Heterodyne Spectrum Analysis
The emphasis in this chapter is on achieving more dynamic range in spectrum analyzers by using heterodyne-detection techniques. Other benefits, such as the ability to measure both the magnitude and phase of a signal, also accrue as a result of heterodyne detection. As we learned in Chapter 8, the photodetector current produced by a power spectrum analyzer is proportional to the input rf signal power. In heterodyne detection we combine the signal spectrum with a reference beam, called a local oscillator, the magnitude and phase of which are known, so that the photodetector current is proportional to the signal voltage instead of to the signal power. The result is a significant increase in the dynamic range.
King et al. (94) described heterodyne detection techniques for recovering both the magnitude and phase information of a light distribution. In their system, shown in Chapter 9, Figure 9.3, the interference of an unmodulated reference beam with a spectrum F(α, t) produces a temporal frequency proportional to the input signal frequency f. Because the wideband input signal is typically centered on a frequency of several hundred megahertz, the interference term occurs at a high temporal frequency that varies as a function of the spatial frequency. As a result, implementing the postdetection filter design for each discrete photodetector element, each with a different center frequency, is not cost effective.
In this chapter we ...