[SI-LIST] Re: 回复: Re: [SI-LIST] differential signal in g (common-mode)

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Bi Han <mike_bihan@xxxxxxxxxxxx>, List` Si <si-list@xxxxxxxxxxxxx>
  • Date: Sat, 30 Jul 2005 03:50:42 -0700

Bi, well I think you have answered your own question:
1. There is a real physical common mode path.
2. If you want an accurate result out of simulation you need to correctly 
model that common mode path.

As an aside, if you are doing package work, a 2D solver is going to be very 
limiting.  You should really be looking at Ansoft or CST.

Steve.
At 06:36 PM 7/30/2005 +0800, Bi Han wrote:
>Steve:
>
>Thanks for the reply.
>
>I agree that there is going to be a CM return path anyway, and there is no 
>way to ignore it since 2D extractor require a reference conductor.
>
>It will be cool that I can terminate it. However, it is hard to find the 
>return path inductance since I did not provide clear path. Therefore, I 
>can not know the return path loop impedance.
>
>In another word, this lead to a question, that whether it is a must to 
>provide the underneath CM return path?
>
>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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