PHASE LOCKED LOOP TUNING CONTROL SYSTEM

摘要

In a phase locked loop tuning system including a phase comparator for producing a pulse type error signal representing the phase and frequency deviation between a frequency divided version of the local oscillator signal and a reference frequency signal and a mode switch for coupling the error signal to an integrator for developing a tuning voltage when a channel is initially selected and thereafter for coupling an AFT voltage to the integrator to modify the tuning voltage to correct for a frequency deviation of the IF signal, switching duty cycle control apparatus is provided for controlling the duty cycle at which the AFT voltage is coupled to the integrator in accordance with the selected channel and more particularly with its band to make the AFT response more consistent throughout the tuning range.