Signals in the time and frequency domain - Signals, Processes, and Systems - page 36

τ

Pulse duty cycle /T = 1/4

τ

T = 1s = 0,25 s

τ

Pulse duty cycle /T = 1/5

τ T = 1s = 0,2 s

Illustration 33: Periodic square wave signals with different pulse duty factors

This Illustration shows how the information from the time domain is to be found in the frequency domain.
The period length T is to be found in the distance I/T of the lines of the frequency spectrum. As in this
Illustration T = 1s a line distance of 1Hz results. The pulse duration is 1/4 in the upper representation

and in the lower 1/5 of the period length T. It is striking that every fourth harmonic above (4 Hz, 8Hz etc,
and every fifth harmonic below (5 Hz, 10 Hz etc) has the value 0. The zero position is in each case at the
point l/

τ

τ . It is also possible to determine the period length T and the pulse duration τ in the frequence

domain.

As from a physical point of view there are no sinusoidal signals "of a infinitely high
frequency", in nature there cannot be signals with "infinitely rapid transitions".