As we discussed in Chapter 10, heterodyne spectrum analysis is information preserving because each photodetector is read out on an instantaneous basis. Heterodyne detection, however, places severe demands on the circuitry associated with each photodetector because the rf electronics are not easily implemented with integrated circuits. In this chapter we describe spectrum analyzers that support the sampling rate required to avoid missing signals and yet operate with a significantly reduced number of photodetector elements. The basic idea is to decimate an Nr-element photodetector array by retaining only every Mth element. We then time share the remaining elements by scanning the spectrum across the decimated array. Each photodetector therefore produces, as a time sequence, the spectral content of the received signal over a small frequency range.
In this chapter we also introduce a signal-processing operation called cross-spectrum analysis to determine the angle of arrival of a signal. For example, if we use two antenna elements to receive a signal from a common source, we can use a dual-channel acousto-optic cell to determine the source direction relative to the receiver geometry through a phase-comparison technique. The phase information, obtained by forming the cross spectrum of the signals received, provides a measure of the angle of arrival of cw emitters at each frequency.