[SI-LIST] Re: Even mode, common mode, and mode conversion

Eric,

Please let me copy two sentences, both referring to even mode,  from your
message below:

1) If you put a +1v on each line, wrt the return plane below, there will be
no voltage difference
between the two signal lines and the voltage pattern will continue
undistorted.

2) The even mode impedance increases as the coupling increases since one
line "shields"
some of the capacitance to the plane of the other line.

I don't see any hint of coupling in 1) whereas in 2) coupling is the key
issue. What did I miss?

Thanks,
Sainath


----- Original Message -----
From: "Eric Bogatin" <eric@xxxxxxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>; <doug@xxxxxxxxxx>
Cc: <garyo@xxxxxxxxxxxx>; <eric@xxxxxxxxxxxx>
Sent: Monday, February 25, 2002 5:06 PM
Subject: [SI-LIST] Even mode, common mode, and mode conversion


>
> Doug and others-
>
> you have hit upon one of the most common sources of
> confusion in the industry, about odd and even and common and
> differential.
>
> The problem is, most folks use these terms incorrectly, and
> the error has propagated so much, it has turned into fact.
> This isn't even one of those myths, it's just a blatant
> error.
>
> In a nutshell, odd and even modes, and any modes in general,
> are special voltage patterns that propagate undistorted down
> a pair of transmission lines. For example, in a pair of
> microstrip traces, if you send a +1 v on one line and a 0v
> signal on the other, the actual voltage on the two lines
> will change, as the signals move down the line. The 0v line
> will see a growing negative signal as the far end cross talk
> builds up and the +1v signal will drop and distort as it
> looses energy to the quiet line. This voltage pattern is not
> a mode. It is just a particular driven voltage pattern.
> There is nothing special about it.
>
> However, there are two special voltage patterns that you can
> impose on the lines which will not change as the signals
> propagate down the lines. If you put a +1v on each line, wrt
> the return plane below, there will be no voltage difference
> between the two signal lines and the voltage pattern will
> continue undistorted. The other voltage pattern is a +1v and
> a -1v applied to the two lines, wrt the return plane.
>
> These special voltage patterns are called modes. We
> typically call the +1v, -1v pattern the odd mode and the +1v
> and +1 v the even mode. The odd mode impedance is the
> impedance of one line when the pair is driven in the odd
> mode. The even mode impedance is the impedance of one line
> when the pair is driven in the even mode. As the coupling
> between the lines increases, the odd mode impedance gets
> smaller, since the line has a higher current needed to drive
> a signal in the odd mode, as there is more current between
> the lines due to the coupling. The even mode impedance
> increases as the coupling increases since one line "shields"
> some of the capacitance to the plane of the other line.
>
> We can drive the pair of lines anyway we want. When we drive
> the signals with a differential signal, and we have
> symmetric lines, we happen to drive them in the odd mode.
> There is no such thing as a differential mode. There is odd
> mode and there is driving a signal differentially. Many of
> us have gotten in the bad habit of saying "differential"
> mode when we really mean odd mode.
>
> Differential impedance refers to the impedance the
> difference signal sees as it propagates down the
> differential pair of lines. What makes it very confusing for
> most folks is that if you are brainwashed into thinking of
> differential mode and odd mode, then its hard to understand
> what the difference between the odd mode impedance and the
> differential impedance is.
>
> In fact, the differential impedance is just 2 x the odd mode
> impedance. It is the voltage of the difference signal
> divided by the current going into one trace, when there is a
> differential signal between the two traces.
>
> The common impedance is the impedance the common signal sees
> propagating down the differential pair of lines. The common
> impedance is actually 1/2 the even mode characteristic
> impedance.
>
> If these are topics you are interested in, we cover them in
> great detail in our signal integrity training classes. The
> issue of odd and even mode impedance and differential and
> common driving is covered in detail in our GTL122 SI 101
> class. Check our web site for details: www.gigatest.com
>
> --eric
>
>
>
>
> *******************************************
> Dr. Eric Bogatin
> GigaTest labs
> www.gigatest.com
> *******************************************
>
> From: "Doug Brooks" <doug@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Monday, February 25, 2002 9:31 AM
> Subject: [SI-LIST] Even mode, common mode, and mode
> conversion
>
>
> >
> > Gurus,
> >
> > I've been following the thread on mode conversion and
> suspect I am not the
> > only one with this question. Can someone explain, in
> layman's terms:
> >
> > The distinction between even mode and common mode?
> >         between odd mode and differential MODE (not to be
> confused with
> > differential traces)
> >
> > And does the existence of differential traces add any
> complications to
> > these distinctions?
> >
> > Thanks for bringing the dummies along with you!
> >
> > Doug Brooks
>
>
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