Calibrating out initial offset of TCXO - Page 1
Forum section: electronics / projects

author: smoothVTer
author_rank: Regular Contributor
timestamp: February 05, 2018, 04:58:45 pm
content: Hello Everybody and happy Monday!I've been using these TCXO devices to achieve MCU clocks running at nearly an identical rate, +/- whatever the initial tolerance of the part is:https://www.ctscorp.com/wp-content/uploads/2015/12/008-0527-0.pdfThe task is to tune / calibrate / cancel the initial offsets from each TCXO using their VC pins, which allow frequency pulling +/- 5ppm or so, by adjusting the pulling voltage and comparing the output to a stable, calibrated frequency reference. The TXCO frequencies in use are 10MHz, 16MHz, 20 MHz and 24 MHz. My target is to achieve < 500ppb frequency difference between all my TXCO's.(1) What kind of equipment would I need to do this kind of tuning in the lab? What I mean is: I see many, many rubidium standards out there but they are all 10MHz. Is a frequency standard the equipment I need? If so, how do I get 16, 20, 24MHz out of it?(2) Frequency counters: Is it possible to achieve 0.5 Hz accuracy from OCXO-based counters? If so can anyone recommend a counter which could be used and is common enough to be found secondhand?

author: ConKbot
author_rank: Super Contributor
timestamp: February 05, 2018, 05:38:27 pm
content: For your reference, 10MHz goes into a counter as a reference frequency, and you use that to measure all your other frequencies. It's used by a lot of test equipment, and has become a defacto standard for reference oscillators.But the more important question is "why?" If they need to be so close, use a PLL and dividers to have them all locked to one source at an exact ratio. Get a 4 MHz tcxo, x4 for 16MHz, x5 for 20, x6 for 24, and either x2.5 for 10, or divide the 20MHz clock by 2.They will maintain the exact ratios, and if you need to calibrate your reference still, you only have to calibrate the 4 MHz source, so only one source of drift, rather than trying to get 4 oscillators to drift together over time.

author: CopperCone
author_rank: Super Contributor
timestamp: February 05, 2018, 05:50:34 pm
content: His option may theoretically have less phase jitter which multipliers and dividers introduce. This would be small however. Phase nuts?

author: mino-fm
author_rank: Regular Contributor
timestamp: February 05, 2018, 07:31:36 pm
content: If you like it you can build your own simple 'firsthand' reciprocal counter using a STM32F429-Discovery board with TFT display:http://mino-elektronik.de/FM_407/fmeter_407.htm#c1(please use translation GER->GB).All you have to do is downloading a program to this board and add some connections and a potentiometer for adjusting the 8 MHz mainclock. Calibrating is done by an 1pps GPS signal. After that you get seven digits resolution of frequency and period of an external signal at the same time.All around this link there are much more 'firsthand' frequency counters.

author: smoothVTer
author_rank: Regular Contributor
timestamp: February 05, 2018, 09:05:43 pm
content: Thanks everyone for replying. I will take your advice and proceed.

author: ConKbot
author_rank: Super Contributor
timestamp: February 05, 2018, 09:07:30 pm
content: Quote from: CopperCone on February 05, 2018, 05:50:34 pmPhase noise inside the loop bandwidth will be primarily the reference (with some added by the PLL) phase noise outside the loop bandwidth will be primarily the VCO. If you use the TCXO w/ control input as your VCO, and keep your loop bandwidth narrow enough, your phase noise will be that of the secondary TCXO, with it frequency disciplined by the master freq reference. A modern PLL/VCO combo should have a VCO optimised for far-out phase noise with the assumption that your close-in phase noise performance will be derived from your reference oscillator.

author: David Hess
author_rank: Super Contributor
timestamp: February 06, 2018, 12:46:40 am
content: Quote from: smoothVTer on February 05, 2018, 04:58:45 pmFor a reference, TCXOs will get you to 1000ppb so something better like an OCXO at 10ppb or Rubidium at 0.5ppb is needed unless you use a GPSDO and you should have a GPSDO anyway for checking the calibration of whatever reference you do use.QuoteGood frequency counters support using an external reference. Or for your requirements, a frequency counter with a built in OCXO would be suitable if you had a way to verify its accuracy.QuoteReading down to 0.5Hz is trivial and only takes 2 seconds of gate time on a simple frequency counter although the delay can be annoying. 0.5Hz out of 24MHz is 20ppb which is a good match for a frequency counter with an OCXO. That leaves lots of options as long as you either use a calibrated OCXO or have a 10 MHz GPSDO for the frequency counter's external reference input.I do not know what your budget is so I am just going to go by counters which I have personally worked with that are available inexpensively used.Something like an inexpensive used HP5316B with its external frequency input could handle it but would be a little slow with a 2 second gate time to get to 0.5Hz. Its option 004 OCXO would be stable enough if you kept a close eye on it but using an external reference would be a sure thing.A Racal-Dana 1991 or 1992 generates 9 digits per second so would be much faster than you could read. Its option 4 built in OCXO is about the same as the HP5316B but its 10MHz external reference input could be used instead.The other old counters I would look for which are similar to the Racal-Dana 1991/1992 are the HP5334A/B and HP5335A. My notes on HP counters are not complete but they have a lot of suitable ones after those.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 06, 2018, 03:11:42 am
content: If you can find one at a reasonable price the old HP5370B is an excellent universal time interval (frequency) counter. Super accurate and stable and 16 digit display!. Mine hasn't been calibrated in years and varies by 0.04 Hz from my GPS locked frequency standard. You can sometimes find these on Ebay for $500 or so. In time interval mode the resolution is 20ps! The newest Keysight units are also spec'd at 20ps. Amazing performance for the vintage. Does null, averaging and standard deviation. Max measurement frequency of 100Mhz however.

author: mino-fm
author_rank: Regular Contributor
timestamp: February 06, 2018, 05:43:06 pm
content: Quote from: Insatman on February 06, 2018, 03:11:42 amTo pay $500 for a simple test? Why?An easy way could be using a fast 3 digit counter with a gate time of 1 s generated by GPS-modul. Resetting the counter, counting for 1 s and reading out the display shows the last 3 LSBs. A value of "000" will display a well tuned VCTCXO. "9xx" is to less and "1xx" is to much. Resolution of the last digit will be 0,1 ppm at 10 MHz. That's enough.

author: Insatman
author_rank: Frequent Contributor
timestamp: February 07, 2018, 02:14:43 am
content: Quote from: mino-fm on February 06, 2018, 05:43:06 pmIt doesn't make sense to buy an instrument for one simple test in most cases I agree. But...if you are looking for a great instrument for you lab that can do this kind of measurement it's a good choice. Personally I think it's a great value considering it's specifications.

author: jbb
author_rank: Super Contributor
timestamp: February 07, 2018, 06:44:34 pm
content: Depending on your location etc. you could also see if a calibration lab can help you out. If you make the test procedure simple, and ask for a ‘debug’ without an official cal certificate, it might be affordable.

author: cdev
author_rank: Super Contributor
timestamp: February 07, 2018, 06:59:54 pm
content: You can find very nice vintage HP universal counters like the 5335A (from around 1980) for around $150. If you pair it with a modern GPSDO bought from telco surplus (to give you a dependably good reference) you can buy a unit without the most expensive options (since that would duplicate the GPSDO's capabilities - a good buy right now are telco surplus TruePosition GPSDOs - they can be had on ebay for $40-45)and then you will have a solid reference 10MHz signal, so that makes it much easier/cheaper to have a respectably accurate time and frequency measurement capability.How much? Its hard to say but I am guessing a bit more than $200 would likely get you there, assuming you already have a computer that has USB so can talk to the GPSDO card.You just have to pick carefully to make sure you get a working unit.You might be able to do it for even less than that. But you might be taking a higher risk.Disadvantage is that particular HP is physically large. (but it seems to be reliable from what I have heard) The Racal units mentioned by David Hess look like in some respects more powerful/faster but they are old and there is an issue with the elastomeric buttons they use. It might be hard to know if the unit you are looking at is stricken with that or not unless you can actually see it in the flesh.One nice thing about using a GPSDO with an ovened oscillator as your frequency standard is less power needs to be used.A GPSDO isnt that large. Nor do they draw a lot of current once they have started up and are running. If you have a GPSDO as your standard you can leavethaton all the time so you don't have to leave the counter on all the time. (any ovened oscillator is most accurate and lasts longer if you just let it run continuously)there are other HP units which are newer and much more expensive and some that look affordable that I don't know so much about.I don't own any of them, I am just at the surveying the solution space stage. I do have a GPSDO and a PIC frequency counter which is all I need right now.

author: David Hess
author_rank: Super Contributor
timestamp: February 07, 2018, 09:39:53 pm
content: Quote from: cdev on February 07, 2018, 06:59:54 pmThese instruments are all old and may require maintenance.My Racal-Dana 1992 had the problem with the buttons but I found it really easy to repair just by cleaning the contacts. They had what looked like rosin residue on them as if the entire board was cleaned after the switches were soldered into place.

author: Squiddaddy
author_rank: Regular Contributor
timestamp: February 10, 2018, 08:59:38 pm
content: I have a Racal Dana 1992 also, and some of the buttons are getting loose. They will eventually fail. Fortunately, I got a good replacement set from China that I will replace them with once they fail. Check out TheDefPom on Youtube in his 1991/1992 repair videos, and he has a link to the switches.

author: cdev
author_rank: Super Contributor
timestamp: February 10, 2018, 09:22:43 pm
content: Of all the counters out there, the Racal/Dana ones in that series seem the best match to what I want to do that I can afford, IF I got a good unit.I came very close to buying one but I didn't have such a good feeling about it because the seller was reticent to do the tests from the service manual I wanted him to do, which just involved simple things like connecting the output of the internal frequency standard to one of the inputs and that made me worry.So I didn't get it.It seems like such an interesting project to try to make one, though, but that kind of project, at least if it involves any design decisions, is still way over my skill level.

author: David Hess
author_rank: Super Contributor
timestamp: February 10, 2018, 10:20:40 pm
content: Quote from: cdev on February 10, 2018, 09:22:43 pmIf you have a PIC or AVR or other small micro, you can drive its clock pin from the TCXO after dividing if necessary, and then count the duration between pulses of the one pulse per second output to determine the clock frequency. Without interpolation this will not return a high resolution in a short time but the GPS pulse per second output may not be much better than 50ns so extra gate time will be necessary anyway without a GPSDO.