Sunday, July 8, 2018

Frequency counter calibration

In two previous posts I described how I first repaired and then checked the calibration of a Racal-Dana 1992 frequency counter. The oscillator of the counter seemed to be within 100 ppb (parts per billion), which based on the aging specification of the oscillator is about as close as it needs to be calibrated. However, I don't think the unit had been calibrated in a very long time, which hints that the aging specification may be very pessimistic. In any case, calibrating the unit seemed like an interesting thing to do.

The 1PPS output of the GPS receiver I was using to check the calibration is specified at 50ns RMS error. This figure alone isn't quite accurate enough for my needs. However, the mean value of the output should be almost exactly 1 second (ignoring any possible gross error, such as loss of GPS fix). Averaging over a number of measurements should then give a much increased accuracy, which can then be used to tune the oscillator. Averaging 100 measurements should give one tenth of the error, which in this case would be about 5 nanoseconds, assuming a normal distributed error.

To easily get averages over the measurements the easiest route was to use GPIB. Unfortunately I didn't have a GPIB interface on my computer, so I needed to build one. See my previous post on building the interface.

Now that I could get readings from the counter, I wrote a simple piece of software for averaging the counts. The calibration procedure would then be simply:
  1. Acquire 100 measurements
  2. Compute average
  3. Adjust accordingly
After doing this, I collected data over one night (around 7.5 hours) to check the calibration. The results of which is shown below.
PPS period measurement over one night.
There are a few outliers during the night, perhaps from short GPS coverage losses. They do not contribute much to the average, since there are only a few, and the distribution seems to otherwise be very close to the normal distribution.
The mean value of the measured PPS period is just 4.04 +- 0.64 nanoseconds shorter than 1 second, which corresponds with the oscillator running just 4 parts per billion too fast.

The next thing I want to investigate with this setup is the long term stability of the oscillator. For this purpose I'll record the measurements over a long period of time, preferably at least some months. This should give a general understanding of the actual aging behavior of the oscillator, since the specified aging seems to be very pessimistic. This setup should also be accurate enough to capture GPS timing errors like the one observed not too long ago (in Finnish), albeit those are very rare.

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