[time-nuts] ADEV from phase or frequency measurement

[Volker Esper]

Bob,

I sometimes indeed do arc welding, as well as forging... But that's
another story.

When waiting for an ADEV measurement I sit still, trying not to move a
millimeter. Seriously, I try to avoid any rf noise, e.g. ham radio, any
airflow, any temperature change. I log supply voltages and check, if
there's anything odd. I avoid any switching supply. I regularely check,
if any switching voltage regulator (TV, monitors, PC,...) disturbs my rf
ether...

As the legends of my plots tell, I do use a passive splitter to get two
similar signals (start and stop) for phase measurement. But I haven't
used it for splitting the 10MHz for frequency measurement (reference and
input).

Lower level is a concern, since it increases phase jitter.

Volker


Am 09.03.2014 23:32, schrieb Bob Camp:
> Hi
>
> Do you routinely do arc welding while waiting for an ADEV plot to finish :)… (I drink beer, but not everybody is into that …)
>
> You might consider trying a passive splitter on one of the outputs of the GPSDO. There aren’t many ways they will mess up a signal other than by dropping it’s level. If level is a concern then indeed they could be an issue.
>
> Bob
>
> On Mar 9, 2014, at 6:17 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>
>> 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
>>
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