[time-nuts] 5370B practical question

Jeff Mock jeff at mock.com
Sat Feb 2 14:03:39 EST 2008


Usually this sort of thing is the caused by settings on the input side 
of the counter, termination, trigger level, etc.  Since the anomaly is 
so consistent and assuming that you've checked all of the usual 
suspects, I would look at the VCOs on a spectrum analyzer and see if 
there are any unusual frequency components confusing the counter.  Even 
something like 50/60 Hz hum could screw up the counts.

jeff


Christophe Huygens wrote:
> Hi,
> 
> I am trying to start some Allen Deviation measurements on some VCXOs
> I have built those for 117, 106.5 and 123MHz (amateur radio use).  The 
> phase noise
> it not my primary concern (for now) but short term stability is (systems 
> are locked
> to GPS etc).  I would like to more formally characterize the performance 
> hence
> my interest in AD.
> 
> I am using the 5370B. For the moment, to get my feet wet, I am just 
> measuring
> frequency - I don t want to deal with TI yet. I think I understand the 
> limitations of
> both the method used and the counter.
> 
> Upon measuring the 106.5MHz results are more or less as expected: I see 
> excursions
> around center of +/- 20milliHz... (1second measurement time) I think 
> these are due
> to hitting the frequency performance limit of the instrument.
> 
> But at 123.5MHz, about 1 in 5 measurments, there is a 1Hz jump... the 
> instrument
> showing 122.999998005 in stead of 999005. I never notice 100, 500 mHz 
> jumps,
> only +/-20 and (+/-20 - 1000).
> 
> I don t know much about the internals... is this the counter hitting the 
> 100MHz limit,
> is something broken, or not likely is my VCO jumping by 1Hz?
> 
> Thanks for any insight,
> Christophe
> 
> 
> 
> 
> 
> 
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