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Design >> RF Design >> Transmission Line Periodic Loss
https://designers-guide.org/forum/YaBB.pl?num=1329451795

Message started by aaron_do on Feb 16^th, 2012, 8:09pm

Title: Transmission Line Periodic Loss
Post by aaron_do on Feb 16^th, 2012, 8:09pm

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?

Title: Re: Transmission Line Periodic Loss
Post by loose-electron on Feb 17^th, 2012, 1:20pm

Looks to me like a phase relationship between your input signal period and the "down and back" period of the transmission line.

Title: Re: Transmission Line Periodic Loss
Post by aaron_do on Feb 18^th, 2012, 3:29am

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