Spectral Shaping by Simultaneous Amplitude and Frequency Modulation

It has been known that complete elimination of one of the sidebands in FM can be achieved by modulating the envelope by a signal related to the baseband modulation. This scheme is known as SSB-FM.1,2 In general, beneficial spectral shaping in FM is achieved only if the amplitude modulating signal is related in some manner to the frequency modulation. Also the amplitude modulation must be positive so that the zero-crossings of the original FM wave will not be altered. For random data, the spectrum of FSK is known to be symmetrical about the carrier frequency, and it appears intuitively clear that 557 reducing the carrier amplitude during the period while the higher frequency is being transmitted reduces the energy of the spectrum in this frequency range. We have observed experimentally that the amount of amplitude modulation critically affects the energy distribution. These preliminary experimental observations prompted us to examine this subject analytically and to verify the analysis by further experimentation. Section II outlines the derivations of the AM-FM spectral density when the baseband modulation is a synchronous digital data signal. Section III repeats the derivation for asynchronous data. Section IV discusses numerical and experimental results