebrew unit with a really good 10MHz OCXO from MORION. Aligned to GPS and Rubidium every year. Drift was never more tha ca. 1Hz. For my everyday lab use.- a GPS receiver from Leo Bodnar for other frequencies than 10MHz- a source built around an EFRATOM Rubidium standard (this is a Rubidium governed OXCO) in case super high accuracy is needed.

author: james_s
author_rank: Super Contributor
timestamp: October 09, 2018, 01:04:33 am
content: You can get a distribution amplifier for next to nothing. I've seen guys using old analog video distribution amps. If you'd rather build something, I have several reels of Maxim video amp ICs, I'd send you a strip of them for the cost of a stamp and envelope.

author: cdev
author_rank: Super Contributor
timestamp: October 09, 2018, 03:31:13 am
content: I use an Extron video amp, worked great out of the box. Cost me almost nothing.

author: texaspyro
author_rank: Super Contributor
timestamp: October 09, 2018, 04:25:04 am
content: Some GPSDOs (like the Thunderbolt) let you disable disciplining and let the OCXO free run. In this mode the phase noise will be lower than when disciplined. For phase noise critical applications (like up-converting to microwave frequencies) this can be quite useful. A typical application would be to let the GPSDO discipline the oscillator to get the freq dead-nut on and then disable disciplining when you need low phase noise.Note that disabling disciplining is different than putting the GPSDO into "holdover" mode. In holdover mode most GPSDOs still control the OCXO freq using predicted aging / temperature corrections. Generally (but not always) holdover mode causes less phase noise than disciplining to GPS.

author: ArthurDent
author_rank: Super Contributor
timestamp: October 09, 2018, 04:48:52 am
content: Here is a photo of an Inline model 1N3021 video amp I modified to use as a 10Mhz distribution amp. Each output has its own wide bandwidth opamp to give good isolation and produce a good clean 10Mhz sine wave, except the bottom six outputs which are square wave. The 75 ohm resistors on input/output were paralleled with 150 ohm resistors to change the in/out impedance to 50 ohms.Under the distribution amp is a Thunderbolt GPS I built into a case, then a modified HP designed GPSDO that has a built-in power supply and display, then a Datum GPSDO with multiple frequency outputs. Each GPSDO drives one of the 3 groups of 6 outputs. These three GPSDOs are powered 27/7 and receive their signal from one of two outside active GPS antennas feeding 1575.42Mhz L1 distribution amps that allow me to run or test about 16 GPS units at once. To the left of the GPSDOs is a nice Ball/Efratom FRK-HLN based rubidium standard with 0.1Mhz, 10Mhz, and an added 1.0Ghz buffered sine wave output. It produces good stable and accurate outputs even though it is about 25 years old.I use one sine 10Mhz output to trigger my scopes, another for my HP 53132A counter, and another for the timebase in my modified FY6600 function generator. The scope and counter make it easy to check the frequency drift of any device under test or comparing one GPSDO against another. The GPSDO drift stays within 5ns over a long period of time.

author: Housedad
author_rank: Frequent Contributor
timestamp: October 09, 2018, 08:44:27 pm
content: This has been a very informative thread. I really appreciate the replies.It looks like I'll find a GPSDO and just use that. Probably way more than I'll ever need, but I'll just consider it TEA.I see from what you guys are saying is that if I ever need to distribute the signal, it is best to use Sine wave. Interestingly, the RF generator I'm working on now has a sine wave on the INT Ref OUT line, but uses a square wave internally. I'm thinking that a reference that puts out both would be useful. Thoughts?

author: tautech
author_rank: Super Contributor
timestamp: October 09, 2018, 08:50:00 pm
content: Quote from: Housedad on October 09, 2018, 08:44:27 pmThe link I posted earlier to Leo's GPSDO offers a US seller:https://v3.airspy.us/product/lb-gpsdo-1/$219, two configurable outputs, antenna and all ready to go !

author: Housedad
author_rank: Frequent Contributor
timestamp: October 09, 2018, 09:03:52 pm
content: When you use a video distribution amp, how do you reconcile that the video amplifier ouputs are 75 ohm vs the test equipment being 50 ohm impedance?

author: jpb
author_rank: Super Contributor
timestamp: October 09, 2018, 09:29:22 pm
content: The two options are to replace the 75 ohm resistors with 50 ohm ones, or do as ArthurDent posted above and parallel them with 150 ohms.

author: Gyro
author_rank: Super Contributor
timestamp: October 09, 2018, 09:42:59 pm
content: Quote from: tautech on October 09, 2018, 08:50:00 pmLeo's single output one is even cheaper, from the same site...https://v3.airspy.us/product/lb-gpsdo-mini/

author: Housedad
author_rank: Frequent Contributor
timestamp: October 09, 2018, 09:59:11 pm
content: Quote from: jpb on October 09, 2018, 09:29:22 pmGotcha. So a very, very mild mod of the amp.

author: ArthurDent
author_rank: Super Contributor
timestamp: October 10, 2018, 02:10:09 am
content: If you try to remove the 75 ohm resistors to replace them with 50 ohm resistors you run the risk of lifting pads or doing other damage to the board. It is much easier and safer to just solder a 150 ohm resistor across the existing 75 ohm resistors.Not all video amps are created equal. The one I bought came with one amp driving all six outputs through resistors so it wouldn't have the best isolation or drive. It did have the 8-pin IC layout for individual amps for each output and I installed amps with a wide bandwidth in those locations to get good isolation and better protection against shorted outputs, etc.. The problem is you don't know what the video distribution amp has inside until you buy it and take it apart.

author: cdev
author_rank: Super Contributor
timestamp: October 10, 2018, 03:18:48 am
content: The Leo Bodnar unit doesn't have an oven, right?What makes it different than, say a ublox timing GPS, some of which can be found on ebay for $20.They have two internal user programmable square wave outs which can be adjusted typically one is set to 1 pps and the other is set to 10 MHz.I think with adjustable duty cycle.One could slap one of them in a box with a Raspberry Pi and a screen or a serial port and have a standalone adjustable DDS.It has the expected amount of jitter.Am I wrong about something here?

author: hamster_nz
author_rank: Super Contributor
timestamp: October 10, 2018, 03:59:38 am
content: Quote from: cdev on October 10, 2018, 03:18:48 amA GPS's PPS signal jumps up and down 'like crazy' (maybe a bit of an overstatement), I collected 10 hours of readings and got +/- , due to the noise in the receiver's location. It is also usually at the nearest cycle of a 48MHz internal clock - But over time it is exactly one pulse per second.A crystal is relatively stable in the short term - from second to second, but over the long term will not be X pulses per second, and can wander with changes in temperature, but it's frequency isn't that accurate.I did an experiment, and with a 100MHz nominal crystal as a reference, over 10 hours I got between 100,000,489 and 100,001,193 counts between the rising edges of the PPS pulses. With an average of 100,000,877 counts I am sure that my Xtal is about 8.77 parts per million fast at standard lounge operating temperatures.If I was able to feed back this measured error to adjust the speed of the crystal, it would be on frequency (thanks to the GPS's PPS reference), stable over the short term in both frequency and phase (due to the nature of the crystal), and also stable over the long term (once again due to the GPS input).

author: metrologist
author_rank: Super Contributor
timestamp: October 10, 2018, 04:23:28 am
content: I've been happy with a couple UCCM trimble units by this guy. This one is a symmetricon for under $120.https://www.ebay.com/itm/GPSDO-Symmetricom-Inside-GPS-10MHz-1PPS-GPS-Send-GPS-antenna/263458624856?hash=item3d575b9558:g:bk8AAOSw9vlaael4:rk:42:pf:0I am also building the Lars GPSDO project posted here. It uses an Arduino Uno type controller, OCXO and 1pps GPS of your choice, and a few cheap components.Lars was very nice to have provided all of the project details:https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/?all

author: MrW0lf
author_rank: Frequent Contributor
timestamp: October 10, 2018, 08:41:25 am
content: Quote from: cdev on October 10, 2018, 03:18:48 amIt's from Leo, therefore it rocks. That's for starters. Second look here:http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=234450 Hz to 800 MHz independent output with tunable (common) level and phase shifts (decent square). It has proven massively useful on occasions.Then look overviews which include Allan deviation etc:http://leobodnar.com/files/Microsoft%20Word%20-%20Investigation%20of%20Leobodnar%20GPSDO_rev2.pdfhttp://leobodnar.com/files/Informal%20Evaluation%20of%20a%20Leo%20Bodnar%20GPS%20Frequency%20Reference.pdfand this:Looks like $20 device or perhaps bit better? My only trouble with it that I have nothing to check it's accuracymyselfBesides possibly Aim TTi TF960.Edit: If read between lines it seems he can offer OCXO version also, sine TCXO one is referred as "standard version".

author: macboy
author_rank: Super Contributor
timestamp: October 10, 2018, 02:31:28 pm
content: Quote from: jpb on October 09, 2018, 09:29:22 pmThose are two options but not the only two. Other options include:- Use 75 Ohm cable, and let the 50 Ohm termination at the instrument cause some reflection back toward the amp. It should be absorbed by the amplifier's 75 Ohm impedance and end there.- Use a 50 Ohm cable, and let the instrument terminate the signal. The level will be a little lower than nominal due to the 75 Ohm source, but there should be minimal reflection back into the cable.In either case, I'd do a sanity check for signal quality, but you will likely not cause any significant impairment of the signal at just 10 MHz. The fact that it is a single fixed frequency sine wave is even better, as reflections will only cause some amplitude error rather than distorting the shape of the waveform.Personally, I have two 1-to-4 video amps that I will use for distribution. I also have many very high quality 75 ohm cables which are not suitable for general 50 Ohm use. I plan to use them in this application, and I'm not worried about the mismatch in the least. I am open to learn why I'm wrong, if someone knows better...

author: jpb
author_rank: Super Contributor
timestamp: October 10, 2018, 05:57:03 pm
content: On the subject of 50/75 ohm termination - a lot of instruments aren't either for the ref in anyway. I think my counters are more like a few kohms so you can locally make them either 50 ohm or 75 ohm with suitable additional resistors.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 10, 2018, 07:15:38 pm
content: Quote from: jpb on October 10, 2018, 05:57:03 pmThat isn't usually necessary. AFAIK, the intent is that you run a single daisy-chain from unit to unit with BNC T-connectors and cables. Terminate the end of the line with a terminator or a device that happens to have a 50 ohm RefIn connection. The high impedance of the other RefIn connections won't load down the line. If you find that a particular device is sensitive to this situation you might have to arrange a dedicated connection for it.Ed

author: cdev
author_rank: Super Contributor
timestamp: October 10, 2018, 10:55:27 pm
content: Several things.The Skytraq GPSs use a Leon3 "Sparc" architecture core running on a chip design that was originally designed for the European space program - to be a highly reliable CPU platform for space vehicles. Also I am pretty sure the clock speed is significantly higher than 48 MHz.The manual for the Leon 3 is available for download on the net somewhere as well as a toolchain being available.It seems to me that the same or similar code base is burned onto significantly different hardware for various different applications. I'm not familiar with exactly what factors are involved but it can be found on the Navspark web site. Both platforms are quite nice in their different ways it seems.Both Skytraq and Ublox sell dev boards and the developer typically uses their configuration applications (windows only but both work under wine) which open up significantly more in the way of interactions with the chip and its internals.Ublox is particularly good in this respect in some ways, their U-center (Windows) application has the ability to configure what seems like hundreds of different settings, for a great many different applications, via multiple UARTs and USB/UART too. Also all Neo 8 series UBLOX GPSs have at least one, and sometimes two timepulse outputs which are changeable. However to do that you need to send commands to the module by some means. Somebody would have to implement a control interface in a microprocessor that spoke to the chips. The protocol is available, so its likely not very difficult. If you want to experiement with it you need a Ublox timing GPS, though which start at around $70 for a current Neo M8T unit, or you can find an older one for much less. For timing its not at all necessary to have multiple GNSS systems available (what buying an 8 series gets you) so its fine to use a Neo 6 timing unit which still has the two programmable time pulses. They sell for as little as $20-25. One can talk to your module via the UART interface using a USB-UART converter if you need to do it from a separate PC, or over SPI or i2c.Skytraq approaches the maker community's needs slightly differently because their chip actually is a CPU so it can do double duty as such and run a limited amount of additional code, toggle GPIOs, etc. This makes it possible to use a single chip to do things which otherwise require a GPS plus additional hardware, usually another board. You can also talk to them over UART, SPI or i2c.Because you can run a limited amount of code on the Leon3 32 bit CPU, you can with skytraq, use that programmability to implement anything that will fit in the space you get, a GPS logger, time stamper, or a clock that displays the location heading, time, etc. with almost no additional parts other than the display and the antenna, battery, SD card, etc.As far as implementing a GPSDO oven control loop and logic in that additional space, I don't know. It seems that some of the needed info would be available. It depends on how complicated your design is.------Also, as far as distributing a signal. Seems to me like having an RF transformer for isolation of the input on any amplifier would likely be useful in reducing noise which might throw readings off. One might even be able to get away with no amplifier using video transformers.. (baluns) as splitters. Especially if your GPSDO has a healthy output level. Use a trifilar winding to split a signal two ways, quadrifilar winding to spilit it three ways and so on. This would probably work best with very thin insulated wire and small balun cores.

author: texaspyro
author_rank: Super Contributor
timestamp: October 11, 2018, 12:21:41 am
content: Navspark does not make the GPS code available...Oh, and Lady Heather can configure both the Ublox and Navspark/Venus chips.

author: cdev
author_rank: Super Contributor
timestamp: October 11, 2018, 03:38:17 am
content: Science and the scientific method could be a life saver. Where we're going most of our young people wont have much if any income/money.People will need low cost tools. Science has a joy component to it, it could be a gateway drug to prevent people from sinking into despair.Enabling scientific thinking.Accurate measurement of time *really* cheaply could be a game changer. There is no point reinventing the wheel if it isn't cheaper.

author: texaspyro
author_rank: Super Contributor
timestamp: October 11, 2018, 04:02:28 am
content: Quote from: cdev on October 11, 2018, 03:38:17 amNope, Heather is blissfuly ignorant of PPS signals.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 11, 2018, 11:38:43 am
content: Quote from: cdev on October 10, 2018, 03:18:48 amNo, it does not really need it for its intended use.QuoteUblox output has 21 nanoseconds of jitter pk-pk. GPS clock has few hundred femtoseconds jitter. This is the biggest difference. I have attached the phase noise plot from mini GPS clock. Notice the lack of spurs - they have not been removed, they are simply absent in the signal.I will try to get phase noise plot directly from Ublox, this will be fun.QuoteAll four statements are technically correct. But the Ublox jitter is still 21ns pk-pk atanyfrequency. Consider that at 10MHz signal period is 100ns and at 100MHz it is 10ns.Leo

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 11, 2018, 02:53:07 pm
content: Here is phase noise ofdirect timepulse output of Ublox M8 modulein comparison withmini GPS clock.I can't add much more to this apart from the fact thatphase noisemaths usually assumes jitter to be much less than signal period. With 10MHz 100ns period signal having 21ns quantisation jitter this is probably pushing it but it is what it is.These two traces will merge somewhere around 0.1Hz on the dark side (in ADEV land.)CheersLeo

author: tggzzz
author_rank: Super Contributor
timestamp: October 11, 2018, 03:35:14 pm
content: Quote from: Leo Bodnar on October 11, 2018, 02:53:07 pmCan you configure the Ublox M8 it so the 21ns jitter isn't apparent every cycle, e.g. with an output frequency of 1MHz or 100kHz etc.That measurement might be more interesting than a 10MHz output.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 11, 2018, 03:56:02 pm
content: Quote from: tggzzz on October 11, 2018, 03:35:14 pmMy kit does not go below 5MHz at the moment.Not sure what you mean by "apparent every cycle" but you can probably extrapolate correlated phase noise levels with rule-of-thumb 6dB per octave (20dB per decade.) Spurs will probably stay where they are.Ublox M8 is a Cortex-M3 core running at 48MHz and using timers and phase accumulation to generate its timepulse output frequencies. The quantisation steps are side effect of timer timebase being 48MHz.

author: cdev
author_rank: Super Contributor
timestamp: October 11, 2018, 04:46:00 pm
content: Leo, how does your frequency generator work, do you have a block diagram anywhere of it?

author: tggzzz
author_rank: Super Contributor
timestamp: October 11, 2018, 05:35:46 pm
content: Quote from: Leo Bodnar on October 11, 2018, 03:56:02 pmApologies for my loose wording.Perhaps measure ublox output at 8MHz (48/6) or 6MHz (48/8) - not 10MHz with its non-integral divisor.

author: cdev
author_rank: Super Contributor
timestamp: October 11, 2018, 05:47:33 pm
content: Leo, I'm just curious if you enable the Ublox setting that incorporates the SBAS satellite data, (thats its name in Europe, in US we call it WAAS) into the timing solution?

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 11, 2018, 06:34:11 pm
content: Quote from: cdev on October 11, 2018, 05:47:33 pmIt will not affect quantisation jitter at all, only underlying receiver local time corrections.Correction:Ublox donotrecommend enabling SBAS for timing applications.QuoteEuropean SBAS system is called EGNOS. US SBAS is WAAS. We are running out of acronyms!GPS clock is, basically, a GPS receiver with clock cleaner. It merges long term stability from GPS and low phase noise from local TCXO. There are a few articles around with its block diagrams.Leo

author: cdev
author_rank: Super Contributor
timestamp: October 11, 2018, 08:14:30 pm
content: The improvements they have been making with GPS are scary. Useful in disasters, for sure, but also quite scary.

author: hamster_nz
author_rank: Super Contributor
timestamp: October 11, 2018, 10:38:56 pm
content: Quote from: Leo Bodnar on October 11, 2018, 06:34:11 pmJust found this about a SBAS trial in Aus (& NZ),http://www.ga.gov.au/scientific-topics/positioning-navigation/positioning-for-the-future/satellite-based-augmentation-system$160M sounds a little bit expensive...In the "Can I use it" FAQ:QuoteandQuoteI might start a new thread.....

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 12, 2018, 06:47:53 am
content: Uh-oh, just noticed that yesterday I wrote the opposite of what I meant... Ubloxdo notrecommend enabling SBAS for timing applications.Quote

author: NivagSwerdna
author_rank: Super Contributor
timestamp: October 12, 2018, 08:28:40 am
content: Quote from: hamster_nz on October 11, 2018, 10:38:56 pmI'm so used to having SA turned off and SBAS augmentation in the UK... I guess you just get used to it. How do you find the small camouflage geocache in the ivy otherwise!

author: blackdog
author_rank: Frequent Contributor
timestamp: October 12, 2018, 01:43:07 pm
content: Hi Leo,Here a happy user of your LeoNTP device.I also lookt at your GPS lockt frequency standard, but is it to expensive to use a better TCXO in your device so the phase noise wil be lower?If i have to pay 30 to 50 Euro's more fo a better phase noise i will buy it :-)Kind regards,Bram

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 12, 2018, 08:21:31 pm
content: Bram,TCXO I use is as good as they come. Current phase noise is at the limit of what you can get with AT-cut crystal that most (all?) TCXOs use.Lower phase noise needs SC-cut crystal which inevitably means OCXO. What sort of improvement are you looking for? Or, at least, at what distance to carrier area? Are low spurs important?Yes, you can get PN improvement but OCXOs that are suitable frequency, output, consumed power and availability are pushing €100 already just on their own.If I know a bit more of what you want I might be able to come up with a solution.LeoQuote from: blackdog on October 12, 2018, 01:43:07 pm

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 13, 2018, 01:17:46 pm
content: Quote from: tggzzz on October 11, 2018, 05:35:46 pmHere is 8MHz output result.It does sound counter-intuitive but integer ratio does not matter. You get same amount of jitter, it is just spread over different time period and its noise density distribution has different shape. RMS of sawtooth does not depend on its period, just amplitude.It is important to note that 48MHz core clock is coming from local Ublox 26MHz TCXO or XO - it's free running and usually has about 0.5-2ppm error. 8MHz timepulse is coming from GPS timebase. These frequencies are completely asynchronous.I had to use John's brilliant KE5FX Noise toolkit and E4406A to get 8MHz plot since HP3048 refuses to lock to such a noisy signal. I have superimposed 8MHz noise trace onto original image. There is so much going on and you can argue what's noise and what's spur but all unwanted energy is still there.Leo

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 21, 2019, 01:36:28 am
content: I know this response is some eight months late now but I thought I'd explain my vote of thanks anyway.Having seen just how much I have written below (TL:DR), I'll summarise by saying that you've shown rather neatly in graphical form just why you can't just simply take the 10MHz output from a NEO-M8N directly for use as a reference other than as quick 'n' dirty substitute for the 10MHz WWV broadcasts which, outside of the USA, are virtually impossible to receive due to the non-existent propagation conditions that prevail during this current period of Sunspot minima.The 60KHz and 198KHz frequency standard broadcasts that can be received here in the UK, are inconveniently too low for direct use as calibration references, needing to be multiplied up which is a project of difficulty on a par with that of constructing a GPSDO in itself. Those graphs demonstrate just why you need the expense and complication of (typically) an OCXO to turn a GPS receiver into a stable frequency reference that can be used by high grade test and communications equipment where stability and low phase noise is of paramount importance.The following is effectively my 'back story' in how I came to appreciate those graphs you made of the NEO-M8N's performance as a jittery 10MHz reference source. Whilst this is probably a story you've heard many times over, others more fresh to the subject, might be able to get something useful out of it.I didn't join the eevblog community until just over six months ago when a posting in sed mentioned the ultra low cost of Chinese test gear, notably Feeltech's ridiculously priced FY6600 (circa $100 at that time - I paid just £75.66 for my, now very much modified, 60MHz version after reading all 69 pages of the FY6600 thread twice over as a result of my purchasing a Siglent SDS1202X-E the week before, also prompted by what I'd read in that thread).As they say, "One thing leads to another..." and here I am with a revived interest in hobby level electronics, only this time with ambitions of building stuff with three or more magnitudes of calibration accuracy than I could ever have hoped to achieve three decades ago.For the past couple of months, my timing accuracy mods to the AWG and my fledgling GPSDO project have taken on the character of an "Arms Race" since they complement each other's development. Initially, I'd upgraded the cheap 'n' nasty SMD XO chip that Feeltech had cursed their FY6600 with to a 0.1ppm 50MHz TCXO oscillator board which has in turn since been usurped by a CQE 10MHz OCXO with a 3N502 clock multiplier which is now, for the time being, running around 30ppt adrift from another CQE 10MHz OCXO currently being disciplined by a fake M8N module. I have another five of these CQE OCXOs to hand, thanks to a chance purchase of a 5 volt 13MHz CQE OCXO at a hamfest over seven weeks ago (my searching for manufacturer's data had led me to the chance discovery of a very cheap source for the 10MHz units, hence the "Lifetime's Supply").One of the earliest conclusions about the nature of the 10MHz output from the M8N module was that it didn't really make any real difference overall whether an even integer divider ratio was chosen over any non-integer ratio simply because the effects of the sawtooth adjustments against the 48MHz TCXO couldn't be escaped, only masked at best. Your test results very nicely and formally confirmed what I'd already managed to conclude simply by gathering the evidence of my ears by monitoring the effects with 25 year old Kenwood HF transceiver.It was so refreshing to see this point about the NEO M8N being made by such a well respected expert as yourself.At the end of the day, the random phase shifting due to the satellite signals having to traverse an ionosphere of varying electron density makes such short term variations pale into insignificance, disappearing below the noise level after the necessary hours long averaging to attenuate this final system deficiency has extended to weeks and then ultimately to years.I find all mention of these "non-integer division jitter" and "sawtooth adjustments" 'defects' with any GPS receiver capable of offering a 10MHz or other frequency alternative to the PPS output rather irritating when it should be blindingly obvious that these are side effects that have little to zero detriment to the basic navigational function of the GPS system.Utilising the long term accuracy of the PPS feature embedded within virtually every GPS receiver, simply requires the use of very long integration periods to average or filter out the effects of jitter and sawtooth noise from the long term timing accuracy of the system to translate it into a an extremely accurate low phase noise frequency reference courtesy of a quality design of GPSDO whether a commercial ready made unit or a DIY fabrication.I have to say, in the four months since I purchased that first M8N module (a genuine u-blox module BTW, now sadly unable to output anything on its PPS line thanks to rank carelessness on my part in an experiment involving a 12v OCXO), I've learnt an awful lot about the issues involved in utilising a GPS receiver to discipline a high quality OCXO into a trustworthy low phase noise frequency reference. I've no doubt that there's yet more to learn, especially once I've actually committed my basic breadboard layout to a soldered up PCB (most likely a dead bug on groundplane bodge) fitted into a cheap metal enclosure.When I first availed myself of the 'luxury' of getting a "10MHz reference" directly out of the M8N module, I did question the need for a 10MHz XO of any sort at all. A few weeks of experimentation later and my question was answered. Now, I realise that unless you're after a quick 'n' dirty alternative to the 10MHz WWV broadcasts, the first priority of any DIY GPSDO project is an electrically tunable XO, preferably an OCXO over a(n electrically tunable) TCXO (butanyelectrically tunable XO will do for an initial attempt).As far as building "Your Very First GPSDO" goes, almost any GPS reciever with an accessible PPS signal can be pressed into such service given enough determination to lock a 10MHz oscillator to a 1Hz reference. However, a word of warning, if you choose one of those cheap £/$4.00 NEO-6Ms, there's a chance you might get one which whittles its PPS down to nothing before immediately reincarnating itself as the full fat pulse you'd initially programmed.The symptoms had been so confusing, it nearly drove me right round the bend... bibble bibble bibble...It hadn't helped that I'd had to divide an OCXO with a tuning range of just 1ppm down by a factor of ten thousand. Getting a phase lock at 1KHz becomes a rather glacial process on the order of 15 to 30 minutes, plenty of time for the PPS to disappear up its own backside two or three times and cause disruption to the phase locking process.My example with a 1KHz squarewave setting would run at the programmed 50/50 ratio for the first 5 to 7 minutes of a 10 to 14 minute cycle before starting to narrow down, eventually to disappear up its own fundament to then instantly reincarnate as a 50/50 squarewave pulse again. The timing of this cycle seemed to be tied to a bug related to the sawtooth adjustments used to keep the rising edge synced to the nearest 48MHz rising clock edge.I tried reprogramming this pulse to the narrowest possible (1μs on a 1ms interval - the only way I could get a 1KHz PPS) and use the PD2 output from the 4046 I was using in the hope the phase/frequency detector could tolerate such insults. Zooming in on the 'scope trace to see the final second of the pulse's disappearing act revealed it literally vanishing altogether, albeit briefly (at a guess, for a whole one millisecond) which not even the PD2 circuit could tolerate. So it was "Goodbye NEO-6N" and "Hello (fake) NEO-M8N" a week or so later.It's just possible that this is an effect that only shows as a result of programming a higher frequency pulse. Leaving it set to the default 1 PPS 10ms wide pulse may avoid this issue but I haven't tested for this possibility. It just might prove to be fine at the default setting when I do eventually find the time to test this hypothesis. I specifically mentioned "even" since odd divisors such as 3 (16MHz output), 5, 7 and 9 (I didn't bother going any further down this particular rabbit hole) result in a 1:2 mark/space ratio which alternates on every sawtooth alignment to a conveniently placed 48MHz clock edge.Quite frankly, in view of the sawtooth jitter, there's little point in worrying about using a non-integer divider ratio to get a 10MHz (or 100KHz in my case) unless nice clean 'scope traces are your over-riding objective. It all comes out in the wash anyway, once you've applied some filtering to the PD's output (typically a CR based LPF with a cut off in the region of a few milli Hertz to be followed by a DSP LPF with a cut off frequency of that's gradually reduced to a micro Hertz or so to filter out the GPS/GLONASS system errors - ionospheric variations etc).JBG

author: aix
author_rank: Regular Contributor
timestamp: June 21, 2019, 04:36:04 am
content: Quote from: cdev on October 09, 2018, 03:31:13 am+1 to that. I've done exactly the same. Lots of very cheap used Extron units on ebay.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: June 21, 2019, 10:06:28 am
content: Quote from: aix on June 21, 2019, 04:36:04 amIt's worth keeping in mind that if you're terminating the 50 ohm cable feeds with 50 ohm loads, there's no urgent need to modify the 75 ohm matching resistors (replacing with 50 ohms or paralleling them with 150 ohm resistors) as long as it can provide sufficient output voltage (typically in this case, it's unlikely to be a problem). When it comes to the issue of transmission line matching where the signals only go one way, the impedance of the driving source isn't critical, provided the loads at the other end match the cable impedance and there's sufficient headroom for the amps to make up for the extra volt drop in the terminating resistors.The transmitting amps can range from zero impedance (constant voltage source) to very high impedance (constant current source) as long they can supply the required voltage or current to the load. In practice, the normal alternative to matching the line driver impedance to the load is to make them constant voltage rather than constant current. However, this exposes the amp outputs to possible fault conditions (short and open circuit faults which can cause overloads and/or instabilities).The matching resistors not only protect the amps from these hazards, they also improve stability by acting as snubbing resistors as well as making any fault finding easier, hence their presence in a typical distribution amplifier. Also, there's the fact that any reflections from a shorted or open circuited cable will only be making one trip back to the amp's terminating resistor limiting any deleterious effects such faults might otherwise have on the rest of the distribution network. The only downside to wasting 50% of an amplifier's output in such resistors is the need to find an extra 6dB's worth of headroom. In distribution amps dealing with signals at the milliwatt level, this is not usually regarded by the designer as an expensive luxury.JBG

author: tkamiya
author_rank: Super Contributor
timestamp: June 24, 2019, 04:42:26 am
content: To do what you said, frequency standards are really not necessary. Most reputable test equipment already comes with good enough standard.Having said that, GPSDO is hard to pass by. It's cheap and it's more than reasonably accurate. While short term stability may be questionable, certainly, for my use, sufficient. When you can spend $200 and have a complete set, why not?Myself, I started there, added a second one, rubidium, rubidium+GPS, and now cesium. Out of all, GPSDO is the one I use most often. One thing to think about is, frequency standard is something you keep it on at all times. Power consumption and warming of the room is a concern. If I turn all of them on, my A/C cannot keep up. Rubidium gets quite warm.