RE: [SI-LIST] : Jitter measurement

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From: sweir (weirsp@atdial.net)
Date: Tue Mar 20 2001 - 20:06:01 PST


The Amherst software is good stuff, and quite reasonably priced at a few
$K. For this type of work it is very valuable. It will let you see
ugly things like your power supply, or nearby loads modulating your
clock.

Regards,

Steve.

If you really want to measure jitter that accurately, one combination of

instruments and software that I know work are:
TEK 694C Scope (best time base they make)
Amherst M-1 Jitter Software (I think sold or recommended by TEK)
These were developed for Rambus technology.
The TEK scope has a better time base than what you are using.
The Amherst software reduces the aperture uncertainty to a few femtosec
by
using a huge number of samples on each rising edge (via the IEEE 488 bus

output) and (I think) curve-fitting to decide when the waveform passes
through the trigger point.
I used this when our TIA (Time Interval Analyzer) gave up because of its

overall 50 ps sampling uncertainty.
The bad news: These items will run you about $50k. However, this combo
is
used by certain big time SONET gear companies working on OC-192 and
higher...
Larry Miller

-----Original Message-----
From: Sunil Kumar [ <mailto:sunilb@cdotb.ernet.in>
mailto:sunilb@cdotb.ernet.in]
Sent: Tuesday, March 20, 2001 5:53 AM
To: si-list
Subject: Re: [SI-LIST] : Jitter measurement

Hello Paul and Mike..
Thanks a lot for your valuable inputs.
Let me add some more information. I am measuring jitter of 62.5 MHz
crystal oscillator (LVPECL). This clock is passing through a clock tree
and finally is given to a Gigabit Transceiver (SERDES). My aim is to
find
the jitter at the clock input of the transceiver, and I am looking
for few picoseconds sigma jitter. The real time oscilloscope
(Lecroy) which I am using, is having bandwidth of 1GHz and sampling rate

of 8Gs/s. I have found out that in addition to single shot (real time),
it
supports Random Interleaved Sampling (RIS). It performs RIS at
25Gs/s. Hence sampling step is 125ps in single shot and it is 40ps
in RIS. Now the jitter measurement results for oscillator output are:
sigma peak-to-peak
RIS 7ps 65ps
Single shot 11ps 100ps

So, I am getting more jitter in single shot in comparision of RIS. Does
higher sampling rate of RIS give better results, or something is wrong?
Thanks..
Sunil Kumar
On Mon, 19 Mar 2001, Paul Levin wrote:
> Mike,
>
> I grant you that amplitude variation in the osciallator's output could

> mask any jitter effects, but in single-frequency oscillators operating

> from clean supplies supplies, I haven't encountered AM (amplitude
> modulation) that was anything the like amount of PM (phase modu-
> lation.) If this is truly a concern, and it may be for some *crystal*
> oscillators, one could always use a limiter stage or two between the
> oscillator and the spectrum analyzer in order to clobber the AM.
>
> Paul
>
> Mike Jenkins wrote:
>
> > Paul, Sunil,
> >
> > Using a spectrum analyzer to measure jitter assumes that there is
> > negligible variation in amplitude of the signal. I think this can
> > be a very tough assumption to make valid.
> >
> > Regards,
> > Mike
> >
> > Paul Levin wrote:
> > >
> > > Dear Sunil,
> > >
> > > Better than either of these would be to get your hands on a good,
> > > old-fashioned analog spectrum analyzer. Center your display on
> > > the nominal frequency and then decrease your resolution bandwidth
> > > (and everything that goes along with it) so that you can see the
> > > detail in the first few KHz on either side of the center
frequency.
> > >
> > > You'll have to study up on jitter mathematics and dBc offsets, but

> > > once you've got that straight, you can integrate the flanks of
your
> > > oscillator between any f1 and f2 and determine the likely jitter
> > > arising from noise in that frequency range.
> > >
> > > Paul Levin
> > > Senior Principal Engineer
> > > Logic Innovations
> > > __________________
> > >
> > > Sunil Kumar wrote:
> > >
> > > > 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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> >
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > Mike Jenkins Phone: 408.433.7901 _____
> > LSI Logic Corp, ms/G715 Fax: 408.433.7495 LSI|LOGIC| (R)
> > 1525 McCarthy Blvd. <mailto:Jenkins@LSIL.com>
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