Problem in designing VHF IF filter for a FM broadcast radio receiver

[Annirak]

Hi,
I'm designing an FM broadcast radio receiver front-end as a method to teach myself about RF design. I'm having some trouble designing the bandpass filter for my IF stage.

To increase selectivity and eliminate image frequencies, I decided to design a dual-IF converter, using one up-converter and one down-converter. The second IF will be the FM standard 10.7MHz, and uses a standard 10.7MHz crystal filter.

The frequency synthesizer I've designed in will generate up to 204.75MHz at my target resolution, which limits my receiver to 284MHz. The down mixer can operate at half the resolution of the up-mixer, so the LO reference for the down converter is not limited to the 204.75MHz.

I originally intended the first IF to operate at 200MHz, so that I could use a simple low-pass filter prior to the first mixer to eliminate image frequencies. I discovered that there are no crystal, ceramic, or SAW filters in the 150-300MHz range, so that makes my original design more difficult.

The solutions I've come up with are using a shorted coax resonator or using a lumped element bandpass filter. Is there another solution to my problem? Could someone point me at some information on how to simulate a shorted-coax resonator? The design process seems pretty straight forward.

Thanks,
Annirak

 

[Annirak]

Re: VHF IF filter design

I've had a look at the filters here, and I'm planning to use ansoft designer sv to do some simulations, since I don't think that pspice will do an adequate job of simulating this.

Is there any other information out there about building narrow-band filters?

 

[biff44]

Re: VHF IF filter design

Really? < 1 minute to find.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=583-1079-1-ND

 

[freeman T]

Re: VHF IF filter design

Hi Annirak, I am not familiar to system design. Regarding to the IF filter, maybe you can try helical filter.

 

[Annirak]

Re: VHF IF filter design

Really? < 1 minute to find.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=583-1079-1-ND

I didn't post quite enough information on this. The inter-channel spacing is 200kHz, so a narrow bandwidth is also necessary. The 500kHz bw filter you found would mean that my first IF includes three adjacent channels.

This post was mostly looking for filter architectures that I might have missed, or coax filter design methodology--which seems to be trivial upon further inspection--and simulation techniques--which are anything but trivial in pspice.

Do you know if it is possible to operate a ceramic filter in its 5th overtone? Does it simply require an LC tank tuned to the 5th overtone frequency?