[SI-LIST] Re: Cross talk and Trace length
- From: Doug Brooks <dbrooks9@xxxxxxxxxxxxxxx>
- To: "Fred Balistreri" <fred@xxxxxxxxxxxxx>, "'rajesh kumar'" <msrajesheee@xxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Wed, 17 Aug 2011 13:19:30 -0700
The terms "far end crosstalk" and "near end crosstalk" can be confusing.
Near end crosstalk is almost always backward crosstalk (unless you
want to add the complication of reflections from the far end!)
Far end crosstalk can include both forward crosstalk (a fairly sharp
rising pulse, if it exists) and backward crosstalk, after it has
reflected from the near end and propagated to the far end. To
complicate matters, backward crosstalk reflected to the far end
arrives at the far end after the forward crosstalk does (sometimes
significantly so) (and looks quite different.) How much backward
crosstalk reflects to the far end depends on how the transmission
lines are terminated at the near end.
In my seminars I show simulations of severe crosstalk coupling in a
stripline environment, with the victim trace properly terminated at
the near end, resulting in NO crosstalk at the far end. Again, easily
simulated in Hyperlynx, and shown in the referenced article, below.
In a real-world circuit, it can be very difficult to separate and
identify these effects unless you are really skilled at what you are
looking at.
Doug
At 10:54 AM 8/17/2011, Fred Balistreri wrote:
>This is true in theory for matched impedance lines. In practice forward
>crosstalk can get larger if the lines are not matched due to reflections
>which add to the problem.
>I've seen the terminology where it's called far end xtk. Far end xtk can get
>large enough to cause issues especially for microstrips. Striplines less
>so.
>
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
>Behalf Of Doug Brooks
>Sent: Wednesday, August 17, 2011 10:25 AM
>To: rajesh kumar; si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Re: Cross talk and Trace length
>
>
>Actually, it depends on whether you are talking about forward or
>backward crosstalk.
>Backward crosstalk: The backward crosstalk pulse WIDTH increases with
>coupled length. But the backward crosstalk AMPLITUDE increases with
>coupled length UP TO THE CRITICAL LENGTH, at which point the
>amplitude stops increasing and peaks out. The peak amplitude is a
>function of coupling, which (among other things) is related to the
>rise time and to the spacing. The critical length is similar to the
>one we use in transmission line analysis --- when the propagation
>time along the coupled length is one-half the rise time.
>
>Forward crosstalk: The pulse WIDTH of the forward crosstalk signal is
>related to rise time. The AMPLITUDE of the forward crosstalk signal
>is related to the coupling, which is related to the rise time, the
>separation, and the coupled length. For practical purposes, for PCB
>design, traces in a stripline environment exhibit no forward
>crosstalk. Microstrip traces can exhibit forward crosstalk, but the
>coupled regions have to be relatively long. In general, forward
>crosstalk is not often a problem on PCBs. It might be, however, on
>multiconductor cabling.
>
>These things can be easily simulated with Hyperlynx. I have done show
>and shown the results of the simulations in an article
><http://www.ultracad.com/mentor/mentor%20crosstalk%20part%202.pdf>Crosstalk
>Part 2; Simulating Crosstalk Effects available on our web site at
>http://www.ultracad.com
>
>Doug Brooks
>UltraCAD Design, Inc
>
>
>At 12:19 AM 8/17/2011, rajesh kumar wrote:
> >Hello SI Gurus,
> > Please share your thoughts on the relationship between
> >cross talk and trace length. In general i consider that the spacing
> >between trace is the main factor that influences cross talk. Still now
> >i assume that 4X spacing between traces (X is a trace width) is a good
> >way to avoid cross talk irrespective of the length of trace. Please
> >share your views on this.
> >
> >Thanks,
> >Rajesh
> >
> >
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