**From:** Larry Miller (*[email protected]*)

**Date:** Mon Mar 26 2001 - 06:10:57 PST

**Next message:**Hassan Ali: "RE: [SI-LIST] : How to model effect of vias on nearby traces?"**Previous message:**ªL´Â·×: "RE: [SI-LIST] : Any standards of the derating of Capacitors,Resit ors, and inducto rs?"**Maybe in reply to:**Sunil Kumar: "[SI-LIST] : Jitter measurement"**Next in thread:**[email protected]: "Re: [SI-LIST] : Jitter measurement"

Tom,

That is not being a contrarian.

That is precisely how a counter-based Time Interval Analyzer (TIA) works.

Basically, it is comprised of a comparator which, when the threshold is

crossed, stores the count ("the time") of a very precise counter in a large

memory. Zillions of these samples are collected (in accordance with the

Zillions of Samples Parameter Dialog) and then can be analyzed for

statistical properties. The Good Old Central Limit Theorem clicks in, and

the measurement uncertainty of any single zero crossing gets swamped out by

the large number of samples averaged.

This is great for measuring long-term (low-frequency) effects like

frequency, drift, and so on, and is the standard tool for synchronous SONET

systems.

However, when you are looking at cycle-to-cycle effects one of two things

happens:

1) you have to decrease the sample population down to the point where

statistics fail you and the sampling uncertainty of individual measurements

comes back into play.

2) In a TIA you get cycle-to-cycle data by essentially taking the derivative

(OK, finite differences) of the time interval data and this greatly

increases the mathematical "noise" in the calculations. Also, the time bases

in counters do not necessarily have great short-term jitter (that's why the

right hand 4 or 5 display digits are a blur); they are mostly designed for

longer interval (0.1s, 1s, 10s) stability and long term frequency accuracy.

The oscilloscope-plus-software packages described heretofore address these

problems and provide much greater accuracy for cycle-to-cycle measurements,

which was the subject of the question. "If the oscilloscope has a lot of

jitter" as you say disqualifies it for measuring, but the ones noted like

the TEK 694 have most excellent time bases (and price tags to match).

Your counter notion is fine (and is a standard product) for longer time

frame effects. One thing a TIA can find is anomalies in PLL-based

oscillators, for example. Many of them have quite a bit of frequency

deviation and limit-cycling down in the audio frequency range.

Larry Miller

-----Original Message-----

From: Thomas Jackson [mailto:[email protected]]

Sent: Monday, March 19, 2001 10:39 AM

To: 'Sunil Kumar'; si-list

Subject: RE: [SI-LIST] : Jitter measurement

Let me be a contrarian.

I think the best way to measure oscillator jitter is with a good counter,

ideally a time interval measurement system. The accuracy of most counters

is much, much better than the timebase in oscilloscopes, real-time or

sampling. Make multiple measurements of the period and calculate some

statistics: max, min, mean, sigma, etc.

If you do use an oscilloscope, over the long-term, both should provide

equivalent results, assuming they both have the same timebase accuracy and

stability. If I understand sampling oscilloscopes correctly, they make one

(1) sample per trigger and build up a trace from a number of widely spaced

samples. Real-time oscilloscopes take a number of consecutive samples when

triggered. If the oscillator has a lot of jitter, it seems to me that a

real-time measurement will converge on the measurement faster.

Tom

Thomas L. Jackson, P.E.

Staff VLSI Design Engineer

Network Access Development

Systems Solutions Group

FUJITSU MICROELECTRONICS, INC.

3545 North First Street

San Jose, CA 95134-1804

telephone: (408) 922-9574

facsimile: (408) 922-9618

http://www.fujitsumicro.com

-----Original Message-----

From: Sunil Kumar [mailto:[email protected]]

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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**Next message:**Hassan Ali: "RE: [SI-LIST] : How to model effect of vias on nearby traces?"**Previous message:**ªL´Â·×: "RE: [SI-LIST] : Any standards of the derating of Capacitors,Resit ors, and inducto rs?"**Maybe in reply to:**Sunil Kumar: "[SI-LIST] : Jitter measurement"**Next in thread:**[email protected]: "Re: [SI-LIST] : Jitter measurement"

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