[time-nuts] ADEV from phase or frequency measurement

[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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