[time-nuts] ADEV from phase or frequency measurement

Bob Camp lists at rtty.us
Mon Mar 10 11:49:06 UTC 2014


Hi

I’ve played with both the 6 db loss resistive splitters and the transformer based 3 db loss splitters. They both seem to be fine for splitting precision 10 MHz signals. For a while I avoided the transformer based parts, but not for any reason I could prove.

Bob

On Mar 10, 2014, at 7:41 AM, Volker Esper <ailer2 at t-online.de> wrote:

> Even if the -3dB were an issue, I'd have much more faith in a clear, passive
> resistor network than in a complex amplifier with all it's unknown non linear
> characteristics, temperature dependent time delays, noise sources, etc., etc.
> This simple home made network is a useful, manageable gadget.
> Volker
> 
>> Bob Camp <lists at rtty.us> hat am 10. März 2014 um 01:47 geschrieben:
>> 
>> 
>> 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
>>>>>>>>> _______________________________________________
>>>>>>>>> time-nuts mailing list -- time-nuts at febo.com
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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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