TC1043 datasheet(8/20 Pages) MICROCHIP

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TC1043

DS21347B-page 8

Capacitor C1 will charge up through R4. When the volt-

age at the comparator’s inverting input is equal to VH,

the comparator output will switch. With the output at

ground potential, the value at the non-inverting input

terminal (VL) is reduced by the hysteresis network to a

value given by:

EQUATION 4-7:

Using the same resistors as before, capacitor C1 must

now discharge through R4 toward ground. The output

will return to a high state when the voltage across the

capacitor has discharged to a value equal to VL.The

period of oscillation will be twice the time it takes for the

RC circuit to charge up to one-half its final value. The

period can be calculated from:

EQUATION 4-8:

The frequency stability of this circuit should only be a

function of the external component tolerances.

Figure 4-8 shows the circuit for a pulse width modulator

circuit. It is essentially the same as in Figure 4-7 with

the addition of an input control voltage. When the input

control voltage is equal to one-half VDD, operation is

basically the same as described for the free-running

oscillator. If the input control voltage is moved above or

below one-half VDD, the duty cycle of the output square

wave will be altered. This is because the addition of the

control voltage at the input has now altered the trip

points. The equations for these trip points are shown in

Figure 4-8 (see VH and VL).

Pulse width sensitivity to the input voltage variations

can be increased by reducing the value of R6 from

10k

Ω and conversely, sensitivity will be reduced by

increasing the value of R6. The values of R1 and C1

can be varied to produce the desired center frequency.

4.8

Voice Band Receive Filter

The majority of spectral energy for human voices is

found to be in a 2.7kHz frequency band from 300Hz to

3kHz. To properly recover a voice signal in applications

such as radios, cellular phones, and voice pagers, a

low power bandpass filter that is matched to the human

voice spectrum can be implemented using MIcrochip’s

CMOS op amps. Figure 4-9 shows a unity gain multi-

pole Butterworth filter with ripple less than 0.15dB in

the human voice band. The lower 3dB cut-off frequency

is 70Hz (single order response), while the upper cut-off

frequency is 3.5kHz (fourth order response).

4.9

Supervisory Audio Tone (SAT)

Filter for Cellular

Supervisory Audio Tones (SAT) provide a reliable

transmission path between cellular subscriber units

and base stations. The SAT tone functions much like

the current/voltage used in land line telephone systems

to indicate that a phone is off the hook. The SAT tone

may be one of three frequencies: 5970, 6000 or

6030Hz. A loss of SAT implies that channel conditions

are impaired and if SAT is interrupted for more than 5

seconds a cellular call is terminated.

Figure 4-10 shows a high Q (30) second order SAT

detection bandpass filter using Microchip’s CMOS op

amp architecture. This circuit nulls all frequencies

except the three SAT tones of interest.

-----------

FREQ

-----------------

2 0.694

() R4

() C1

()

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