TABLE OF CONTENTS As discussed in Chapter 3, the fast Fourier transform can be used to determine the frequency domain representation (spectrum) of a time domain signal. The signal must be digitized in the time domain; then the FFT algorithm is executed to find the spectrum. Figure 4-4 shows a simplified block diagram of an FFT analyzer. The input signal is first passed through a variable attenuator to provide various measurement ranges. Then the signal is low-pass filtered to remove undesirable high-frequency content which is beyond the frequency range of the instrument. The waveform is sampled and converted to digital form by the combination of the sampler circuit and the analog-to-digital converter. The microprocessor (or other digital circuitry) receives the sampled waveform, computes the spectrum of the waveform using the FFT, and writes the results on the display.
The FFT analyzer accomplishes the same thing that the bank-of-filters analyzer does, but without the need for many bandpass filters. Instead, the FFT analyzer uses digital signal processing to implement the equivalent of many individual filters. Thus, when considering the operation of the FFT analyzer, it is appropriate to think in terms of a bank of parallel filters, each filtering a portion of the frequency spectrum. A typical FFT spectrum analyzer is shown in Figure 4-5.
Conceptually, the FFT approach is simple and straightforward-digitize...
