[time-nuts] ADEV from phase or frequency measurement

Magnus Danielson magnus at rubidium.dyndns.org
Sun Mar 9 14:46:07 EDT 2014

On 04/03/14 01:05, Volker Esper wrote:
> Am 03.03.2014 23:04, schrieb Magnus Danielson:
>> Volker,
>> 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 closer.
>> 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.
>> Cheers,
>> Magnus
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> I already did these measurements, I stick the plots at this posting.
> - The plot with the long file name shows frequency mode measurement:
> 10MHz external reference put into channel A.
> - "...Plot2" shows a phase measurement, where I fed the reference signal
> via a home made 50 ohms splitter into both input channels. (I prefer
> feeding the channels symmetrically)
> Both plots show linear negative gradients, but the phase plot is
> steeper. The frequency plot transitions into a horizontal at about 1000s.

The 1/sqrt(tau) curve is higher than the background noise of the 
counter. That flicker phase noise is more typical of a buffer or source 
than of the counter.

For shorter taus I would expect the white noise to dominate.

I'm just surprised about the level of flicker phase noise. What is the 


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