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

[TheInfamousFlavio at hotmail.com]

Here's a thought.  Perhaps there is a problem with the rear reference input; 
A and B channels are fine since they read the same error.  The question is 
"Is there a way of adjusting the Reference Input so that it correctly read 
the 10Mhz input?"

I tried the same setup with my other counter and it read 10,000,000.0000 or 
thereabouts correctly. So rule out the GPSDO and the cables.



----- Original Message ----- 
From: "Dr Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Wednesday, February 21, 2007 19:47
Subject: Re: [time-nuts] SRS SR620 External Source Issue -- Help Request


> Hej Magnus
>
> The Frequency measurement logic (before the interpolators) appears to
> use a pair of 2 bit synchonisers, one for the rising edge and one for
> the falling edge of the internally generated frequency gate. The
> synchronisers synchronise the respective transitions of the frequency
> gate signal to the external frequency being measured.
> One synchroniser uses U508A + U509A the other uses U508B + U509B, so
> that any mismatch in propagation delays between U509A And U509B will
> result in a small error in the time interval. Even for ECL flipflops a
> propagation delay difference between U50(A and U509B of 150ps wouldn't
> be unusual. As far as I can tell the calibration procedures do not
> address this issue.  The uncalibrated differential delay through the
> interpolation circuit source selection multiplexer will also contribute
> an offset.
> Perhaps a better assignment of propagation delay differences may be
> U509A and U509B 50ps, multiplexer and wiring delay mismatches 100ps.
>
> Bruce
>
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