Parasitic Passbands in distributed filters

Thread Starter

Kinka-Byo

Joined Apr 22, 2019
11
Hello there,
it is known that if we realize a filter (for instance a low pass filter) by using distributed elements (transmission lines), there will be some parasitic passbands (which moreover show a behaviour which seems "periodic").

For instance I did this simulation of a Butterworth filter of 5th order:





Note that it behaves as a perfect low pass filter only for few GHz after the cut-off frequency. What is the cause of this phenomenon?
 

SLK001

Joined Nov 29, 2011
1,533
It's called the "re-entrant mode". A transmission line that resonates at F will also resonate at 2F, 3F, 4F ... etc. And they aren't parasitic. You can minimize the effect of the re-entrance by using "defective" transmission lines.
 

Thread Starter

Kinka-Byo

Joined Apr 22, 2019
11
It's called the "re-entrant mode". A transmission line that resonates at F will also resonate at 2F, 3F, 4F ... etc. And they aren't parasitic. You can minimize the effect of the re-entrance by using "defective" transmission lines.
Precisely, what do you mean with defective transmission lines?
 
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