Radio Frequency by Steve Winder and Joe Carr - HTML preview

14.1 Pulse amplitude modulation

The digitization of analogue waveforms by pulse code modulation is accomplished in two stages. First the waveform is sampled to produce pulse amplitude modulation (PAM). Short-duration samples are taken at regular intervals which are long compared with the sampling time but short in relation to the highest signal frequency. The result is a train of pulses whose amplitude envelope is the same as the analogue waveform. The envelope (Figure 14.1) will contain:

• clock frequency f_c, the sampling rate
• all the signal frequencies contained in the waveform from the lowest, f₁, to the highest, f₂
• upper and lower side bands, (f_c − f₂) to (f_c − f₁) and (f_c + f₁) to (f_c + f₂)
• harmonics of f_c and the upper and lower side band frequencies
• a DC component equal to the mean value of the PAM waveform.

The envelope contains the original signal frequencies and can be demodulated by a low-pass filter which will pass f₂ but not the clock frequency. The clock frequency must therefore be higher than 2f₂. For line communications in the UK a clock frequency of 8 kHz is used with a maximum modulating frequency of 3400 Hz.