Principles of Amplitude modulation
Amplitude Modulation (AM) plus frequency division multiplexing (FDM) is one way of solving above problem. Each conversation is shifted to a different part of the frequency spectrum by using a high-frequency waveform to "carry" each individual speech signal. These high frequencies are called carrier frequencies .
Amplitude modulation is the process of varying the amplitude of the sinusoidal carrier wave by the amplitude of the modulating signal, and is illustrated in Fig.
The unmodulated carrier wave has a constant peakvalueand a higher frequency than the modulating signal , but, when the modulating signal is applied, the peak value of the carrier varies in accordance with the instantaneous value of the modulating signal, and the outline wave shape or "envelope" of the modulated wave's peak values is the same as the original modulating signal wave shape. The modulating signal waveform has been superimposed on the carrier wave.
When a sinusoidal carrier wave of frequency fc Hz is amplitude - modulated by a sinusoidal modulating signal of frequency fm Hz , then the modulated carrier wave contains three frequencies .
1) fc Hz : Original carrier frequency
2) ( fc + fm ) Hz : The sum of carrier and modulating signal frequencies
3) ( fc - fm ) Hz : The difference between carrier and modulating signal
This is illustrated in Fig
It should be noted that two of these frequencies are new, being produced by the amplitude-modulation process, and are called side-frequencies. The sum of carrier and modulating signal frequencies is called the upper side-frequency. The difference between carrier and modulating signal frequency is called the lower side-frequency. This is illustrated in the frequency spectrum diagram of Fig.
The bandwidth of the modulated carrier wave is
( fc + fm ) - ( fc - fm ) = 2 fm
i.e. double the modulating signal frequency
The complete amplitude-modulated wave band of lower sideband plus carrier plus upper sideband shown in Fig. 8 takes up more frequency bandwidth than is really necessary to transmit the information signal since all the information is carried by either one of the sidebands alone . The carrier component is of constant amplitude and frequency so does not carry any of the information signal at all . It is possible by using special equipment to suppress both the carrier and one sideband and to transmit just the other sideband with no loss of information. This method of working is called single sideband working ( SSB ) . This method is not used for domestic radio broadcasting , but it is used for some long-distance radio telephony systems and for multi-channel carrier systems used in national telephone networks.