bad return loss in band pass filter

[hamid_h552]

hello dear friends.
I've designed a bandpass filter in 140 MHz and 20MHz bandwidth. I've casscaded two 3th order eliptic filter for this purpose.
the resulting insertion loss is about 3 dB in simulation , but Its about 12 dB practically.
it's return loss is very bad in pass band (4~6 dB) and this is the reason for high insertion loss.
So how I can improve the return Loss?
tnx

 

[flatulent]

There are two basic causes. One is the parasitic resistance of the parts. The other is the parasitic capacitance in the inductors.

Is the passband shape what you expected?

 

[Filtronetics]

It's a very simple filter you are designing, I bet you are using LC filter designs. You need to tune inductances (coils) or capacitors (trimmers), or both. You should be able to obtain return loss better than 12 dB easily, if you try hard, you should be able to obtain 20 dB RL for such a simple filter.

Good luck!

 

[hamid_h552]

hi.
yes the pass band shape is exactly the same as designed but there is a 10 dB return loss in pass band.
how I can improve this?

 

[aswinkumarv]

The reason for the poor insertion loss while cascading two filters is due to mismatch uncertainty.

This mismatch uncertainty is high if the ports going to be cascaded have poor return loss


The ports to be cascaded has to be matched not only for amplitude but for phase also (complex impedance)

In simulation The effect of phase mismatch may be not taken care