Dear Time-Nuts,
I played around with my SR-620 counter and TimeLab and got
strange ADEV curves, so I started to investigate a bit.
The SR-620 is equipped with Option 1, the high performance
Wenzel OCXO - specified with 5E-12 at tau = 1s.
I tried to evaluate the counters noisefloor. I fed 10 MHz
signal to the start channel and, via a BNC Tee, the same
signal to the stop channel. A 1 pps pulse does the arming
once a second.
I tried 3 diffferent combinations - please see the attached
TimeLab plot:
1. 10 MHz from a Trimble TB to start and stop, reference
internal (blue line)
2. 10 MHz from the counters REF OUT to start and stop,
reference internal (pink line)
3. 10 MHz from Trimble TB to REF IN, start and stop,
reference external (green line)
All clocks should be abt. one magnitude better than the
noise floor which I would expect from a TIC with 25 ps
resolution, so I wouldn´t expect any difference between
those 3 curves, but only the measurement in which the
internal OCXO was not used shows the expected performance
with abt. 2E-12 at 1 second. The measurement which used only
the internal clock was abt. one magnitude worse, and the
adev curve shows some ripple in the 8 to 40 seconds range.
I had a look at the ref out with a spectrum analyzer, but
the smallest resolution bandwith I have available at home is
6 Hz - no spurs noticeable in that region.
Is it reasonable to argue that the oven is out of spec? Did
anyone on the list encounter a similiar effect with a SR-620
before? Or am I simply overlooking something?
Thanks a lot for any comments and have a nice weekend,
Matthias
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