Posted on Apr 4, 2012 6019
For some applications it is important to have a tone decoder with narrow bandwidth and fast response time. This can be accomplished by the dual time constant tone decoder circuit shown. The circuit has two low-pass loop filter capacitors, C2 and C'2. With no input signal present, the output at pin 8 is high, transistor Qj is off, and C'2 is switched out of the circuit. Thus, the loop low-pass filter is comprised of C2, which can be kept as small as possible for minimum response time. When an in-band signal is detected, the output at pin 8 will go low, will turn on, and capacitor C'2 will be switched in parallel with capacitor C2.
The low-pass filter capacitance will then be C2 + C'2. The value of C'2 can be quite large in order to achieve narrow bandwidth. During the time that no input signal is being received, the bandwidth is determined by capacitor C2.