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

Magnus Danielson magnus at rubidium.dyndns.org
Mon Apr 9 17:00:49 EDT 2018


Hi David,

On 04/09/2018 03:29 AM, David Burnett wrote:
> 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.

Really good! Welcome!

> 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.

OK, first off, one hides the dead time of the counter by using the
time-interval mode and use a reference signal such as a PPS or other
suitable rate as start/Channel 1 signal where as the measured signals
goes into stop/Channel 2. That works up to the rate of the counter, and
you don't want to miss samples.

You should also consider what the time resolution you would need. A
HP5372A for instance can do 8000 samples as every 100 ns with 200 ps
resolution for instance. There is more modern counters that do much better.

> 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.

Well, start with what you have, learn what limits it and then you can
motivate better toys/tools.

That you mention dead-time is refreshing, so you get an extra point
right there, besides asking how to get started.

Now, how much drift do you expect that your oscillators would do? Do you
have a rough idea?

i have done lot's of measurements using a 53131A and hold-over, but
usually the first 10-100 s is relatively easy, but depending on the
details of the lock mechanism it can be more or less "interesting".

Cheers,
Magnus

> 
> 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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