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Re: [BP] Wed August 4th Agenda: 802.3ap Channel Model Adhoc Conference Call - Phase Delay Distortion



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-----
From: Mellitz, Richard [mailto:richard.mellitz@intel.com]
Sent: Wednesday, August 04, 2004 5:48 PM
To: Brian Brunn; STDS-802-3-BLADE@listserv.ieee.org
Subject: RE: [BP] Wed August 4th Agenda: 802.3ap Channel Model Adhoc Conference Call

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
Sent: Wednesday, August 04, 2004 3:03 PM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: Re: [BP] Wed August 4th Agenda: 802.3ap Channel Model Adhoc Conference Call

 

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

 

 

PhaseDelayDistortion_040805.pdf