Question

Difference between amplitude spectrum and magnitude spectrum

Answer 1

Answer:

Any signal whose amplitude is a function of time has a corresponding frequency spectrum. When the signals are viewed in the form of a frequency spectrum, certain aspects of the signals or the underlying processes producing them are revealed. In some cases the frequency spectrum may include a distinct peak corresponding to a sine wave component. And additionally there may be peaks corresponding to harmonics of a fundamental peak, indicating a periodic signal which is not simply sinusoidal.

The power spectral density (PSD) of the signal describes the power present in the signal as a function of frequency, per unit frequency. Sometimes one encounters an amplitude spectral density (ASD), which is the square root of the PSD. This is useful when the shape of the spectrum is rather constant, since variations in the ASD will then be proportional to variations in the signal's amplitude itself. But it is mathematically preferred to use the PSD, since only in that case is the area under the curve meaningful in terms of actual power over all frequency or over a specified bandwidth.

In short, when we use the word spectrumor frequency spectrum, we are referring to the magnitude squared of its Fourier transform, which is a bit different from the PSD (See the third paragraph of this section). You may also refer to this question for more clarification of the terms.

Answer 2

Answer:

I.1.6 Limitation of t Domain or f Domain Representations

The amplitude or power spectra of the signals (deterministic or random) shown in Fig. I.1.4 gives no indication of how the frequency content of a signal changes with time, information that is important when one deals with FM signals, or other kind of nonstationary signals. This frequency variation often contains critical information about the signal and process studied in applications. The magnitude spectrum cannot therefore suitably represent.

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