tunable LC filter using varactor in VHF

[cddxdlk]

tunable filter varactor vhf

hi
i want to design the tunable LC filter using varactor in VHF, i choose BB809 in my circuit.

the input P1dB of the LC filter is 10dBm, can you tell me how to improve the input P1dB, how to choose the varactor to realize the high P1dB

 

[biff44]

tunable filter diode

The filter measures at 10 dbm compression, and you want what P1dB instead? Do you have plenty of tuning range, or are you starved for tuning range?

 

[cddxdlk]

tunable rf filter design varactor

3xs tiff44
I want P1dB>25dBm
in my design,I have enough tuning range.
I don't know how to choose varactor to improve P1db.
in varactor datasheet , which character can illustrate the power_capacity of the varactor?

 

[biff44]

Re: tunable filter help

A varactor diode changes its capacitance as a function of the DC voltage across it. But they are fast diodes, and can respond to RF voltages superimposed on the DC control voltage. So if it were me, I would try to minimize the RF voltage across the varactor diode!

If the instantaneous voltage (RF + DC) swings to a place where the diode can forward bias itself, you will have some clipping of the RF waveform. Also, if the instantaneous voltage (RF + DC) is too high and exceeds the breakdown voltage of the varactor, you can conduct current too and have clipping.

So, choose a varactor with a high breakdown voltage (>30 volts), do not operate with the DC control voltage too close to forward biasing (i.e. keep at least 2 VDC on the varactor, and avoid the temptation of going to 0 volts to get more capacitance change). Also, a high breakdown voltage implies a thick region of lightly doped silicon--where, due to transit time effects, it will be harder to forward conduct current at high microwave frequencies.

Keep in mind that the filter topology has a lot to do with what RF voltage each varactor sees. If you are in a narrowband filter, and have loosley coupled resonators, you will have a big resonant voltage across the varactor!

You could try having two varactors in series for RF, but in shunt each for the DC control voltage. That would reduce the RF voltage by 1/2.

You could try to redesign your filter. If you designed a bandpass for 50 ohms in and out, try redesigning it for 25 ohms in and out, and use 50 to 25 ohm transformers at the input and output. That will lower the RF voltage swing.

You should study varactor diode physics, including things like "punch through".

Rich

 

[vimalagowri]

i need a circuit regarding varactor tuned rf filter it is a passive filter not combline or meme filter only passive components +programable varactor