What is this part called in a filter? What causes it?

 

hi vas,
Welcome to AAC.
Please post a diagram of the filter.
E

 

hi vas,
Welcome to AAC.
Please post a diagram of the filter.
E

This is it.

 

Stray wiring capacitance has more capacitance than your tiny capacitor values.
Stray wiring inductance of your circuit might have more inductance than your tiny inductor values.
Is it a 5.8GHz filter?

 

If the filter is active using an opamp then the high frequency breakthrough is caused at high frequencies where the opamp has no gain.

No Sir it is 1.2GHz filter. This is just the testing of it. I am getting this sort of curve in all the other designs as well.

 

It would be a resonance peak in the stopband. It looks like it is 25 dB down from the passband response. It can happen in some types of filters (elliptical) in exchange for a steep rolloff in the transition band. Parasitic elements may also be in play.

 

hi vas,
This is what LTSpice shows.
E

 

Wow. That's a ton of gain in the passband for a passive circuit @ 650 MHz.
Oh right no source or load impedance.

 

hi,
Using the general purpose 50R, gives this, looks closer to the scope image.
E

 

I agree that looks closer to what I would expect, but there is something else going on in the actual circuit that we cannot see. Maybe the TS could explain how the "lumped" components were formed and/or confirm that "lumped" components were even used. If I were doing this I would be using a different approach.

EDIT: because the original magnitude plot shows very little attenuation in the passband I would be tempted to try a small source impedance and a 50 Ω load impedance. I'm not sure what is causing the passband ripple, but I see no evidence of the expected 6 dB attenuation in the passband from equal source and load impedances in that original magnitude plot.