[time-nuts] How to measure Allan Deviation?
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Oct 23 23:20:56 EDT 2006
Didier Juges wrote:
> Dr Bruce Griffiths wrote:
>> Didier Juges wrote:
>>> I do not understand the signal on the rear trigger outputs. At the
>>> moment, I have a single 10 MHz sine signal fed to the START channel, and
>>> the 5370 is set to TI, MEAN, SAMPLE SIZE 1, + TI ONLY, START channel
>>> triggers on rise and STOP channel triggers on fall, and START COM is
>>> selected. The instrument displays about 60 nS (fairly stable, 150 ps
>>> jitter) or so at the moment. The rear START trigger shows a negative
>>> going pulse (400uS wide, at 62 mS rep rate), with the rising edge
>>> (positive going) synchronous with the 10 MHz signal and the phase is
>>> adjustable using the *STOP* trigger level! When I trigger the scope on
>>> the falling edge of the START trigger output, the 10 MHz signal seems to
>>> drift, and the trigger setting has no effect, except that if adjusted
>>> too far, the instrument stops updating the display.
>>> Apparently, the START trigger adjustment has no effect on the timing of
>>> the rear START trigger output, but will cause the display to freeze if
>>> the START trigger is set to either extreme, even though the START
>>> trigger output on the rear does not change.
>>> The STOP trigger output works the opposite: it responds to the START
>>> trigger level. Could it be that the outputs are reversed on my instrument?
>>> I am not sure this is a complete description, and maybe there is an
>>> obvious answer that I am missing, but I am perplexed.
>> The 5370A processor can swap the front panel START and STOP inputs over
>> via an ECL multiplexer on bord A22.
>> The rear panel outputs reflect the actual selected START and STOP events
>> not necessarily the signals connected to the front panel START and STOP
> Good catch, but I thought that only came to play when +/-TI was
> selected, and I have selected +TI ONLY?
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That's not the way I interpret the manual or the article on the 5370 in
the HP journal.
+TI ignores all STOP inputs until after the START event.
+-TI START before STOP is a positive time interval STOP before START is
a negative time interval. Internal logic implements a precedence detctor
so that the relative order of accepted start and stop events can be
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