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

• From: "Eric Bogatin" <eric@xxxxxxxxxxxx>
• To: <si-list@xxxxxxxxxxxxx>, <doug@xxxxxxxxxx>
• Date: Mon, 25 Feb 2002 19:06:26 -0600

```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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```