Digital modulator and synchronous demodulator system: an alternative to the analog phase detector

A digital modulator and synchronous demodulator system has been developed which more effectively recovers signals buried in noise than does its analog counterpart. Circuitry of integrated design is described for controlling the modulation in physical measurements and the simultaneous, synchronous demodulation and conversion of the signal into a binary number. This system is especially useful when interfaced with a laboratory computer. The square wave which produces the modulation has purposely lengthened transition times to minimize modulation transients. Demodulation is accomplished by feeding the amplified AM signal into a voltage-controlled oscillator and counting the resulting pulses with counters gated, by the modulation reference circuitry, to count up and down on alternate half-cycles. The number accumulated by the counters is proportional to the true integral of the demodulated signal over a preset number of modulation cycles. The period of the modulation cycle is, in turn, determined by a quartz crystal oscillator. The demodulator can be gated to ignore excessively noisy portions of the modulated signal. Four demodulators with relative gains in ratios of eight operate simultaneously to provide a large dynamic range of more than 10(6). Application of this system to a computer-controlled microwave spectrometer is described. A sample spectrum is given to illustrate the wide dynamic range.