Digital Communications Corporation

Rixon Inc.

Lester R Querry

Ajay Parikh

An improved phase-locked loop includes a phase detector, active loop filter and a VCO, in conventional phase-locked loop relationship. In addition, a positive feedback path couples active loop filter output to active loop filter input. In the absence of loop lock, the positive feedback provided by the positive feedback path satisfies the conditions necessary for the production of an oscillating output from the active loop filter circuit. The oscillating output generated by this 'oscillator' drives the VCO in a sweeping action allowing the loop to acquire signals outside of its normal capture range. On capture, phase detector output provides an additional negative feedback path which overrides the effect of the positive feedback path and therefore, oscillations cease and the loop locks. The positive feedback path can comprise a relatively simple passive frequency selective circuit.

Phase locked loop with automatic sweep

A direction phase shift keyed communication system provides the capacity to transmit any one of two symbols for each modulation level. The modulator produces a substantially constant frequency signal which includes a phase transistion in respect of each information bit. In a system with a single modulation level, corresponding to a phase transistion .phi., one of the two symbols is represented by a phase transition +.phi., i.e., an advancing phase transition. The other symbol is represented by a phase transition -.phi., i.e., a retarding phase transition. A demodulator responds to a substantially constant frequency signal with a phase transition included, and produces an output signal of a first kind in response to an advancing phase transition +.phi. and an output signal of a second kind in response to a retarding phase transistion -.phi.. The system is capable of operating with multiple modulation levels, each of which can represent any one of two symbols, depending on the sense, i.e., advance or retad, of the transistion. Preferably, each modulation level is 180.degree. or a multiple thereof.

Directional PSK modulation and demodulation system

A phase-locked demodulator for demodulating a phase-difference modulated carrier having a repeated phase offset increment introduced therein. The demodulator includes, in common with conventional phase locked demodulators, a mixer having a first input connected to receive the modulated carrier and a reference carrier generator such as a voltage controlled oscillator (VCO) connected in a phase-locked feedback loop forming the second input to the mixer. In order to reduce the number of output levels, the VCO is tuned to a frequency Fr=Fc.+-.Fb[(.alpha.)/360] where Fc is the carrier frequency, Fb is the baud rate and .alpha. is the repeated phase offset. In this way the phase offset increment introduced into the modulated carrier is substantially cancelled out thereby leaving only the phase changes containing data information.

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Laurel, MD, United States of America

Lester R Querry