FreeLists / si-list / [SI-LIST] Differential bus emission

[Dorin <dorin.oprea@xxxxxxxxxxx>]

Hi,

The circuit topology is module-card-backplane-cardIO with 40" trace
length. The signal frequency is 400MHz. The transmitter Tx1 from the
module is driving the receiver Rx2 from the cardIO and Tx2 from cardIO
is driving Rx1 from the module in the same time. The routing clearance
between the two differential links is 0.015" (edge coupled traces routed
on the same layer). Tx1 and Rx1 are on device 1, Tx2 and Rx2 are on
device 2; the two devices are different technologies.
I have calculated the common mode voltage, Vcm=(Vp+Vn)/2, at Tx1 and Rx1
on the module (same thing on cardIO).
The simulation shows +-50mV noise between the two links (Vcmtx1+Vcmrx1,
Vcmtx1-Vcmrx1).
Question: what are the options to minimize emission caused by this
noise?
I see two possibilities:
1) mangle the links so that loop area is minimum (diff1 - diff2 - diff1
- diff2 - ....), but some add xtalk
2) isolate them by dedicating a signal layer for each direction (diff1
on one layer and diff2 on a different layer)
I always appreciate your opinion.

Thanks,
Dorin
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