component present in the analog signal. The relation of sam- pling frequency and the highest frequency of the signal to be sampled is stated as follows: If the sampling frequency, fs, is higher than two times the highest fre- quency component of the analog signal, W, the original analog signal is completely described by these instantaneous samples alone; that is, fs > This minimum sampling frequency is sometimes called the Nyquist rate. We can describe it in other words as an analog signal with the highest frequency component as WHz. It is completely described by instantaneous sample values uniformly spaced in time within a period: Ts =1/fs <1/(2W) (3.4) Introduction to Telecommunications Network Engineering Figure 3.8 represents the operating principle of a sampling circuit and an analog signal before and after sampling in both the time and frequency domains. The sampling circuit contains a generator, G, that produces short sampling pulses at the sampling frequency fs These sampling pulses close the switch of a relay at each sampling instant for a short period of time. The original analog signal x(t) is sampled each time the switch is closed and a sampled signal y(t) is produced. The sampled analog signal y(t) contains short pulses that represent signal x(t) values at discrete points in time. This sampling process that produces y(t) is known as pulse amplitude modulation (PAM) because the amplitudes of the pulses contain the values of x(t). The time-domain curves in Figure 3.8 show the original continuous analog signal x(t) and the sampled signal y(t). The sampled signal y(t) con- tains values of an analog signal at sampling instants. We can imagine that if the sampling frequency fs is high, that is, the distance between sampling instants Ts is short, the sample pulses describe the original signal quite well. We could draw a line that connects the peak values of the pulses and the shape of this curve would be close to the original signal shape of x(t). The changes in x(t) are related to the frequency content of 