[SI-LIST] 回复： Re: differential signaling (common-mode)

Steve:
 
In twisted pair, is there any predictable common mode return path?
 
If there is not, how they control the CM reflection? In Dr. Howard's book,
there is transformer and common mode choke between Tx and cable. I guess that 
there might be huge CM resonance in the cable, however, they are rejected at 
Tx/Rx side, right?
 
thanks,
Han
 


steve weir <weirsi@xxxxxxxxxx> дµÀ£º
Bi,

Whether you model it or not in a real chip and board there will be a CM 
return path. You would be well advised to control it within the package so 
that it can match what is achievable on the board. Then you can terminate it.

In unshielded twisted pair applications the transformer buys a huge CMR.

Steve.
At 11:32 AM 7/30/2005 +0800, Bi Han wrote:
>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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