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

Bob kb8tq kb8tq at n1k.org
Sun Apr 8 23:00:38 EDT 2018


The basic fact is that oscillators drift on *all* time scales. How much they drift 
depends on the type of oscillator. A free running VCO based on a PCB resonator
will drift differently than a bare crystal oscillator. An OCXO will have different drift
characteristics than a bare oscillator. 

A 53131 by its self is not capable of measuring the ADEV of a crystal oscillator over
short time periods. The counter’s ability scales by the gate time so eventually if the
gate time is long enough, it will catch up. That might be beyond 1,000 seconds for a
good OCXO. By that point, your dead time concerns are not as significant. 

There are a number of methods out there to get higher resolution than a bare counter
will provide. The NIST Time and Frequency archives have quite a bit buried in them. 
That includes information that digs into your dead time concerns. 

One alternative to the 53131 is a TimePod from Symmetricom. A somewhat less 
expensive approach is a DMTD setup. Both require a “better than” device to compare
your oscillator under test to. Unfortunately that’s a drawback to pretty much all ways
to do this. If your reference is more noisy than your DUT, you will just see the reference. 


> On Apr 8, 2018, at 9:29 PM, David Burnett <db at berkeley.edu> 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.
> 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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