[time-nuts] ADEV from phase or frequency measurement

Bob Camp lists at rtty.us
Sun Mar 9 20:47:35 EDT 2014


Hi

So far, I have not found the 3db lost in a a passive splitter to be a problem with anything I have played with. With proper termination , they seem to be a good thing to use.

Bob

On Mar 9, 2014, at 7:55 PM, Volker Esper <ailer2 at t-online.de> wrote:

> 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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>>>>>>> and follow the instructions there.
>>>>>> 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
>>>>> _______________________________________________
>>>>> time-nuts mailing list -- time-nuts at febo.com
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>>>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>>> and follow the instructions there.
>>>> 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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