[SI-LIST] Re: AW: [SI-LIST]: Differential mode noise source in PCB
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
- Date: Mon, 24 Mar 2014 08:22:38 -0400
Vinod,
Gert is correct, differential mode noise comes from two sources on the PCB:
common mode to differential conversion, and crosstalk to other signals.
common-to-differential conversion happens because of asymmetry in the
transmission path of the two conductors in a differential pair. It happens
because of different instantaneous return paths on each leg of the
differential pair when passing through packages, vias, connectors, or close
to other traces. It also occurs due to the natural difference in trace
skew, because of the anisotropic behavior of laminate weaves.
Crosstalk occurs whenever the differential signal path passes close enough
to other signal paths that coupling occurs. This happens in the same
places that differential mode conversion occurs.
Minimization of the differential noise problem involves maintaining
signal-to-ground symmetry throughout the channel, and minimizing average
signal density in all regions of the PCB, but especially through vias.
Regards,
Scott
On Mon, Mar 24, 2014 at 6:52 AM, Havermann, Gert <Gert.Havermann@xxxxxxxxxxx
> wrote:
> Vinod,
>
> Noise is crosstalk from an unknown source. Usually noise is distributed in
> common mode, that's why the differential signaling was invented as the
> differential mode isn't much affected by common mode signals. But in cases
> of mode conversion this noise will become differential and disturb your
> signal. Then there are Digital sources of noise that mainly happen on the
> silicon itself.
> The "coupling coefficient" of the common mode noise coupling into the
> differential pair increases with distance between the traces of the
> differential pair, where the worst case is that the noise is completely
> coupled into a single trace of the differential pair (as done in PCIe3
> testing).
>
> Differential mode noise is not much affected by return paths because it is
> differential and has a very small effect on EMI because it can't radiate as
> long as it runs on differential traces.
>
> If your PBC design converts a lot of common mode into differential mode
> onto the signal traces, then you do have an EMI Problem, but that's not a
> caused by the differential mode noise itself.
>
> BR
> Gert
>
>
>
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> -----Ursprüngliche Nachricht-----
> Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> Im Auftrag von vinod ah
> Gesendet: Montag, 24. März 2014 11:00
> An: SI-LIST
> Betreff: [SI-LIST] [SI-LIST]: Differential mode noise source in PCB
>
> Hi All,
> I have a basic query on Differential mode noise in High speed signals
> board.
>
> 1) As far as i understand, Differential mode noise is noise which is
> injected on one of the line of differential pair and the noise returns on
> other line of differential pair (like in PCIe3 where we inject differential
> mode interference for Rx circuitry BER testing).
> So whats the problem caused by differential mode noise as the return path
> is very clear for this kind of noise.
>
> 2) What is the source of differential mode noise in boards/pcb's i.e.
> trying to understand what sources on board can generate differential mode
> noise?
>
> 3) Will differential mode noise cause EMI as return path is very short? If
> EMI caused, what needs to be done to reduce the EMI without affecting the
> actual differential signals integrity.
>
> Regards
> Vinod A H
>
>
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