On "Distortion of Single-Sideband Transmission Due to AM/PM Conversion"

In a previous paper, this author developed an expression for the intermodulation distortion generated in single-sideband (SSB), amplitude modulation (AM) systems carrying frequency-division multiplex (FDM) telephony, due to AM/PM conversion. For the purpose of the theory, the SSB signal is replaced by an equivalent system consisting of a carrier simultaneously phase and amplitude modulated. To simplify the analysis, the AM/PM conversion factor was assumed constant over the significant range of variation of the envelope of the effective carrier. This is not, in fact, a very realistic assumption since the AM/PM factor varies quite, considerably with the input power level. Particularly in the case of a traveling wave tube working well below saturation, the dependence is likely to be linear It has subsequently been realized that the analysis is still quite tractable when the AM/PM factor is proportional to the input power level. In fact, on this assumption, it turns out that the expression given previously for the intermodulation distortion (expression (14) of Medhurst et al. [ibid., vol. CS-12, pp. 166-176, June 1964]) applies, if the AM/PM conversion factor is given its value at the rms level of the multichannel SSB signal. This is a useful result, since it appears that for acceptable distortion levels in TWT's used in satellite system ground stations, for example, it would be necessary to operate the tube in this linear region. The analysis is presented.