[time-nuts] ADEV from phase or frequency measurement

Volker Esper ailer2 at t-online.de
Mon Mar 3 19:05:59 EST 2014

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.

Thank you

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