abt filters

when we talk abt filters we talk only abt insertion loss but we dont talk abt return loss why?
when iam designing a combline or interdigital using tapped line input i directly give feed line( 50 ohm) to resonator which is having an impedance of greater than 50 ohm ...so there is a mismatch between feed line and resonator impedance..how will it affect the response?

There are many parameters like rejection, delay etc. In this scenario, your manager may be interested perticulary on IL.

A lot wrt ripple BW etc and may not be acceptable.

when there is a mismatch then what i have to do? whether i should put any impedance inverter to match 50 ohm and high impedance line

This is the all idea of tapped line, to provide an impedance transformation.
So you have to tune the filter design (length, spacing, width, etc, of the resonators) in the way that at the tapped point the impedance is nearly 50 ohms, meeting in the same time all the other parameters of the filter, insertion loss, rejection, bandwidth, ripple, group delay, etc.
There is no reason to use extra impedance networks to match these kind of filters.

u said that at tap point location there should be matching of 50 ohm and resonator impedance .. how can i locate the tap point on resonator that the impedance is 50 ohm ....actually by parameter extraction technique (i take 2 resonators as they are in the basic filter and they are loosely coupled to feedline so that coupling between resonators dominate the coupling between feedline) and by adjusting the spacing between resonators i got the spacing for required coupling coefficients......but my question is how to make a location at first resonator such that at tap point the impedance is 50 ohm.........?
Q2)i have the external quality factor between the tap line and first resonator and again by EM simulation i change the tapline position onto the resonators to get the required external quality factor.......is it the way to predict the tap point or not?
Q3)since iam designing a combline filter that the resonators are short circuited to ground through via hole grounding ..........can u plz explain in layout window of ADS how to change the momentum substrate(create/modify) for making a vial hole grounding

We definitely talk about return loss as well as insertion loss. Basiclly insertion loss is caused by several reason: reflection(return loss), radiation and thermal issues (dissipation loss).

So the mismatching will cause large reflection, which would worse your return loss, and insertion loss correspondingly.

There are several ways to making matching, the most common way is to add a λ/4 transformer between feedline and your resonator. By adjusting the transformer, you can achieve the best match.

Hope it helps.

hi frnd u said to add quarter wave transformer between tapline feed and first resonator ......some people are saying not to use any quarter wave transformer ............so what to do?so what to do

Using additional quarter wave transformer of course will increase some conducting loss and the size, but can decrease the return loss dramatically, as well as making insertion loss better. If you don't have any size limitation and don't have much tuning room in your original structure, why not just use one?

hi frnd is it necessary to place a lumped capacitor to combline filter when we are implementing in microstrip......( i have read in some text book that when we are implementing in strip line configuration.......then we have to place a lumped capacitor) but when we are implementing in microstrip then lumped capacitor is not possible.........then how can i perform tuning to that bandpass filter.............?
plzzz help