varactor tuned rf bandpass filter

[vimalagowri]

hi iam designing a tunable bandpass filter using varactor diode its frequency is 100Mhz-400Mhz and the varactor here is programmable its capacitance varies by varying the reverse voltage can u give me the basic idea regarding this ..it is not a combline filter or rf mems filter it is just a passive circuit which uses varactors for tuning

 

[kalbun]

Hello... what do you need exactly? From your message is not clear if you need to understand how a varactor works (but I suppose you know) or how to use a varactor in your filter.

Giuseppe

 

[RCinFLA]

100-400 MHz is quite a wide tuning range for a varactor sweep. If you are doing a multi-pole filter you have to account for coupling coefficient across the tuning range.

Q of varactors degrade at lower reverse bias (higher capacitance). You can view it as an approximately constant Rs in series with varactor capacitance.

 

[FvM]

Varactor diodes intended for TV VHF band III would basically fit the frequency range (e.g. NXP BB181), but the achievable capacitances ratio is too low for 1:4 frequency variation, apart from the problems mentioned by RCinFLA. This means, you most likely need to split the frequency range in two bands 100-200 and 200-400 MHz. Todays wideband varicap tuners are often utilizing individual voltage control of each varicap with digital adjustment of the characteristic. You may want to provide a similar technique, if you are intending higher filter performance, and also think of an additional Q adjustment.

 

[vimalagowri]

thanks for ur reply can u give me any circuit which tunes upto 100-175Mhz using varactors.and i also need information regarding rf tunable bandpass filter using varactors

 

[FvM]

I'm only aware of the usual tuner designs, which are baiscally tuned amplifiers rather than passive circuits. Expect for a (now reduced) frequency range requirement, you didn't tell any specifications for the intended filter (bandwidth, passband/stoppband attenuation, dynamic range).

 

[vimalagowri]

I'm only aware of the usual tuner designs, which are baiscally tuned amplifiers rather than passive circuits. Expect for a (now reduced) frequency range requirement, you didn't tell any specifications for the intended filter (bandwidth, passband/stoppband attenuation, dynamic range).

passband is 100-175Mhz

 

[FvM]

passband is 100-175Mhz

According to your previous description, it's the tuning range rather than the passband. Other important filter parameters are still unsaid.

By the way, don't start multiple threads on the same topic, otherwise administrators will transfer them to the trash can. In my opinion, cross-posting is way to fool users that are willing to help you. Whem I'm get aware of excessive cross-posting, I'll stay away from answering the respective posts.