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

  • From: "Christman, Timothy (STP)" <Timothy.Christman@xxxxxxxxxxx>
  • To: "'haedge@xxxxxxxxxxxx'" <haedge@xxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
  • Date: Tue, 26 Feb 2002 16:42:32 -0600

Each propagates undistorted, but at different velocities?

Timothy J. Christman
Test Engineer
Tel 651.582.3141  Fax 651.582.7599
timothy.christman@xxxxxxxxxxx
Guidant Corporation 
4100 Hamline Ave. N.  
St. Paul,  MN   55112  USA 
www.guidant.com


-----Original Message-----
From: David G Haedge [mailto:haedge@xxxxxxxxxxxx]
Sent: Tuesday, February 26, 2002 3:56 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Even mode, common mode, and mode conversion




Eric and all,

My understanding of a transmission line that has  +1 volt signal on one
line and
0 volts on the other is actually the superposition of an even mode signal
of +0.5 volt / +0.5volt and an odd mode signal of +0.5volt/-0.5volt, giving
you
the +1volt/0volt signal on the line, in which case each mode should
propagate
undistorted.  Is this not what in fact is occurring in a line excited in
this nature?

David Haedge
Raytheon






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.








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