**From:** Mike Jenkins (*[email protected]*)

**Date:** Mon Aug 14 2000 - 12:12:08 PDT

**Next message:**Fred Balistreri: "Re: [SI-LIST] : Tools for high-speed interconnect design"**Previous message:**Charles Bishop: "RE: [SI-LIST] : LVDS Skew"**In reply to:**S. Weir: "RE: [SI-LIST] : LVDS Skew"**Next in thread:**Greim, Michael: "RE: [SI-LIST] : LVDS Skew"

To those discussing the effect of skew between two nominally

complementary signals (LVDS or whatever) on the resulting

differential signal, a small comment: The phrase "prevent

a crossing" has been used (several times, I think) to describe

the effect of skews greater than the risetime. The piecewise

linear model gets a bit "kinky" at that point, but a crossing

still occurs. That is, the differential waveform does still

go from + to - and back again: (monospace font, please)

_________

\

+sig \

\______________

_______

/

-sig /

_______________/

_________

\

\

dif'l \___

sig \

\

\______________

Real signals are, of course, a bit smoother, but skew > Tr does

slow the dV/dt thru the transition (i.e., thru zero), with the

associated increased sensitivity to threshold noise.

FWIW, skew < Tr (by this model) only rounds out the corners of

the data eye without affecting the zero crossings at all. (This

assumes that Tr is less than half of the bit interval. Otherwise,

intersymbol interference will occur -- i.e., the transition is

affected by the previous transition.)

As has already been pointed out, skew on dif'l signals does

induce a common mode component, which is a very serious noise

consideration in some applications.

Sorry to inject my 2 cents. I didn't want anyone thinking that

such skew caused the signal (i.e., zero crossings) to disappear

entirely.

Regards,

Mike

"S. Weir" wrote:

*>
*

*> Michael,
*

*>
*

*> It is easy to get fooled by this, but Vittorio is correct in his suspicion,
*

*> assuming equal rise and fall times, a crossing occurs so long as the skew
*

*> is less than the total rise time. If you offset by 50% of the rise time,
*

*> then the crossing occurs at 25% or 75% amplitude. This is easily shown by
*

*> solving for Tcross:
*

*>
*

*> Given Tr = Tf, and a positive Tdelay for the rising waveform relative to
*

*> the falling waveform:
*

*> Tcross occurs when Vrising = Vfalling
*

*> Tcross cannot occur before 0, nor after the falling waveform reaches 0,
*

*> therefore Tcross is bound between 0 and Tr
*

*>
*

*> Vr = Vlow + (( T - Tdelay )/Tr * ( Vhigh - Vlow ))
*

*> Vf = Vhigh - ( T/Tr * ( Vhigh - Vlow ) )
*

*>
*

*> Solving for T where Vr = Vf:
*

*>
*

*> Vlow + T/Tr * ( Vhigh - Vlow ) - Tdelay/Tr * ( Vhigh - Vlow ) = Vhigh -
*

*> T/Tr * ( Vhigh - Vlow )
*

*>
*

*> Tdelay = 2T - Tr
*

*>
*

*> Since T is bound between 0 and Tr, the maximum value of Tdelay is Tr.
*

*>
*

*> If you use Scott's paper model, you can easily see this. The rising
*

*> waveform must be shifted to the left or the right of the falling waveform
*

*> by the whole Tr interval in order to prevent a crossing.
*

*>
*

*> Regards,
*

*>
*

*> Steve.
*

*> At 01:58 PM 8/11/00 -0400, you wrote:
*

...

-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mike Jenkins Phone: 408.433.7901 _____ LSI Logic Corp, ms/G715 Fax: 408.433.7461 LSI|LOGIC| (R) 1525 McCarthy Blvd. mailto:[email protected] | | Milpitas, CA 95035 http://www.lsilogic.com |_____| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~**** To unsubscribe from si-list or si-list-digest: send e-mail to [email protected] In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****

**Next message:**Fred Balistreri: "Re: [SI-LIST] : Tools for high-speed interconnect design"**Previous message:**Charles Bishop: "RE: [SI-LIST] : LVDS Skew"**In reply to:**S. Weir: "RE: [SI-LIST] : LVDS Skew"**Next in thread:**Greim, Michael: "RE: [SI-LIST] : LVDS Skew"

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