ES366790A1 - Improvements in or relating to automatic frequency control circuits - Google Patents

Abstract

In an afc system of the type in which the oscillator to be controlled is swept over a frequency band until it reaches the lock-in range of the control loop, the discriminator comprises two selective amplifiers 4, 5 symmetrically detuned relative to the discriminator centre frequency, and means are provided to discontinue the sweep only when the signal sweeps through the two amplifiers' passbands in a predetermined direction. This ensures that, if the oscillator forms the local oscillator of a receiver, the oscillator is only locked on the correct side of the received carrier. As described, the oscillator 1 to be controlled is frequency swept by sawtooth generator 17, and its output is mixed at 3 with the output from a reference oscillator 2 to produce a difference frequency which is applied to the two tuned amplifiers 4, 5. The output from the two amplifiers is fed to a differencing circuit 6 which provides the error control signal UF. The amplifier outputs are also fed to respective threshold circuits 7, 8 which produce outputs as the signal from mixer 3 sweeps through the amplifier passbands. If the sense of sweep is such that threshold 8 is triggered before threshold 7, the output from threshold circuit 8 is passed through AND gate 15 to set bi-stable 12 which enables AND 14. The subsequent triggering of threshold 7 then produces a signal which is passed by AND 14 to trigger bi-stable 11, the output from which actuates a changeover switch 16 to disconnect the sweep and connect the error signal UF to the oscillator 1. With the opposite sense of sweep, threshold 7 triggers before threshold 8, gates 14, 15 remain inhibited and bi-stable 11 is not triggered. Changeover switch 16 therefore remains in the position in which the sawtooth generator 17 provides a sweep signal to oscillator 1.