ocxo, rbxo, gpsdo calibration - Page 1
Forum section: electronics / general

author: metebalci
author_rank: Frequent Contributor
timestamp: February 17, 2022, 08:15:07 pm
content: I am a little confused about how to calibrate these. In other words I think I understand some basic concepts but not sure if I understand them correct. Not in a particular order:- for GPSDO, I mean a Leo Bodnar unit at the moment, which has a TCXO, does any answer for the questions below change if GPSDO had a OCXO?- GPSDO needs to be locked to the satellite(s) for some time for its output to be better than ocxo and rbxo (correct?). Is this time in 1000s range or more (eg >24h) ? When it loses the gps lock for a short period (shorter than the answer to previous question at least), would it require the same time or less to have the same output stability ?- what is the correct way(s) to calibrate ocxo and rbxo against gpsdo ? Is it enough to have gpsdo reached to its stability (after X seconds/hours), use it as ref input to freq. counter and just use a single reasonably high accuracy single measurements ? For 10MHz, assuming 0.1ppb (not sure 100% why I say this number), so 1mHz, so 11 digits which can be done under a second. Or should I use average measurements or time interval measurements or else (and why) ?

author: Picuino
author_rank: Super Contributor
timestamp: February 17, 2022, 08:30:41 pm
content: Usually the GPSDO measure the phase shift between OCXO and Satellite signal.You can measure a phase shift of 1ns with relative ease, so you can get 9 decimal places of precision per second, 12 decimal places every 1000 seconds.The way to measure the phase shift can be as simple as plotting the signals on an oscilloscope (for manual adjustment)For more advanced methods, please let more experienced users comment.

author: metebalci
author_rank: Frequent Contributor
timestamp: February 17, 2022, 09:34:52 pm
content: Quote from: Picuino on February 17, 2022, 08:30:41 pmThat is a good information thanks. I might have used a few words incorrect maybe. What I meant was calibrating ocxo and rbxo with/using a gpsdo (and a freq counter or a scope etc.). I say gpsdo because it is the only thing I can trust as an absolute reference. I dont have another officially calibrated freq. standard.

author: donlisms
author_rank: Frequent Contributor
timestamp: February 18, 2022, 02:24:22 am
content: I think one basic technique is to put both signals to be compared on the oscilloscope, triggering on the reference signal. The other signal will show its drift before your eyes; if it takes 10 seconds to drift one complete cycle, it's off by 0.1Hz, relative to the reference. Very much like what is shown in the video (which I think is maybe not so interesting, because I would expect to let the standards all warm up for a very long time before expecting them to produce stable frequencies).

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 18, 2022, 08:07:54 am
content: The GPS signal has a short time jitter of about 10-10, depending on signal strength, satellite constellation, and your receiver.Therefore this is just sufficient for adjustment of an OCXO at about 1 or 10 sec averaging time, as latter is stable (mid and long term) to about <10-9.A GPSDO with a good internal OCXO might improve this, but you need a good one, with averaging, T.C. learning and so on, like the Trimble Thunderbolt.Then you get the good short term stability of its OCXO, and the superior long term stability of GPS, but still a mediocre mid term stability.The learning needs about 2 days, and then you survive a failing GPS signal, recovering within short time, maybe minutes only.For calibration purposes, you need a longer period of time of uninterrupted signal AND of surveillance time.The GPS signal has a mid term variance, maybe on the order of 10-11due to e.g. the permanent change of its satellite configuration.For the whole picture, please look at internet pages of some time-nuts, who provide Allan deviation diagrams for several receivers. (*)Therefore, as a RB clock has to be adjusted down to about 10-12, any Lissajous methods, or short termed measurements are useless and lead to wrong results.You need to compare the RB frequency vs. the GPSDO frequency, best at a down converted 1pps with a phase / T.I. comparison method, i.e. by means of a T.I. counter, and monitoring that over several hours to average the short and mid term variations. Use TimeLab (**) which supports commonly used counters and provides appropriate comparison methods, as phase or direct frequency.Frank(*)http://www.thegleam.com/ke5fx/(**)http://www.miles.io/timelab/beta.htm

author: Picuino
author_rank: Super Contributor
timestamp: February 18, 2022, 05:55:23 pm
content: To calibrate my OCXO with the signal from a GPS, I started by setting up a 16-bit DAC circuit to control the frequency of the OCXO, and I made a frequency divisor by 10000 (with a PIC uC). I compared a 1000Hz signal from the GPS with the 1000Hz signal from the OCXO divisor.I then hand searched for the frequency closest to 10MHz for the OCXO, trying to make the phase shift equal to zero.After that, I compare directly the 1000Hz from GPS with 10MHz from OCXO, to be able to extend the resolution of the oscilloscope to nanoseconds.I then measured the remaining phase shift in 100 seconds (PS_1 in nanoseconds).I changed the DAC setpoint by 100 points and again measured the phase shift in 100 seconds (PS_2 in nanoseconds).I calculated the DAC gain = (PS_2 - PS_1)/100 (nanoseconds / DAC points)With the DAC gain, it is easy to adjust the DAC so that the OCXO has zero phase shift, with a simple linear calculation.Edit: The lower frequency for manual calibration was 1kHz

author: metebalci
author_rank: Frequent Contributor
timestamp: February 19, 2022, 09:32:49 am
content: Quote from: Picuino on February 18, 2022, 05:55:23 pmI have a similar setup, OCXO with 16-bit DAC and working on the PIC now to have a divider. The RbXO only has a trimmer.Is there a particular reason you choose 1kHz ?

author: metebalci
author_rank: Frequent Contributor
timestamp: February 19, 2022, 09:56:16 am
content: Quote from: Dr. Frank on February 18, 2022, 08:07:54 amThanks Frank. I will do measurements with Timelab. What is the reason for downconverting to 1pps ? Is it to have more resolution (minimum measurable time interval is fixed so decreasing the freq. increases the relative resolution ?) ?So the aim is on the long run (~a few hours) to have no phase deviation between RbXO and GPSDO, correct ?RbXO has better short term stability anyway, so if the long term deviation is minimized, that is the calibration for RbXO. and I guess because this can only be measured in a few hours after an adjustment on RbXO, each iteration of such adjustments will take ~a few hours to see the effect of it.

author: Picuino
author_rank: Super Contributor
timestamp: February 19, 2022, 10:32:45 am
content: Quote from: metebalci on February 19, 2022, 09:32:49 amIt is the maximum output frequency of my GPS. It is used to adjust the OCXO with the oscilloscope when the frequency deviations are very large.If you have a good frequency meter, it is better to do a first frequency adjustment with it.