[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: Sun, 31 Jul 2005 07:39:25 -0700
Bi, no, the common is considered to be far enough away from the conductors
that they have negligible coupling to it. That's not all that accurate,
because in a wiring duct there are many parallel cables that will all act
like the common. Note that an application like Ethernet, common mode
magnetics tend to be very important in suppressing unbalanced common
currents to a common, ( ie victim antenna ). I believe that Dr. Johnson
published at least one article on this with a nice illustration.
If you examine an Ethernet transformer configuration, you will see that it
includes a common mode termination. Often this is a high voltage capacitor
in series with a termination resistor to common.
When we use a transformer, up to the point that the transformer breaks
down, a perfect transformer would block all CM. However, any real
transformer has imperfect coupling match. Real transformers suffer the
problem of mode conversion, where some of the signal energy actually gets
converted into common mode noise, and some common mode noise ends up
getting converted to apparent signal energy. The CM choke helps with both
of those problems within a limited frequency range. But ultimately, we
have to do something to dissipate the CM energy.
Steve
At 07:53 PM 7/31/2005 +0800, Bi Han wrote:
>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
>
>7976450e.jpg
>
>
>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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