Comparator and filter based FSK and DPSK modem demodulator - uses 32 tap CCD or BBD transversal filters with adapted weights supplying outputs to paired comparators and AND gate

摘要

The comparator based demodulator uses transversal filters produced in CCD or BBD devices with weighted tops adapted to the type of modem in use, comparators and logic AND gates to replace the more complicated voltage squaring circuitry in conventional modems. The filters have 32 weighted tops and the comparators thresholds at plus and minus 0.7 maximum input voltage. The fitters are designed to produce null or quadrature outputs. A DPSK demodulator, consisting of two fitters produces outputs corresponding to the correlation of the input over a period less than a band. One has sine coefficients, the other cosine. The outputs are coupled to two comparators which are fed into a logic AND gate. An FSK demodulator has two channels each with two fitters, both fitters have the same output at one modem frequency and one fitter has a zero at the other frequency. The technique may be applied for example to DPSK demodulation at 1200 bands with a 1800 Hz carrier using an input sampling frequency of 28800 Hz. The comparator threshold levels give maximum immunity to parasitic noise induced errors. The circuit elements used are easily integrable.