[time-nuts] ADEV from phase or frequency measurement

Mon Mar 10 07:41:32 EDT 2014

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
> >>>>>>> _______________________________________________
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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
> >>>>> _______________________________________________
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> >>>>> 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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