Re: [SI-LIST] : USB Slope matching and RFI/EMI

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Tue Oct 26 1999 - 08:26:37 PDT


"Yehuda D. Yizraeli" wrote:
>
> Hi,
>
> Just wondered if someone knows the nature of the requirement for
> matching of the rise and fall slope time in the USB spec.

Shalom Yehuda!

The rise and fall times, as well as the crossover point, need to match
for two reasons:

1) Receiver performance. Mismatch would take the receiver out of its
   best common-mode range and cause increased jitter. Thanks to SE0
   conditions USB can't require large receiver common-mode ranges.
2) EMI. Mismatches between the rise and fall, esp. in unshielded
   cables, result in large common-mode energies that radiate like crazy.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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