[time-nuts] ADEV from phase or frequency measurement
ailer2 at t-online.de
Sun Mar 9 17:36:30 EDT 2014
Am 09.03.2014 19:46, schrieb Magnus Danielson:
> On 04/03/14 01:05, Volker Esper wrote:
>> Am 03.03.2014 23:04, schrieb Magnus Danielson:
>>> 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
>>> 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
>>> 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.
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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
> 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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Source in both cases is an HP Z3805 GPSDO. I didn't insert an additional
buffer, however, the Z3805 has two (buffered) outputs. I used one for
counter reference, the other one for counter input.
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