Side locked loop with circuit for preventing sidelock

Details Side locked loop with circuit for preventing sidelock Side locked loop with circuit for preventing sidelock

H - Electricity

03

D

329/3

H03D 3/02 (2006.01)

Patent

CA 1072189

Phase Locked Loop With Circuit For Preventing Sidelock Abstract of the Disclosure A side lock detector to prevent side lock in coherent PSK demodulation. For demodulation purposes it is necessary to recreate the carrier at the receiver. This may be accomplished with a phase-locked loop. However, with plural phase PSK mod- ulation the input signal to the phase-locked loop has energy at side bands spaced at modulation rate intervals from the desired lock frequency as well as at the desired lock frequency. To detect side band lock, a band pass filter monitors the phase detector output of the phase-locked loop. The center frequency of the filter is equal to the modulation rate and the filter band width is very narrow. Only during side lock will signifi- cant amounts of energy pass the filter. This energy may be detected by a simple diode detector or the like. A lock inhibit means is provided, responsive to the output of the detector, to drive the voltage controlled oscillator out of a side lock condition. The lock inhibit means may comprise a relay which, when energized, closes its contacts to ground the phase detector output in order to inhibit a locked condition. By making the relay slow to drop away the oscillator is sufficiently removed from a side lock frequency by the time the relay drops away to prevent oscillating into and out of side lock. Alternatively, the relay contacts may ground the phase detector input with similar results. As a further alternative, a sweep voltage may be added to the low pass filter output of the phase locked loop to drive the oscillator through a range of frequencies. A lock detector de-energizes the sweep voltage at lock. In the case of side lock, however, the detected side lock condition again energizes the sweep voltage to drive the oscillator away from the side lock frequency.

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