Design of digital signal processing quadrature modulator for digital fm and analog FM signals

Abstract

In the field of mobile communication, the transmission of picture and data signals in addition to conventional speech transmission requires modulators that are suitable to both analog and digital FM. In the present paper such a quadrature modulator is proposed which provides stable transmission characteristics by digital signal processing (DSP) techniques. The analysis is mainly aimed at additional noise effects due to the digital generation of analog FM signals. The relations between the sampling frequency used in the D‐A conversion of analog speech signals and the sideband noises accompanying modulation are developed. The relations are given between the number of bits used for quantization in the A‐D conversion and the baseband signal/noise ratio in the case of FM or PM reception of the analog FM wave generated by the proposed modulator. Based on the results of this analysis, a modulator design method providing sufficiently high noise suppression is presented in a flowchart form. The results of measurement of a quadrature modulator realized by the proposed method are shown to agree well with the results of analysis. The agreement demonstrates that the required modulator characteristics can be obtained without tuning.