GPSDO vs Rubidium reference - Page 1
Forum section: electronics / testgear

author: mskobier
author_rank: Regular Contributor
timestamp: April 17, 2015, 06:29:22 pm
content: All,Along with working on the Tektronix oscilloscopes, I have also been dabbling in time references. In particular a surplus 10mhz rubidium reference, and a china made GPS disciplined clock with 10Mhz sine wave output. Having a fair amount of measuring equipment, but nothing that approaches the theoretical accuracy of eith the rubidium reference or the GPS disciplined clock. So today, I hooked both up , let them warm up for a few hours, and input the output of both references to my 2445B, and took a look at the display. Both had a nice sine wave display, but the displays were shifting position in reference to each other. It takes right at 5 minutes ( as best I can measure) for the displays to shift one cycle. That works out to .00333 hz difference. The question I am asking myself, is which one is actually the most accurate? The stand alone surplus rubidium reference or the GPS disciplined reference. Normally I would believe the GPS one is the most accurate, but it is one of the relatively inexpensive ones from China that they are selling on -bay. One of the interesting things about the GPS reference, is that when I opened it up to see what was inside, the OCXO was made in the USA. I have searched the net and can find no reviews on these GPS based references. Do any of you guys (and gals) have one of these units (GPS disciplined Clock, BG7TL 2014-12-09), and just how accurate do you think it is, and how did you test it?Mitch

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: April 17, 2015, 07:50:17 pm
content: I bought a BG7TBL GPSDO (awaiting delivery), but it seems that this device doesn't really output 10MHz:http://www.ke5fx.com/gpscomp.htmBTw, since you have one, how are you satisfied with it? How's the build quality?

author: Electro Fan
author_rank: Super Contributor
timestamp: April 17, 2015, 07:55:56 pm
content: +1 for same and similar questionsie, is either a GPS or rubidium reference inherently more accurate and/or stable?What is the lowest cost generally available solution for getting an accurate and stable 10MHz signal?- saw this - any comments on it?http://www.ebay.com/itm/GPS-DISCiPLINED-CLOCK-GPSDO-10MHz-10M-OUTPUT-SQUARE-WAVE-RS232-OUTPUT-GPS-NMEA-/281649576001?pt=LH_DefaultDomain_0&hash=item41939f7841(EDIT: I was writing as Wuerstchenhund was posting - so maybe similar/redundant question/interests)Any reason to think that a Trimble Thunderbolt would be better?What type of antenna (how long and where does it need to be placed inside/outside) is required for a GPS?How much difference does the power supply for a GPS make, if any?Will cable length between the GPS and the test equipment have an impact on performance?Can a 10MHz clock drive multiple test equipment devices at the same time (or will that degrade performance)?Thanks!

author: G0HZU
author_rank: Super Contributor
timestamp: April 17, 2015, 08:26:43 pm
content: QuoteBoth will be fallible in the real world so there won't be a definite answer to this but I don't think it matters unless you are working in something like a high end research lab or a calibration house.

author: Mr Simpleton
author_rank: Unknown
timestamp: April 17, 2015, 08:39:25 pm
content: I would put my money on the GPS diciplined XO...It is locked against GPS which relies on caesium clocks. This has excellent long time stability but sucks for short time, and phase noise. Now the XO will have best of both worlds. The rubidium keeps its frequency a lot better than the XO but over time and restarts it will not be as exact as the locked XO, especially if you only had it on for a few hours. Most likely it needs 24+ hours to reach its final frequency.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 17, 2015, 08:41:05 pm
content: Quote from: G0HZU on April 17, 2015, 08:26:43 pmHi G0HZU,Thanks. Ok - if both are "fallible but comparable and sufficient"which would be most cost-effective/practical/easiest to use/live with? Do you have a preference?Also, I tripped across one of these:http://www.ebay.com/itm/EXTRON-ADA-3-80-DISTRIBUTION-AMPLIFIER-/371204748687- would this enable the clock to drive multiple units, or can simple T-Connectors do the job (I'm guessing something might be impedance sensitive and it can't be that easy).Thanks, EF

author: Electro Fan
author_rank: Super Contributor
timestamp: April 17, 2015, 08:43:57 pm
content: Quote from: Mr Simpleton on April 17, 2015, 08:39:25 pmMr. S.,Just to clarify, are you saying that the GPS disciplined XO will or won't have phase noise issues? Thanks, EF

author: G0HZU
author_rank: Super Contributor
timestamp: April 17, 2015, 09:05:56 pm
content: I'm the wrong person to tell you which to buy because I would say neither. I don't have either of them here.However, A year or two ago I did borrow the company GPS disciplined OCXO made by Quartzlock and spent some time trying to see how reliable it was and I wan't impressed with it (in terms of how it would suit me)http://quartzlock.com/product/frequency-reference/gps/E8-YIt's easy to get the E8-Y to show 'locked' even if it is nowhere near the quoted specs way after it shows locked. This is due to the quality of reception from the various satellites it can detect. I ended up connecting to it via RS-232 via some Motorola diagnostic SW to help me put the antenna somewhere where it could receive lots of satellites cleanly. The SW gave me qty and signal quality for every satellite it could detect. Then it worked a lot better but I could easily make it misbehave by moving or twisting the unit. It also took about 30 minutes to lock.The main reasons I don't have either is that they take a long time to stabilise from a cold start and they are usually too noisy to use as a precision reference clock for decent GHz capable test gear (although the E8-Y above is pretty good in this respect) The typical Rb standard is quite power hungry too and I really don't see the point of having one of these running all the time in my workroom at home. For me, the high stability OCXOs in my RF test gear are good enough and I rarely need to lock all my test gear to just one OCXO to sync them up better.

author: chicken
author_rank: Frequent Contributor
timestamp: April 17, 2015, 09:22:06 pm
content: Quote from: Wuerstchenhund on April 17, 2015, 07:50:17 pmNot sure what you mean with "doesn't really output 10MHz". From the linked article:QuoteSounds close enough for me.

author: mskobier
author_rank: Regular Contributor
timestamp: April 17, 2015, 09:26:53 pm
content: All,The build quality looked as good as the pictures on -bay showed. I did not see any issues , and it appears to be a nice compact design. The GPS antenna that comes with it is quite long, approx. 20ft (6m), and the unit runs on 12VDC.Thanks for the link to the KE5FX web page. That answered a lot of questions I had. I can live with a 2E-11 frequency error. Far better than I can even think of measuring. I do not plan on operating at 100ghz, but for me, a 2hz error at that frequency is acceptable. Just trying to calibrate a frequency counter that goes to 7 decimal places, and have a good reliable frequency reference.For <$150.00 delivered, (paid over $100.00 for the surplus rubidium reference) I think it is reasonably priced.Looking at the performance of the Trimble Thunderbolt it appears to be a bit more accurate, even if it is limited in the number of satellites it can see at any given time.Mitch

author: Wuerstchenhund
author_rank: Super Contributor
timestamp: April 17, 2015, 09:41:39 pm
content: Quote from: chicken on April 17, 2015, 09:22:06 pm9.999 999 999 800 Hz != 10MHz .Now, that wasn't difficult. was it?

author: Mr Simpleton
author_rank: Unknown
timestamp: April 17, 2015, 09:49:29 pm
content: What I am saying is that a XO do have better phase noise than a caesium oscillator, and depending on offset from carrier can be better than a rubidium too.Long term, short term stability and phase noise are not that easy to compareIf you really need/want accurate frequency and time you really have to run the reference all the time... Rubidium or OCXO! Now a GPS diciplined oscillator may do a bit better but most high performance uints do take more than one hour to settle.Most non-timenuts are just as good with a calibrated double oven crystal oscillator, or if you are in a hurry and have the ref swtiched off most of the time a TCXO is a lot faster.I ended up with a used Trimble Thunderbolt which I plan to run against our Symmetricom GPS-Rb clocks (swtiched on 24/7) at the office, just for fun. And once verified this will help me to calibrate a few HP reference oscillators I have.

author: Mr Simpleton
author_rank: Unknown
timestamp: April 17, 2015, 10:02:05 pm
content: A GPS locked crystal oscillator can have excellent performance and still be pretty simple...http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htmhttp://www.jrmiller.demon.co.uk/projects/ministd/stab.htmGiven its price, the performance is not bad at all! And consumes 3W, so can be left on 24/7 even if you care about the environment

author: Electro Fan
author_rank: Super Contributor
timestamp: April 18, 2015, 09:47:26 pm
content: Just throwing this question out there for discussion in case anyone has any thoughts - might be others who have a similar question or interest....Given the choice of either a square wave or sine wave output from a GPS 10MHz clock, what would be the various reasons for choosing one or the other? (My primary reason for the clock would be to gain accuracy and consistency across various pieces of test equipment, but there might be other purposes/applications and considerations.) Thx

author: johnny_canuck
author_rank: Contributor
timestamp: April 19, 2015, 01:54:31 am
content: A square wave contains (theoretically) infinite harmonics. This may or may not matter to the instrument that's using the 10 MHz signal as it'stimebase. All of my gear (frequency counter, RF sig generator, spectrum analyzer) is looking for a 10 MHz sine wave. I use a Trimble Thunderbolt GPSDO, a DownEast Microwaves 4-port distribution amp and a very clean linear power supply for the lot.Ken

author: Electro Fan
author_rank: Super Contributor
timestamp: April 19, 2015, 02:38:06 am
content: Quote from: johnny_canuck on April 19, 2015, 01:54:31 amWhat spec would show (how would you know) that a particular piece of equipment is looking for a sine wave vs a square wave (or vice versa)? Thx

author: G0HZU
author_rank: Super Contributor
timestamp: April 19, 2015, 01:24:33 pm
content: In my experience wrt RF test gear the ext ref input is AC coupled and will accept either a low level sine wave and most will also accept a square wave. However, some of my lab grade RF test gear outputs a 2V pkpk square wave biased at 2.5V as a master 10MHz reference output.But there is risk from spurious effects from the harmonics from a square wave for some types of test gear.The advantage with a square wave is you can buffer/feed it directly into a divider if you need to divide down to 5MHz or 2MHz or 1MHz to suit some types of test gear. But I think a sine wave would be better if you wanted to daisy chain 'lots' of test gear to a common 10MHz reference.If you have decent RF test gear then you also run the risk of degrading the spectral purity of the test gear if you feed an external reference that has poor spectral purity to it. Some of the cheap Rb references on ebay would be totally unsuitable for this and these are best used as a frequency reference for calibration rather than a master clock for a whole load of test gear.Most counters have an AC coupled input for the external reference and will accept either a square wave or sine wave as an external reference in my experience.

author: johnny_canuck
author_rank: Contributor
timestamp: April 19, 2015, 09:00:44 pm
content: Perhaps I should clarify. I'm using the GPSDO sinewave output as the external timebase reference input for my instruments. The manuals forthese instruments specify a certain RMS voltage for that input. Doesn't that suggest a sinewave?

author: G0HZU
author_rank: Super Contributor
timestamp: April 19, 2015, 10:04:30 pm
content: Maybe... but we are in danger of opening a real can of worms here because not everyone will interpret things the same way.Most test gear is tolerant of square wave or sinewave input for the external reference in my experience.However, I have some RF test gear here that asks for a particular power level range at the ext reference input. eg 0dBm to 10dBm at 10MHz. So this implies 50R and a sine wave to me.But I have other high end RF test gear that asks for about 1.8V rms at the input. The same item of test gear outputs a 5V pkpk square wave as its internal 10MHz reference output.In my experience most RF test gear is much more tolerant of sine/square wave/drive level changes than the datasheet describes. But I usually just tie sig gens together via the external reference input or feed external reference signals to frequency counters.I rarely feed an external reference to anything else. If I need to sync up a spectrum analyser to a sig gen (or two) I would use the analyser OCXO reference as the master and this has always worked very well for me.However, I can't imagine a (realistic) scenario where I would want to do this with a GPSDO or Rb standard to sync them all up instead of using the analyser OCXO as a master. I can't see where the added value comes from. I can forsee the potential downsides of attempting this though...

author: TSL
author_rank: Regular Contributor
timestamp: April 20, 2015, 01:24:55 am
content: Quote from: johnny_canuck on April 19, 2015, 09:00:44 pmNo... RMS can be used for nearly any wave shape.For the math of it for square, pulse, triangular, go here...http://masteringelectronicsdesign.com/how-to-derive-the-rms-value-of-pulse-and-square-waveforms/regardsTim

author: TSL
author_rank: Regular Contributor
timestamp: April 20, 2015, 01:37:29 am
content: +1 for what G0HZU said.Locking a range of instrumentation to a single master standard must be done with consideration of what each instrument requires as a reference.While most require 10Mhz of some sort, some have 50 ohm impedance others might have 10k. This in it self can lead to mismatches and signal reflections so its a good idea to feed instrumentation with a fully isolating distribution amplifier so that no one instrument can affect the other.Additionally that standard must meet or exceed the phase noise and harmonic content of your best equipment lest you muddy the results. i.e. My HP8566 manual has this to say..."Analyzer performance will be degraded unless frequency reference phase noise and spurious signals are < - 140dBc single sideband (1 Hz) referred to 10 MHz at a 100 Hz to 10 kHz offset"So like G0HZU , if an instrument needs to be in sync with the SA, the SA becomes the master oscillator.Normally my sig gens and counters are all attached via a distribution amp from my always on GPSDO standard using double shielded equal length coaxes.As to the question of why sine and not square, johnny_canuck's statement of infinite harmonics is an important consideration especially if its analog/RF stuff your doing.The LPR manual also has this to say..."Transmitting rf output signals over long distances is less of an EMI issue for the user whenthe signal is a sine wave instead of a square wave because a sine wave lacks harmonics. In addition,the power consumption of the sine wave driver into 50 ohms is lower than for a square wave driverinto 50 ohms, especially when providing short circuit protection."regardsTim

author: johnny_canuck
author_rank: Contributor
timestamp: April 20, 2015, 01:51:38 am
content: The Thunderbolt frequency accuracy is specified as 1.16 E-12 (one day average). No OCXO is going to come even close to that. After a couple of hours from a cold start, the T-bolt monitor software shows the 10 MHz output flickering around +/- .05 ppb. That's accurate! That's why they were used to sync up mobile phone sites in the days of CDMA. Of course, you need an outside aerial ($5 on e-Bay) and a run of cheap cable-tv coax to pick up the GPS satellites. And it never has to be sent out for calibration. Phase noise is spec'd as -135 dBc/Hz at 100 Hz offset.

author: G0HZU
author_rank: Super Contributor
timestamp: April 20, 2015, 08:20:40 pm
content: QuoteSure, but if you sync all three bits of test gear to the analyser OCXO then they are all going to be sync'd to each other.The added value of having sync is the goal here and not the absolute accuracy that GPS offers.A typical requirement for me would be to sync up two sig gens so I can investigate phase related phenomena in a system. Typically this would be with harmonically related signals. I can adjust the relative phase of the two signals because they are in sync via a common master reference.Having a GPDSO doesn't add any real world value here because sync is sync. The OCXO in a decent spectrum analyser will be good enough as a master for this type of work. There will be people out there in industry who 'need' to do dev work with RF test gear that demands the accuracy that GPSDO can offer but I'm suggesting that there aren't many other people outside this group. However, there will be plenty of people who 'want' this capability even though they don't 'need' it. Hence the abundant sales of stuff like this on ebay to keen hobbyists.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 21, 2015, 05:47:29 pm
content: Hi G0HZU,Just checking to see if you could help me sort out a basic voltage (amplitude) measurement question?On a HP 8648C the dial that sets amplitude can be configured (button pushes) to read mV or Vemf or dBm or dBuVemf or dBuV. Is it reasonable to assume that some such setting (like mV or Vemf) would produce a voltage that would match the voltage reading on an oscilloscope? For some reason (might have to do with AC vs DC coupling or impedance, but I've tried those) I can't get a voltage output from the sig gen to match what I'm seeing on the scope (not peak, or peak to peak, etc.).It's probably just operator error but I'm slightly wondering about the sig gen.Thanks! EF

author: G0HZU
author_rank: Super Contributor
timestamp: April 21, 2015, 06:15:57 pm
content: Hi EFYou should maybe start a thread about your sig gen but if you set it to +10dBm (10mW) at maybe 10MHz and feed this via some RG58 coax to a scope and terminate the scope in a 50R load then you should see 2Vpkpk on the scope.You should see the same level of 2Vpkpk on the scope if you set the sig gen to 1.414V emf because the emf is the open circuit rms voltage but you are terminating the output in 50R so the (terminated) rms voltage will be 0.707V (which corresponds to 2V pkpk)The Vemf reading confuses a lot of people because it refers to the rms voltage BEFORE you terminate the output in 50R. My Marconi sig gen here has the option to select Vemf or Vpd and Vpd is the rms voltage when terminated in 50R. I've set mine to Vpd mode because I don't like to use Vemf. So to get 2Vpkpk with this sig gen I would select 0.707Vpd. But you would select 1.414Vemf for the same result on your HP8648C when fed into a 50R load. This is the same as selecting a power of +10dBm using dBm instead of Vemf etc.Also, try entering 707mV (and not 707mVemf) and you may still see 2Vpkpk across the 50R load at the scope because the mV mode is likely to be the same as my mVpd mode. Both of my Agilent sig gens offer mV or mVemf options and maybe yours does too?Hope this makes sense.

author: Electro Fan
author_rank: Super Contributor
timestamp: April 21, 2015, 08:37:36 pm
content: Hi G0HZU,Thanks! At +10dBM, 10MHz into 50R it is very close to 2Vpkpk; I am using a 30" BNC cable (RG223/U); a longer BNC cable (RG58A/U) reduces the voltage readout, so I suspect a slightly shorter cable might reclaim the .015 or so Volts I might be off (or the sig gen or scope might be a tad off). 1.414V emf also produces a very similar measurement. Likewise with 0.707V.It makes senses when I think about it but I don't have enough experience with test equipment and the principles to have it all line up automatically without thinking through it. But this is why I love learning with test equipment - when stuff doesn't line up you have to wonder why, and when stuff does line up you gain some confidence that you are headed in the right direction. Test equipment + EEV = great way to learn! Thanks again G0HZU, EF

author: Electro Fan
author_rank: Super Contributor
timestamp: April 21, 2015, 09:16:01 pm
content: According to this cable loss calculator the difference of a few feet shouldn't be making any difference (with either RG58 or RG223), so whatever caused the longer cable to show a lower voltage is not clear.The calculator also indicates that the typical loss for a pair of connectors could be on the order 0.01dB - but I'm guessing that is way too low level to show up in the voltage reading.http://www.timesmicrowave.com/calculator/?productId=131&frequency=10&runLength=2.5&mode=calculate

author: G0HZU
author_rank: Super Contributor
timestamp: April 21, 2015, 09:35:41 pm
content: I think your sig gen is behaving just fine in these tests. Don't worry if it isn't exactly 2Vpkpk because there are several reasons why it is likely to be slightly off in terms of Vpkpk and none of them mean anything is wrong.Eg the load impedance might not be an exact 50R. Your scope won't be perfectly accurate either and the sig gen will not be perfect in terms of accuracy so if you saw 2.1Vpkpk or 1.9Vpkpk instead of 2.0Vpkpk into a decent 50R termination then it's nothing to worry about.I could measure several of my sig gens in a similar way and I'd expect to see small errors in all of them. eg +/-0.4dB wouldn't be a surprise. But this doesn't bother me because this is still a typical result for this type of sig genMaybe these last few 'sig gen' posts need to be moved (by a mod?) to a new thread?

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: April 22, 2015, 08:03:43 pm
content: Have several Ref stds. The Rubidium standalone stays within 1 cycle easily of GPSDO Rubidium once adjusted, and heated up. Both a GPSDO OCXO, and Rubidium, both barely drift in/out of phase. You can never have too many...

author: TSL
author_rank: Regular Contributor
timestamp: April 22, 2015, 11:08:12 pm
content: Quote from: SoundTech-LG on April 22, 2015, 08:03:43 pmLOL this can lead down the time or frequency nut path all too easily! I've got two Tbolts and 3 Rubidiums and a couple of timing GPS's. All the GPS's hook through a distribution amp to a Symmetricom antenna on the roof.And i've been looking to expand the collectioncheersTim

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 22, 2015, 11:34:05 pm
content: Quote from: TSL on April 22, 2015, 11:08:12 pmI don't think there is anything wrong with this, so long as it gets turned into a productive effort. I'mstillknee deep in the stuff, and I feel like I have barely made a dent in the frequency business, but I have a specific purpose. I'm needing an excellent frequency standard, not only for synchronizing my test gear, but much of my radio design work revolves around precision frequency work. ±20ppm of a decent crystal oscillator is 20time too loose for the off-the-shelf stuff I'm aiming at, and ideally ±100ppb would be the norm and anything better is gravy.

author: mskobier
author_rank: Regular Contributor
timestamp: April 24, 2015, 11:43:13 pm
content: All,Since I started this thread, I figure I better keep you all up to date on what's going on.A friend has the same GPSDO that I do (he had his first). He has his his set up and running 24/7 for the past few weeks. I had been telling him about using the oscilloscope with the Rubidium reference on one channel and the GPSDO on the other and watching the traces frequency drift between the two. I had read the KE5FX link provided by Wuerstchenhund. Especially in reference to the frequency not being exactly 10Mhz. So today, I took my scope and GPSDO over to his house and set up a test. Since his has been up and running for a number of days, we selected his as the reference frequency on channel 1 of the scope. We hooked my GPSDO up to channel 2 and let it achieve lock. Needless to say the frequencies were off by quite a bit. After a couple of hours, the frequency difference had settled down to about .003hz. I know it need to run longer to completely stabilize, but we had thought it would be closer than that after a couple of hours. I left the GPSDO and scope hooked up over at his house, and we will check on it again tomorrow or the day after to see if the frequency between the two GPSDOs stabilize out to the same frequency, or if there is a bit of frequency drift between the two units. Unfortunately, we do not have a more precise reference or a way to measure that precise of frequency. My best counter only goes to 0.1hz resolution, and trying to calibrate it started this whole frequency measurement mess. We are both having a lot of fun with our experiments and actually learning some things along the way. Anyway, I'll post again in another day or two once things have an opportunity to stabilize out properly.Mitch

author: G0HZU
author_rank: Super Contributor
timestamp: April 25, 2015, 01:02:15 am
content: Do you know anyone local with an old HP8405A vector voltmeter? These old 1MHz to 1GHz RF Voltmeters date back to the 1960s and can be used to measure relative phase between two oscillators if they lie in this frequency range.I've got one here and although I use mine for other things it could be used for comparing two 10MHz oscillators. You can measure phase and log the output using a DMM because the meter outputs DC voltage vs phase at the rear or you can just look at the dials on the front. Just connect one oscillator to channel A and connect the other to channel B.These old meters are quite cheap now. They were once a common sight in any versatile RF lab but you may find a radio ham who has one locally? It's going to be a lot better than using a basic old school analogue scope...

author: TSL
author_rank: Regular Contributor
timestamp: April 25, 2015, 02:48:40 am
content: Quote from: mskobier on April 24, 2015, 11:43:13 pmFor GPSDO's I doubt it, you really need to leave them on for a month to get their true values. There is an interesting discussion going on over on the TimeNuts list about tuning the Thunderbolt and GPSDO behavior in general. Similarly someone is trying to do tests similar to yours and elicited this response..."Expecting that unit to meet holdover after only being locked for 12 hours is nota reasonable thing.Let it run for a week. Let it lock up for at least 4 or 5 days and get a good surveyon the location. It should run < ± 5ns one sigma with a good survey."and this..."> My> Thunderbolt has been on for a week and had been locked to the GPS> signal for at least 12 hours prior to the test.Quartz crystals are creatures of habit. They need to be on continuously for months before they settle down to their best behavior. And the Kalman filter needs to train long enough to see significant temperature changes and long periods of steady (that is, settled-down) drift. Thinking in terms of days and hours is orders of magnitude shorter than any realistic expectation of good behavior. If a Tbolt has been running locked and normally for months, and is turned off for a day and then turned back on, it may settle down in a day or three. But if you bought it surplus, it may have been off for years, and banged around considerably (or much worse) between the last use and you receiving it. In that case, you may well need to leave it alone for months before the crystal settles down and the Kalman filter is well-trained."Profiling behavior of devices that have performance measured in fractional PPB requires a stable setup, time and patienceOh.. and it looks like you're definitely becoming a time or frequency nut now!!regardsTim

author: Electro Fan
author_rank: Super Contributor
timestamp: April 25, 2015, 05:14:19 am
content: If they ideally need to be on for months to train toward their best and they might need a few days to recover from an outage after being trained does that imply they should be on a UPS?

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 25, 2015, 01:26:11 pm
content: Quote from: TSL on April 25, 2015, 02:48:40 amAs someone who has started becoming a frequency nut over the last year or so, are there any good books you might recommend?

author: German_EE
author_rank: Super Contributor
timestamp: April 25, 2015, 02:14:38 pm
content: One thing to remember is that GPS receivers have two modes of operation. The first mode ensures that your location is as accurate as possible and the second is used to make the time signal accurate. Check your receiver data sheet and make sure you are in the right mode.

author: G0HZU
author_rank: Super Contributor
timestamp: April 25, 2015, 02:38:24 pm
content: Quote from: XFDDesign on April 25, 2015, 01:26:11 pmhttp://www.theworks.co.uk/p/health-issues/overcoming-obsessive-compulsive-disorder/9781849010726?CAWELAID=720011340001810940&CAGPSPN=pla&gclid=CjwKEAjw9uypBRD5pMDYtsKxvXcSJACcb9AYWqUckO_QR0LNv6iS2aGkcjESTJx1YV0xxkBadZmIwRoCDNvw_wcB

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 25, 2015, 02:41:50 pm
content: Quote from: G0HZU on April 25, 2015, 02:38:24 pmDon't you lot call this an ASBO?

author: G0HZU
author_rank: Super Contributor
timestamp: April 25, 2015, 03:08:57 pm
content: To show how useful the old HP8405A can be for stuff like this I dug it out and let it warm up and then tried connecting a couple of Agilent sig gens to it with both generators set to 10MHz and 0dBm.Both generators have the 1E5 high stability OCXO option fitted.Using the reference adjusting menu in one generator I was able to set the two generators to achieve the level of agreement seen in the short youtube video below.It shows 1 degree of phase shift in 8 seconds which is about 3.5e-11 short term drift. This is about 0.00035Hz error? If I lock both generators together then the HP8405A will show no meter movement at all as it can show <<0.1degree shift over 1 hour typical once it's been allowed to warm and settle. So every bit of jitter and drift is down to the relative differences between the two sig gens.I would expect that if I left the adjustment settings alone and repeated this test tomorow the generator OCXOs will have aged apart to be much worse than this. eg maybe a factor of 10 worse.HP suggest that a healthy HP8405A can be configured to show down to 5e-13 if used over a longer time frame and the sensitive phase meter output at the rear is logged by a DMM.In the video below look at the Phase dial on the right and the +/- 6 degree scale markings.https://youtu.be/Eo2k6mWZ2VM

author: bingo600
author_rank: Super Contributor
timestamp: April 25, 2015, 04:24:34 pm
content: Thanx for the video.Does this thing use normal BNC's or dedicated probes/Bingo

author: G0HZU
author_rank: Super Contributor
timestamp: April 25, 2015, 04:52:59 pm
content: Speaking from bitter experience it's probably best to borrow an HP8405A that is known to work...They come with two very long probes that are incredibly easy to damage both electrically (overload) and physically because the tips can snap off. But you can get accessory kits that allow the fitting of BNC adaptors for the probes and this helps protect them from knocks at the expense of a few extra pF of capacitance.The meter works like a crude sampling oscilloscope and it is quite a complex beast inside. I bought mine to measure negative impedances for a project I was working on and I paid just £80 for it. But it took a LOT of hard work to get this meter functioning properly as it didn't work when it was delivered.You can read about my plight in the link below and this should be enough to put most people off buying a used/untested example:Mine is now working really well but it had numerous niggly faults in it that degraded the performance...http://www.vintage-radio.net/forum/showthread.php?t=102117Note:There's a chap on that thread that is an ex HP design engineer and he quoted this:QuoteI've heard the same from other sources but I think there will be better solutions used todayThe HP8405A is a classic bit of test gear and I used one a lot in my youth (in an RF lab at work) as they can be used for so many things.

author: G0HZU
author_rank: Super Contributor
timestamp: April 25, 2015, 05:31:10 pm
content: You can read about the HP8405A in the May 1966 HP Journal here:http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-05.pdfOn page 7 (the section highlighted with a sky blue background) they seem to claim it can get down to a precision of 1e-13 within a few minutes when used as a frequency comparator. This seems a bit optimistic to me but I think 1e-12 can be quickly/easily realised.However I don't have any oscillators here that are stable enough to test it this harshly. I kind of lose interest when dealing with stuff this 'precise' as I have no need for it.

author: MarkL
author_rank: Unknown
timestamp: April 25, 2015, 07:52:14 pm
content: Quote from: mskobier on April 24, 2015, 11:43:13 pmHey Mitch,I just can't help pointing out that this is a great use for the CTT in that 2465 scope you have, since the question came up in another thread.You can measure the drift over time between the two GPSDOs with 10ps resolution. It's much faster and more accurate than watching the two signals on the screen.Auto Trigger A on Ch1, trigger B on Ch2, set resolution to 10ps, manually set both triggers level to 0.00V (or whatever's needed), select CTT delay function.Screen shot of two 10MHz signals and the measured delay between them (+1 cycle, since there's a minimum wait before B can trigger).

author: mskobier
author_rank: Regular Contributor
timestamp: April 25, 2015, 07:58:07 pm
content: MarkL,I was thinking the same thing. Of course, I did not know how to go about setting it up for the test. Now I do (thanks for the help!). See my latest post in regards to the 2465CTS, and you will see I have resolved the channel 2 triggering issue I have been trying to sort out. Hopefully, next weekend, That scope will be functional again.Mitch

author: TSL
author_rank: Regular Contributor
timestamp: April 26, 2015, 01:28:20 am
content: Quote from: XFDDesign on April 25, 2015, 01:26:11 pmProbably the definitive place to start for any time or frequency nut would be here...http://www.leapsecond.com/time-nuts.htmregardsTim

author: TSL
author_rank: Regular Contributor
timestamp: April 26, 2015, 01:33:54 am
content: Quote from: Electro Fan on April 25, 2015, 05:14:19 amYes - and if you look at some of the units from both Symmetricom and TRAK you will find them with DC inputs to facilitate running them from batteries. Mostly 48v since they'd buss off a Telco's DC in the exchange.regardsTim

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 26, 2015, 03:02:53 am
content: Quote from: TSL on April 26, 2015, 01:28:20 amThanks!Incidentally, my projects with WSPRnet wound up leading me down this path. Those ±20ppm crystals suddenly stop being so great when your frequency tolerance has to be better than 1.48Hz/f_carrier. On 20m this is <10.5ppm. All well good and fine, because you can adjust that crystal frequency! ... and then temperature effects come in and walk the carrier from 14.0956MHz to 14.0974MHz. Well there is 127.6ppm.Oops. It's these kinds of projects which feed various interests XD

author: TSL
author_rank: Regular Contributor
timestamp: April 26, 2015, 03:27:19 am
content: Quote from: XFDDesign on April 26, 2015, 03:02:53 amExactly - I play WSPR on 630m, 30m and occasionally 2m and its precisely this problem that caused my to modify my IC-7400 so I could lock it up.regardsTim

author: G0HZU
author_rank: Super Contributor
timestamp: April 26, 2015, 11:48:51 am
content: Quote from: XFDDesign on April 26, 2015, 03:02:53 amI've never used the ham whisper modes but I assumed they are very narrowband. eg <10Hz.So if you wanted to operate on 14MHz (I'm guessing that) you would need stability of about 0.2ppm to keep well inside this BW. So I don't understand the 10.5ppm number from XFDDesign. Are there different bandwidth and stability requirements for these modes?On 2m I would 'guess' you would need +/-0.02ppm for the duration of the contact. But I have no experience of using WSPR or its requirements. A decent OCXO can manage +/-0.0005ppm across one day.Probably the reason a commercial ham transceiver 'fails' badly here is because the master reference is probably something like a cheap TCXO inside the radio. So it will get heated up by heat from the transmit power amplifier. This will cause frequency drift during the contact due to the limited compensation performance of the TCXO wrt temperature gradients inside the transceiver.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 26, 2015, 06:07:32 pm
content: Quote from: G0HZU on April 26, 2015, 11:48:51 amWSPR itself has a band of 500Hz for a window, with a baud rate of 1.465symbols/sec, which comes out to 1.46Hz. The carrier tolerance gets tighter the higher up in frequency you go. The actual band of interest is carrier + 1500 through carrier +2000Hz through SSB modulation. I had a typo earlier though, I wrote 10.5ppm but the correct answer was 1.05ppm. With 20m, 14.0956MHz and a +10ppm error, you basically have a 141Hz fudge factor. At 80m, 3.5926MHz, that same 10ppm error becomes 35.9Hz. Both of these can actually be calibrated out with the software. Just as it gets easier at lower frequencies, it gets quite a bit harder at higher frequencies like you suspected. 10ppm on 2m is 1.4kHz, which exceeds the range the software can calibrate out, and gives you such a wide window of uncertainty it's problematic.The OXCO and similar standards solve the much bigger problem: drift.Drift is the killer. You can calibrate out most of the "dc offsets," but you cannot solve the problem of drift over time and temp. This is where I really got a serious appreciation for the precision business. 20ppm DC can be dialed out. 20ppm/ºC when your PA has heat creep through your PCB and raises the board temp 8C? Well at 14MHz, that's 2.24kHz. This is when I went to a TCXO at $16 a pop. 1ppm/ºC which gives me no more than ±14.1Hz drift/ºC - this is fine forfield gear. Once I added thermal isolation to the board, it was also fine for fixed-station stuff too -- but only for 20m. Just as you mentioned, push it to 2m, and that same 1ppm is 144Hz error. Pretty problematic. I finally read up some things in QEX on the FEI Rb standards, and "ooh, ~0.02ppb/day stability." With that as a standard, my 20m work has an error of 282uHz, which gives me a good level of certainty. It's all of these incremental steps that really drove home the point. If one tries to do the 23cm work, at 1296.5MHz, 1ppm alone is 1.3kHz, so one is likely needing <100ppb at a minimum and ideally 5ppb or better.There are also some interesting subtleties with locking everything to a good, clean standard. It's mainly with RF measurements, but you can get some "phantom" ip3 measurements from having your gear run asynchronous from one another due to ever so slight timing errors. It getsfarmore noticible at the 10+GHz range where the errors get multiplied up.

author: G0HZU
author_rank: Super Contributor
timestamp: April 26, 2015, 07:41:44 pm
content: Thanks for the clarificationBeing a bit pedantic over this, I'd still argue that the short term drift of a decent OCXO will often be just as good as a GPSDO (if not better) so in terms of achieving good stability in the timeframe of a radio contact I don't really see a real advantage of the GPSDO in this case.For a bit of fun whilst litening to the Arsenal V Chelsea game today I measured the residual noise/stability of my old HP8405A to see if it could get close to HP's claim of 1e-13 over several minutes.Sadly it didn't quite achieve it so mine probably still needs a few parts renewing.But it did manage +/- 0.015 degree of jitter over 100 seconds and could do this reliably, test after test.I logged the phase output connector with a DMM and it has an output of 500mV for 6 degrees. Or 1mV = 0.012degree.I typically see +/- 1mV at the DMM in this test and the plot below is about +/- 0.015degree over 100seconds which is in the ballpark of 1e-13 but I suspect a healthy HP8405A will be a bit better than this.The test below measured the SAME oscillator on both channel A and B so ideally there should be a flat line. The chart display constantly autoscales to the data so the chart scaling is a bit strange but it is in degrees rather than mV.

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 26, 2015, 08:18:26 pm
content: Back OT,If I understand correctly, all of the GPS satellites are synchronized against a terrestrial cesium clock. The clock is transmitted up to the satellites, corrections are made. If this is the case, the timing reliability is dependent upon the GPS system you're getting your clock from. In this sense, then, what is our certainty that the GPS clock is better than a Rubidium standard? I would certainly take a Hydrogen Maser or Cesium clock over the Rubidium, but I don't have any objective means to validate that the GPS clock is a superior one to the Rb. What we can identify is that there is some error in time, but that doesn't automatically infer which source is right.If I had a few of the Rb Standards being used in this comparison, I would power all of these up first and see how well behaved they are against eachother. If they're incredibly good this at least confirms that the Rb standards drift together reasonably well. Knowing this, I could then try to infer quality against other sources.

author: mskobier
author_rank: Regular Contributor
timestamp: April 26, 2015, 11:39:01 pm
content: You guys (and Gals) are way over my head in regards to time and frequency measurements. However, I do have some rudimentary data to share with the group.As I posted earlier, I took my GPSDO over to my friend house where he has his GPSDO up and running for a couple of weeks to see how close they were in frequency to each other using an oscilloscope to display the two signals and measuring the time it took for the signals to drift one cycle. The initial results were not so good. I left my scope and GPSDO at my friends with both his an my GPSDO turned on for a total of about two complete days(~48hrs). I went and visited today to check how the two GPSDOs were doing after the extended operation time. What we saw was very promising. Over the course of one hour, we saw very little drift. Probably on the order of 1/10 (or less) of a cycle drift in that period of time. It was so slow, that we chose not to try and wait for the signals to shift one cycle, and called it quits for now. What we did learn, is that the two GPSDOs were very close to each other infrequency. If I had to guestimate, I would say they were less than 3.0E-5hz difference between the two references. I know that is not saying much, but with the limited equipment we have at our disposal, that is as good as it is going to get. All I can say, is that in 48hrs, the two references stabilized very close to each other in frequency. What the actual frequency is, I do not know. One of my concerns was that the cheap (china made) GPSDOs were going to have significant differences in frequency. That does not appear to be the case from the limited testing we have done.Mitch

author: G0HZU
author_rank: Super Contributor
timestamp: April 27, 2015, 12:26:34 am
content: That result sounds more like what you can expect. If you get your other scope working and can log it with a PC you could build up a profile of the short term stability over a few minutes. If not, then maybe find someone with an old HP8405A and log it that way?I hope nobody minds another HP8405A plot but I logged the residual phase drift for a 10MHz oscillator fed to both channels over 460 seconds (approx 7.5 minutes) and the result is as below. It shows the performance potential for this instrument for comparing oscillators over time although I don't usually use it like this so I hope I'm doing it correctly. Again, I only saw about +/- 1mV of drift on the DMM in the whole 7 minutes.The result looks pretty good to me and the 'trend' over the 7 minutes is only about 0.015 degrees in 460 seconds which is much closer to what HP were claiming it can do way back in 1966. i.e. it is approaching an order of magnitude better then 1e-13 over this timeslot so maybe mine is working better than I first thought. The Y scale is again in degrees and not mV.However, I have no two oscillators here that can maintain 1e-13 over 7 minutes to do a 'real' test and I doubt many other people will have either.

author: TSL
author_rank: Regular Contributor
timestamp: April 27, 2015, 01:26:04 am
content: Quote from: XFDDesign on April 26, 2015, 08:18:26 pmGPS satellites each have 4 atomic clocks on board, 2 Cesium and 2 Rubidium. These are not the RB's we might buy and this combination has 1ns accuracy.Those clocks are synchronized ( after accounting for relativity ) from the Naval Observatory Master Clock which comprises the following...4 USNO rubidium fountains20 hydrogen masers23 5071 cesiumsHigh stability and precision is required or you just couldn't plot your location properly. i.e. 1ns in time is ~1 foot.For more info on GPS, go here...http://www.metaresearch.org/cosmology/gps-relativity.aspFor info on the master clock, go here...http://tycho.usno.navy.mil/frontpage.htmlGiven I have 3 RB units, I might be temped to fire them all up and plot them against each other with my Thunderbolt as the base ref. The fun part here is accounting for cable lengths etc. since they will introduce notable delays.Might have to start a time-nutty thread for thisregardsTim

author: TSL
author_rank: Regular Contributor
timestamp: April 27, 2015, 01:30:48 am
content: FYI.If you really want to understand the accuracy and timing involved in the GPS constellation, go here...http://www.usno.navy.mil/USNO/time/gps/usno-gps-time-transferregardsTim

author: XFDDesign
author_rank: Frequent Contributor
timestamp: April 27, 2015, 03:17:14 am
content: Quote from: TSL on April 27, 2015, 01:26:04 amAh okay, I had bad information then.I knew of the National Observatory, but I was informed that the satellites all synchronized to that -- not that they had high-precision clocks on-board in addition.On the experiment, might I suggest not worrying so much about the cable lengths initially and simply tying all of them together to the same source and determining their phase skew against eachother? If you're using the approach I will be taking for phase-noise measurements to determine OSC-to-OSC stability, you should be able to get a DC error out of your phase-detector and factor that in as a calibration coefficient. Since what we're collectively going after is the time-variance, I wouldn't imagine seeing the skew as being a moving target.Additionally, do you have anything like WWV in Australia?

author: TSL
author_rank: Regular Contributor
timestamp: April 27, 2015, 05:28:38 am
content: Quote from: XFDDesign on April 27, 2015, 03:17:14 amMy RB's can't be GPS synchronized but it would be interesting to plot their aging and their deviation from 10Mhz sourced from the Thunderbolt.As for phase noise, I use KE5FX's program on my GPIB attached PC to get phase noise plots from my HP8566B.WWV in Aus ? - we used to. It was call VNG from, Lindhurst in Victoria, but our stupid government decide not to fund it any more so now we don't have an on-air time standard. We can hear WWV on 5 and 10Mhz and sometimes we can JJY too.regardsTim

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: April 29, 2015, 02:11:13 pm
content: Interesting to note tracking two Symmetricom GPSDO's RFTG-u (OCXO), and RFTG (Rb), a RFG RB (non GPSDO Rb), and maybe the GPS calibrated gigatronics sweeper @ 10Mhz. All on screen at once. Hardly any phase shifting whatsoever over 4 hour time frame.eLoran may be resurrected (in U.S.), and in a much more accurate form. My Stanford Research FS700's are waiting. Nice built in Phase detector LCD meter. Looks like it will be backward compatible with Loran C.

author: Chevacc
author_rank: Newbie
timestamp: July 17, 2015, 12:38:57 am
content: Hi all! Who buys GPSDOs? What for these devices need?

author: Lightages
author_rank: Unknown
timestamp: July 17, 2015, 01:38:04 am
content: They are very accurate clocks, basically. They are used to keep cell hone towers synchronized, or to have very accurate clocks for astronomical observations, navigation, etc. They are also used to provide a stable frequency reference for radios and other circuits.

author: Chevacc
author_rank: Newbie
timestamp: July 17, 2015, 02:17:01 am
content: Thank you. Do you design GPSDO?

author: Lightages
author_rank: Unknown
timestamp: July 17, 2015, 02:26:57 am
content: No, I don't design GSPDOs. I am just an user

author: Chevacc
author_rank: Newbie
timestamp: July 17, 2015, 06:59:44 am
content: Are there most popular/the best GPSDOs? How can I estimate/check quality of these devices? It costs about 1000 - 15000 $. Why so difference in prices?Can you send me information about your last time GPSDO?

author: Lightages
author_rank: Unknown
timestamp: July 17, 2015, 07:06:26 am
content: Actually I am future user. I have one on the way from a ebay purchase. If you want to know more I am not the person to ask. There are many more on this forum who have much more experience than I do.

author: TSL
author_rank: Regular Contributor
timestamp: July 17, 2015, 07:14:22 am
content: Quote from: Chevacc on July 17, 2015, 06:59:44 amIt depends on what you want to use one for.Many people here use one to provide the 10Mhz reference for their test gear, others such as myself, also use one as a basis for local oscillator locking of UHF and up radios. The primary difference being that for test gear locking, a square wave output is just fine but for RF LO you want sine wave and exceptionally low phase noise.Some people are using them in precision timing applications and then the 1 pulse-per-second jitter becomes a concern. Cheap GPSDO's might have +- 90ns of jitter whereas better units might only have +-2nS of jitter.So, the question is... What do you want one for ?regardsTim