[time-nuts] SRS SR620 External Source Issue -- Help Request

Magnus Danielson cfmd at bredband.net
Wed Feb 21 18:11:53 EST 2007

From: <TheInfamousFlavio at hotmail.com>
Subject: [time-nuts] SRS SR620 External Source Issue -- Help Request
Date: Wed, 21 Feb 2007 17:57:22 -0500
Message-ID: <BAY128-DAV486B32E20C06D80748D6CA9880 at phx.gbl>

> I have an SR620 counter that I set up with a Z3801A as an external reference.  If I put a bnc T connector at the output of the Z3801A and use two equal length bnc cables, one to the ext. ref input on the back and the other to channel A then do a frequency measurement, I get a mean that is about .0015 Hz below 10,000,000.0000Hz.
> Does any know why this might be happening? I would expect it to read 10,000,000.0000 exactly give or take a couple on the last digit.

The first thing that sprung to my mind was maybe some form of mathematical
error in the software. There might be some other form of error such as
non-linearity in interpolators, but that should not show up as a frequency
error like that. It seems strange.

There is really two very nasty tests you can do:

1) Measure with the time-base time on one or both channels, at various delays.
2) Measure a stable source which is just slightly off the time-base in
   frequency either in one channel or time-base on one channel and other source
   on the other.

This really reveals "interesting" behaviours, such as how one channels signal
affects the timing of the other.


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