The two most common approaches for analog phase modulators have been full phase deviation modulation at RF and frequency (and phase deviation) multiplication. An example of the second approach: 1) phase modulate a 10 MHz sub-carrier at .05 radians; 2) pass this signal through a nonlinear device (comb generator/step recovery diode) and 3) filter it with a bandpass filter centered at 200 MHz. The resultant RF signal has a one radian deviation @ 200 MHz. Both the sub-carrier frequency and phase deviation are multiplied by twenty. These traditional analog approaches have drawbacks. The full phase deviation approach suffers from limited deviation range and an exponential, not linear, transfer function from control voltage input to phase deviation output. The frequency multiplier approach is less affected by these limitations, but requires a complex bandpass filter at the final frequency to eliminate equal power harmonics that are offset by the sub-carrier frequency (190 MHz and 210 MHz in the example).

The Phase Modulator discussed in this paper utilizes a mixed signal approach combining both analog and digital signal techniques. The Mixed Signal Phase Modulator (MS PM) does not have the drawbacks of purely analog phase modulators. The hardware complexity and real estate requirements are about the same for both the traditional analog modulators and the MS PM. In addition, the MS PM has several advantages such as easily and accurately adjustable frequency and modulation index.