[SI-LIST] Re: Mode conversion

  • From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxx>
  • To: <dmckean@xxxxxxxxxxxxxxx>,"Signal Integrity Group" <si-list@xxxxxxxxxxxxx>
  • Date: Wed, 20 Feb 2002 08:30:10 -0700

A simple explanation that I've always liked is a diff line with unequal
"lengths" (physical and/or mode induced)causes the currents to have
different phases or arrival times, and therefore the currents do not cancel
completely, i.e., are not exactly 180 phase from each other.  Due to the
high impedance of the receiver a very small amount of current can develop a
large common mode voltage across the receiver input, producing common mode
radiation.  Microstrips have more issues than striplines due to
inhomogeneous permittivties and surface effects.  I haven't found right
angle "corner" capacitance to have any measureable effect ....yet.  If
meander sections are tightly coupled, you could have issues even at slow
speeds.  If you lay them out correctly they pose no problem.

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Doug McKean
Sent: Tuesday, February 19, 2002 4:20 PM
To: Signal Integrity Group
Subject: [SI-LIST] Re: Mode conversion



In terms of EMI, the full name for "mode conversion" is
"differential-to-common mode noise conversion".  It's
when differential mode noise gets converted to common
mode noise. The exact mechanics of this escapes me,
but does happen to varying degrees with just about
any differential buss layout.  The worse the layout, the
more common mode noise produced. For instance,
put lots of equal to or greater than 90 degree bends in
a differential buss layout between points A and B and
you will get lots more common mode noise as opposed to
a straight buss configuration between the same two points.
Not sure if it's some odd/even mode phenonmenon or
some additive crosstalk thing. I'm sure someone here
would know.  - Doug McKean


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