I want FM selectivity!!!

[mstechca]

I finally got one FM station in which is at 107.9Mhz.

BUT, that 1 FM station seems to be tackling several bands.

I have the frequency of the oscillator set between 2 and 4Mhz.

If I lower the carrier frequency and lower the oscillator frequency, the same station appears.

What should I set the oscillator frequency and the carrier frequency if I wanted 102.9Mhz?

102.9Mhz is a local station which is about 5km away. 107.9Mhz is a local station that is about 3km away.

How can I get 102.9Mhz clearly and get rid of 107.9Mhz that has been hogging my radio?

 

[stevez]

In most radios it requires several stages of amplification/conversion to get the degree of selectivity required in all but the most remote environment. Each of the stages must be sharply tuned in and of themselves.

What is usually done is to design/build RF stages at fixed frequencies that are very selectively tuned so that only a very narrow "window" is available (high selectivity) for RF to pass. These are traditionally known as IF (intermediate frequency) stages. In front of this is placed a mixer where the frequency of interest and a local oscillator are mixed to generate both the sum and difference of those frequencies. The idea is to have either the sum or difference land right on the "window" - a whole bunch of sums and differences are there at the beginning of the IF but because of the selectivity only the frequency of interest gets thru.

For what it's worth, it is my understanding that DSP(signal processing) can do a lot of this work too but is likely to be too complex to bother with.

 

[stevez]

I neglected to explain that the output of the IF is still at radio frequency so other stages must follow to derive the desired audio or data.

Worth noting is direct conversion but AM/CW/SSB are only modes that work with this. The frequency of interest is mixed with a local oscillator that is very close to but not exactly on the desired frequency. At the output of the mixer appear the sum and the difference. The good part is that the difference is at audio frequencies (the intent) and the sum is at RF and is easily removed.