[SI-LIST] Re: Pk-Pk jitter
- From: Eddy <eddyvk@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 22 Dec 2007 16:02:40 -0800 (PST)
Currently most jitter measurements take about 10,000
samples to build the distribution. Working out the
odds of 1:10,000 in a perfect Gaussian distribution
then the PkPk value is at 8.1X RMS.
Bi-, tri- or higher -modals make things a bit more
complex. These distributions are a mix of multiple
gaussians and the overall RMS value has lost its
meaning. What is often done with these distributions
is try find a gaussian slope at the edge with a "tail
fit" algorithm and estimate things from there.
In a lot of applications it's not so much about the
energy in the tails but rather about the freak short
pulse that hangs the system or the freak long/short
pulse that makes you lose a data packet.
Jitter is like a 'mythical' phenomenon where
spec-manship is indeed used a lot to try make numbers
look better. An easy trick is to use just "Pk" instead
of "PkPk" to make numbers look only half as big.
When comparing jitter spec's from different vendors,
carefully check what type of jitter is specified and
what units are used.
Eddy
> --- olaney@xxxxxxxx wrote:
> >
> > The gaussian assumption is often not a good one.
> > For instance, take the
> > case of a control signal with crosstalk from a
> > nearby sine wave. If high
> > enough, the jitter will begin to take on the
> bimodal
> > distribution of the
> > sine wave. Statistical tools are quite useful for
> > troubleshooting cases
> > like this. Also note that gaussian tails do not
> go
> > on forever in circuit
> > applications. They can't, because once the signal
> > excursion hits the
> > rails it is clamped by the nonlinearities of the
> > protection networks.
> > Even for infinite rails, a reasonable observation
> > period must be used,
> > lest the observer grow old and retire before the
> > product is shipped. 100
> > seconds is probably equivalent to forever,
> depending
> > on the bandwidth of
> > course. Various standards have specifications for
> > this, whether explicit
> > or implicit (e.g. a number of bits at a given bit
> > rate).
> >
> > Since some data sheets quote only the lower and
> > therefore more impressive
> > looking RMS value, a useful rule of thumb is that,
> > absent contamination
> > such as mentioned above, a working number for
> > 'random' P-P jitter is ~7X
> > RMS. Where both figures are shown on data sheets,
> > the ratio is rarely
> > less than 6 or greater than 7. The energy in the
> > 'lost' tails is
> > meaningless.
> >
> > Orin
> >
> > On Sat, 22 Dec 2007 00:43:11 -0800 Hal Murray
> > <hmurray@xxxxxxxxxxxxxxx>
> > writes:
> > >
> > > > The vendor often provides PK to PK jitter
> > parameter of PLL,
> > > but as
> > > > we know, a reference clock on board will be
> feed
> > to PLL via ASIC
> > > > package and a input IO buffer, so we how to do
> > simulation to get
> > > > reference clock jitter requirement. could you
> > help to give
> > > simulation
> > > > methodology? thanks!
> > >
> > > What does Peak-to-peak mean in this context?
> > >
> > > If the distribution is Gausian (which is the
> usual
> > assumption), then
> > > the
> > > tails go on forever. It's just a matter of how
> > long you wait.
> > >
> > > Is there an implied cutoff at 6 sigma or
> something
> > like that? I
> > > think that
> > > would make Pk-Pk a useful description.
> > >
> > > Is the typical distribution actually Gaussian?
> > All sorts of things
> > > get more
> > > interesting if it isn't quite Gaussian.
> > >
> > >
> > > --
> > > These are my opinions, not necessarily my
> > employer's. I hate spam.
> > >
> > >
> > >
> > >
> >
>
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