OCXO Calibration - Page 1
Forum section: electronics / beginners

author: arielberschadsky
author_rank: Newbie
timestamp: February 23, 2023, 06:57:36 am
content: I recently purchased a 10MHz OCXO that has a voltage reference pin that permits slight adjustment of the oscillation (a few Hertz in either direction). I plan to use this as the timebase for a digital clock of high accuracy and precision. The input voltage on the OCXO is 5V, and the reference voltage must be between 0 and 4 Volts. I have already made a simple voltage divider using two standard resistors to achieve 4 volts, and then I use a 15-turn potentiometer to select the 0-4 volts that achieves exactly 10MHz as measured with my frequency counter. My question is if this is the best way to generate the reference voltage, given that the resistors could vary slightly with temperature, and vary over time, thus causing a slight drift from the initial reference voltage selected. For this application, is it considered better practice to use a variable voltage regulator to select the exact voltage I need on the reference pin, or perhaps another method I haven't considered?

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: February 23, 2023, 08:03:22 am
content: Could you please post the part number of the OCXO for reference

author: bingo600
author_rank: Super Contributor
timestamp: February 23, 2023, 12:52:10 pm
content: @OPAre you sure you don't have a reference voltage already , comming out on an OCXO pin ?Some of them actually have a dedicated Vref output pin.Re: Resistors ... They can be bought with a very low PPM change per degree - PPM/CHow many $$ are you willing to spend ?

author: tggzzz
author_rank: Super Contributor
timestamp: February 23, 2023, 01:11:07 pm
content: Consider thermally coupling the voltage divider resistors, since that will reduce the effects of temperature change.Obviously, ensure the 5V is stable, and that any potentiometer is not affected by mechanical and thermal shocks.Ensure no noise is coupled onto the control pin, since that would affect the oscillator output.

author: Picuino
author_rank: Super Contributor
timestamp: February 23, 2023, 02:23:38 pm
content: The OCXO has some frequency drift over time, so there is a limit to the accuracy you can achieve with resistors.The OCXO I use has a maximum drift of 0.1ppm per year (now that it is old the drift is lower).The frequency control range is about 3ppm total when changing the input from 0 to 5V.This means it has a maximum yearly drift that needs to be corrected with a maximum of 170mV change in control voltage.Assuming that the actual values are less, note that you will need a correction of about 50mV/year. This can be achieved with relatively normal resistors connected to a normal voltage regulator.If you want better accuracy, you will have to improve the voltage reference (e.g. with an ADR4540 reference) the accuracy of the resistors or, better yet, use a DAC and make frequent adjustments e.g. with a GPS signal.Note that the accuracy of your frequency meter will probably have an error of several tenths of a ppm, unless it is newly calibrated.

author: arielberschadsky
author_rank: Newbie
timestamp: February 23, 2023, 04:49:11 pm
content: Thanks for all the responses. The part number is CTI OSC5A2b02.

author: wasedadoc
author_rank: Super Contributor
timestamp: February 23, 2023, 05:27:14 pm
content: Quote from: arielberschadsky on February 23, 2023, 04:49:11 pmThat does not have Vref out.https://dl6gl.de/media/files/ocxo-cti-osc5a2b02-datasheet.pdf

author: arielberschadsky
author_rank: Newbie
timestamp: February 23, 2023, 09:25:35 pm
content: I never said it did. It has VRef in.

author: wasedadoc
author_rank: Super Contributor
timestamp: February 23, 2023, 10:16:41 pm
content: Quote from: arielberschadsky on February 23, 2023, 09:25:35 pmI never said that you said that. Another poster wrote that some modules do have a Vref out and asked if you were sure that yours does not.

author: Zenith
author_rank: Frequent Contributor
timestamp: February 23, 2023, 11:20:10 pm
content: In my experience of that particular OCXO bought used from Aliexpress, they are pretty damned good ( < 1 in 10^8 over a couple of months compared against a GPSDO) even if fed from a regulated, but not so great, 5V supply and the adjustment voltage comes from a dubious multiturn preset directly across the 5V supply. Adjustment can be twitchy with that arrangement. I found that using a preset which was recovered from something else, needed safe cracker fingers to get right.I found my five could be brought to spot on 10MHz with adjustment voltages between 1.6V and 2.3 V comparing them against a GPSDO. The practical adjustment range, allowing for ageing, is probably between 1.5 and 3V. You could experiment with yours and see how it behaves with various reference voltages.Obvious improvements are to use a dedicated 5V supply - no need to go overboard on that. Then to use a voltage reference chip to supply a dead stable voltage over 3V. Then to use low tempco resistors (which are quite cheap) to reduce the range over which the high quality (Bournes or such) multiturn preset has to operate.What accuracy are you looking for with this clock? Exactly how good is the frequency counter's standard? Have you thought of cutting to the chase and using a GPSDO or off air frequency reference for the clock?

author: donlisms
author_rank: Frequent Contributor
timestamp: February 24, 2023, 03:20:17 am
content: May I share how I'd think about it? I don't know if it will or not. Maybe. This is all ball-park, mental approximations with simplifications.If it's a few Hz adjustment plus or minus, I'll round up and say 10Hz adjustment range. That's 1ppm.Then it's 4v for 1ppm frequency, so if I divide voltage changes by 4 I get ppm Hz changes. 1% voltage change gives me 0.25% ppm change, or IOW 0.0025 ppm change in frequency.Armed with this very approximate view, perhaps I can draw some conclusions. A typical fixed resistor at 100ppm temperature coefficient, gives worst case 0.0000025ppm drift in the frequency, per degree C. Seems okay to me.And so on for the other stability factors of the ref voltage.

author: hvontres
author_rank: Contributor
timestamp: February 24, 2023, 05:14:14 am
content: I just installed a 10Mhz reference for my HP5316A (built from this thread:https://www.eevblog.com/forum/testgear/hp-5316a-teardown-and-frequency-standard/msg531885/#msg531885)I don't have a gpsdo (yet) but I did set up a ublox LEA-5T-0-003 GPS in the garage and let it survey itself in. After that, I used the A/B mode on the 5316 to tweak the reference to read exactly 10e6. I don't know what frequency counter you have, so this trick may not work for you. One other trick I found on the 5316 is that if you make the gate time long enough, you can get it to overflow, so you get an extra digit on the display. If you want to really go crazy, set up a way to log you frequency counter over a fairly long period (I used the GPIB interface) and check to see if there is any dither in the readings. Using this method, I was able to get it settled in the sub-ppm range. As mentioned earlier, you will occasionally need to re-adjust the trim for long term drift.

author: 807
author_rank: Frequent Contributor
timestamp: February 24, 2023, 12:51:30 pm
content: Quote from: arielberschadsky on February 23, 2023, 06:57:36 amAre you using the frequency counter's internal reference for your readings? If so, your 10MHz OCXO will only be as accurate as the counter's internal oscillator.Unless your counter can use an external reference from a rubidium or GPSDO then you can't be sure that tweaking every last mHz out of the OCXO is worth it.

author: Zenith
author_rank: Frequent Contributor
timestamp: February 24, 2023, 02:54:16 pm
content: Quote from: 807 on February 24, 2023, 12:51:30 pmYes, the only practical way to set these things up to perform to their specification is by using a known high quality reference such as a GPSDO. Especially if the frequency counter is old, its reference can be up to a couple of ppm out and beyond adjustment. Also 25 plus year old OCXOs can take many hours to stabilise properly and stabilise at a slightly different frequency every time they are powered up.Then frequency counters sometimes came with several frequency reference options; an expensive high grade OCXO, a basic OCXO, and just a pretty good crystal oscillator which could be good to a few ppm. The last was for cost sensitive customers or those who intended to use the counter with an external reference. It's worth checking which one you have and what its specification is.I suppose if it's a newish high grade frequency counter, recently calibrated, it would be different, but the GPSDO/rubidium/off-air route seems better.

author: fourfathom
author_rank: Super Contributor
timestamp: February 24, 2023, 03:42:26 pm
content: Quote from: Zenith on February 24, 2023, 02:54:16 pmThat "different frequency" issue is true for all crystal oscillators, oven OCXOs, admittedly some more than others. I've also seen this in MEMs oscillators (SiTime, etc.) The phenomenon is called "retrace". I learned about this when evaluating TCXOs for a radio project, and at first thought it was due to a lag in the thermal equilibrium times of the oscillator on the board and the temperature sensor in my temp. chamber. But no, it turned out to be a well-known (just not to me) phenomenon called "retrace", or thermal hysteresis.For all typical calibration and reference purposes, one of the Bodnar GPSDOs is probably the way to go.

author: arielberschadsky
author_rank: Newbie
timestamp: February 24, 2023, 04:06:05 pm
content: Thank you for all the helpful comments. In my particular case, I don't have a modern, high precision (Rubidium/GPSDO or even OCXO-based frequency counter), so my effort to calibrate this OCXO to any degree of precision or consistency won't be very successful. But the discussion of temperature immune resistors and a high precision voltage reference IC was very interesting and useful.

author: fourfathom
author_rank: Super Contributor
timestamp: February 24, 2023, 04:23:58 pm
content: Quote from: arielberschadsky on February 24, 2023, 04:06:05 pmIf you have a radio that can pick up WWV on 10 MHz, you can attach a small "antenna" wire to your OCXO output and listen for a beat-note. You can get at least sub-Hz resolution by listening to the audio phasing effects.

author: Zenith
author_rank: Frequent Contributor
timestamp: February 24, 2023, 04:34:26 pm
content: Looking at the notes I made when I looked at a few CTI OSC5A2B02 OCXOs from Aliexpress, I found the frequency was about 9,999,987 Hz for 0.1V control voltage, spot on 10MHz for 2.03 to 2.08V control voltage (there was one outlier that was spot on at 1.7V) and 10,000,014 to 10,000,019 Hz for a control voltage of 4.0V.So if you don't have access to a suitable high quality frequency reference, I suggest you use decent quality metal film resistors to form a voltage divider, preferably fed from a voltage reference, to give a 2 volt control voltage and that should be a good enough guess.

author: Zenith
author_rank: Frequent Contributor
timestamp: February 24, 2023, 04:38:15 pm
content: Quote from: fourfathom on February 24, 2023, 03:42:26 pmIt seems to be particularly bad with ancient (30+ year old OCXOs). It may not be entirely down to the crystal but other components, such as presets, which are showing the effects of age.

author: Picuino
author_rank: Super Contributor
timestamp: February 24, 2023, 07:32:56 pm
content: In any case, keep in mind that with an OCXO it is relatively easy to get it set to an accuracy of 0.1 ppm.This error is equal to an error of 3 seconds per year. How accurate do you want your watch to be?