[time-nuts] ADEV from phase or frequency measurement

Magnus Danielson magnus at rubidium.dyndns.org
Mon Mar 3 17:04:11 EST 2014


On 03/03/14 00:50, Volker Esper wrote:
> Sorry for the "time delay"...
> TIC: SR620 with Z3805 as external reference; signal source
> Nortel/Trimble GPSTM (GPSDO) 10MHz output
> Enclosed two plots (SW: "Plotter"):
> - one is sigma(tau) calculated from phase samples (SR620 TIME mode),
> - the other one is sigma(tau) from frequency data (SR620 FREQ mode)
> Whole equipment had a power up time of several days/weeks. Room
> temperature was stable over both measurements (within about 2 degrees C).

The SR620 uses a bit different path through the logic when doing TI and 
FREQ measurements. The frequency measurement has a "feature" that means 
that the time error between start and stop signal needs to be calibrated 
out. This can be done using the calibration routines given in the 
manual. This should not affect the ADEV measure, but as a precaution.

Try doing a pair of noise-floor measurements. That is, feed the 
reference 10 MHz to the A input for the frequency noise measurement.
Then, for the TI noise-floor measurement, put a T on the A input, put it 
in high-Z mode and then use a 1 m cable to put the signal onto the B 
input which is terminating.

You indeed have a higher level. Your initial shape makes me wonder. I 
would really like to get the TimeLab measurement files and eye-ball them 

If you plot the phase or frequency, it may be easier to spot systematic 
wobbles. TDEV would also help, as it provides a general *tau scaling to 
the ADEV plot.


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