Signals from phase-modulated satellite transmitters usually exhibit some degree of incidental amplitude modulation. The effects of incidental AM are analyzed when this type of signal is demodulated by a phase-lock receiver which does not employ a limiter preceding the loop phase detector. The presence of incidental AM causes a reduction in the receiver output signal-to-noise ratio. The tolerable level of AM decreases in proport ion to the phase modulation index Beta. For a square-wave modulating sig nal, a 1 db reduction results at the receiver PM channel output when Beta = 1 radian and the percentage of AM = 23, Beta = 1.2 radians and the percentage of AM = 16, or Beta = 1.5 radians and the percentage of AM = 4. Although only the PM channel of the receiver is used ordinarily, utilizing both the AM and PM channel by summing offers an improvement in SN relative to the SN ratio of the PM channel if the percentage of foi HC incidental AM is greater than fifteen. (Author)
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