Lars DIY GPSDO with Arduino and 1ns resolution TIC

[cdev]

I just saw this post by Bob ,KB8TQ, over on Time-Nuts on how to test an OCXO, this looks like a useful thing to add to this thread. He says that most salvaged OCXOs on EBAY etc, have issues, and this is a way to weed out the ones that are obviously defective from the good ones, (and the ones with more subtle issues, it seems).  You need a multiple bench supply with adjustable voltage and current.

http://lists.febo.com/pipermail/time-nuts_lists.febo.com/2020-August/101216.html


[time-nuts] Datasheet for TeleQuarz 20TQG01?
Bob kb8tq kb8tq at n1k.org
Sat Aug 22 09:18:58 EDT 2020
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Hi

Regardless of who made it, OCXO’s often don’t do well after being knocked around.
The process of puling them off pcb’s is stressful to the internals. The shock and vibration
in normal “junk box” storage can be an issue. As a result, the original performance data
may not be of much use.

I’ve tested a lot (like hundreds) of surplus OCXO’s. Well under 10% of them met their original
performance specs. That’s just for the stuff I can test in the basement, with better test systems,
the total would be lower.

Pinout wise, the ground pin *should* be easy to spot. Normally it’s bonded to the case. Once you
have it identified, start looking at pinot diagrams for similar sized OCXO’s. It may fit a standard
pattern. That would give you a guess for the +12V pin.

Hook up with a supply set to about a 1.2A current limit. Apply 12V to the assumed supply and
ground. The part probably pulls < 1A and almost certainly pulls < 1.5A. After 10 minutes on power
the current should start to cut back. If the case gets really hot (like you can’t touch it for the count
of 5) then something probably is wrong ….

Once you have it on power, a ’scope hopefully will spot the output pin. If it’s a sine wave, running
without a load may give you a pretty distorted output. 50 ohms is a good guess for the load.

The EFC pin should look like a high resistance on an ohm meter. It may or may not have a bias
on it when the device it powered. Feeding it 0 and 2.5V while watching the output on a frequency
counter should let you ID it. After running the device for a day or three, “tune” the EFC to whatever
voltage gives you 10 MHz. That’s the center voltage for what you have, regardless of what it
was when it left the factory ….

Lots of fun !!

Bob

> On Aug 22, 2020, at 8:34 AM, Paul Boven <p.boven at xs4all.nl> wrote:
>
> Hi again,
>
> Replying to myself:
>
> More online searching has revealed that 20TQG01 is likely a date code (week 20 of year 2001, charge G?). The other identifiers printed on the OCXO are '7368' and 'EO10391410'.
>
> On 8/22/20 11:56 AM, Paul Boven wrote:
>> Dear time nuts,
>> In my surplus drawer, I found an OCXO which I would like to use for a home-built GPSDO. The part in question is a 10 MHz, 12V OCXO, type 20TQG01. It came from a flea market, and looks to have been removed from a PCB. It's fairly large metal case (h = 2.5cm, w = 4cm, l = 5cm) and has 7 pins. According to the seller, it achieves 1.2 ppb (!).
>> Unfortunately, the Telequarz group no longer exists, and I've been unable to find anything resembling a datasheet. Any further information would be very welcome, especially pinout and performance parameters.
>> Regards, Paul Boven.
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com

[Mike99]

Please could someone explain how to optimise the time constant, as I don't really understand the instructions that Lars wrote. Set the GPSDO into hold mode, but then what?

Thanks

Mike

OK I decided to do the decent thing and read this thread before asking any more questions. Several days later I have a better understanding 

I have built Lars' original circuit with an NDK OCXO that came out of an IFR signal generator. It's very sensitive to control voltage (1500ppb/volt) but I managed to get the gain up to 500 with a voltage reference, potential divider and a 4.7M resistor in the DAC line.

With default EEPROM values it locks quickly and stays locked over long periods. However there are quite a few "10MHz Missing?" errors during warm up. Once it's locked they disappear forever except if I use the h command (h0, h1 etc.) when they appear quite frequently again. This means I can't use the hold mode to estimate the optimum TC. I'm confident in the quality of the 10MHz input to the divider, so could this be a software issue?

Instead I plotted MDEV in locked mode with TC=4. Am I right that it's suggesting around 1,000s for the optimum TC?

Thanks,

Mike

[thinkfat]

Provided you have not tinkered with the data by removing the drift, yes, it suggests that. But you should display the error bars and maybe take more samples, right and the "end" of the graph the uncertainty is very high.

[Mike99]

Provided you have not tinkered with the data by removing the drift, yes, it suggests that. But you should display the error bars and maybe take more samples, right and the "end" of the graph the uncertainty is very high.

OK, thanks. I will do a longer run but I get very impatient  .

I changed the TC to 200 and it took 85 minutes to lock, even though the TIC value was below 100 at all times.

Still confused about the 10MHz Missing message.

Mike

[thinkfat]

Provided you have not tinkered with the data by removing the drift, yes, it suggests that. But you should display the error bars and maybe take more samples, right and the "end" of the graph the uncertainty is very high.

OK, thanks. I will do a longer run but I get very impatient  .

I changed the TC to 200 and it took 85 minutes to lock, even though the TIC value was below 100 at all times.

Still confused about the 10MHz Missing message.

Mike

Oh, patience is a much needed trait when you tune a GPSDO

The "10MHz Missing" message is printed when the ADC value is at 1023, i.e. at maximum. It should not get higher than 1000. Looks like the interpolator is not OK.

Take a look at the output of the 4046 and check the components from there to the ADC input pin. Maybe you need a larger series resistor.

[Mike99]

The "10MHz Missing" message is printed when the ADC value is at 1023, i.e. at maximum. It should not get higher than 1000. Looks like the interpolator is not OK.

Take a look at the output of the 4046 and check the components from there to the ADC input pin. Maybe you need a larger series resistor.

OK will do.

Mike

[Mike99]

I checked the series resistor and it measured 3.8k. I replaced it with a 4.7k and there were no "Missing 10MHz?" messages during warmup 

I've started another run with TC=4s (I'll leave it running longer this time). I notice that the values for TIC and diff_ns are much closer to zero than before.

Mike

[Johnny B Good]

 Hi cdev,

 I've been preoccupied with my latest addition, an Efratom LPRO-101, this past week so only spotted your post just now. That article on testing unknown OCXOs by Bob offers good advice but there is one worrying aspect of it concerning the choice of Vcc/Vdd voltage where he suggests you just blunder in directly with a 12v Vdd voltage. If he's talking about an OCXO for which there's seemingly no published datasheet to provide any firm guidance on the required voltage (and this seems to be the case), you could land up applying 12v to a 5v part which is guaranteed to make said 5v part instantly emit its 'magic smoke'.

 This was exactly the situation I was facing just over 18 months ago with my very first (one and only) 13MHz CQE OCXO. Although it was a dead ringer for a 13MHz Vectron (the only datasheet I could download for any guidance), I was still unsure about risking 12v on what may have possibly been a 5v part (it looked like NOS rather than a carefully extracted OCXO removed from a decades old PCB where 12v was the most common Vdd specification).

 I wasn't prepared to take any risk of 'smoking my 4 quid radioham rally bargain' with a 12v jolt so tested it on a 5v supply which proved sufficient for it to start outputting a 3v p-p square wave after about 10 to 20 seconds delay (it turned out that the voltage regulation with load of the humble 1117-5 is rather crap - I was actually only getting 4.82v at the 280mA (current limited) warm up loading upon the cheap LDO used on the solder-less breadboard power adaptor board).

 After an 8 minute warm up time, the frequency settled down to 13MHz exactly and the current had dropped to 180mA. Using a potentiometer across the Vcc to supply a tuning voltage to the EFC pin proved it could be electrically tuned and thus a candidate for my GPSDO experiments (I'd already figured out a way to generate a 10MHz output locked precisely to a 13MHz square wave).

 I tried powering it from a 6.9v rail (half of the 13.8v smpsu made up with a pair of 5v 10A Farnell switchers in series modified to output 6.9v each), monitoring the square wave output voltage which had shot up to 6v p-p. I decided to risk another volt's worth using a charged up NiCad in series (I didn't possess a 32V 10A max variable bench supply back then - I do now ) and saw the output had risen by yet another volt strongly hinting that the oscillator/buffer was seeing the full Vdd voltage rather than the 5v of an internal LDO which would be mandated by a 12v design.

 I'd spotted a trend whereby I could expect to see a 10 or 11 volt p-p output off a 12v Vdd, assuming no loss of magic smoke, an output voltage I'd never ever seen specced by any of the many OCXO datasheets I'd perused so decided it was best to play safe and assume for the time being that it actually was a 5v part (after all, on a 5.2v supply, it was outputting a 4v p-p square wave within seconds of applying power and functioned exactly as an OCXO would be expected to behave. The only hint that it may have been a 12v part - the now slightly less protracted 7.5 minutes warm up time, wasn't strong enough evidence at the time to risk destroying my one and only OCXO to date.

 One advantage in assuming a Vcc of 5v being the simplification of the power rails in my MK I GPSDO down to just a single 5.17v. The protracted 7 minutes or so warmup was no impediment since it meant less overshoot when it finally did get up to temperature - you really need at least half an hour either way for an OCXO to be reasonably settled down before the GPS stops chasing a moving target.

 By the time I had more evidence of it being a 12v part ( a bunch of 10MHz sine output CQE OCXOs of exactly the same form factor and pin out I'd fortuitously chanced upon as a result of a continued fruitless search for a datasheet) by virtue of having enough spare 10MHz CQE OCXOs to risk blowing one up for just a modest 6 quid investment which revealed the exact same 280mA warm up current which, at 12v, dropped to just 90mA to give an identical warmed up power consumption of around 900mW, I'd already settled on a single 5.2v rail design, devoid of the need of another DC boost converter or the need to tie the power supply to a 12v only option in place of the 7 to 24 v requirement allowed by the buck converter I was using for the single 5.2v rail.

 Apart from the protracted warm up time, that 13MHz OCXO worked just fine in my MK I GPSDO (essentially a variant of the excellent James Miller design). I only retested that OCXO with a 12v rail less than a fortnight ago after I had successfully commissioned and proved my MK II variant as its replacement, proving what I had long suspected that it had been a 12v OCXO all along - still, better safe than sorry. It had served me well as an ersatz 5v OCXO these past 6 months or so. Now it can serve as a memento of my first experimental efforts at designing and building a GPSDO - I doubt I'll find another use for it.

 Anyhow, for anyone checking out their very first and only OCXO for which they have no absolute and incontrovertible evidence as to its actual voltage requirement, rather than plunge straight in with a 12v supply as Bob suggested (not a problem if you've bought a job lot dirt cheap), try a 5 or 6 volt supply to start with and monitor the Vref pin to check whether it stabilises at either 3.3 or 5 volts (the more ancient 12v OCXOs may have oddball 6 or 7 volt reference voltages). A 3,3v reference would strongly hint at a 5v OCXO whereas a 5v reference would suggest a 12v unit (that 13MHz CQE didn't stabilise at its 5.1xx volt set point until Vdd had hit the 11 volt mark).

 Unfortunately, checking the Vref pin (if it has one - why would you even bother with an OCXO if it doesn't have a Vref pin?) isn't a reliable indicator - some OCXO's with a "Vref pin" are no better than those without by virtue of it not being connected to anything, hence the advice to monitor the output pin to look for it reaching a fixed p-p limit as the test voltage is ramped up.

 Also, though I didn't appreciate it at the time, if the output takes 15 to 20 seconds or more to stabilise into a recognisable waveform on a 5v supply but appears much quicker off a 6v supply (2 or 3 seconds), that's a pretty good indicator that it's more likely a 12 rather than a 5 volt part. This was a more obvious indicator that the 10MHz units weren't 5 volt parts - they did eventually stabilise at a 3v p-p sine output after a 20 or so seconds delay on a 5.2v supply (slower than the 13MHz unit had done on a 4.82 volt supply - hence my doubt as to it being a 12v part).

 A variable bench supply (which I didn't have when I was first trying to determine that 13MHz OCXO's voltage requirement) is a very useful tool in this case. Although the accuracy of their built in voltage and current meters (particularly the current meter) leave something to be desired in the cheaper (affordable) Chinese models, they're still good enough to indicate unexpected behaviour perhaps just in time to terminate the test before lasting damage has occurred.

 However, a better strategy is to monitor the current demand at a 5.5 or 6 volt setting and wait for the warm up current to drop to a lower steady reading after maybe as long as 10 minutes if the part is a 12v one. The heater current is normally a constant current during the warm up phase so you can reconfigure the current limit setting on your bench supply to somewhere between the heating up current and the settled at temperature current and restart the test, allowing the current to stabilise at its set point temperature before increasing the voltage in small increments, looking for any departures from the expected small reductions in heater current demand to compensate for the voltage increase to maintain the steady energy consumption required to hold a constant oven temperature, not forgetting to monitor the clues provided by the Vref pin and the p-p output voltage level.

 If you keep in mind that the absolute maximum allowed supply voltage stress of 7 volts typically quoted in datasheets for 5v parts, if you reach the 8v mark with no obvious symptoms of distress being shown, then it's almost certain you're dealing with a 12v OCXO where you can use one volt increments to reach a 12v target. Once you're confident you're testing a 12v part, you can 'ice the cake' with a 13.2v test to definitely confirm this (standard voltage tolerance for 12v parts being +/-10% - I took the test voltage on my 13MHz OCXO right up to 14v for good measure: I was curious and had no foreseeable use for it so wasn't concerned as to whether this would prove to be 800mV too many).

JBG

[Mike99]

Wise advice from JBG.

I only currently have one OCXO, which was made by NDK and came out of an IFR signal generator. There is no data sheet for it but the IFR schematic shows it fed with a supply of 11V so I think it's safe to assume it's a 12V part.

I have a Trimble 65256 on order from China. It's advertised as a 12V part but I know that many of them are 5V so I will be adopting your cautious approach when it arrives.

As for a Vref pin, I wouldn't use one even if it was present as I think an external precision regulator would perform better.

Mike

[BobPDX]

I prototyped up the Lars GPSDO from the schematic in his original post, set a Ublox 7N to 1 Hz with a 10% duty cycle and connected the GPSDO to a BG7TBL Crystal Board dated 2016-09-25 with a DAPU 055A-HSDD OCXO. I had previously removed the trimpot on the OCXO board and replaced it with a 3 wire header for the ground, voltage reference and control signals.  Lars original software was downloaded and used unmodified.  I did use the F1 D command to set the DAC nearer the approximately 3.3 Volts needed to give a 10 mHz signal.  The control voltage was monitored by a Fluke 45 DVM and the output frequency measured by a BG7TBL FA-2 Frequency counter with a gate of 1 second.  A LPRO-101 rubidium oscillator, which had been calibrated to a Cesium Standard 2 months ago when I purchased it, was used as the FA-2 clock.  One of the purposes of building the GPSDO was to provide ongoing assessments of accuracy of this clock, and perhaps eventually discipling it also.  The GPSDO was run continuously for more than 24 hours before collecting the data shown in the attached.  The Lars GPSDO appears to be accurate (assuming the clock is still on frequency) with a median value of 10,000,000.000 078 Hz and a mean value of 10,000,000.000 222 Hz  with a n=25112.    The frequency does vary over several milliHz as shown in the FreqTimePlot.pdf attached.  Perhaps someone who is familiar with what this GPSDO  could advise me how much the precision could be improved with better settings.  By comparison and on the same scale is a similar plot from a BG7TBL GPSDO 2014-12-09 input with the same clock.  Thank you.  Bob

[cncjerry]

 Hey, I read thru all the notes, Lars did a great job and Jim Harmon took it a step further with the 7200 and DAC.  I plan to use Jim's design to discipline an Rb unit but instead of all the EFC stuff, the Rb I'm using takes a tuning word of .2e-11 offset so I can just send that over another serial line.  I bought two of those M8T GPS units a while back, one for the Rb and another for a DOCXO.

Question: how are you getting the data for the Adev/Mdev charts? Is that coming off the Aduino in relation to the GPS 1PPS phase?  I use a reference, either a Cs or another GPSDO, with Riley's DMTD as realized by Bert feeding a TIC.  I want to compare this to my Lucent GPSDOs. 

Thanks,

Jerry

[Dbldutch]

Root cause found for the temp/ns/DAC jumps.

For several months I have been hunting for the cause of the strange temperature jumps of the three different OCXO ovens I have. For far too long, I suspected the temperature jumps to come from the ambient temp, or the OCXO oven and this would influence the ns/DAC jumps and so I was hunting in that direction.

I have finally found the root cause.
Below is what happens with my Oscilloquartz version. It is no longer using the full OCXO isolation, to avoid "over-heating" but the whole GPSDO unit is now in an extra enclosure with isolation to avoid sudden room temperature drifts.

These graphs run from midnight to midnight and are sampled every second as they are coming from the Lars firmware. I collect the output by a Raspberry Pi connected to the Txd/Rxd pins of the Arduino Nano. These daily log files are emailed to me.
To see what is going on during the day, I also use a USB-mini cable connected between the Arduino and my PC. I use the Arduino serial monitor to show the second by second Lars' report on my screen.

The cause of the temperature jumps, that literally changes from one second to the next, is due to my PC waking-up from sleep in the morning or going to sleep in the evening. This going to sleep or wake-up will activate or deactivate the power on the USB port of the PC, and hence will apply 5V to the Arduino through the USB serial link, or not.

This is interesting, because I feed a regulated 8V DC to the VIN power input of the Arduino, and I do not have any other logic connected to the 5V and 3V3 power outputs from the Arduino board. I only have a 100uF cap attached to the 5V output, and nothing to the 3V3 output. You would think that the 8V supply and the on-board voltage regulator would sufficiently power the Arduino, regardless of the presence of 5V coming from the USB connection. The Arduino circuit uses a Schottkey diode (D1) to "OR" the on-board regulated 5V supply with the 5V coming from the USB input.

For a reason I cannot explain yet, the Arduino ADC1 and ADC2 inputs that are used to sample the temperatures are influenced when you apply the USB cable with power. You can also see in the graphs that ADC0, used for the TIC input, gets the same treatment, and this influences the TIC measurement (ns graph) and hence the DAC.

Because of the DAC jumps, we have a "falsifying" event in the TimeLab reports.

The solution would be to use a USB cable with the 5V power removed, or remove D1 on the Arduino board. Unfortunately, this diode is located on the bottom of the PCB and is not easy to get to when you already soldered the Arduino on a main PCB, like I did.

[Fennec]

Hey,

my english is not the best but I found this. Maybe someone can use it.

/attached
comes from https://ieeexplore.ieee.org copyright maybe? idk

[Johnny B Good]

Wise advice from JBG.

I only currently have one OCXO, which was made by NDK and came out of an IFR signal generator. There is no data sheet for it but the IFR schematic shows it fed with a supply of 11V so I think it's safe to assume it's a 12V part.

I have a Trimble 65256 on order from China. It's advertised as a 12V part but I know that many of them are 5V so I will be adopting your cautious approach when it arrives.

As for a Vref pin, I wouldn't use one even if it was present as I think an external precision regulator would perform better.

Mike

 Hi Mike,

 thanks for that 'vote of appreciation'.  The advice really only covers the situation where you just can't find a datasheet or any hints whatsoever of the Vcc / Vdd voltage requirement for a one and only example of OCXO. If I'd found a datasheet for the 10MHz CQE OCXOs, I might have dared to test the 13MHz unit at 12v sooner rather than much later using the additional clue given by both types exhibiting the exact same 280mA constant current warm up characteristic.

 I never did find any datasheets for the 10MHz units and the vendor had no idea either so I only discovered they were a 12v type by risking one at the higher voltage (the difference being that I had another two to spare at that stage). I'd already committed to a 5 volt only design by then and since the 13MHz OCXO seemed to be coping much better with a 5.0v supply than any of those 10MHz variants ever did, I decided to play safe and carry on assuming the 13MHz OCXO was a 5 volt part with just a rather protracted warm up characteristic (I didn't have a variable voltage bench supply back then otherwise it might well have been a different story).

 My thinking behind the use of an OCXO Vref instead of a separate high precision reference IC was that even if a cheap 'n' cheerful 1117-5 LDO had been used, the fixed loading from the oscillator and thermal controller and another fixed 1mA external load would eliminate the issue of voltage variation with current demand and the fixed 12v supply, likewise the voltage regulation versus input voltage variations and ripple leaving only the question of the effect of its TC. Guess what? With a precisely regulated 'oven temperature', this was simply going to be yet another non-issue! 

 Until I saw this blog posting:-

http://syncchannel.blogspot.com/2016/03/schematic-of-cts-1960017-10mhz-ocxo.html

 I would have expected the Vref pin option to always be an excellent alternative to a separate (and expensive) high precision voltage reference IC. In this case, I'd agree with you regarding an external precision voltage reference IC being the much superior option. 

 I suppose I should now qualify my recommendation with the phrase "provided it is a 5v reference in the case of a 12v OCXO or a 3.3v reference in the case of a 5 volt one and you run a basic verification test to prove they will be as temperature stable as you have every right to expect".

 It seems at least one brand (CTS) cheats on this critical aspect of OCXO design so I can see why you have a different view to my own (so far I've only had experience of the CQE brand which doesn't cheap out on the Vref circuit). It seems I touched lucky with that batch of seven CQE 10MHz 12v OCXOs I'd purchased at just £4.99 each off a UK seller last year. If they're good enough for the Symmetricom GPSDO described here:- https://youtu.be/zkzMtLyqju4?t=793

 I think I can safely say they must be one of the better brands for use in a DIY GPSDO.  That one is almost identical to mine in that the DOC number matches exactly that of all of the 10MHz units in my own modest collection (the 13MHz one has a different DOC number). Even the little blue circular stickers are the same except for the fact that on mine they appear to be a day/month code rather than whatever the 90/4 is supposed to represent. (on very close inspection, the number seems to be 30/4 - another day/month code, maybe the colour of the sticker codes for the year?). The layout on the label is different but all the numbers are present on both mine and that Symmetricom one. The 9850/0529 looks to be some sort of serial number since they're all unique on mine eg 0042/nnnn, 0047/nnnn and 0050/nnnn on the five remaining spares I have to hand.

 I was rather pleased to see a CQE OCXO used in that Symmetricon GPSDO since I seem to be the only one mentioning this particular brand in these EEVBlog topic threads. They must be good quality OCXOs otherwise Symmetricom wouldn't be using them in any of their kit.

 In my case, stability of this voltage reference trumps absolute accuracy since I just need a very stable meter DC offset to allow a cheap 9999 counts DMM to display the millivolts portion to within a tenth of a millivolt, allowing me to observe the effects of ionospheric and tropospheric timing errors, the latter being about a tenth of the former, inherent in the GPS navigation system which impose those annoying subsonic (mHz) phase shifts that become ever more evident when trying to use them to calibrate RFSs to within a few uHz in even the best of the single frequency (L1) GPSDOs.

 Using a more expensive dual frequency timing receiver can dynamically cancel the ionospheric timing errors for each of the SV signals used in the navigation fix calculations but are a rather expensive solution for hobbyist use.

 A GPSDO, whether a simple cheap DIY effort or a sophisticated expensive microntroller based one disciplining a high quality double ovened OCXO, might be accurate enough over days long intervals out to tens of years but they all stink when it comes to the minutes long periods of stability required to calibrate an RFS to within a few uHz of its potential accuracy within a reasonable span of time. To do that, you need to be disciplining another RFS which could be done with the cost effective Lars based design that doesn't have to cope with the unpredictable behaviour of OCXOs at this scale of precision.

 Whilst a microcontroller based GPSDO can, in principle use a Kalman filtering algorithm to smooth out these annoying phase shifts from data collected over periods of 24 hours and longer, this seems like a "Catch 22" situation in that you're relying on what you're disciplining to carry on following a predicted trajectory by which to refine the tuning voltage to match an average of the previously collected data.

 Unfortunately, even the best quality of double ovened OCXOs can't be guaranteed to always follow the plan and will depart from their predicted trajectory from time to time, requiring a contingency algorithm to promptly drag them back on frequency. It strikes me that the Lars design either lacks such an algorithm or else it isn't being properly tuned by those attempting to configure their own implementations (or, possibly overlooking unexpected shortcomings in their choice of components and circuit layout).

 Disciplining a RFS is a lot easier. You only have to compensate for ageing once you've stabilised the base plate temperature and eliminated the effect of barometric variations (using a barometric sensor to compensate via the external tuning voltage input being the most practical alternative to a vacuum containment or, a little less impractical, a pressure regulated containment).

 Of course, the main worry by hobbyists who've blown close to 200 quid on a twenty year old RFS is the remaining lamp life. Most hobbyists will put up with the limitations of an OCXO based frequency standard and only run the RFS for a day or three at a time for occasional use as a calibration standard in order to extend the service life of this precious commodity..

 Considering the exorbitant pricing of a "High Quality" GPSDO, I think I'd prefer to take my chances and run a disciplined RFS full time - I'd like to see just how much longer a 20 year old RFS with a "Design life of at least ten years" will run for before it needs replacing.

JBG

[Mike99]

I would have expected the Vref pin option to always be an excellent alternative to a separate (and expensive) high precision voltage reference IC. In this case, I'd agree with you regarding an external precision voltage reference IC being the much superior option. 

Yes I'd rather use an external reference with known characteristics than the unknown internal circuitry of the OCXO.

I think your requirements are rather more demanding than mine. All I need is an accurate affordable 10MHz frequency reference for occasional use to calibrate my frequency counter and other amateur radio equipment.

Mike

[thinkfat]

Of course, the main worry by hobbyists who've blown close to 200 quid on a twenty year old RFS is the remaining lamp life. Most hobbyists will put up with the limitations of an OCXO based frequency standard and only run the RFS for a day or three at a time for occasional use as a calibration standard.

I'd not trust a RFS that hasn't warmed up and settled for at least a couple of days...

[cncjerry]

Hats off to Lar's.  I breadboarded his circuit, hooked it to one of my HP 10811s that sit on a board from an HP 5328 counter and gave it a shot.  It wouldn't lock until I found I had to invert the EFC.  So I made a simple change in the programming and it locked in about a minute (changed the positive to the negative terminal - Young Frankenstein) .  Yet to see if it sticks but I'm optimistic.

I wish I had wired up the Lock LED to make it easier to watch.  Anyway, lots of fun with a simple circuit.

Jerry

[Mike99]

I've just finished a 5 day run of my Lars GPSDO with a 4 second time constant. Some results are attached, and I have some questions for the experts please.

1) Does the MDEV plot suggest an optimum time constant of around 500 seconds?
2) Given that my use for this is as a frequency standard for occasional calibration of test equipment I have no need to run it 24/7. If I chose a shorter time constant for a faster lock time what effect would it have on frequency accuracy?
3) Is it possible to deduce the quality of the OCXO from these results (e.g. good/average/poor)?

Thanks!

Mike

[thinkfat]

Is that really the DAC column you imported, or is it that TIC values? TIC values would be mostly useless, as they don't tell you enough about the OCXO.

[0xFFF0]

The DAC is in the 3rd column. But the value jumps a lot.

[thinkfat]

The DAC is in the 3rd column. But the value jumps a lot.

Yes, but with TC=4 that's expected. The DAC follows closely the TIC and the ADEV or MDEV will eventually expose the point where the GPS beats the OCXO with regards to stability.

[Mike99]

Is that really the DAC column you imported, or is it that TIC values? TIC values would be mostly useless, as they don't tell you enough about the OCXO.

Yes, I imported the DAC column.

Mike

[thinkfat]

Is that really the DAC column you imported, or is it that TIC values? TIC values would be mostly useless, as they don't tell you enough about the OCXO.

Yes, I imported the DAC column.

Mike

Did you by chance edit the series in TimeLab? Like, remove linear drift or drift curve? The MDEV graph looks a bit indecisive here, after 5 days of collecting samples, you'd expect a very clear uptick that signifies the OCXO drift. But it's going down and further down only.

[Mike99]

The DAC is in the 3rd column. But the value jumps a lot.

Here's an extract from an overnight run with TC = 200s. The DAC value rises and falls slowly. Is that correct?

(I don't have any temperature compensation yet and the GPSDO is not enclosed).

Mike

[Mike99]

Did you by chance edit the series in TimeLab? Like, remove linear drift or drift curve? The MDEV graph looks a bit indecisive here, after 5 days of collecting samples, you'd expect a very clear uptick that signifies the OCXO drift. But it's going down and further down only.

Yes I subtracted the global linear frequency trend. Here is the unedited plot.

Mike