Lars DIY GPSDO with Arduino and 1ns resolution TIC

[MIS42N]

So that's my thoughts. What's your target? what are you going to be happy with?

I'm using a timing receiver and external antenna which give me a positional accuracy of better than 1 metre. The OCXO is a new old stock Vectron that has been running for about 3 months. It claims +/-10ppb stability over 0-70degC and +/-1ppb aging per day.

The most accurate piece of kit I have is a 9-digit Racal 1991 frequency counter, so calibrating that would be my ultimate goal. I would then be using that to calibrate HF transmitters and receivers, so nothing too demanding.

Mike

If you are feeding your output to the Racal, after it has had an hour or two to warm up, I would expect the reading to be stable, maybe varying the last digit by 1, no more. It may not be accurate, but it should be stable. What you describe is capable of 10 digit accuracy and if you aren't getting it then something is wrong. But sorting out if it is instability in the Racal, or something else, might require another frequency counter. Sounds like you are where you want to be. Congrats.

[Mike99]

If you are feeding your output to the Racal, after it has had an hour or two to warm up, I would expect the reading to be stable, maybe varying the last digit by 1, no more. It may not be accurate, but it should be stable. What you describe is capable of 10 digit accuracy and if you aren't getting it then something is wrong. But sorting out if it is instability in the Racal, or something else, might require another frequency counter. Sounds like you are where you want to be. Congrats.

Thanks  All due the the excellent assistance from the experts on this forum.

My default position is "near enough is good enough" but unfortunately I have a streak of perfection and the shift due to the overnight drop in temperature annoys me.

Mike

[MIS42N]

I have a streak of perfection and the shift due to the overnight drop in temperature annoys me.

Mike

Could it be variation in supply voltage? It may be non linear so simple temperature compensation wouldn't work. I think someone previously said plot the average DAC against temperature. I did it with my last rig, fed the dac/temp into a spreadsheet, created a plot. You may see a trend. To make sense of it, I asked for least squares linear interpretation. Might be an exercise in frustration, it was for me. Good luck.

[Johnny B Good]

So that's my thoughts. What's your target? what are you going to be happy with?

I'm using a timing receiver and external antenna which give me a positional accuracy of better than 1 metre. The OCXO is a new old stock Vectron that has been running for about 3 months. It claims +/-10ppb stability over 0-70degC and +/-1ppb aging per day.

The most accurate piece of kit I have is a 9-digit Racal 1991 frequency counter, so calibrating that would be my ultimate goal. I would then be using that to calibrate HF transmitters and receivers, so nothing too demanding.

Mike

 Those temperature stability figures seem outrageously high for any OCXO of worth to my mind. Quite frankly, I'm surprised no one else has picked up on those figures. TBH, those figures look more in line with that of a TCXO than an OCXO.

 The CQE 10MHz OCXOs I'm using (going by the temperature and ageing stability of the one used in my FY6600) are at least an order of magnitude better on temperature stability (admittedly only a guesstimate) and a good two orders better on ageing.

 I installed a small 12v 6W smpsu board removed from a Linksys wallwart into the AWG to power the OCXO independently of the power up/down state of the AWG itself in order to keep retrace and protracted power on warm up at bay (it takes just 1.3W off the mains supply to keep it at temperature), versus the 5 to 6 watts 'standby power' of an 8 watt AWG switched into 'standby mode').

 I've got it wrapped in a sponge rubber overcoat to protect it from cooling fan turbulence and to knock about 100mW off its naked 850 to 900mW consumption. Even so, it still surprisingly shows a two or three hour warm up drift of circa 150ppt due to powering the AWG up from stone cold.

 When I was trying to use it as a substitute RFS by micromanaging its small shifts in frequency due to changes of room temperature using the 2GT effect by altering the tilt angle. Whatever daily ageing it had, seemed to be swamped by the room temperature variations. There was no discernible change of an average angle of dangle requirement from day to day over periods of a week or more.  I rather suspect it had been a matter of pure dumb luck that I'd picked out that single OCXO from a collection of seven which happened to have a very low ageing rate.

 Now that I have a real RFS to test my basic GPSDO against (essentially a modern respin of the James Miller design taking advantage of an M8T to simplify the design a little more), I no longer obsess about the stability of the FY6600's OCXO upgrade which now includes an external 10MHz reference input socket to injection lock the OCXO, specced to lock within +/-10ppb of the OCXO's frequency (I consider an error of 1ppb to be 'beyond the pale', requiring an immediate recalibration to get it back within 100ppt of my GPSDO reference).

 From casual observation of the GPSDO's behaviour during the ten or so minutes post OCXO warm up, these CQE OCXOs look to have a 2 to 3 Hz per volt tuning rate equating to a 3mHz per millivolt change in the VFC voltage. Making some basic test measurement with my current MK II GPSDO, I see a VFC voltage change of roughly 100uV per degree C change in external case temperature equating to an uncorrected variation of some 2 or 3 ppb for a 70 deg C room temperature delta.

 In the case of a James Miller respin, such external temperature variations are of no to little consequence in frequency stability even under the more extreme ambient temperature variations typical of a home environment. Of greater significance in a GPSDO relying on a single frequency GPS receiver such as the M8T, are the minutes to minutes variation in phase due to variations in the electron density of the ionosphere, typically +/- 3 to 5ns over periods of 5 to 15 minutes or so.

 I had been running my setup using both the GPS and the Galileo constellations in recent weeks where the phase variation would keep within +/- 3ns for hour long periods before ambient temperature changes took their toll on my naked, yet to be installed in a thermally regulated enclosure, RFS. About a week ago, I decided to remove the Galileo constellation to see whether there would be any improvement using only the GPS constellation. My impression so far is that, if anything, there has been a slight but definite deterioration in regard of ionospheric induced phase variations.

 I'll be trying the effect of using the Galileo constellation on its own but it has now become obvious that I really need to stabilise the base plate temperature of my RFS before I can get more reliable data on this issue of ionospheric perturbations which leaves me pondering the best way to achieve such thermostatic control... I'm still pondering.

John

[Mike99]

Could it be variation in supply voltage? It may be non linear so simple temperature compensation wouldn't work. I think someone previously said plot the average DAC against temperature. I did it with my last rig, fed the dac/temp into a spreadsheet, created a plot. You may see a trend. To make sense of it, I asked for least squares linear interpretation. Might be an exercise in frustration, it was for me. Good luck.

I'm running the GPSDO off a 15V linear power supply that came with a GPS amplified splitter unit. The GPSDO itself contains the usual assortment of LM7805/LM7808/LM78L05/REF02EZ regulators.

I didn't see a trend when I plotted DAC vs temp but I'm doing another uncompensated run and will try again shortly.

Mike

[Mike99]

Those temperature stability figures seem outrageously high for any OCXO of worth to my mind. Quite frankly, I'm surprised no one else has picked up on those figures. TBH, those figures look more in line with that of a TCXO than an OCXO.

Well, it's marked MC2001X4-046W and I found the attached data sheet which I presume is applicable.

I measured the tuning range from 0 to 4V as 1.8ppm which according to the data sheet denotes an SC cut crystal.

That's pretty much all I know about it.

Mike

[Johnny B Good]

Those temperature stability figures seem outrageously high for any OCXO of worth to my mind. Quite frankly, I'm surprised no one else has picked up on those figures. TBH, those figures look more in line with that of a TCXO than an OCXO.

Well, it's marked MC2001X4-046W and I found the attached data sheet which I presume is applicable.

I measured the tuning range from 0 to 4V as 1.8ppm which according to the data sheet denotes an SC cut crystal.

That's pretty much all I know about it.

Mike

 I downloaded that document and compared it to other OCXO datasheets. It seems comparable to other makes, reminding me just how generously wide a tolerance the "OCXO Club" gives its products to forestall any customer complaints of the product failing to meet specifications.

 I think you'll find it'll more than exceed its given specification in actual use simply because those specifications are so far from being 'Stellar'. I daresay the specifications for my collection of CQE 10MHz OCXOs will show similarly wide tolerance figures should I ever track down any datasheets for these now obsoleted by successive changes of manufacturing ownership. Still, if they were deemed good enough to go into Symmetricom GPS time and frequency reference kit, then that's good enough for me. 

John

[Mike99]

I downloaded that document and compared it to other OCXO datasheets. It seems comparable to other makes, reminding me just how generously wide a tolerance the "OCXO Club" gives its products to forestall any customer complaints of the product failing to meet specifications.

 I think you'll find it'll more than exceed its given specification in actual use simply because those specifications are so far from being 'Stellar'. I daresay the specifications for my collection of CQE 10MHz OCXOs will show similarly wide tolerance figures should I ever track down any datasheets for these now obsoleted by successive changes of manufacturing ownership. Still, if they were deemed good enough to go into Symmetricom GPS time and frequency reference kit, then that's good enough for me. 

John

I also have an NDK END3032A which came out of an IFR2025 signal generator but it's impossible to find any data for it other than what's quoted in the signal generator manual, which is:

Aging rate: Less than±2.5 in 10⁷ per year, less than ±5 in 10⁹ per day after 2 months’ continuous use.

Stability: Better than ±5 in 10⁸ over the temperature range 0 to 50°C.

Mike

[Mike99]

I've just discovered something interesting ...

All my previous measurements were taken with the NEO-M8T tracking three satellite systems: GPS, Galileo & Glonass.

I ran the GPSDO for 2 days with no temperature compensation and only GPS enabled and the results appear to be much better.

Why would that be?

Mike

[Johnny B Good]

 I suspect mainly on account you dropped the glonass constellation from your bouquet of SVs. 

 If you add the Galileo constellation to the GPS one, you may find a slight improvement over just the GPS alone. However, that's just the impression I get after switching to GPS only a week or two back after running with both Galileo and GPS for about a month or so.

 It could simply be the greater room temperature variations afflicting my Rubidium reference by which I've been monitoring the ionospheric induced phase shifts on the GPSDO's 10MHz output which arose when I decided to go solo with the GPS constellation - bad timing on my part and I'm now more actively interested in giving my RFS a thermostatically controlled enclosure to eliminate the effect of room temperature variations.

John

[thinkfat]

I suspect mainly on account you dropped the glonass constellation from your bouquet of SVs. 

 If you add the Galileo constellation to the GPS one, you may find a slight improvement over just the GPS alone. However, that's just the impression I get after switching to GPS only a week or two back after running with both Galileo and GPS for about a month or so.

 It could simply be the greater room temperature variations afflicting my Rubidium reference by which I've been monitoring the ionospheric induced phase shifts on the GPSDO's 10MHz output which arose when I decided to go solo with the GPS constellation - bad timing on my part and I'm now more actively interested in giving my RFS a thermostatically controlled enclosure to eliminate the effect of room temperature variations.

John

I agree about GLONASS. Any configuration that included it had degraded timing stability. GPS and Galileo are quite acceptable especially if you don't have perfect clear sky view.

[tchiwam]

Funny, I tried to get a decent 10MHz and 1PPS for my lab.

I have a symmetricon GPS shooting the PPS in a PRS10. I then use the disciplined 1PPS and 10MHz out from the PRS10 for my systems and triggers.

Even this has some jitter between the PPS and 10MHz...

[Mike99]

I agree about GLONASS. Any configuration that included it had degraded timing stability. GPS and Galileo are quite acceptable especially if you don't have perfect clear sky view.

Thanks guys  . In my ignorance I had assumed that more satellites would give a better result.

I've disabled GLONASS and run survey-in again and my positional accuracy has improved from 0.75 metres to 0.2 metres.

So as someone said a few posts back it looks like my work here is done. Except ...

What I need now is another GPSDO so I can compare results. I think I'll build Yannick's design next.

Curse this addiction 

Mike

[Johnny B Good]

 Hi Mike,

 Last night/this morning, I finally got round to rerunning an experiment I did a few months ago to compare the timing offsets between the GPS, the Glonass and Galileo constellations where I discovered (but didn't make hard copy notes of) the offsets involved. I seem to recall that the Galileo offset from GPS had been in the 3 to 5 ns region whilst that of the Glonass had been a massive 18ns or so.

 I reran only the GPS versus Galileo experiment (the Glonass had been so far out, I didn't see any point in repeating that experiment ) and my best estimate of the offset this time round was that it was somewhere around the 1 to 2ns mark.

 Since I'd gained the impression that using the GPS on its own had definitely, if only slightly worsened the VFC variations to circa 600uV pk-pk versus the 300 to 400uV when using both GPS and Galileo together, I decided to go back to that combination after running this test.

 The daily  room temperature mediated VFC voltage excursions are around the the 1mV at present - not an issue as far as a basic James Miller styled GPSDO design is concerned. Quite frankly, a basic, microcontroller free GPSDO can deal with this sort of temperature variation (a half to one degree per hour rate of change) without any measurable detriment to its phase stability. With a single frequency timing module like the M8T, the biggest issue is that of the ionospheric induced phase shifts at rates of 1 to 5ns per 5 to 15 minutes and longer time periods which totally swamp the slower room temperature variations which are almost totally compensated for by being part of the PLL error correction process anyway.

 The upshot of this being that I'm not at all surprised at your improved stability and tighter positional survey in results after ditching the GLONASS constellation. 

John

[MIS42N]

What I need now is another GPSDO so I can compare results. I think I'll build Yannick's design next.

Mike

I had a look at Yannick's schematic https://www.instructables.com/GPSDO-YT-10-Mhz-Lcd-2x16-With-LED/

Some things to think about:
(1) It looks like he is using the 10MHz clock as the clock for the processor (which I think is a good idea) which is therefore the base clock for the PWM. Using the PWM with a period of 65536 counts (16 bits) gives a frequency out of 10,000,000/65,536 = approx 153Hz. That's rather low and I tried using LTspice to figure the response of the 10K ohm + 100 uF capacitor and I got a figure of about 7mV p-p (very rough, but ballpark). Perhaps someone better at number crunching can verify this. That may modulate the frequency with an unwanted 153Hz tone.
(2) The control voltage is only as stable as the power supply to the processor
(3) I think 16 bits may limit the Allen Deviation (which I still struggle to understand) - assume a sensitivity of the OCXO to the control voltage of 1V/Hz. 1 bit change of the control yields a 1 cycle difference in less than 4 hours. I think if I understand it right that means the system can't fix the frequency to better than 1 part in 10^-11. Of course, that is not big deal unless you want better.

So I'm not sure there's any benefit over Lars solution (actually the 153Hz tone would make it worse if it is a real thing).

A simple solution is to use a stable voltage reference (the higher priced OCXOs have it) and a potentiometer to provide 99% of the control voltage (which will need manual adjustment) and let the output of the processor provide the other 1%. If you already have one reference it shouldn't be too hard to set up - fix the PWM to a 50% duty cycle and feed the output of the OCXO to your referenced frequency meter. Fiddle the pot until as close to 10MHz as you can get, then unleash the PWM to make the finer adjustment.

More theoretical speculation. May not be right but worthy of thought?

[Mike99]

I had a look at Yannick's schematic https://www.instructables.com/GPSDO-YT-10-Mhz-Lcd-2x16-With-LED/

Some things to think about:
(1) It looks like he is using the 10MHz clock as the clock for the processor (which I think is a good idea) which is therefore the base clock for the PWM. Using the PWM with a period of 65536 counts (16 bits) gives a frequency out of 10,000,000/65,536 = approx 153Hz. That's rather low and I tried using LTspice to figure the response of the 10K ohm + 100 uF capacitor and I got a figure of about 7mV p-p (very rough, but ballpark). Perhaps someone better at number crunching can verify this. That may modulate the frequency with an unwanted 153Hz tone.
(2) The control voltage is only as stable as the power supply to the processor
(3) I think 16 bits may limit the Allen Deviation (which I still struggle to understand) - assume a sensitivity of the OCXO to the control voltage of 1V/Hz. 1 bit change of the control yields a 1 cycle difference in less than 4 hours. I think if I understand it right that means the system can't fix the frequency to better than 1 part in 10^-11. Of course, that is not big deal unless you want better.

So I'm not sure there's any benefit over Lars solution (actually the 153Hz tone would make it worse if it is a real thing).

A simple solution is to use a stable voltage reference (the higher priced OCXOs have it) and a potentiometer to provide 99% of the control voltage (which will need manual adjustment) and let the output of the processor provide the other 1%. If you already have one reference it shouldn't be too hard to set up - fix the PWM to a 50% duty cycle and feed the output of the OCXO to your referenced frequency meter. Fiddle the pot until as close to 10MHz as you can get, then unleash the PWM to make the finer adjustment.

More theoretical speculation. May not be right but worthy of thought?

Thaks for those insights, I will give them some thought. However, this is an addiction remember, so facts and logic have little to do with what happens next ... 

Seriously, though, I don't need spectacular accuracy. Plus I have a spare OCXO and all the bits I need to build it so it will make for an interesting project during the dark winter evenings. And it has a display 

My Lars GPSDO uses a stable voltage reference and potential divider to trim the gain.

Mike

[Johnny B Good]

 Season's greetings to one and all.

=====snipped=====

 BTW, I did swap the OCXO out that evening and had it up and running for initial tests after checking the 'other usual suspects' I'd had in mind as possible contenders for the sudden EFC transient and 3% phase shift in the PLL's 100KHz input that I'd provoked with my finger tapping test.

 However, after letting the EFC voltage settle to its new value of 2.4210, I did another finger tapping test and got an unpleasant surprise as it suddenly shifted half a cycle or so at 10MHz before stabilising. It hasn't responded to the many vibration tests  over the past two days since, so I suspect I may simply have relieved some pent up stress in the "Stress Compensated Cut" crystal inside my "NOS" OCXO. I guess only time will tell but apart from that little 'surprise', it seems to be behaving itself just fine.

 The EFC has dropped another 8mV and I'm expecting it to come to a halt and start going up again based on my previous experience with the 13MHz unit. I'm assuming that the 13MHz unit's retrace behaviour is typical - it may well not be. I'm just going to have to keep my eye on the EFC voltage (as I normally do) for the next few days and see what happens before I start running any power cycle tests.

John

 Following up on my OCXO swap out just over six weeks ago (how the time just flies by!), here's an update on how the replacement has performed in that time.

 From my experience with my MK I GPSDO build using the 13MHz OCXO which had exhibited a rising VFC trend, indicating a reducing frequency with age, I had assumed the replacement 10MHz OCXO I'd used in the MK II GPSDO might follow a similar trend (both being CQE units) with the initial reducing VFC being simply a retrace effect similar to what I'd seen with the MK I,

 It would seem that ageing can go in either direction, even with same brand units. In this case, the replacement OCXO has relentlessly continued its downward VFC trend to the point where it slowly settled to 2.4018v from an initial 2.4210v just over six weeks back (I never did get round to power cycling it since I was hoping to see the trend halt and finally reverse). However, early this afternoon, It had suddenly dropped to 2.3998v +/-100uV or so.

 After confirming my meter connections were still ok and that I was observing a real VFC reading which was behaving as if the PLL was still in control (I was in the middle of my RFS boxing up project so decided to forgo the half hour or so to set it up to compare the GPSDO output against), I tapped the GPSDO with my fingers and observed an immediate upward change in the VFC voltage (by around 10mV or so) before it eventually settled down around the 2.4010v mark where it has been hovering ever since, +/- the usual 2 or 3 tenths of a millivolt due to ionospheric effects.

 So, after some six weeks of monitoring the replacement OCXO's behaviour without any untoward 'events' other than that initial response to my finger tapping at the start of the test run, I got to see a less dramatic repeat of the instability I'd experienced with the original OCXO, leaving me in a similar situation I'd had to start with.

 The only thing I can say this time is that it does seem to be an OCXO issue rather than some obscure intermittent fault elsewhere (dry joint or whatever). I guess 'll just have to accept this as the downside of disciplining an OCXO which can't truly be guaranteed to be completely free of such anomalous behaviours no matter how good the quality is supposed to be. The only thing to be said in this case is that there isn't a microcontroller algorithm to be 'caught on the hop' since, for a start, there isn't a microcontroller to begin with.

 The "Take Away" for those of you struggling with the Lars design, is that any odd events you might see in your data may well  simply be down to your OCXO 'being an OCXO, unruly behaviour from time to time included". At the current rate of ageing, it seems to have settled to around 1mV drop per week in the average VFC voltage so I'll keep it powered until the new year when I expect it to finally drop below the 2.4000 volt mark before I try power cycling it to observe its retrace behaviour.

 My own 'Take Away' from this is that these OCXO based GPSDO's need a little TLC to help with their disciplining. I'm pretty much resigned now to the need to administer a 'gentle love tap' every other day or so as part of its routine (percussive) maintenance to forestall any sudden frequency jumps (I'd been a little lax of late in providing the required TLC).

John

Update 2021-02-13 :-

 The above speculation turned out to be completely off the mark (i.e. a load of "Bullshit" ). Those rogue phase shifts had started to become more frequent over the past few weeks, indicating that a second 'jumpy' OCXO showing identical behaviour to the first would be unlikely in the extreme, therefore pointing the finger of suspicion to a dry joint or a bad connector contact elsewhere.

 I opened it up to check, specifically, for a dry joint between the centre pin of the PCB mount SMA socket and the ref input pin on the 74HC74046. I couldn't provoke the symptoms other than for the briefest of responses to finger pressure stress testing (which in hindsight points to insufficient spring contact pressure in the SMA socket itself).

 I'd had great difficulty through difficult access to get a fine tipped soldering bit onto the pin to effect a good solder joint and this had been my prime suspect. Thorough and careful examination with a jeweller's loupe and varying angles of illumination revealed an almost text book perfect joint, much to my surprise. Careful examination of the socket contacts however, suggested the lack of contact pressure as the most likely issue so I used the tip of a pin to close the two contact fingers up.

 Further vibration testing before sliding the board back into the enclosure failed to provoke a response but immediately after sliding it back in and temporarily fitting a couple of the screws to test the virtually fully assembled unit, resulted in a slow climb of VFC voltage free of the normal PLL variations seen during a normal startup. It seems I had disturbed a very weak solder butt joint between the adjacent PC1 output pin and the one and only 2M resistor I had to make up the 1200s TC filter. I resoldered this joint, adding additional solder to give it the much needed structural strength it had lacked from my initial soldering.

 Whilst this makes it another possible source of trouble, I think it's an unlikely rank outsider since I don't think it became an intermittent solder joint until after I'd disturbed it when pushing it slightly to one side to gain better access to the adjacent ref input pin. Besides which, the symptoms hadn't exactly matched the original symptoms I'd experienced. My money is still on it being a low spring contact pressure in that SMA socket which, provided I don't disconnect it again, should now no longer cause any further trouble (fingers crossed ).

[Mike99]

Happy New Year everyone (better late than never  ).

I have now built two GPSDOs. Does that make me a timenut?

The Lars/Paul unit uses a NEO-8MT timing receiver while the Yannick one has a 7M. Both read exactly the same freqency on my 9-digit counter.

I've also perfected making my own double sided PCBs using a laser printer, toner transfer paper and a laminating machine.

Happy days.

Mike

[thinkfat]

Next you'll need a distribution amplifier for the 10 MHz. I made one over the holidays. I'll post a link if anyone is interested.

[nealix]

@Thinkfat:

Yes, I'd be interested in learning about the distribution amp.

Happy New Year.

Neal,  N6YFM

[thinkfat]

@Thinkfat:

Yes, I'd be interested in learning about the distribution amp.

Happy New Year.

Neal,  N6YFM

I've started a topic here: https://www.eevblog.com/forum/projects/my-10mhz-distribution-amplifier/
Github repository with the design files here: https://github.com/thinkfat/dist-amp

[Dbldutch]

I also build a kind of 10MHz "distribution amplifier".
It's actually only two isolated channels to avoid ground issues with non-earth connected equipment (which I have).
The PCB fits inside my GPSDO enclosure.
The description is at the end of my blog describing my version of the Lars GPSDO.
http://www.paulvdiyblogs.net/2020/07/a-high-precision-10mhz-gps-disciplined.html
Files are available on my Github, link is in the Blog

[iMo]

@Dbldutch: interesting reading your blog!
My similar experience while experimenting is the system is pretty sensitive to EMI. For example switching on/off a lamp or any other device unlocked my XOR based loop such I had to wait the entire time constant (aprox 20 minutes) to get it "locked" again.

I think you have to isolate galvanically the system completely off the environment, ie:
a) optocouplers for serial,
b) 1pps pulses from NEO via an optocoupler too, and
c) the 10MHz output via a broadband transformer. I would not connect the BNC socket to the pcb ground via that RC as you did.

The power supply must resist the EMI and the mains noise pulses as well (linear one with a transformer with very good filtering, capacitive multipliers). Thus complete galvanic isolation, no common grounds.. And a metallic box too..

Would be interesting to see how the operation of this GPSDO changes with such an isolation!

[MIS42N]

An update on my experiments. I had a disconnect between the need to have a GPS receiver with a good view of the sky, and the need to have the GPSDO in a friendly environment. This was solved by putting the GPS antenna on a roof about 6 meters up. The GPS receiver is cable tied to a roof rafter. The 1ppS and NMEA serial data is pushed through a UA9638 differential line driver down 16 meters of Cat 5 Ethernet cable to a UA9639 receiver then to the GPSDO. This works well.

I have trouble reconciling the measurements bandied about in this topic and the results I get from my methods. In my setup the microprocessor sets a fixed control voltage on the OCXO then monitors the difference between the OCXO and the incoming 1ppS. When they diverge by some limit (currently about 100nS) a correction is applied to the voltage to bring the two back in synchronisation. The setup now has a good signal and the OCXO is in a reasonably stable temperature, and the time between adjustments is typically between 20 minutes and several hours. The change in control voltage can be interpreted as a change in frequency (the relationship is about 0.1V/Hz), the frequency changes are now less than +-0.002 Hz so I assume the OCXO frequency is always 10 MHz +- 0.002 Hz. How do I interpret that as an MDEV or ADEV or any of the other acronyms?

I note people plot their DAC values against time and they are continually changing. Doesn't that cause larger short term frequency variations? Is it a natural outcome of using a PLL (real or simulated in software). I believed so (and still do) and decided that a PLL is not the answer. The OCXO could not be reliably said to be any particular frequency as it is affected by the instantaneous DAC value. I don't see how this is useful - e.g. if used as a method of locking a 49.152MHz signal for an ICOM receiver. It would be forced to follow the DAC swings too.

I conclude that for best results, maximum effort should go into building a very stable OCXO. Then really loosely couple it to the GPS system. For example if an OCXO could be guaranteed to run uncorrected for 24 hours and not lose or gain more than a couple of cycles, then a burst of 1ppS for a minute a day would be enough to work out a correction for the next 24 hours. It wouldn't matter if the 1ppS was out by 100nS, it represents an error of little more than 1 part in 10^-12 over a day. Any cheap GPS module would deliver that sort of result, all the effort to get 1ppS accurate to 10nS is unnecessary.

Something to discuss?

[Dbldutch]

Hi IMO,
Thank you for the comments, I always appreciate your insights.

To clarify some of your questions and remarks, also for others reading this, the following:

The GPSDO circuit itself is completely isolated inside my metal enclosure. Nothing connects to the metal. Only the already isolated outputs of the distribution amp are connected through the SMA chassis parts to the enclosure.
The 12VDC going in (coming from a charging lead battery) is not connected to earth ground and the negative is isolated from the enclosure.
The SMA cable from the antenna is not connected to earth ground.
The BNC connectors for the 1PPS and 10MHz out on the front-panel are isolated from the chassis, although the ground of the BNC is connected to the GPSDO ground, of course.
The serial connections to the Neo and to the Nano are fed to the measuring and logging enclosure (all plastic) that sits on top of the GPSDO is also isolated from earth ground and fed by the same 12V supply as the GPSDO. The serial leads go to a Raspberry Pi Nano that is also not connected to earth ground.

I have been using my three at first, and now two GPSDO's for almost a year now and I never experienced an EMI issue that I didn't cause myself. That only happened when I connected other floating (not connected to earth ground) instruments to the not yet isolated 10MHz output. That's why I added the isolated outputs.

My Bliley based GPSDO is now permanently connected through the isolated outputs for the 10MHz signal to my Function Generator and my Frequency Generator for a few weeks now and I have not had a single issue so far. As of interest, I look and graph the Lars, NMEA and Counter logs daily. No issues whatsoever.