OCXO calibration

[Codebird]

I've built myself a little precision frequency reference. It uses an OCXO. Specifically, this one:

http://www.isotemp.com/wp-content/uploads/2011/10/OCXO143-3.pdf

Sure, it's less stable than a GPS disciplined oscillator, but it's less than a tenth the price and it's ready to go after just a minute to warm up. The rest of the electronics in my reference box are just a CD4020 to divide it down into convenient outputs, a 50ohm driver, voltage regulator and some level/frequency selection controls.

But, I got that reference off of eBay. And according to the datasheet, it's only guaranteed accurate to 3.5e-7 after ten years - which isn't far off the age on it now. That's still pretty good though, I think? I'm not sure if my calculations are right, but I think it comes out to about the same as most TCXOs? 0.35ppm. Unlike a TCXO though, I have the option to calibrate it.

What I'm wondering now is how I might go about calibrating such a thing. I'm in the UK, so I don't know if there are any good frequency reference stations I might be able to pick up. Any suggestions? Or anyone else in the country with access to the right equipment who would like to help? I don't know how much a laboratory would charge, and I doubt they'd be happy about touching anything hobbyist-made.

The purpose of this box is testing cheap, super-low-cost frequency counters off of eBay so that I can either be impressed with their low-cost performance or mock them for including useless extra digits, depending on results. I am curious to see how the El Cheapo bare-PCB counters compare with the thousand-pound-plus professional counters.

[MK]

You could build a long wave reciever for Rugby (60KHz) or Radio 2 longwave (Droitwich 198KHz) and divide down to 1KHz and compare the two by looking at the beat note?

[Gyro]

Buy yourself a cheap Ublox GPS module off ebay. In brief:

- Divide your OCXO down to 1MHZ.
- Set one of the Ublox module frequency outputs to 1MHz.
- Compare the two signals on a scope, ignoring any jitter, just look at the long term roll between the two traces.
- Tweak the frequency adjustment voltage of the OCXO for zero roll.
- Repeat as often as you feel necessary.

GPS module teardown here...
https://www.eevblog.com/forum/projects/ebay-u-blox-lea-6t-gps-module-teardown-and-initial-test/msg886887/#msg886887


P.S. To configure / monitor the Ublox module you use their u-center s/w... https://www.u-blox.com/en/product/u-center-windows

[DimitriP]

The purpose of this box is testing cheap, super-low-cost frequency counters off of eBay so that I can either be impressed with their low-cost performance or mock them for including useless extra digits, depending on results. I am curious to see how the El Cheapo bare-PCB counters compare with the thousand-pound-plus professional counters.

The "cheapo" counters don't have enough digits.
For all practical purposes, old, aged and all, you have a 10.000.000.x Hz oscillator


Illustrated exambles fro current ebay listings:
http://www.ebay.com/itm/High-Accuracy-RF-1-to-500-MHz-Frequency-Counter-Tester-measurement-For-ham-Radio-/281423258318?epid=1147058638&hash=item41862222ce:g:hCsAAMXQRPRTHhjB


http://www.ebay.com/itm/High-Accuracy-1-500MHz-Frequency-Counter-meter-Antenna-for-Ham-Radio-Interphone/321820986581?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D41376%26meid%3De58eba64db0d41bdaa6e36fba88e4c50%26pid%3D100005%26rk%3D6%26rkt%3D6%26sd%3D281423258318&_trksid=p2047675.c100005.m1851


You can build a cheap GPSDO, for it, but that's another rabbit hole entirely !

[texaspyro]

Buy yourself a cheap Ublox GPS module off ebay. In brief:

The freq outputs of the Ublox module are derived from the (undisciplined) oscillator on the module (could be a XO or a TCXO) that is further degraded by the GPS sawtooth error and frequency divider jitter.   So what you are trying to do is to adjust a OCXO (with maybe 1E-11 stability) against a cheap TCXO with maybe 1E-8 stability / accuracy... a VERY losing proposition.  To properly set a OCXO on freq requires a GPSDO or cesium beam oscillator.  Or maybe a hight quality rubidium oscillator that was recently calibrated against GPSDO or cesium oscillator...  and all those telecom grade rubidiums are NOT particularly high quality rubidiums.   

[edpalmer42]

Your calibration method will depend on how close you want the frequency to be.

Note that the daily drift for your OCXO is spec'ed at 1e-9, i.e. 0.01 Hz/day.  Do you want to adjust it every day?  Probably not.  Once a year?  That would mean that the frequency will be within 1e-7 if it meets its spec., i.e. 100 PPB or 1 Hz for your 10 MHz OCXO.  As texaspyro said, GPS by itself has a stability of about 1e-8, i.e. 10 PPB for a one second measurement with no overall frequency error.  A 10 sec. gate time would give you 1 PPB.  That sounds like a good match.  A bare GPS receiver could be used as a reference.  If these numbers aren't good enough for you, you'll have to upgrade to a better OCXO, a GPSDO or a Rubidium.

You said "it's ready to go after just a minute to warm up".  No, it isn't!  The aging specs only apply if you keep it powered up all the time.  A single power failure will wreak havoc with the aging.  You'll have to try it to see how big the effect will be for your particular unit.  OCXOs typically age fast after a power cycle.  Aging drops slowly until it eventually levels out to some low value.  For your unit, that could take a few months, maybe more.  They also may not return to the exact same frequency after a power cycle.  This is called 'retrace'.  It's not listed on the datasheet for this model.

Ed

[cdev]

I have a GPSDO thats on a bare PCB and I am debating how to enclose it. What are peoples feelings on ventilation? Should the box be completely enclosed? It has a Bliley OCXO and I have noticed that when I have my fan going near the GPSDO it seems to go into holdover more frequently. Right now its actually in a cardboard box. cardboard is actually a better insulator than metal when its shut but I likely want a metal box ultimately, when I have figured out exactly what is going to be in there, I may add some DDS functionality..

Im wondering if it should be completely closed. Likely it should be, I am guessing.

The temperature when its operating normally (as recorded by its onboard sensor) is usually between 41 and 50 degrees depending on how its enclosed. The voltage going to the oven changes quite a bit.

[texaspyro]

As texaspyro said, GPS by itself has a stability of about 1e-8, i.e. 10 PPB for a one second measurement with no overall frequency error.  A 10 sec. gate time would give you 1 PPB.

No, the frequency outputs of the Ublox GPS module are not locked to anything.  If they are off 1E-8,  averaging them for however long will still be off 1E-8.   You cannot use the Ublox frequency outputs to meaningfully set an OCXO.

The Ubkix 1PPS output is slaved to GPS.  Averaging that will reduce the uncertainty.  To use the Ublox to set an OCXO, you will need to divide the OCXO down to 1 Hz and compare it to the Ublox 1PPS and deal with the sawtooth error.

The UCT-8663 DOCXO (at the time 10 for $100) that I put in my HP-53132A on one of Gerry Sweeny's boards is drifting less than 1E-10 per month.  Some of that drift is the reference and DAC on the board.   Some of the 8663's from that same lot have drifts in the low E-11 / month range.   It took it around 2 months for it to settle down to that rate.

[edpalmer42]

As texaspyro said, GPS by itself has a stability of about 1e-8, i.e. 10 PPB for a one second measurement with no overall frequency error.  A 10 sec. gate time would give you 1 PPB.

No, the frequency outputs of the Ublox GPS module are not locked to anything.  If they are off 1E-8,  averaging them for however long will still be off 1E-8.   You cannot use the Ublox frequency outputs to meaningfully set an OCXO.

The Ubkix 1PPS output is slaved to GPS.  Averaging that will reduce the uncertainty.  To use the Ublox to set an OCXO, you will need to divide the OCXO down to 1 Hz and compare it to the Ublox 1PPS and deal with the sawtooth error.

Thanks for the clarification.  I should have mentioned that I was thinking about the 1 PPS.  Are you sure about the frequency error of the other outputs?  I've got a ublox app note "GPS-based Timing Considerations with u-blox 6 GPS receivers" that says that the frequency error of an 8 KHz output is only 6.2e-11.  I'd assume (yeah, I know) that newer timing receivers would be at least that good.  Numbers like that will change from test to test and will eventually average out to zero.  I don't have a lot of experience with ublox boards.

The UCT-8663 DOCXO (at the time 10 for $100) that I put in my HP-53132A on one of Gerry Sweeny's boards is drifting less than 1E-10 per month.  Some of that drift is the reference and DAC on the board.   Some of the 8663's from that same lot have drifts in the low E-11 / month range.   It took it around 2 months for it to settle down to that rate.

I hate those UCT oscillators!  I've never been able to figure out if they're actually clones of the Oscilloquartz units or not.  The Oscilloquartz 8663 isn't a DOCXO, it's just an OCXO.  Have you found an actual datasheet for the UCT unit?  In any case, your tests show that the UCT-8663 sounds like an upgrade path if Codebird's Isotemp OCXO isn't adequate.

Ed

[4CX35000]

I've built myself a little precision frequency reference. It uses an OCXO. Specifically, this one:

The purpose of this box is testing cheap, super-low-cost frequency counters off of eBay so that I can either be impressed with their low-cost performance or mock them for including useless extra digits, depending on results. I am curious to see how the El Cheapo bare-PCB counters compare with the thousand-pound-plus professional counters.

Whatever you do it will involve a external reference to do the check which receives on of the standard services such as GPS, MSF, DCF77 (Germany) or Droitwich. There are a number of circuits which are used to receive the Droitwich (Radio 4) reference on the internet, one below is one I came across a while back.

Article which appeared in a magazine is probably a good project to build.
http://www.epemag3.com/lib/free_projects/lab_equipment/0602-%20Frequency%20Standard%20Generator.pdf

If you do build the above circuit then for checking simply use a frequency counter with the 10 MHz from the OCXO on the counter reference input and the 100 KHz from the Droitwich standard on the input. The reading should be 100 KHz exactly and adjust the reference OCXO to the until the counter reads 100 KHz exactly. I suggest you leave the OCXO and Droitwich reference to run for a while for it to fully warm up and then do the check and adjust every so often until your happy with he result.

Afterwards as a future hobby project you can look at using the Droitwich reference as a active reference which automatically adjusts the OCXO as this will drift depending on operating conditions such as room temperature and OXCO oven stability.

As for a counter they tend to be similar. I have a Keithley 776 here and a Thurlby Thandar TF830, and both are reasonable for what I'm doing. But there are plenty of Racal counters on eBay going for around £100.

[Gyro]

As texaspyro said, GPS by itself has a stability of about 1e-8, i.e. 10 PPB for a one second measurement with no overall frequency error.  A 10 sec. gate time would give you 1 PPB.

No, the frequency outputs of the Ublox GPS module are not locked to anything.  If they are off 1E-8,  averaging them for however long will still be off 1E-8.   You cannot use the Ublox frequency outputs to meaningfully set an OCXO.

The Ubkix 1PPS output is slaved to GPS.  Averaging that will reduce the uncertainty.  To use the Ublox to set an OCXO, you will need to divide the OCXO down to 1 Hz and compare it to the Ublox 1PPS and deal with the sawtooth error.

https://www.u-blox.com/sites/default/files/products/documents/Timing_AppNote_%28GPS.G6-X-11007%29.pdf

The OP's calibration requirements aren't particularly demanding.

[CJay]

Those Trueposition GPSDOs that are popping up on eBay are looking like a good proposition right now if yu need a budget GPSDO, accuracy would seem to be good (are they supported by LadyHeather yet?).

Racal counters, you can pick up bargains, I got two 1998s for ~£120 inc shipping, they needed a 630mA slow blow fuse each and both had the high stability OCXO fitted, last cal dates were around 2007, they've been checked against various 'standards'  (Radio 4 off air, three different GPSDOs, two rubidiums) and, despite being ten years out of cal, still remarkably accurate to the best of my ability to check thought i realise I can't provide calibration crtificates for any of my standards, my logic tells me that if they all came from different sources and all agree to a few tenths of a Hz then they're reliably accurate enough for my needs.

The U-Blox was the least stable of the lot when triggering my coutners, there was a good amount of wander but an interesting snippet of information in the datasheet and the U-Blox software indicates that the can be configured to discipline an off board OCXO if you add a cheap DAC to them and know the specifications of the OCXO

[texaspyro]

I hate those UCT oscillators!  I've never been able to figure out if they're actually clones of the Oscilloquartz units or not.  The Oscilloquartz 8663 isn't a DOCXO, it's just an OCXO.  Have you found an actual datasheet for the UCT unit?  In any case, your tests show that the UCT-8663 sounds like an upgrade path if Codebird's Isotemp OCXO isn't adequate.

I don't think UCT every built their own oscillators.   My guess is they are re-branded Oscilloquartz units.  My UCT rubidium standard has an LPRO-101 in it.

I've put the UCT units in a lot of frequency counters (mostly Tek DC5010's) and have been very surprised at how little they drift.  Also how little they seem to be affected by temperature.

[G0HZU]

The purpose of this box is testing cheap, super-low-cost frequency counters off of eBay so that I can either be impressed with their low-cost performance or mock them for including useless extra digits, depending on results. I am curious to see how the El Cheapo bare-PCB counters compare with the thousand-pound-plus professional counters.

Your OCXO really only has to be stable over time and temperature to test cheapo counters. It doesn't have to be calibrated to be very close to 10MHz. In other words, in your house, your OCXO defines what 10MHz is. Just warm up and align the cheapo counter references to agree with your OCXO and then watch how much they drift the next time you switch them on. Also see how much they drift on a cold day to a hot day or a cold morning to a warm evening. The cheapo counter will probably drift over 10Hz at 10MHz in these tests. Your OCXO should drift over 100 times less than this so I don't see the point of calibrating your OCXO or even buying a GPSDO. It won't add much value to these tests because a cheapo counter will have so much drift anyway.

[texaspyro]

Lady Heather supports the Ublox and Venus/Skytraq devices.  The Venus timing receivers have an advantage in that their internal clock is 120 MHz and the sawtooth error is in the +/- 6 ns range.

There can be issues using multiple GNSS systems for precision timing.  The different GNSS systems have slightly different implementations of the second / UTC.   I would avoid enabling multiple GNSS systems for a precision timing application.   I seem to remember Ublox mentioning this.

[cdev]

Yes, I've read that too.

 Ublox's defaults to the GPS (USNO) system but lets people pick another one (but not two at the same time) because of the timing difference.

Quote from: texaspyro on Today at 08:57:53

Lady Heather supports the Ublox and Venus/Skytraq devices.  The Venus timing receivers have an advantage in that their internal clock is 120 MHz and the sawtooth error is in the +/- 6 ns range.

There can be issues using multiple GNSS systems for precision timing.  The different GNSS systems have slightly different implementations of the second / UTC.   I would avoid enabling multiple GNSS systems for a precision timing application.   I seem to remember Ublox mentioning this.

[cdev]

Frequency counters should be adjustable so that youre not displaying extra digits when you don't have accuracy to base them on.

Quote from: G0HZU on Today at 08:46:58

Quot

e

[lars]

About GPS module jitter:

The Venus per datasheet for 838-T has an internal (free running) oscillator at about 81 MHz and places the frequency outputs including the PPS on the nearest edge of the 81 MHz. This gives a ripple of +-3ns (6ns p-p). This is also what I have measured, but be aware that it might do some nasty excursions sometimes.

See: https://www.febo.com/pipermail/time-nuts/2016-October/101037.html

The uBlox NEO-6M, NEO-7M and LEA-6T I have tested all have about 21ns p-p jitter. As far as I know both frequency outputs are locked to the GPS but have the 21ns p-p jitter and doesn´t get lower if you use 8MHz instead of 10Mhz out but it looks better on an oscilloscope but you are just cheated.

About the 8663 OCXO:

As far as I known it is a very good double oven XO. In several data sheets for GPS clocks from Oscilloquartz it is mentioned as double oven. My own tests also indicate a double oven from current consumption and temperature stability.


Lars

[lars]

For me calibration is about trust. If I measure something I want to trust the result to be as good as I think. To know that, I check (calibrate) my instruments or references against something else that I trust (that is know how accurate it is or said in another way know the measurement uncertainty). If I know someone else has promised how accurate it is and I trust the source I can use that. That is if I buy something from a manufacturer with a specification I can trust it if I trust the manufacturer. It will of course be a small risk my unit is bad for some reason. But this is also true if I calibrate something (by myself or external). Almost all external calibration use a confidence of 95% for that reason. That is, it is 95% chance it meets the uncertainty specification.

For the Isotemp 143 OCXO we have a datasheet from a reliable manufacturer but the problem is the source (used from eBay). Can we trust it?? If I bought it new I could be quite sure it met the spec of slightly better than 1ppm (1uHz/Hz). If that is enough for me I would trust it for further measurements like checking frequency counters. If I need better than the spec I must check it against a better source.

For me the best cheap source is a GPS module as the uBlox NEO6M, LEA-6T or similar and a 10MHz OCXO. As Gyro says in reply #2 you just need to divide down the 10MHz and compare it to the output of the GPS module with an oscilloscope. If you have just the 1PPS I would recommend a digital oscilloscope that you can trigger on the 1PPS. I have done this with just the PPS out from ublox modules and it works well if you divide down the 10MHz at least 10 times. Dividing down 16 times also works well (the output of the divider needs to be an integer of 1Hz).

Using radio or TV signals also worked for me but you need to be sure the transmitter really are locked to eg GPS. I learned this the hard way before I went to only GPS. Another problem I experienced is radio transmitter closing down in a quite short time. For the TV the change from analog to digital made the before long term stable signals almost useless.

After having used GPS to calibrate and adjust my 10MHz references I decided to build a GPSDO as I was tired of adjusting (and I had an Arduino I wanted to learn to program). With a GPSDO I don´t have to adjust or know the error. Of course a GPSDO is not perfect and you need to know the measurement uncertainty even for your GPSDO see for example:

Two links about GPSDO´s as frequency standards and traceable calibration:
http://tf.nist.gov/general/pdf/2289.pdf
http://www.gps.gov/cgsic/meetings/2016/gust.pdf

If you need 1ppb (1E-9 or 0.001ppm) most GPSDO with OCXO´s if showing locked will be ok. But if you need more with good confidence you probably have to understand the specs, if they say enough ? or try to understand the TIC and DAC values to get a feel for the stability.

Note that ADEV is stability and not measurement uncertainty. But as the GPS is locked to a much more accurate reference ADEV can be used to estimate the uncertainty. My own rule of thumb is that the expanded uncertainty (95% confidence) is 2-10 times the worst ADEV at Taus longer than the gate time. To be conservative I normally use 10 times.

Lars
?

[lars]

If you have a GPS module with 1PPS and an 10MHz oscillator with voltage control input I should say it is easy to build a GPSDO for less than 10USD of components for the GPSDO controller.

Here is my simple but well working design that works for all kinds of 10MHz oscillators from XO to rubidiums:

https://www.eevblog.com/forum/projects/lars-diy-gpsdo-with-arduino-and-1ns-resolution-tic/

In the last pages of the instruction you can also see examples of what stability I have achieved with two different oscillators. On page 15 you find a list of useful links to general GPSDO topics, other designs and measurements. On page 8-9 the software including the PI-loop is described.

In hold mode the controller works as an 9 digit per second 10+-0.05 MHz counter and output the delta frequency every second on the serial line, making it easier than using an oscilloscope, with just two HCMOS IC and an Arduino Pro Mini and a few passive components.

Lars

[edpalmer42]

About the 8663 OCXO:

As far as I known it is a very good double oven XO. In several data sheets for GPS clocks from Oscilloquartz it is mentioned as double oven. My own tests also indicate a double oven from current consumption and temperature stability.
Lars

Can you share any of these documents (or links to them)?  The Oscilloquartz 8663 data sheet doesn't specify single or double oven, but other Oscilloquartz data sheets, e.g. 8660, 8682, and 8863, specify double oven.  Read those model numbers carefully - they're easy to mix up.  From this, I assumed that the 8663 was a single oven design.

Ed

[lars]

Here are three links to datasheets from Oscilloquartz describing 8663 as a double oven oscillator:

http://www.sungwhatech.com/product/pdf/08.Time%20&%20Frequency%20Systems/OSA%204500%20OEM%20Clock.pdf
http://www.sungwhatech.com/product/pdf/03.GPS%20Systems/OSA%205200B%20GPS%20Clock.pdf
https://telcogroup.ru/files/pdfs-vend/oscilloquartz/Star-glonass_da_ed03.pdf

Lars

[orin]

Buy yourself a cheap Ublox GPS module off ebay. In brief:

- Divide your OCXO down to 1MHZ.
- Set one of the Ublox module frequency outputs to 1MHz.
- Compare the two signals on a scope, ignoring any jitter, just look at the long term roll between the two traces.
- Tweak the frequency adjustment voltage of the OCXO for zero roll.
- Repeat as often as you feel necessary.

I second the use of a scope to do the comparison.  No need for fancy counters.  In fact, it's the method used by HP in some of their adjustments.

[edpalmer42]

Here are three links to datasheets from Oscilloquartz describing 8663 as a double oven oscillator:

http://www.sungwhatech.com/product/pdf/08.Time%20&%20Frequency%20Systems/OSA%204500%20OEM%20Clock.pdf
http://www.sungwhatech.com/product/pdf/03.GPS%20Systems/OSA%205200B%20GPS%20Clock.pdf
https://telcogroup.ru/files/pdfs-vend/oscilloquartz/Star-glonass_da_ed03.pdf

Lars

Thanks for the links.

Boy, that's confusing.  What are you supposed to believe?  The datasheet for the oscillator that doesn't say double oven, or the datasheet for other devices that does?? 

Ed

[texaspyro]

As far as I know both frequency outputs are locked to the GPS

The frequency outputs are not locked to GPS.  They are just divided down from the free running GPS board oscillator via a DDS.   If they could lock the osc and GPS together (like the Thunderbolts do), then there would not be any 1PPS sawtooth error.

[Gyro]

The frequency outputs are not locked to GPS.

Do you have a reference for that statement?

There's sawtooth error on the frequency outputs, whatever frequency you program them to (there's no dedicated 1pps output). That's why you have to visually ignore the jitter when comparing on the scope - just watching the long term trend.

[edpalmer42]

As far as I know both frequency outputs are locked to the GPS

The frequency outputs are not locked to GPS.  They are just divided down from the free running GPS board oscillator via a DDS.   If they could lock the osc and GPS together (like the Thunderbolts do), then there would not be any 1PPS sawtooth error.

Is this a question of 'definition'?

Saying that the frequency isn't locked to GPS suggests to me that over time, the 10 MHz and the 1 PPS could drift apart.  I don't think that's true.  My understanding is that the rising (usually) transition on the 1 PPS output happens as close as possible to the actual time as broadcast by GPS.  Since the resolution is limited by the software and the clock speed of the GPS receiver, there is jitter which, over time, generates the sawtooth waveform.

The method of generating the 10 MHz signal will vary from model to model, but again I would expect something that relates back to the 1 PPS.  One receiver I looked at output shorter or longer pulses so that the average came out to exactly 100ns (i.e. 10 MHz).  I've sometimes seen warnings that these outputs should be cleaned up with a phase-locked oscillator to remove the jitter before trying to use them.

As you stated, a processor clock that was locked to GPS should be able to minimize, if not remove, sawtooth error.

Ed

[Gyro]

Yes, you have it exactly Ed.

I've sometimes seen warnings that these outputs should be cleaned up with a phase-locked oscillator to remove the jitter before trying to use them.

Yes, that's in the Ublox timing app note that I posted the link to. The human eye does a pretty good job though. I've done it, Lars has done it, Uncle Bob has done it.


EDIT:

As you stated, a processor clock that was locked to GPS should be able to minimize, if not remove, sawtooth error.

Of course the GPS is only able to output sawtooth correction data at low rate, iirc ic can be configured upto a few times a second on the ublox. Above that, you're down to removing the  sawtooth with a pll (or eye).

[texaspyro]

The Ublox output freqs are best described as "derived from" GPS rather than "locked to" GPS.   Adding and dropping pulses to keep the average frequency close to the desired value is a far cry from actually locking the oscillator frequency to the GPS data via something like a EFC input to steer the GPS TCXO.

[cdev]

How about "inspired by actual GPS signals" ?

[edpalmer42]

Or maybe "distantly related" to GPS.   

I've seen that output described as a Numerically Controlled Oscillator.

On the receiver I mentioned, I measured the 10 MHz output with an HP 5372A Time Interval Analyzer and got the following results:

"Measuring the period of 50M cycles shows two normal distributions, one centered at 94.0 ns (~20M readings) and
the other at 104.4 ns (~30M readings).  Apparently, this load 'brackets' the 100 ns point by sending short and long cycles on
an almost one-to-one basis that average to 100ns."

Later it was pointed out that the ratio of short and long pulses would vary from unit to unit and with temperature.  Note that the difference between the two periods of 10.4 ns represents a frequency of ~ 96 MHz.  This is sort of in the ballpark of the 'up to 120 MHz' note regarding the clock speed that was in the spec sheet.  The output might work if you were measuring frequency because the gate time of 1 to 10 seconds would be long enough to average out the jitter.  Other uses might not work as well.  The manufacturer didn't try to hide this behaviour and, in fact, had an application note on using a phase-locked oscillator to clean up the output.

I would still say that it was locked to GPS, but be careful - here be dragons!

Ed

[texaspyro]

I think the Ublox is running at 48 MHz and clocks the output on both edges -> 96 MHz.

Also, I don't know if their NCO actually attempts to compensate for TCXO drift or (I suspect) they are just attempting to align the 1PPS output to the nearest edge and report the difference via the sawtooth message and are not also adjusting the divisor for TCXO drift.

[lars]

The data sheet for the NEO-7M says this on page 10:

”The TIMEPULSE output generates pulse trains synchronized with GNSS or UTC time grid with intervals configurable over a wide frequency range. Thus it may be used as a low frequency time synchronization pulse or as a high frequency reference signal.”
https://www.u-blox.com/sites/default/files/products/documents/NEO-7_DataSheet_%28UBX-13003830%29.pdf

So maybe it is just a definition as Ed says. I call it locked, uBlox say synchronized and Mark derived from.

What I know from practical measurements is that the time pulse outputs stay enough close to other GPSDO's over a day to achieve at least 12 digits of accuracy.

Also the jitter on the time pulse is about 21ns p-p with a saw tooth like pattern (for short times as minutes). This is what you get with a free running 24MHz oscillator and the time pulses aligned to both edges of the oscillator. Another possibility is a 48MHz oscillator only aligned to one edge but I guess more on a 24Mhz oscillator.

The output pattern is also what you get with a NCO or a DDS with a 1 bit DA and a 48Mhz clock. I prefer to see it as a 1bit DDS as I have worked quite a lot with DDS’es and it makes it easier for me to think of jitter and spurious.
https://en.wikipedia.org/wiki/Direct_digital_synthesizer

If you say that the oscillator in the Trimble Thunderbolt is locked to GPS (Trimble probably should use even another word: disciplined ) I would also say that the free running oscillator + 1bit DDS is locked to GPS. The difference is the much lower jitter of the oven controlled oscillator in the TBolt compared to the DDS in the NEO-7M. The second difference is that the Tbolt has analog control of the oscillator and the NEO-7M digital of the DDS (oscillator). When of course the Tbolt has a timing receiver but that involves a lot of other errors outside of the internal GPS engine-oscillator “phase locked loop”.

By the way it is possible to read the frequency offset of the free running oscillator in the NEO-7M to nine digits (1ppb) every second with the UBX protocol.

Lars

[Gyro]

The Ublox output freqs are best described as "derived from" GPS rather than "locked to" GPS.   Adding and dropping pulses to keep the average frequency close to the desired value is a far cry from actually locking the oscillator frequency to the GPS data via something like a EFC input to steer the GPS TCXO.

Well I suppose that description is a getting a little bit closer than:

They are just divided down from the free running GPS board oscillator via a DDS

Progress indeed. 

There would be very little (zero) point in a GPS module outputting free running frequencies. As I explained, the frequency outputs, at whatever setting experience sawtooth error, yes that means that the closest edge of the 48MHz clock (yes I believe both edges are used, hence the 21ns P-P). Again that is very different from "Adding and dropping pulses" (citation definitely needed). What it results in is a clock with edge jitter (sawtooth error + if you are silly enough, any additional jitter resulting from non interger divide ratio).

If you really want, you can configure the frequency outputs to produce a free running clock in the absence of GPS lock (one of the many settings). Once the GPS lock is achieved, you can see the edge jitter start as it locks / 'derives' / synchronises / corrects the frequency. Of course in most cases it is more sensible to configure the outputs to stay at logic low or logic high until GPS lock is achieved.

Also, I don't know if their NCO actually attempts to compensate for TCXO drift or (I suspect) they are just attempting to align the 1PPS output to the nearest edge and report the difference via the sawtooth message and are not also adjusting the divisor for TCXO drift.

Again, there is no dedicated 1pps output, the mode of operation doesn't sudenly change when you go to 1pps.  To get 1pps, you just configure one of the frequency outputs to a 1000000uS period and set the mark/space to 10% or whatever you want. The only advantage of going down to low pps is that the serial or USB port can then output sawtooth correction data (configurable up to 10 times/sec iirc). [Edit:] I don't know how they do this either, presumably the former of your two suggestions as it applies at all frequencies, not just 1pps.

[texaspyro]

The Thunderbolt is rather unique in that the RF chain is derived from the OCXO.  That results in a 1PPS output that has no sawtooth error.  It also means that if the OCXO is off frequency by a small amount that  it won't track satellites.  You can set the DAC voltage to a setting that will cause loss of lock.

When I wrote the code that Lady Heather uses to "autotune" the disciplining parameters, I had to limit the DAC step it uses to calculate the EFC gain to around +/- 10 (?) mV in order to avoid losing lock.  I think I originally used +/- 500 mV.

[cdev]

These super affordable PIC based devices look potentially useful in helping ascertain the accuracy of a GPSDO given a decent frequency standard to use as a reference.

http://www.leapsecond.com/pic/

[Yannick99]

Codebird... you can build my oscillator. Easy and very cheap

https://www.instructables.com/id/GPSDO-YT-10-Mhz-Lcd-2x16-With-LED/

Just put your ocxo and the uC will adjust the vco by itself. No need of display or led if you want.

[usagi]

bg7tbl gpsdo master reference