DS00795A-page 9

© 2002 Microchip Technology, Inc.

AN795

FREQUENCY/TONE DECODER

The frequency, or tone, to be detected is converted into a proportional analog voltage by the TC9400 F/V converter. The quad

comparators sense when the voltage (frequency) exceeds any of the four preset frequency limits. A logical "1" at any of the five

outputs indicates the frequency is within those limits.

This system is useful for determining which frequency band a signal is in, or for remote control, where each frequency band

corresponds to a different command.

FM DEMODULATION WITH A PHASE-LOCKED LOOP

The high linearity of the TC9400 (0.01%) is used to greatly improve the performance of a phase-locked loop, resulting in very precise

FIGURE 19: Frequency/tone decoder.

FIGURE 20: FM demodulation with a phase-locked loop.

+

–

TC9400

F/V

+

–

+

–

+

–

Quad

Comparator

Frequency Set

TC9400

V/F

Frequency

Comparator

Loop

Filter