[SI-LIST] Re: Does differential signalling really need complimentary signal ??
- From: "Rohit Mishra" <Rohit.Mishra@xxxxxxxx>
- To: "Tom" <tom_cip_11551@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 1 Aug 2013 23:05:04 -0700
Tom,
When I said complementary signal, I meant antipodal signal that's one
signal lie on 1 and other on -1.
Differential signaling doesn't necessarily need -ve voltage to cancel
ac component of common mode current.
Regards,
Rohit
From: Tom [mailto:tom_cip_11551@xxxxxxxxx]
Sent: Friday, August 02, 2013 3:46 AM
To: Rohit Mishra; all.si.list@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Does differential signalling really need
complimentary signal ??
"it is not complementary signal per se that is important but the signal
that goes opposite directions on each transition "
Rohit, are not two signals that go in "opposite directions on each
transition" the very definition of a complementary signal? How is that
different from a complementary signal?
Thanks
Tom
________________________________
From: Rohit Mishra <Rohit.Mishra@xxxxxxxx>
To: all.si.list@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Sent: Thursday, August 1, 2013 8:17 AM
Subject: [SI-LIST] Re: Does differential signalling really need
complimentary signal ??
Rajan,
it is not complementary signal per se that is important but the signal
that goes opposite directions on each transition as Steve pointed out.
Remember, It's AC component of common mode current that creates all
EMI/noise and when signal in two wire line changes direction on each
transition, it effectively cancels this AC common mode current flowing
between line and reference.
Hope that helps !
Rohit
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Rajan Hansa
Sent: Thursday, August 01, 2013 6:03 PM
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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