Tuesday, February 2, 2016

Checking the calibration of a frequency counter

In the previous post I went through how I repaired a Racal-Dana 1992 frequency counter I had obtained from the dumpster. After the repair the instrument seemed to function correctly, but I wasn't sure if it still were anywhere close to calibrated. Given that I don't have an atomic frequency standard, and I'm not planning on buying one, the options I was left with were to either borrow an atomic standard or to use some other frequency source to check the calibration on the 1992.

I've been interested about synchronizing clocks using GPS signals and have been wondering if the cheap GPS receivers (Fastrax uPatch100) I have are any good in this respect. Their data sheet lists their PPS (1 pulse per second) output accuracy as 50 ns RMS, which is 50 ppb (that is 50*10^-9 relative error). The aging rate of the oscillator in the 1992 is specified at 3 ppb/day. The accuracy of the PPS output would thus seem to be adequate for determining whether calibration is required.

Fastrax uPatch100 GPS receiver with a DC/DC converter to power it from a lithium battery. The coax carries the PPS signal.
The GPS receiver had to be placed outside to get a signal. The coax goes back inside and connects to the frequency counter.
An interesting feature of the 1992 is what is called (R-X)/Z mode. In this mode the unit subtracts some programmable constant X from the measurement and divides the result by Z. Programming X=1 and Z=1 allows to easily check the deviation from 1 PPS. At first this deviation was reading around 3 microseconds, which was due to the lowish accuracy of operating only from the crystal oscillator in the GPS receiver. Once the receiver got a position fix, the deviation dropped down to between 10 and 100 nanoseconds, which is about what could be expected with the 50 ns error specification of the GPS receiver. However, I'm somewhat confident that the error in the PPS output of the GPS receiver should have zero mean value. As I'm almost exclusively reading positive values, it suggests the 1992 is running a tiny bit too fast.
Racal-Dana 1992 showing the period error. Here the PPS output is measured as being 23 nanoseconds longer than a second (or what the 1992 considers to be a second).
In conclusion, the unit seems to have an accuracy of around 100 ppb. That is very good considering the aging specification of the oscillator as it would still be within spec even if it were calibrated just a couple of months ago. Though this in fact hints that the oscillator may be much more stable than what is specified. I might investigate this more later.

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