[time-nuts] Question about effect of sample interval on ADEV

Mark Spencer mspencer12345 at yahoo.ca
Sun Jul 7 22:40:59 EDT 2013


Hi, to echo the comments John and Magnus made, I've encountered similar issues in the past when using my HP5370B's to compare my FTS 1050 to other standards.  The fix involved using double shielded "RG 400 style" cables in my lab (I realize the RG 400 standard is not really a standard any more..)   I had previously terminated all of the relevant unused inputs and outputs of my time nuts gear with 50 ohm terminators.
 
I've also encountered similar issues with several standalone Datum 1000B's and needed to add ferrite chokes to the power and double shielded signal cables connected to the Datum 1000B to solve the issues.  I suspect the Datums would work better inside of a shielded enclosure with bypassed power leads vs sitting on a bench in my lab.   My recollection is that the internal oscilator inside of the FTS 1050 is typically similar to the Datum 1000B. 
 
My BVA 8600 seems relatively immune to these issues.
 
As a side note I've also found I get better results when using 3 to 6 dB of attenuation between the output of my particular FTS1050 and the inputs of my particular HP5370B's (I seem to recall there are some other threads in the archives about the need to optimize the signal levels to get the best results from the HP5370 series counters.)    Your results may differ.
 
Best regards
Mark S

Message: 5
Date: Sun, 7 Jul 2013 14:38:09 -0700
From: "John Miles" <john at miles.io>
To: "'Tom Van Baak'" <tvb at leapsecond.com>,    "'Discussion of precise
    time and frequency measurement'"    <time-nuts at febo.com>
Subject: Re: [time-nuts] Question about effect of sample interval on
    ADEV
Message-ID: <0b8501ce7b5a$47ea8cf0$d7bfa6d0$@miles.io>
Content-Type: text/plain;    charset="us-ascii"

This won't be a sampling-interval issue.  It sounds like a beat note.   To
diagnose it, you can use the 5370B in frequency mode (with its internal
timebase) to measure the frequency of the 5065B and the FTS 1050B.  Subtract
the two readings, then see if the reciprocal of the frequency difference
corresponds to the location and spacing of the periodic ADEV bumps.  If so,
that's likely to be the explanation, and you can confirm it by tweaking the
FTS 1050B's frequency and seeing if the beatnote moves accordingly.

As far as getting rid of the artifact is concerned, it may help to use
double-shielded cables, although I don't know if the isolation between the
START and STOP inputs on the 5370B is good enough to eliminate the
possibility of beatnotes in a TI measurement with HF signals on both jacks.
If you are feeding the 5/10 MHz inputs to both START and STOP inputs, try
using a 1-pps divider on the START source.  

You could also try using the 5370B in frequency-count mode, with the 5065A
as an external reference and the FTS 1050B at the STOP input.   There will
be a reduction in ADEV fidelity due to the dead time but it will probably be
less objectionable than the beatnote ripple.

-- john, KE5FX
Miles Design LLC

> > My assumption, apparently incorrect, was that the software would take
> > the sampling interval into account so that I would get essentially the
> > same plot. When I edit a plot changing the sample interval, the trace
> > remains essentially unchanged and this seems inconsistent with the
> > results noted above. Can anyone explain in relatively simple terms what
> > I'm missing?
> >
> > Thanks,
> > Bob Darby
>





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