[time-nuts] Once again about counter calibration

Dave Martindale dave.martindale at gmail.com
Sun Apr 26 00:30:24 EDT 2015

The problem with using a 1 Hz reference when looking at a nominal 10 MHz
signal is that you will get a stable scope display with no drift when the
input is *any* integer number of cycles/sec.  So 10,000,000 Hz will give a
stable display, but so will 9,999,999 Hz and 10,000,001 Hz.  Unless you
know that your 10 MHz signal is already within 0.5 Hz of the correct
frequency, the drift method is likely to cause you to adjust to the nearest
integer number of Hz, not exactly 10 MHz as you want.

It's better to start out with input and reference that are the same
frequency, or are related by some small integer factor.  So you can compare
a 10 MHz adjustable oscillator to a reference that is 1 MHz or 5 MHz or 10
MHz or 20 MHz.  For a stable scope trace, connect the lower of the two
frequencies to the scope trigger input, and the higher frequency to the
scope waveform input.  Then adjust for zero drift.

Also, use an analog oscilloscope if you have one (or a digital scope with a
high waveform update rate).  An analog scope will clearly show a high drift
rate as a smeared waveform, so you will know that you need to keep
adjusting the frequency trimmer and (probably in which direction).  With a
low-cost digital scope, you can get beat frequencies between the drift
frequency and the screen update rate that make it appear as if the input
waveform is stationary, when it fact it is drifting rapidly.

- Dave

On Sat, Apr 25, 2015 at 11:12 PM, d0ct0r <time at patoka.org> wrote:

> I tried to use 1PPS as Ext. trigger for the oscilloscope. I was able to
> "stabilize" signal movement. Then I tried to calibrate OCXO. However its
> appeared out of range (I reach end of potentiometer limit but counter still
> shows that OCXO is out of 10Mhz). Which kind of "suspicious".
> Then I decide to disassemble my project to take pure 10Mhz directly from
> GPSDO to measure OCXO signal. Its read totally different frequency value
> now. So, using 1PPS didn't work for me. I tried using that GPSDO 10 Mhz as
> Ext. Trigger. And now I got much better result. I was able to calibrate
> 5386A to some extent. But my 5386 has TCXO. So, after few minutes its
> moving out of "perfect" value. May be I need to wait much longer to
> stabilize oscillator. I am not sure what to expect here.
> Using GPSDO 10 Mhz as REF signal, I was able to calibrate OCXO. And now
> its potentiometer position was nether at its both extremes. The reading on
> 5386a (using 10 sec gate) fluctuate from 9.999999993M to 10.000000007M.
> Again, may be I need to wait much longer when OCXO will be stable.
> So, I think the best approach will be using 10Mhz GPSDO as ref. signal for
> this counter. In another case, I'll need to wait to warm it up (the manual
> advised only 30 minutes. But I am not sure). And then re-calibrate it. Its
> time consuming.
> I am curious, if its practical to calibrate something like Morion MV89A
> and use it as signal reference for this counter ? Or OCXO still will drift
> out of desired frequency relatively soon ?
> Regards,
> V.P.

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