[time-nuts] Once again about counter calibration

Charles Steinmetz csteinmetz at yandex.com
Sun Apr 26 02:01:01 EDT 2015


>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, you are using a 5386A with TCXO to meaasure an OCXO, and the 
reading you get drifts 14 e-10 (or 1.4 e-9), nicely centered on 10 
MHz.  This error band includes the error of both the TCXO and the 
OCXO (presumably, the TCXO contributes most of this error, but you 
don't have enough information to conclude that yet).  [This error 
also includes the jitter in the trigger circuitry of the 5386A, but 
if you are using a 10 second gate time that should be small compared 
to the drift of the TCXO.]

I don't believe you can reasonably expect this setup to exhibit less 
drift than what you are seeing.  In fact, I'm surprised the drift 
range is as low as 1.4 e-9.  Waiting will probably change the center 
of the drift range (it will not stay symmetrically +/- 7 e-10 above 
and below 10.000000000), but not the magnitude of the range (14 e-10 
peak to peak).

>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).

Generally, a time nut would let a counter warm up for 24 to 48 hours 
before doing any critical work.  And yes, the GPSDO is probably the 
most stable of the three oscillators you have (I assume you leave it 
running 24/7/365), so the best you will be able to do is to feed it 
into the 5386A's external reference input and measure the OCXO.  You 
should see a smaller drift range than the 1.4 e-9 you are seeing now, 
which will indicate that most of the drift you are seeing now is due 
to the TCXO (and confirm the expectation noted above).

Most time-nuts use a "distribution amplifier" to supply separately 
buffered copies of their best oscillator to various instruments and 
other equipment.  In your case, you would feed the GPSDO into the 
distribution amp and then use the distribution amp's multiple outputs 
to feed the 5386A's reference input, your various radios, etc., etc.

Best regards,

Charles






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