[SI-LIST] Question on ground trace in TL
- From: Bi Han <mike_bihan@xxxxxxxxxxxx>
- To: han.bi@xxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 1 Aug 2005 21:24:24 +0800 (CST)
Dear friends:
I did an experiment on TL simulation. The transmission line configuration is
CPS.
There is no other ground on the chip.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+++++++++ +++++++++++++++++
DIEL + sig + + GND +
+++++++++ +++++++++++++++++
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
SUB
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Case A: I set GND trace as reference conductor in 2D solver.
Case B: I set GND trace as signal trace, and place another reference conductor
far away.
Both cases are simulated in .AC analysis.
In case A, it is a 1 conductor case and the R_term is placed between SIG and
ideal ground (node 0 in hspice).
In case B, it is a 2 conductor case and the R_term is placed between two
conductors and does not connect to ideal ground;
In short, in case B, ground trace is treated as a signal line; all other setups
are the same.
The simulation setup case B is like below:
At far end, a resistor terminates between SIG and GND trace. At near end, same
resistor terminates. In case A, both source and load well terminated and at far
end, see very smooth voltage gain.
In case B, it is assumed matched. However, in frequency sweep, severe peaking
is seen in far-end voltage.
The result is very confusing. Which case is real? Both cases have their
drawback.
Case A assumed ground trace to be very quiet and ¡°0¡± everywhere.
Case B introduced another reference conductor faraway, which induce GND/SIG
trace ¡°common mode¡± resonance.
I hope people could wash my mind. (It is quite stuck inside now.) This
experiment also lead me to doubt that why PCB ground plane could be used as
reference plane. In high speed signal case, most return current will flow just
underneath the signal trace. The EM condition will be very like above
experiment. The voltage distribution should not be ideally ¡°0¡± everywhere.
Understanding above case is the key to me.
Thanks you!
---------------------------------
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