[time-nuts] Considerations When Using The SR620

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 2 22:29:19 UTC 2012

On 12/02/2012 10:25 PM, Charles P. Steinmetz wrote:
> Paul wrote:
>> 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.

Never thought about that part, other than the fact that the fan is 
annoying like hell.

> 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.

Which is why a high stability reference is an option, like most.

In contrast it is interesting to note that the HP5370 had a lower 
stability oscillator as option, so removing the 10811 was thus using a 
negative option.

> 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.

Another source of temperature dependence is the analogue interpolator.

> 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.

This is good advice. Consider what you need to do. You might want to 
consider having a rubidium doing your hold-over. for instance. A PRS-10 
would be an interesting option for instance.


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