[time-nuts] Measuring gpsdo vs itself

David davidwhess at gmail.com
Tue Nov 6 02:22:17 UTC 2012


On Tue, 06 Nov 2012 02:14:04 +0100, Magnus Danielson
<magnus at rubidium.dyndns.org> wrote:

>Hi Bob,
>
>On 11/05/2012 01:30 PM, Bob Camp wrote:
>> Hi
>>
>> As a practical example - a SR620 will look much better reading it's own reference than it will looking at almost anything else. That said, it's still a good idea to make sure the counter looks good reading it's own reference. If it doesn't look good, then you need to fix something.
>
>I agree that this is a good self-test strategy, it usually gives a good 
>clue if something is really bad or not, but as I pointed out, it doesn't 
>give a fair idea of the measurement noise floor.
>
>Cheers,
>Magnus

A lot of counters support a self check mode which counts the internal
reference.

I tracked down an interesting problem in a Tektronix DC505 by wiring
the internal channel A input to measure the internal reference.  Noise
from the display multiplexing was getting into the level shifter
located before the last 4 integrated counter stages and causing
spurious counts.  Based on the documentation and how it did not match
the actual circuit, I suspect Tektronix fiddled with the values in
production until it seemed to work but never figured out the real
problem or back annotated the documentation.



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