Phase-locked loop and method for automatically setting its output frequency

摘要

A phase-locked loop (10) comprises a voltage-controlled oscillator (12) to which a control voltage is applied as produced by a phase/frequency detector (22) as a function of the difference between the frequency (fref) of a reference signal and the output frequency (fvco) of the voltage controlled oscillator (12) and the oscillator contains as a frequency-influencing circuit element a varactor (14) whose capacitance value can be varied over a fine adjustment range by the control voltage for altering the output frequency. A variable capacitance (18) is provided which can be connected in parallel to the varactor (14) when there is a change in the frequency (fref) of the reference signal, the value of this capacitance (18) being adjustable as a function of the control voltage output by the phase/frequency detector (22).