Effects of modulator deficiencies and amplifier nonlinearities on the phase accuracy of GMSK signalling

Both simulated and experimental results are presented for the phase trajectory and spectrum of a GMSK signal generated by imperfect quadrature modulation. It is shown that modulator deficiencies produce phase distortion and, from this, the RMS phase error of an imperfectly generated signal can be predicted. It is also shown that spectral spreading of the bandlimited signal occurs and the RMS phase error increases significantly when the imperfect GMSK signal is passed through any nonlinear two-port such as a class-C HPA. Consequently, both sources of error have to be considered together when evaluating a communications system that employs nonlinearly amplified GMSK.<>