PM, Vector Composing, Armstrong PM, QAM-based PM, Equivalent Analysis Abstract. Both the Armstrong phase modulation (PM) and QAM-based PM are typical vector composing circuits in the course “Radio Frequency Circuits”. Due to having innate defects of nonlinear phase distortion and parasitical AM, the characteristics of Armstrong PM are not good unless the modulation exponent is small enough. Therefore, the Armstrong PM is an approximate circuit. The QAM- based PM is a quite different circuit from the Armstrong PM, and its defects are not given in general teaching material. The equivalence of two circuits is proved in this paper by mathematical method. So, the QAM-based PM has same pros and cons with Armstrong PM and can be replaced with the latter completely.