RE: [SI-LIST] : Jitter measurement

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From: Larry Miller (ldmiller@rhapsodynetworks.com)
Date: Thu Mar 22 2001 - 20:55:27 PST


My view, not knowing anything about your jitter measurement packages:

1. Use the real-time scope. Hopefully you have a lot of trace storage
memory.
2. Set up to trigger near the beginning of memory and view as far away in
memory as you can.

Why?
If you view near the trigger point the scope's trigger jitter will dominate
or be subtracted from the oscillator jitter because you are viewing at the
same instant or point on the oscillator waveform as you are triggering on.
If you view at some distance (time) away from where you trigger the
oscillator's jitter will accumulate and tend to swamp out the scope jitter,
or at least not be correlated with it. This intrinsically assumes that the
scope time base is linear. Also, in effect you are averaging the scope's
trigger uncertainty over a number of oscillator cycles-- however as many as
are in your memory, since you only used one trigger to get a number of
oscillator cycles.

This method is recommended in the FDDI and Ethernet standards, by the way.

The sampling scope assumes that you have a repetitive waveform, which you
don't. You would, except for jitter. Therefore, if the sampling scope is
triggered by your oscillator signal you again cancel out the oscillator
jitter (since you are using the oscillator as the trigger reference) and you
are really mostly measuring the scope sampling jitter. Most high frequency
(sampling, multi-GHz) scopes require that you use an external trigger source
(not always easy to come by).

The newer jitter packages eliminate the scope triggering uncertainty by
mathematically averaging many cycles (similar to 2 above) and using curve
fitting to decide when the "real" trigger point occurred. That leaves just
the scope time base nonlinearity and the oscillator jitter in your
measurement. Good scopes have good time base linearity over the short time
period, so that can be a comparatively small contributor to what you see.

Larry Miller

-----Original Message-----
From: Sunil Kumar [mailto:sunilb@cdotb.ernet.in]
Sent: Monday, March 19, 2001 7:05 AM
To: si-list
Subject: [SI-LIST] : Jitter measurement

Hello everybody..

I want to measure cycle-to-cycle jitter generated by a crystal
oscillator. I have two options:
        
        1) Real time oscilloscope
        2) Sampling oscilloscope

The bandwidth of the real time oscilloscope is enough for my
measurements. Both the oscilloscopes are equipped with jitter
measurement packages. Can anybody suggest which method is better?
And why?

Thanks a lot..

Sunil Kumar

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