A highly frequency selective circuit that includes a twin-T notch filter connected in the feedback path of a transistor is disclosed. Also included in the feedback path is an isolation network formed of a pair of emitter coupled transistors. One of the isolation transistors is fed by the feedback signal from the twin-T notch filter and the other is fed by the input signal to the overall frequency selective circuit.
A combining and filter circuit is provided to connect the acquisition circuit and phase detector circuit to the voltage controlled oscillator in a phase locked loop. The combining and filter circuit includes twin-T filters for removing reference frequency signals. An output circuit includes series resistors and capacitors, and reverse poled Schottky diodes which provide rapidly changing acquisition signals that are relatively noise free.
