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Hi Brian Understand…
unwrapped. How do you think this plays into layer 1-3 via issue on the
backplane? These are highly reflective and prone to resonate with spacing which
will affect phase delay. … Rich Channel Ad hoc
attendee From: owner-stds-802-3-blade@listserv.ieee.org
[mailto:owner-stds-802-3-blade@listserv.ieee.org] On Behalf Of Brian Brunn Hi Rich, It appears you are right onto one of the
issues that highlights why GroupDelay is not the proper quantity of
interest ... namely "you can't add derivatives". Looking at your PhaseDelay plot, it looks
like you used wrapped phase (the result has a sawtooth shape). To
calculate phase delay you need to use unwrapped phase. Attached is a set of Magnitude,
PhaseDelay an PhaseDelayDistortion plots for a series of (mathematically
modeled) lenghts of low-grade FR-4 (6", 13", 18", 24" &
30"). Here PhaseDelay grows linear with distance as you would expect
so as you desire, 'the whole appears to be the sum of the parts'. Also
when normalized for PhaseDelayDistortion (the true quantity of interest), the
distortion grows linearly with distance. These results are just using a
model so need to be verified with measurement. The model also does
not have significant reflections. Significant reflections may do
interesting things to the PhaseDelayDistortion at specific frequencies. The values for PhaseDelayDistortion are in
the 10's to 100 ps. This is significant relative to our data rates
and jitter budgets. The actual mapping to jitter is a complex
function. Thankfully, equalizers can equalize out
PhaseDelayDistortion. The amount of PhaseDelayDistortion encountered in
real channels will effect the complexity of equalizer needed (a future group
discussion). The immediate issue is seeing if switching from GroupDelay
to PhaseDelay will eliminate the disagreements over measurement BW/averaging
and allow us to make faster channel characterizations. Regards, Brian Brunn Channel Adhoc attendee -----Original
Message----- Hi Brian I under that
different group delays at different critical frequencies directly effects
jitter (eye closure and distortion) on a specific channel. It is also a
sensitive measure of one of component of jitter. I tied to make this work
before for compliance channel on another project but discovered you can’t
add derivatives. I can loosely bound a system loss by combination of losses
of the components. However, I found many cases where the group delay of chain
of channels wasn’t even close to a sum of the component channels. So I
agree the Group delay should be deemed questionable at best. Have you done
a similar analysis for Phase Delay? Is the sum of the parts equal to the whole? Just for
kicks I plotted Phase delay (-phi/omega) for two radically different channels.
You can see the mag il response of the two channels in the attachment as well
as the PD. I was trying to get feel how we used group delay. Could you
help me here? … Rich
Mellitz, Intel Corporation Ad hoc
meeting attendee From: owner-stds-802-3-blade@listserv.ieee.org
[mailto:owner-stds-802-3-blade@listserv.ieee.org] On Behalf Of Brian Brunn Hi All, Attached is my recommendation that we
look at changing our quantity of interest from Group Delay to Phase Delay. The criticality of delay distortion is
debatable. However, I want to get this out quickly because switching
to Phase Delay may be the right thing to do *and* help eliminate some of the
repeatability problems people are seeing when taking channel
measurements. Someone on the call mentioned that magnitude repeatability
was fine and that the phase was the problem.. Regards, Brian Brunn |