[time-nuts] Basic question re adev measurements

[Mark Spencer]

Thanks John, to put this in perspective the data sets for the good readings might have phase wraps every 40,000 seconds or so.  The data sets for the bad adev readings might have phase wraps ever 3,000 seconds or so.  Typically I would compare two 10 Mhz signals and take one reading per second.  I didn't realize the phase difference can't exceed 50 ns vs 100 ns.  I have more to learn here (:

This is helpfull.

Regards
Mark Spencer


------------------------------
On Mon, 26 Mar, 2012 6:00 PM EDT John Miles wrote:

>It's a bit worse than that.  The DUT and reference phase difference can't be
>allowed to exceed 50 ns per trigger interval, in the case where two ~10 MHz
>signals are being measured.  If the frequencies disagree enough to create a
>phase slope greater than that -- meaning if they are more than 5E-8 * 10E6 =
>0.5 Hz apart if you are triggering once per second -- your TI counts will be
>aliased.  
>
>You won't be able to unwrap the phase properly if your software isn't aware
>that the data is coming from a different Nyquist zone, so to speak.  I'd
>expect some odd looking ADEV plots in that case.
>
>-- john
>
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-
>> bounces at febo.com] On Behalf Of Mark Spencer
>> Sent: Monday, March 26, 2012 2:08 PM
>> To: time-nuts at febo.com
>> Subject: [time-nuts] Basic question re adev measurements
>> 
>> Greetings,  I was reviewing some older adev plots of mine and noticed that
>> there may be a correlation between lower adev numbers and lower
>> frequency off set between the reference source and the device under test.
>> It's my understanding that the adev calculations remove constant frequency
>> off sets but I'm wondering in practice this degrades the the measurements.
>> 
>> It occurs to me that if I am comparing two 10 MHz signals with a TIC that
>the
>> available dynamic range of each measurements will be 100 ns.   Would a
>> constant frequency off set effectively reduce the precision of the
>> measurements by eating up some of this dynamic range ?
>> 
>> To put this in perspective frequency offsets of say one or two parts per
>> trillion seem to result in better adev readings than off sets of say ten
>or
>> more parts per trillion.
>> 
>> Sorry if I have missed something obvious here.
>> 
>> Thanks in advance
>> Mark Spencer
>> 
>> Sent from my iPod
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