[time-nuts] Considerations When Using The SR620
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Sun Dec 2 21:25:53 UTC 2012
>The following comment appeared on this list recently and it scared
>me a little:
>>Though the SR620 TIC is a great instrument when hunting the pico
>>seconds we have to realize, that it's a thermal design desaster (I
>>have to apologize to all sr620 friends). I have to run it for at
>>least 12 hoursif not 24 to be shure, that every single part is at a
>>more or less stationary thermal state. Some (NERC) say "...never
>>switch it off".
>I assume this instability is due to the instability of the internal
>frequency standard. * * * In fact, in our measurements, we plan
>to use a Cesium frequency standard as the timebase to our SR620.
>Does this anecdotal warning apply generally to the instrument or
>mainly to the use of the internal standard oscillator?
I concur with the comment above that the thermal design of the 620
could have been better -- the sensing thermistor is in an "exhaust
stack" between the fan (which is blowing out) and the rear enclosure
wall. This means that, instead of trying to maintain the internal
instrument temerature at a constant level, it tries to maintain the
exhaust stack temperature constant with a viciously fast response
time that leads to instability at startup. I have more than once
considered moving the thermistor to a location near the TCXO, but
since the fans always run up to full speed rather quickly at room
temperature anyway, I have never bothered to try to improve the fan circuit.
Additionally. the TCXO remains powered during standby, but not
exactly on frequency because the DAC that adjusts it during operation
is not powered. So, there is some settling from that adding to the
temperature drift. Note also that the DAC steps are not very fine,
so you cannot expect to get the internal oscillator trimmed to better
than e-9 or so. SR apparently thought that most users would connect
620s to external standards, so there was no reason to make them pay
for a high-precision internal standard they would not use.
IME -- operating with an external reference that is better than the
specified accuracy of the 620 -- they meet SR's specifications within
a few minutes at most after switching on from room temperature
storage. (The trigger circuitry may drift a bit as it warms up, so
you may want to check the trigger drift if your application involves
slowish sine waves. I have not investigated this.) Ideally, you
would let the instrument warm up for at least an hour and then
perform an internal calibration before starting your measurements.
All that said, the only way you will know for sure how your
particular instrument and standard will perform is to characterize
them before you start your mobile measurements. In doing so, you
should observe a protocol that resembles the actual travel between
measurements, at least with respect to time and temperature. I
strongly urge you to do this so you can have confidence in your measurements.
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