Spectral Shaping by Simultaneous Amplitude and Frequency Modulation
J. Salz, Jean-Jacques Werner · Bell System Technical Journal · 1980
In an attempt to simplify the design of separation filters in frequency-division multiplexed digital data transmission by FSK, it was found that the energy on one side of the FSK carrier can be reduced by suitably modulating the amplitude of the FSK signal. We present a simple technique for controlling the energy on one side of the carrier of an FSK signal. The method entails modulating the amplitude of the FM wave in unison with the frequency. In other words, if the energy in the region above the carrier must be reduced, the envelope is somewhat suppressed while the higher frequency is transmitted. The proposed hybrid modulation technique is applicable in frequency-division multiplexed digital data transmission by FSK and can often ease the design of separation filters. Here we investigate this modulation technique theoretically as well as experimentally. Our theoretical results include closed-form formulas for the various power spectra of interest. We also determine the optimum amount of amplitude modulation to achieve the greatest amount of sidelobe energy suppression for a fixed amount of total average power. The experimental investigations substantiate the analysis, and we make comparisons of the technique with alternative approaches.