DIGITAL COMMUNICATIONS EQUIPMENT USING DIFFERENTIAL QUATERNARY FREQUENCY SHIFT KEYING

摘要

A digital radio transceiver uses a differential 4-ary frequency shift keying (D4FSK) modulation scheme to achieve a radio that is low cost, miniaturizable, low in power consumption and high in spectral efficiency. An adaptive digital "slicer" (31) looks at a digitized FM video signal produced in the receiver to determine if the data sent is a 00, 01. 10 or 11. The digitized FM video signal is first processed by a differential decoder (31), the output of which is input to a peak detector. The peak detector outputs a signal upon the occurrence of a three-step transition. Two and three step transition are separately detected and used to perform digital clock recovery. A carrier detect circuit uses a digital bit filter to produce a carrier detect signal (CD) that is reliable and glitch-free. The digital radio transceiver uses frequency hopping spread spectrum transmission (23). A switchable loop bandwidth frequency synthesizer provides for glitch-free switching between a wide hopping bandwidth and a narrow bandwidth between hops.