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

  • From: "Zhou, Xingling (Mick)" <xlzhou@xxxxxxxxx>
  • To: "'eric@xxxxxxxxxxxx'" <eric@xxxxxxxxxxxx>, si-list@xxxxxxxxxxxxx,doug@xxxxxxxxxx
  • Date: Tue, 26 Feb 2002 10:55:22 -0500

Dear Eric,

Thanks for the clearance. 

I would like to share some different thoughts. They may not be quite
correct.

Actually, if we notice that "mode" is typically a field concept (pattern)
and "voltage" is a circuit concept (based on integral of fields under
certain conditions), it is not difficult to distinguish them. Voltage can be
in both in time domain and frequency domain, mode only in frequency domain.
Although there are some authors using the same word "mode" to describe the
decomposition of voltage vectors, the word appears as "differential mode"
and common mode", not as "even and odd modes". 

Obviously, we do not talk about "ground plane" in optics or waveguide theory
since the quasi-static concept is invalid theoretically and practically. But
"mode" still survives. There are even higher order modes !!

Even we talk so much about current (electrons) flows in electronics, the
fact is that no single electron can physically migrate from one end of a
wire to the other. In stead, electromagnetic field propagates.  

Another question is what are the essences of differential signals ?  It is
the relation between two signals sent and received by the same pair of
driver and receiver. Same magnitude and out of phase ? Cancelling return
currents ?  Looks like, differential signals are described in both time and
frequency domain too. If you look at all the digital design books,
differential signals are described as two step pulses with opposite signs.
However, in analogy design, they are defined as two signals with the same
magnitude and out of phase (like full wave analysis). Of course, it can be
easily shown that they are consistent by doing Fourier Transformation.
However, if differential signals are related simply to (even or odd) mode
(as we did), both must be in frequency domain. 

Now the discussion we are having is about the propagation (transimission) of
differential signals using transimission lines. The purpose is to transmit
differential signals from the driver to the receiver. In principle, the
status in the middle of the transmission is not important if the output
keeps the differential relations. Although we can not prove that using
symmetric lines is the only solution, it is the simplest nominal solution
and the easiest and probably the best practice.

Finally, when we say differential mode and common mode, do we mean the local
(static) (on one plane) relation ? It has nothing to do with the
propagation.  

Regards,

Mick






-----Original Message-----
From: Eric Bogatin [mailto:eric@xxxxxxxxxxxx]
Sent: Monday, February 25, 2002 8:06 PM
To: si-list@xxxxxxxxxxxxx; doug@xxxxxxxxxx
Cc: garyo@xxxxxxxxxxxx; eric@xxxxxxxxxxxx
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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