[time-nuts] periodic jitter , frequency offset in phase lock LeCroy SDA 5000 vs Tektronix AFG 3252

Stefan Brisken stefan.brisken at fhr.fraunhofer.de
Wed Apr 7 07:35:16 EDT 2010


Hi all;

I did a little experiment. It comprised a Tektronix Arbitrary Function Generator 
AFG 3253 and a digital oscilloscope LeCroy SDA 5000. The original intent was to 
see the properties of the phase lock of the oscilloscope's 100 MHz external 
reference input.

I gave a 100 MHz output of the Arbitrary Function Generator onto the digital 
oscilloscope.

a) I recorded data sets up to 10ms at 5GS/s (called "data")
   In post processing I constructed a 100 MHz sinusoid digital
   reference signal (called "ref"). I performed a hilbert transformation
   on both. Then I calculated the phase difference "delta_phi" with respect
   to the time vector:

            delta_phi= angle(hilbert(ref)) - angle(hilbert((data))

   It turned out that delta_phi in all measurements had roughly the same
   slope(envelope) of 36000°/s so I concluded there must be a frequency
   offset of 100 Hz. So far so good. I removed the slope and had a look at
   the residual jitter, which seemed to be Gaussian distributed with a standard
   deviation of 1.8°. But a closer look at delta_phi showed that there still was
   a remarkable deterministic, periodic jitter left. Rather sinusoid than
   sawtooth. With a frequency of 100 MHz.

   A possible explanation for that would be that the sampling time of the ADC
   of the oscilloscope depends slightly on the power of the received signal. If
   this relation is linear, that would explain a sinusoidal phase shift.

   Can anyone confirm or diffirm that theory from his or her experience?

   My aim is to remove the deterministic jitter. But no fit in the time domain
   worked out.

   The periodic jitter was also present in b).


b) Same as above, but I gave CH2 of the AFG on to the external 100 MHz reference
   input oft the oscilloscope. I expected a constant phase difference
   (=same frequency) in average, but it turned out that there is still 1-2 Hz
    mean frequency offset left.

   Is this the normal case? I had expected a non measurable frequency offset due
   to the phase lock. Warm up effects can be ruled out.

To make confusion complete the header file of the oscilloscope data states (for 
both a) and b) ) that the sampling rate was not exactly 5 GS/s but 66 S/s below.
Can I rather trust the header file or the preset 5 GS/s?

Thanks a lot.

-Stefan




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