Tone Detector

FX105A

© 1999 Consumer Microcircuits Limited

6

D/105A/5

1.5

General Description

The input signal to the FX105A is ac coupled to the buffer amplifier input, which is internally biased at

50% of supply voltage. The signal appears at the output of the buffer amplifier as an ac voltage

form four sample-and-hold RC circuit integrators. See Figure 3.

With no input signal level, each capacitor charges to the dc bias level so differential voltages are zero.

When an input signal is applied each capacitor receives an additional charge. This charge is

determined by the integrated average of the signal waveform during the time the capacitor is switched

into the circuit.

Figure 3 shows the operating sequence of the VCO sampling switches and their relationship to a

sample the input as it crosses the dc bias level. Should the signal not be locked to the VCO, a

amplified, is applied to the VCO as an error correcting signal to enable the VCO to “lock.”

Figure 3 also shows the operating sequence of the “Word” sampling switches and their relationship to

These capacitor potentials are differentially amplified and applied to a dc comparator, which switches

output is a logic signal used to control a counter. This counter switches the FX105A output ON when

the comparator output is maintained in the “Word present” state for a minimum number of consecutive

signal samples. The activated output switch reduces the comparator threshold by 50%, introducing

threshold hysteresis. Output chatter with marginal input signal amplitudes is thereby minimised.

Figure 3 Sampling Clocks of Commutating Filters