TABLE OF CONTENTS By varying the sample rate, the frequency span of the analyzer can be controlled but the start frequency of the span is always at DC. The frequency resolution of the measurement can be improved arbitrarily but at the expense of a lower maximum frequency. Band selectable analysis (also known as zoom operation) allows the user to reduce the frequency span while maintaining a constant center frequency. In other words, the displayed frequency range is not limited to starting at DC. This is useful because very narrow spans away from DC can be analyzed.
Band selectable analysis is accomplished by a change in the instrument block diagram (Figure 4-13). The output of the ADC is multiplied by a digital sinusoid, which mixes it down in frequency. [5] Many readers will recognize this as just a digital version of the heterodyne techniques often used in radio receivers and swept spectrum analyzers. The frequency span of interest (Figure 4-14) is mixed with a complex sinusoid at the center frequency, which causes that frequency span to be mixed down to baseband. The digital filter is configured for the proper span by using the appropriate decimation factor. The output of the digital filter is FFT'd to obtain the frequency spectrum. The bandwidth of the digital filter can be narrowed significantly, producing frequency spans as narrow as 1 Hz.
