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 -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> 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 -- [email protected] > To unsubscribe, go to > 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. Volker _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
