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I apologize for sending this before completing it.
I intended to press "save", but apparently hit "send" instead.
See additional notes within. Hopefully, this can be
of some use during tomorrow's con-call. On the other hand, maybe you folks
completed everything after I left last week!
Tom
----- Original Message -----
Sent: Sunday, February 17, 2002 9:47
PM
Subject: [802.3ae_Serial] More about
stressed eye calibration
All
-
Since returning
from Santa Rosa, I have had a few more thoughts on calibrating a stressed eye
for Rx testing.
1. Achieving
vertical closure (ISI) with only amplitude interference will add more pulse
shrinkage than we probably want. To counter this, I referenced a 5 GHz filter
in my presentation Wednesday. I would rather not use 5 GHz filter, but it
looks like something is required to emulate closure from channel dispersion.
7.5 GHz provides less than might be expected in a link, but may be acceptable.
It is certainly more readily available.
2. We can calibrate the 2 sine terms (amplitude interference and
phase jitter) while transmitting square wave patterns. If on a scope, the
interferer amplitude can be observed on the top or base line, or its jitter
can be observed at the waist of the crossing. Obviously the phase modulation
term can only be observed as jitter at the waist of the crossing.
The peak to peak values of these sine terms can be determined by placing
cursors on the two modes or peaks of their histograms.
Either sine term can also be calibrated using RF spectral analysis while
transmitting square wave patterns. I have the appropriate equations somewhere,
and this method is usually quite accurate at least for phase modulation. I
believe spectral analysis is what Agilent currently uses for calibrating the
phase modulation term.
For simplicity, jitter from the sine terms are both sufficiently high
probability that their peak to peak values can be added to predict their
combined peak to peak total. Convolution analysis is not
necessary.
The two sine terms must not be correlated (harmonically
related).
3. As a concern, I am becoming convinced that calibrating final eye
height to sufficient accuracy with short patterns may not be possible. I
expect there will always be residual variation in amplitude and phase response
across the very wide frequency range, and long patterns will inevitably show
measurably worse baseline wander and jitter. So what do we do? Measure an rms
value and extrapolate? As shown by Agilent's bathtub curves, the BLW and
jitter distributions are expectedly truncated Gaussians.
Of course, some such low probability jitter is expected in real systems,
and so perhaps we should welcome it. The E/O and other sources will add more.
Given this, what calibration levels for jitter should be target? Steve Joiner
proposed that jitter from the combined sine terms could be set to W+3sigma in
the absence of low probability jitter.
However, since some low probability jitter will inevitably exist, I
propose...
I stopped here before because I don't really have any good
recommendations. Some measurements by Agilent should help, but as Piers
suggests, perhaps we should not concern ourselves with low probability jitter -
we can't control it, can't measure it, is probably dominated by other sources,
etc. (this all presumes it is kept relatively small, however). So maybe Steve's
suggestion is reasonable.
Thanks,
Tom
Stratos
NW
425/672-8035 x105
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