Commercial AM Broadcasting

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Commercial FM Broadcasting

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Commercial Broadcast TV

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Cellular Telephone

Cellular Telephone
AMPS ( Advanced Mobile Phone System) Standard

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AT&T FDM Hierarchy

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Heterodyne Receivers

There are two basic methods for demodulating FDM signals. The first is the Variable Tuned Filter (VTF) method illustrated in Figure . In this method, the center frequency of the variable band-pass filter is changed to pass only the desired signal and reject all others. The requirements of such a filter are severe. Approximations to the ideal requirements result in rather poor performance.

The second method is the super-heterodyne receiver and is by far the more common method used. The basics of this method are illustrated in Figure . The super-heterodyne receiver employs a band-pass filter at a fixed center frequency (called the intermediate frequency or IF) and a variable local oscillator to frequency translate the desired signal into the pass band of the band-pass filter. The wide-band variable band-pass filter is necessary only to reject the signals centered at the image frequencies.

To understand image frequencies consider the following FDM system consisting of N signals:

where

Suppose the desired signal the message portion of . The superheterodyne receiver must perform a two step process:

Step 1:

Frequency translate from to .

Step 2

: BPF the translated signal and demodulate.

Step 1 is achieved by mixing with a local oscillator . There are two possibilities for :

For high side tuning, let

The mixer output and the band-pass filter output are given by

The input to the demodulator consists of two signals superimposed on each other so that the demodulator will not be able distinguish between the two. The unwanted signal is centered at . Since this undesired signal signal is also mixed to the passband of the band-pass filter, it is termed the image frequency.

For low side tuning, let

The mixer output and the band-pass filter output are given by

In this case, the unwanted signal is centered at the image frequencey .

A wide-band variable band pass filter is used to reject the image frequencies prior to mixing to the intermediate frequency. The requirements on this filter are not severe so that physical realization is practical.

  
Figure: Ideal VTF Receiver

  
Figure: Ideal Super-heterodyne Receiver