What is a spectrum?
So what is a spectrum in the context of this discussion? A spectrum is a collection of sine waves that, when combined properly, produce the time-domain signal under examination. Figure 1-1 shows the wav form of a complex signal. Suppose that we w re hoping to see a sine wav . Although the wav form certainly shows us that the signal is not a pure sinusoid, it
does not give us a definitive indication of the reason why. Figure 1-2 shows our complex signal in both the time and frequency domains. The frequency-domain display plots the amplitude v rsus the frequency of each sine wav in the spectrum. As shown, the spectrum in this case comprises just two sine waves. We now know why our original waveform was not a pure sine wav . It contained a second sine wav , the second harmonic in this case. Does this
mean w have no need to perform time-domain measurements? Not at all. The time domain is bett r for many measurements, and some can be made only in the time domain. For example, pure time-domain measurements include pulse rise and fall times, overshoot, and ringing.
Time domain measurements Frequency domain measurements
Figure 1-2. Relationship between time and frequency domain
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