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Abstract

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.

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