questions about FM Superhetrodyne Receiver

[walid]

Hello

Q1)
If the received signal frequency = 100 MHz, then the local oscillator equal 110.7 or 89.3 MHz?
thanks

[Hero999]

It could be either but normally the local oscillator is higher not lower.

[Kevin Weddle]

Higher or lower? The oscillator should match the incoming frequency as close as possible. Otherwise, your introducing a different frequency, which results in something more indicernable. Fortunately, most broadcasts have plenty of power and a wide enough bandwidth for audio and video.

[Hero999]

Higher or lower? The oscillator should match the incoming frequency as close as possible.

No.

Look up  superheterodyne receiver and you'll see why the oscillator's frequency needs to be different to the signal being received that's all I'm going to say.

[Kevin Weddle]

I think your right, but there are two problems. The incoming signal, let's say centered at 100MHz will not exactly match the oscillator. So the resulting frequency won't be 0Hz. The bandwidth of the incoming signal will result in frequencies maybe to 1MHz. So you could lose something, maybe not.

The problem is that if you set the oscillator at let's say 95MHz, a 95MHz transmission from another station will appear at the lower end of the spectrum, only slighty diminished assuming the correct length of antenna. High pass filtering stages would reduce this unwanted station.

[audioguru]

Kevin,
Please learn about how a super-heterodyne radio works.
It uses a lower frequency fixed frequency IF amplifier (10.7MHz for FM radios) and a radio-frequency mixer. The mixer adds and subtracts the input signal with the local oscillator that is 10.7MHz above or below the input frequency. The IF frequency is only 10.7MHz beause it is simpler to design a lower frequency IF instead of a very high frequency bunch of RF filters.
Also it is much easier to make a variable frequency local oscillator instead of a variable frequency tuned amplifier at the very high frequency.