Demodulation of Wideband, Low-Power FM Signals*

Abstract

Some theoretical aspects of the demodulation of wideband, low-power FM signals are discussed. It is assumed that a band-limited, continuous, analog signal is supplied to the modulator and is recovered to a fidelity suitable for television, telephone, or carrier telephone. Much of the paper assumes that the baseband signal is sampled and clamped before it is applied to the frequency modulator. The combination has been called PAM-FM and is characterized by a piecewise constant transmitted frequency. PAM-FM can be demodulated by spectrum analysis means not suitable for continuously varying frequencies. It is shown that a spectrum generator can be derived from the techniques of radar pulse compression, and is equivalent to an infinite set of correlators or matched filters plus means for scanning their terminals. The spectrum analysis circuit forms are compared with demodulators using frequency detectors, with and without FM feedback, in regard to theoretical noise sensitivities. The theoretical sensitivities are quite similar for spectrum analysis and FMFB under conditions assumed. The comparisons disclose that frequency detectors (followed by filters) enjoy a disguised but efficient use of a differential phase coherence which is a characteristic of FM signals. A combination of spectrum analysis and frequency detection is described which has some of the theoretical advantages of both.