**From:** *MikonCons@aol.com*

**Date:** Thu Aug 10 2000 - 12:32:32 PDT

**Next message:**Perry Qu: "[SI-LIST] : Tools for high-speed interconnect design"**Previous message:**Lynne Green: "RE: [SI-LIST] : Bandwidth/Frequency Response simulations"**Maybe in reply to:**Peterson, James F (FL51): "[SI-LIST] : RE : LVDS Skew"**Next in thread:**Scott McMorrow: "Re: [SI-LIST] : RE : LVDS Skew"**Reply:**Scott McMorrow: "Re: [SI-LIST] : RE : LVDS Skew"

As far as I know, a differential receiver is still a differential receiver.

Even if the individual trace voltages of a differential pair do NOT

transition during the same time frame (as aptly illustrated by Scott's paper

demonstration), a data rate (double-edge clocked or not) whose time period is

substantially longer than the signal rise time will function properly (from

the logic standpoint), but can create potential EMI problems.

To clarify using Scott's example, applying a peak-to-peak amplitude of 100%

(or use mV if you like) to EACH signal (centered about some DC level of

course), you will find the DIFFERENTIAL signal to also be 100% (or mV)

peak-to-peak. For perfect switching synchronism (i.e., no skew with

crossover at the centers of both signal transitions), the transition time of

the differential signal is EQUAL to the rise time of the individual signals

(400 ps for Scott's example). If one signal is skewed by the magnitude of

the rise time (400 ps), and the transition half-cycle (or data rate

half-cycle) is sufficiently long (~800 ps or greater), the differential

signal still achieves full 100% peak-to-peak amplitude, but exhibits a

degradation/doubling in rise time from 400 ps to 800 ps. Arbitrarily

assuming an 800 ps half-cycle, the width of the window would be reduced from

800 ps (ideal case) to 400 ps (no-crossover case) or 50% and the dwell time

at full amplitude would be reduced from 400 ps to 200 ps (in each

half-cycle), again a 50% reduction (only a coincidence). Note, however, that

if the data rate half-cycle is longer than 800 ps, the impact is reduced in

time on a 1:1 basis; i.e., stretching 800 ps to 1200 ps changes the

differential signal full-amplitude dwell time by (1200-800+200) ps = 600 ps,

which is three times the previous value. The rise time is of course still

800 ps for the cited no-crossover case.

As I see it, there is no direct eye pattern correlation in system

functionality (or malfunction) except on differential clock lines or in chips

operating at (or above) the clock frequency. Perhaps someone can cite

exceptions to this (perhaps ignorant) view.

Mike

Michael L. Conn

Owner/Principal Consultant

Mikon Consulting

(408)727-5697

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**Next message:**Perry Qu: "[SI-LIST] : Tools for high-speed interconnect design"**Previous message:**Lynne Green: "RE: [SI-LIST] : Bandwidth/Frequency Response simulations"**Maybe in reply to:**Peterson, James F (FL51): "[SI-LIST] : RE : LVDS Skew"**Next in thread:**Scott McMorrow: "Re: [SI-LIST] : RE : LVDS Skew"**Reply:**Scott McMorrow: "Re: [SI-LIST] : RE : LVDS Skew"

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