[SI-LIST] differential signaling (common-mode)

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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