Exar XR-567 PLL system is connected as de-coder having narrow bandwidth and fast response time. Circuit has two low-pass loop filter capacitors,C2 and C'2. With no input, pin 8 is high,Q1 is off, and C'2 is out of circuit. Filter then has only C2, which is kept small for minimum response time. When in-band input tone signal is detected, pin 8 goes low, Q1 turns on, and C'2 is in parallel with C2 to give narrow bandwidth. Supply voltage can be 5-9 V.-"Phase-Locked Loop Data Book," Exar Integrated Systems, Sunnyvale, CA, 1978, p 41-48.