[SI-LIST] Re: Jitter Analysis
- From: Bob Patel <whizplayer@xxxxxxxxx>
- To: khalida@xxxxxxxxxxx, si_list <si-list@xxxxxxxxxxxxx>
- Date: Mon, 11 Feb 2002 17:07:27 -0800 (PST)
Hi! Khalid, As you mention in your email, the EYE is
completely closed at the receiver and when you try to
measure Jitter the instrument complains because he
cannot find the points where the bits are crossing
i.e. 0 to1 and 1 to 0. And as you reduce the length
the EYE is opening up and now it is able to measure
jitter. I know you are trying to fins out how much
jitter is present at the receiver and how well your
receiver is working to remove this jitter but to
measure this you have to use another approach i.e.
test with random data patterns from the transmitter to
back plane to an instrument for measuring jitter and
now increase ISI by using different data patterns upto
a point where the instrument cannot measure nd this
will give an idea as to how well the receiver is
working.
Also, what kind of BER are you getting with these
trace length and do you notice that as the length
reduces and if you keep the post equalisation constant
your BER increases i.e. over compensation is taking
place for short traces.
Bob
--- Khalid Ansari <khalida@xxxxxxxxxxx> wrote:
>
> Hi all,
>
> I have a driver and receiver talking in an
> asynchronous
> fashion at 2.64 Gbps. The driver is sending out
> random
> data over 64 inches of PCB trace (5 mil of around
> 10" LC
> and 8 mil rest on Backplane). There are a lot of
> SMAs
> on the path too for connecting traces to each other.
> Using
> TDR I determined each to be around 1.5 pF of
> discontinuity.
> My receiver is able to pick this up even though the
> eye looks
> completely closed due to excessive ISI (thanks to
> the
> equilization circuitry). I am trying to look at the
> content of
> the jitter on a WaveCrest jitter analyzer. The
> WaveCrest
> keeps on complaining, "Measured DCD+DDJ data does
> not
> appear to match user specified pattern.
> Sample:2.57e-289
> Limit: 5.00e-001". When I reduce the trace length
> by about
> 20 inches then a distinguished eye can be seen due
> to lesser
> losses. This eye I am being able to analyze using
> the
> WaveCrest.
>
> In past experiences has anyone seen similar
> behavior.
> What are the limitations of the Wavecrest? I can
> measure
> the random jitter by cutting down the trace and
> using the
> Wavecrest but for DJ what should I do? The eye is
> so closed
> that I can't even get a meaningful result using the
> histogram
> on the Tek 11801 B scope.
>
> In short, what is the best way for me to find out
> the amount
> of deterministic jitter on my data. Any ideas will
> be appreciated.
>
> Thanks in advance,
> Khalid
>
>
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