[time-nuts] Considerations When Using The SR620

Charles P. Steinmetz charles_steinmetz at lavabit.com
Sun Dec 2 21:25:53 UTC 2012

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.

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.

Best regards,


More information about the time-nuts mailing list