[SI-LIST] Re: Differential noise
- From: steve weir <weirsp@xxxxxxxxxx>
- To: wcooke@xxxxxxxxxxxxx,"'rajat.chauhan@xxxxxx'" <rajat.chauhan@xxxxxx>
- Date: Wed, 15 May 2002 08:00:37 -0700
Wayne, but as the formula shows, a little extra separation between any
aggressor and either signal in the pair goes a long way. The bottom line
is that a crosstalk budget must be established up front.
Regards,
At 10:29 AM 5/15/2002 -0400, Wayne Cooke wrote:
>Hi Rajat,
>
>When you are routing a differential signal on a PCB, you will not have great
>coupling between the differential pair. At most, you may get around 10%
>coupling between the differential pair, most of the return currents will be
>flowing on the reference planes.
>
>When you are looking at the crosstalk between two signals when you have a
>solid reference plane for your return path (for microstrip for example), the
>crosstalk is proportional to 1/(1+(D/H)^2). D here is the distance between
>the center of the two traces, and H is the height of the microstrip to the
>reference plane. For more details, see p192 of Howard Johnson's book
>"High-Speed Digital Design, a Handbook of Black Magic". Since crosstalk is
>proportional to the SQUARE of separation distance, an agressor trace routed
>on the same layer will have much more contribution on the nearest trace of
>the differential pair than the further trace. Every mil makes a difference
>here. When you subtract out the crosstalk that is common on both traces,
>you will still be left with a sizeable amount of crosstalk that represents
>the differential component.
>
>Wayne Cooke
>Signal Integrity Engineer, Innovance Networks
>19 Fairmont Ave., Ottawa, Ont. K1Y 1X4
>email: wcooke@xxxxxxxxxxxxx
>
>
>-----Original Message-----
>From: rajat.chauhan@xxxxxx [mailto:rajat.chauhan@xxxxxx]
>Sent: Wednesday, May 15, 2002 7:35 AM
>Cc: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Differential noise
>
>
>
>Hello all
> I am confused about, what are the chances of DIFFERENTIAL
>noise to appear on well coupled differential pair, when the
>differential signals are stable (not switching).
> I assume that DIFFERENTIAL noise on the pair is mainly
>aftermath of transitions and remains for sometime after the
>transitions.Noise appearing on the pair at any other time(when
>differential signals are stable) is generaly COMMON-MODE noise.
> How much am I correct to assume this way.
>I need to know the validity of this assumption in one of my design, so
>please give your views.
>
>Thanking in advance.
>Rajat
>____________________________________________
>
>RAJAT CHAUHAN
>STMicroelectronics
>
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