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.
Cheers,
Magnus
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