electrically tunable filter

I want to know how to design electrically tunable filter ?
who can give me some paper about it and some samples?
thanks

http://www.ansoft.com/deliveringperf...ndpassfilt.pdf

How about this papers?

Hi,

i cant upload so many papers and u pls google these papers..hope this will help u


Varactor-loaded split ring resonators for tunable notch filters at microwave frequencies

Tunable split-ring resonators for nonlinear negative-index metamaterials

Tunable Metamaterial Transmission Lines Based on Varactor-Loaded Split-Ring Resonators

Tunable split-ring resonators for nonlinear negative-index metamaterials

Tunable Microstrip Bandpass Filters With Two Transmission Zeros

Tunable Split Rings Resonators for Reconfigurable Metamaterial Transmission Lines