[SI-LIST] differential signaling (common-mode)
- From: Bi Han <mike_bihan@xxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 30 Jul 2005 11:32:01 +0800 (CST)
Hi, SI-listers:
The design task is to design on-chip differential transmission line to send
signal. According to a lot of papers, edge coupled stripline is used.
The cross-section is as below graph(might be lost). One key thing is that there
is no underneath common-mode return path.
___________________________________________________
______ ______
dielectric |__+__| |__-___|
___________________________________________________
Hi-Z substrate
___________________________________________________
Then what is going to happen to common-mode signal? Common mode should not be a
propagating mode, however, its S21 parameter after several reflections should
be as below.
S21=A*Hx*T/(1-R2*R1*Hx*Hx)
R1 is source reflection coefficeint;
R2 is load reflection coefficeint;
Hx is propagation function, Exp(-gamma*L);
Common mode impedance will be quite high if there is no good return path.
Therefore, both source and load are not well terminated. Source could be very
like short, and load could be very like open.
Then R1=-1, R2=1; the S parameter could go infinity if Hx(w)=-1; It could
resonate;
It looks that twisted pair could survive this condition and goes well, why?
Also several IEEE papers also used above configuration on-chip and goes fine,
why?
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