[SI-LIST] Re: Does differential signalling really need complimentary signal ??
- From: "Howard Johnson" <howie03@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 3 Aug 2013 16:15:39 -0700
Dear Rajan,
The quote you provide says, "any AC currents induced... are
counteracted...," but it doesn't say anything about DC currents, which are
not cancelled. Fortunately, we don't have much trouble in high-speed digital
systems with DC effects, so the lack of complementary behavior at DC causes
no difficulties.
At home on the range,
Howie Johnson
P.O. Box 1309, Twisp, WA 98856
509.997.0750
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Rajan Hansa
Sent: Thursday, August 01, 2013 5:33 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Does differential signalling really need complimentary
signal ??
Hello Experts,
I have confusion over differential signalling so please help to clear my
clouded mind.
I was reading a book by Howard Johnson where "differential signaling is
defined as transmission of *two complementary signals* over two-wire
signaling and if both wires/traces are identical & matched *so that both
wires/traces have equal coupling to the reference system*, any AC currents
induced in the reference system by one wire are counteracted by equal and
opposite signals induced by the complementary wire"
That means complimentary signal in differential signalling helps to mitigate
the effect of parasitic coupling between differential trace and reference
system and I think that's the biggest motivation of using differential
signal at high frequency.
Complementary signals if I understood correctly is sending opposite signal *
simultaneously *to both traces that means +V to one trace and -V to another
trace but in many designs I don't see complementary signals are used for
differential signalling like in USB 2.0 I see + 200 mv (dp) and 0 V (dn) for
active high and 0 V (dp) and + 200 mv (dn) for active low ( If it was
complimentary signal it would have been +100 volt (dp) and -100 volt (dn)
for active high and -100 volt (dp) and +100 volt (dn) for active low )
My question is how can we mitigate the effect of parasitic coupling between
differential trace and reference system when signal is not complimentary or
is there something am I missing ??
Rajan
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