[time-nuts] ADEV from phase or frequency measurement

[Magnus Danielson]

On 09/03/14 22:36, Volker Esper wrote:
> 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:
>>>> 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
>> source?
>>
>> Cheers,
>> Magnus
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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.

Hmm... what is the noise when you measure it on the SR620 itself?

It seems a little high here.

Cheers,
Magnus