[time-nuts] ADEV dead time w/ HP 53131A & TimeLab

David Burnett db at berkeley.edu
Sun Apr 8 21:29:25 EDT 2018


Hi time-nuts,

I'm doing oscillator-related research for my PhD and found this list
recently. It's been a great resource in trying to refine my freq
measurement setup and in starting to understand what's really going on
inside my lab equipment.

I've been trawling the archives and have a question about measuring ADEV
accurately with the Agilent 53131A frequency counter I have on hand. From
the comments on this list and elsewhere, and the fact that TimeLab will
talk to my 53131A directly, I have the impression one can use the 53131A
for period measurements with which to calculate ADEV. But from GPIB command
sniffing it looks like there's a lot of dead time between measurements:
-- In period or freq mode* measurements take an extra ~130ms longer than
gate time to return (but this seems to produce the correct measurements for
TimeLab);
-- in time interval mode they take about ~20ms;
-- in totalize mode they take about 5ms, in keeping with "200 measurements
per second" advertised in the brochure, but of course this is a simple
counter and one loses the resolution of a reciprocal counter or anything
smarter added in.

Is it just generally assumed everyone is making period measurements on time
scales long enough that any instrument dead time is ignorable? Or is
TimeLab and everyone else silently applying the correction factor as
described by the Barnes & Allan NIST paper (NIST technical note 1318)? Or
is there a configuration mode I'm missing that prints measurements with
more regularly? TimeLab's GPIB commands seem to be limited to "get current
measurement" so I might not have the box set up right to start with.

My research concerns oscillator drift on time scales of ~1ms to ~10s, so
I'm guessing the 53131A with its 5-130ms of dead time isn't suitable for
what I'm trying to measure. But I'd still like to know how folks are
getting around this dead time issue with the 53131A for their measurements
in hopes it'll shed light on how I can do the same without needing to pick
up more gear and the inevitable shipping delay that will entail. I suspect
someone will recommend that I get a time-stamping/continuous measurement
box, which is probably the best solution. But I'm hoping there's a way
around that in the short term so I can make these measurements sooner.

73,
Dave

* Others on this list have warned about using this mode because the machine
does a lot of averaging but it seems like TimeLab needs the box to be in
this mode regardless. I'm still looking for the part in the manual where
HP/Agilent/Keysight owns up to this and describe how it changes the
measurement.



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