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TC1043 数据表(PDF) 8 Page - Microchip Technology |
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TC1043 数据表(HTML) 8 Page - Microchip Technology |
8 / 20 page TC1043 DS21347B-page 8 © 2002 Microchip Technology Inc. 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. V L V DD 3 ----------- = 1 FREQ ----------------- 2 0.694 () R4 () C1 () = |
类似零件编号 - TC1043 |
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类似说明 - TC1043 |
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