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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