[SOLVED] superheterodyne receiver + FM

[davidjohhn]

Hi there,

I am currently studying about superheterodyne receiver. The local oscillator is set to fix a value when receiving a signal. If the input signal is a FM signal then it is going to have two frequency components. How it is possible if the receiver can receive a FM signal?

Please explain me

Best Regards

 

[keith1200rs]

All modulated RF signals have a finite bandwidth. The receiving bandwidth of the receiver is designed to accommodate that.

Keith

 

[G4BCH]

Not sure what you mean by the two frequency componets but I assume that it is the image frequencies from the mixer rather than the modulation band width of the signal, which as Keith states all receivers are designed to handle.

Consider the two inputs to the mixer as F.LO. The local oscillator, and F.Sig the wanted signal. F.Sig is actually a band of frequencies containing the modulation sidebands( this can be any modulation).
The output of the mixer will be F.LO + Fsig and F.LO - F.Sig. At the output of the mixer you have to put a filter to select the signal you want, usually F.LO-FSig for an FM receiver, this is the intermediate frequency or IF. This filter has to have an bandwidth suitable to pass the modulation.
The rest of the receiver consists of amplifiers and a demodulator designed to operate at the IF.
The mixer has two responses a response at FLO + IF and FLO - IF. In an FM receiver the wanted response is usaully FLO + IF. The IF is 10.7MHz and the wanted signals inthe range 88 to 108MHz so the LO tunes from 77.3 to 97.3MHz. The unwanted response, the image, in the range 66.6 to 86.8MHz is removed by a filter at the mixer input. Thi filter is a tracking filter tuned at the same time as the local oscillator.

Peter

 

[ALERTLINKS]

In AM you have sidebands, which differ acording to modulation frequency. In fm it is about 100kHz frequeny driift in comercial fm band. IF amplifier has particular bandwidth. It let pass the slight difference also.