Improved digital PSK modulator for modem.

摘要

An improved four-phase differential phase shift keyed modulator for use in a modem is shown. A microprocessor (15) generates two vectors of equal frequency and equal magnitude, but different relative phases, by reference to a cyclic sine table contained in a read only memory (22). Values of the two vectors are summed and provided as an output (32) to a digital to analog converter (35). The phase shift of the carrier in a phase plane with reference to a predetermined reference phase shows that the path taken by a vector in the phase plane defines a straight line between two phase values by which dibits are encoded. The result is lower out-of-band transmitted signals resulting in a relatively simple transmit filter (37).