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

Ulrich Bangert df6jb at ulrich-bangert.de
Thu Feb 22 11:24:43 EST 2007

Dear ???,

I encountered pretty much the same effect when I bought my second hand
SR620. As you, I have been extremly surprised by the fact that the
counter did not display exactly 10.0 MHz under the conditions that you

However, the answer is in the software: The built in time base has a
coarse gain adjustment screw (electronic adjustment) and the fine gain
adjustment is done by calibration byte number 4 in the software. Please
note that this calibration byte (in fact it is 12 bits) is NOT
influenced by the autocal procedure so no wonder that autocal brought
you no changes. It seems, that even with an external time base connected
this calibration byte stays active and that you have to set it by hand
to its most correct value as explained under point 5 of "Calibration
Procedure" / "Clock Oscillator Calibration" on page 77 of the handbook.

If you don't have the handbook available: You may download it (but
missing the schematics) from the SRS web site.

Best regards
Ulrich Bangert, DF6JB 

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von 
> TheInfamousFlavio at hotmail.com
> Gesendet: Mittwoch, 21. Februar 2007 23:57
> An: Discussion of precise time and frequency measurement
> Betreff: [time-nuts] SRS SR620 External Source Issue -- Help Request
> 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.
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