Hi all,


I designed a simple microstrip line on PCB for a circuit, and in the measurement, I noticed there is a periodic dip in the insertion loss. So I ran a simulation using ADS momentum, and noticed the same effect (see attachment). Note that this is insertion loss calculated as |S₂₁|²/(1-|S₁₁|²)

When I make the line shorter, the dip moves to higher frequencies. Does anybody know why this periodic dip is occuring?


thanks,
Aaron

EDIT: Seems to be because the ground plane is too narrow. Does anybody know any guidelines on how large the ground plane should be?

Not sure what you mean. Sounds like you're talking about a standing wave...the plot is actual power loss, so it is not due to reflection.

After some careful simulating, it seems to be related to the width of the ground plane underneath the microstrip line. If I make the ground plane very wide, the dips go away. Its kind of strange though because at the moment, the ground plane is already around 20 times wider than the trace.


Aaron