Computing stuff tied to the physical world

TK – Frequency accuracy

In Hardware on May 17, 2012 at 00:01

Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.

(this is again a bit of a side excursion, about checking the quality of a measuring instrument)

I recently visited a friend who had to get his frequency meter’s calibration verified to a fairly high precision. Thinking of the Rubidium clock I got from eBay, he came up with the idea of using a transfer standard.

The thing with accuracy, is that you have to have an absolute reference to compare against. One option is to go to a “calibration lab” and have them test, adjust, and certify that your instrument has a certain accuracy. Awkward, expensive, and usually a bit over the top for “simple” hobby uses.

So the other way to do things, is to “transfer” the calibration in some way. Buy or build a device which can keep the desired property stable, calibrate it to some standard, move it to where the instrument needing calibration is located, and compare the two. Or vice versa: match to instrument, then compare with a standard.

The latter is exactly what we ended up doing. First we created a little Arduino daughter board with a “VC-TCXO” on it: that’s a “Temperature Compensated Xtal Oscillator” which can be fine-tuned through a voltage. Here’s the setup, created and built by Rohan Judd:

DSC 3082

On the left, an SPI-controlled digital potmeter, on the right a VC-TCXO running at 10 MHz.

Via a sketch, the VC-TCXO was fine-tuned to produce exactly 10.000,000 MHz readout on the frequency counter we wanted to verify. This was done at about 18°C, but a quick test showed that this VC-TCXO was indeed accurately keeping its frequency, even when cooled down by more than 10°C.

I took this device back home with me, and set up my frequency generator to use the Rubidium clock as reference. So now I had two devices on my workbench at JeeLabs, both claiming to run at 10 MHz …

Evidently, they are bound to differ to some degree – the question was simply: by how much?

Remember Lissajous? By hooking up both signals to the oscilloscope, you can compare them in X-Y mode:

SCR26

Channel 1 (yellow) is the VC-TCXO signal, some sort of odd square wave – I didn’t pay any attention to proper HF wiring. Channel 2 (blue) is the sine wave generated by the frequency generator.

The resulting image is a bit messy, but the key is that when both frequencies match up exactly, then that image will stay the same. If they differ, then it will appear to rotate in 3D. It’s very easy to observe.

The last trick needed to make this work is simply to adjust the frequency generator until the image does indeed stop rotating. This is extra-ordinarily sensitive – the hard part is actually first finding the approximate setting!

After a bit of searching and tweaking, and after having let everything warm up for over an hour, I got this:

DSC 3080

IOW, the frequency I transferred back to JeeLabs with me was 9.999,999,963 MHz. We’re done!

To put it all into perspective: that highlighted digit is 0.1 ppb (billion!). So the frequency counter appears to be 3.7 ppb slow. Assuming that the transfer standard did not lose accuracy during the trip, and that my Rubidium clock is 100% accurate. Which it isn’t of course, but since its frequency is based on atomic resonance properties, I’m pretty confident that it’s indeed accurate to more than 0.1 ppb.

The real story here, though, is that such breath-taking accuracy is now within reach of any hobbyist!

  1. Another thing to watch out for, is the loading of the oscillator. In general, changing the impedance of the load will change the frequency somewhat (certainly at the ppb level I would think, for a TCXO). If you have a clean 50 ohm load in each case, not a problem, but with that funny waveform I’m not sure.

  2. @jbeale, fair comment. This was a ‘proof of concept’ attempt. The next pass, if there is some interest in the capability, would go to a PCB with individual LDO, output buffer and co-ax connector. The module is surprisingly tolerant of supply/loading, but worth refining to control those elusive ppm/ppb.

    • Starts chanting “We want PPT!”

      ;-)

      Nice result there JC/Martyn and Rohan.

  3. Hmm, you don’t have a GPSDO yet?

    • There’s always more accuracy just behind the horizon :)

      Nope – I was thinking of building that myself one day, but that’s a bit on the back-burner for now. To be honest, yet more accuracy is not truly important for what I’m working on these days…

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