Suppressed clock pulse-duration modulator for direct sequence spread spectrum transmission systems

摘要

In the disclosed digital circuit, a digital direct sequence modulation signal is converted to a suppressed clock pulse-duration modulation signal to thereby suppress the clock feature in the frequency spectra of a spread spectrum transmission system. The disclosed digital circuit includes a parallel output shift register for converting the direct sequence modulation signal to a corresponding series of four-bit digital words. The digital words supplied by the shift register are loaded into a four-bit synchronous binary counter circuit. The counter circuit counts upwardly from the value of the digital word and supplies a carry pulse to a logic circuit which, in turn, produces an output pulse, the duration of which is representative of value of the digital word supplied by the shift register. The pulse-duration modulation signal supplied by the logic circuit is modulo-2 added with a signal having one-half the clock rate of the information embedded in the direct sequence modulation signal to supply the suppressed clock pulse-duration modulation signal.