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Design of a quadrature modulator consisting of gaAs‐FET double‐balanced mixers

In order to realize a mobile communication system using MSK narrow-band digital FM modulation, a high-accuracy and low-dissipation power IC modulator is needed. In this paper a digital IC double-balanced mixer, employing a D-A converter and FET analog switch, is proposed and the design method of a quadrature modulator using this kind of mixer is explained. The spurious level contained in the modulated output signal is analyzed; then the accuracy required of the D-A converter in order to reduce the spurious level is determined. Next, the relationship is obtained between the upper limit of the carrier frequency of the signal, whose spurious level specification is satisfied by the modulator, and the FET parameters. GaAs FET measurements confirm the analytical results, namely, a quadrature modulator with lower than -40 dB spurious level up to about 1 GHz can be achieved.