[SI-LIST] Re: current return-path for differential pair on PCB

Istvan,
given a PCB in which the differential traces are routed near reference
planes the 'differential traces' are not tightly coupled. Most of the
coupling is to the reference plane(s). For example, a 100 ohm differential
trace is close to actually being two non-coupled 50 ohm stripline traces
driven out-of-phase. So, the 'return' path thru the reference plane does
need to be unbroken.

    Dave Cuthbert


On 11/29/08, Istvan Nagy <buenos@xxxxxxxxxxx> wrote:
>
> hi
> for a differential pair, like PCIe or SATA, some people (some of my
> colleagues) say, its not needed to have a continous good current return path
> since the differential voltage is referenced only to the other signal-trace
> in the pair.
>
> I think the current return path has to be continous even for diffpairs,
> because:
> 1.) Common mode noise:
> if the return path is not continous, then it creates loops for the common
> mode noise, so the common mode noise will be bigger. If the CM noise is
> bigger than it introduces jitter into the differential signal. which is bad.
> 2.) Impedance:
> if the return path is poor, the the return current has to flow around to
> get a path between the source and destination, so on some areas the return
> current doesnt flow unterneath the signal traces so basically their
> single-ended (odd mode) impedances (so the diff impedance too) will be
> different than its calculated in the imp.calculator (polar, or MMTL), which
> assumes a perfect ground plane.
>
> am i right, or wrong? why?
>
> Istvan Nagy
> Concurrent Technologies plc, UK.
>
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