# FM preselect filter

Hello I am designing an fm receiver and had some questions about the RF front end. To tune into different channels I know I need a tunable oscillator, but does the bandpass filter connected to the antenna also need to be tunable? or just have a bandwidth of 200MHz. I don't really have any circuit schematics ready yet to share sorry. Also, it may be a stupid question but I also wanted to ask if the image rejection filter and preselector are the same or not.

Thank you

• What frequency range is the receiver intended to receive? You almost certainly want the bandpass filter to be much narrower than 200 MHz. – Peter Bennett Mar 8 '18 at 23:03

## 2 Answers

Yes, you need a preselector of some sort. It is this stage that provides "image" rejection for a superhet receiver.

The superhet works by heterodyning the input signal down to a fixed IF frequency, using a local oscillator that is offset from the signal frequency by the IF frequency. When the two are mixed together, the sum and difference frequencies are produced, one of which is the IF frequency, and the other is rejected by the IF filters.

However, there is a second signal frequency that could also be mixed down to the IF frequency. Suppose your local oscillator is by design higher than the signal frequency, so that $$f_{LO} - f_{signal} = f_{IF}$$

The second (image) signal would be such that

$$f_{signal} - f_{LO} = f_{IF}$$

i.e., it is higher than the desired signal by twice the IF. It is the job of the preselector to reduce the level of this image frequency to an unobjectionable level.

• Thanks! I also have another question, I want to know if channel selectivity is also dependent on this preselect filter or if that is another filter after the RF amp. If so then is it easier to implement a tunable bandpass with ganged capacitors? or try to implement one with varactors. If I choose to use the varactors is there some appnotes you could point out to me for some reference designs? – user7538434 Mar 9 '18 at 0:39
• No, channel selectivity is primarily defined by the IF amplifier. – Dave Tweed Mar 9 '18 at 1:57

Some more modern receiver chips choose to use a lower intermediate frequency than the older 10.7 MHz intermediate frequency that was common years ago. With a low-I.F., image rejection becomes a big problem.
It is solved by using phasing techniques inside the chip that amplify one sideband, and reject the other sideband (which is the image).
The radio-frequency filtering at the chip's antenna connection need not be greatly selective. Here's an example arrangement where headphone leads also serve as the FM radio antenna. A wide-bandwidth fixed-tuned 100 MHz. bandpass filter is the only radio-frequency selectivity. J1 is the audio headphone, whose common connection is grounded through L1: