A Simple Modulator for Sinusoidal Frequency Shift Keying

This paper describes a method of generating a constant-envelop sinusoidal-frequency-shift-keyed spread-spectrum signal from a sequence of phase coded rectangular pulses. In-phase and quadrature pulse streams are combined and passed through a SAW filter. The constant time delay property of the SAW filter is used to transform the rectangular pulses into the proper shape. Time and frequency domain outputs are shown for a modulator operating at 70 MHz and a chip rate of 5 Mchip/s.