[SI-LIST] : Signal traces without reference plane

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From: Heiko Dudek (heikod@cadence.com)
Date: Wed Nov 17 1999 - 08:45:25 PST


Hi,

since being involved in a discussion about "routing pcb traces while having
partially no ground reference", I would love to get some other opinions. So,
assume a setup like this:
(Which you can get on systems with multiple GND / VCC potentials easily.)

 ______________________
                       |
       GND Plane |
                       |
 ________________________________
 (<-Receiver) Signal Trace |
 ____________________________ |
                       | | |
                       | | |
                       | | |
                       | | |
 ____________________________| |
 (<-Driver) |
 ________________________________|
                       |
 _____________________ |
  Second Signal Trace ||
 __________________ ||
                   | ||
                   | ||
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   

Theoretically I would expect some differential mode EMI because of the current
loop the return current on the ground plane forms with the signal current.
Of course, there's some reflection on the signal trace because of the impedance
mismatch; additionally one should see some additional inductance because of the
E field stretching (which goes along with H concentrating) serial to the signal
trace. Finally, there might be some coupling into the second signal trace.

The *BIG* question is, how relevant is this theory (if applicable at all) to
actual PCB design ? What are your experiences on this ? How do you control (or
better, avoid) this configuration while autorouting ? Do you care at all (if yes,
what's the critical edge rate you have to start worrying) ?

Thank you very much,

Hans Betz
Heiko Dudek

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