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| SEC. 23-1 THEORY OF
SINUSOIDAL
OSCILLATION To build a sinusoidal oscillator, we need to use an amplifier with positive feedback. For the oscillator to start, the loop gain must be greater than 1 when the phase shift around the loop is 0°. SEC. 23-2 THE WIEN-BRIDGE OSCILLATOR This is the standard oscillator for low to moderate frequencies in the range of 5 Hz to 1 MHz. It produces an almost perfect sine wave. A tungsten lamp or other nonlinear resistance is used to decrease the loop gain to 1. SEC. 23-3 OTHER RC OSCILLATORS The twin-T oscillator uses an amplifier and RC circuits to produce the required loop gain and phase shift at the resonant frequency. It works well at one frequency but is not suitable for an adjustable frequency oscillator. The phase-shift oscillator also uses an amplifier and RC circuits to produce oscillations. An amplifier can act like a phase-shift oscillator because of the stray lead and lag circuits in each stage. SEC. 23-4 THE COLPITTS OSCILLATOR RC oscillators usually do not work well above 1 MHz because of the additional phase shift inside the amplifier. This is why LC oscillators are preferred for frequencies between 1 and 500 MHz. This frequency range is beyond the funity of most op amps, which is why a bipolar junction transistor or FET is commonly used for the amplifying device. The Colpitts oscillator is one of the most widely used LC oscillators. SEC. 23-5 OTHER LC OSCILLATORS The Armstrong oscillator uses a transformer to produce the feedback signal. The Hartley oscillator uses an inductive voltage divider to produce the feedback signal. The Clapp oscillator has a small series capacitor in the inductive branch of the resonant circuit. This reduces the effect that stray capacitances have on the resonant frequency. SEC. 23-6 QUARTZ CRYSTALS Some crystals exhibit the piezoelectric effect. Because of this effect, a vibrating crystal acts like an LC resonant circuit with an extremely high Q. Quartz is the most important crystal producing the piezoelectric effect. It is used in crystal oscillators, in which a precise and reliable frequency is needed. SEC. 23-7 THE 555 TIMER The 555 timer contains two comparators, an RS flip-flop, and an npn transistor. It has an upper and lower trip point. When used in the monostable mode, the input triggers must fall below LTP to start the action. When the capacitor voltage slightly exceeds UTP, the discharge transistor turns on to discharge the capacitor. SEC. 23-8 ASTABLE OPERATION OF THE 555 TIMER When used in the astable mode, the 555 timer produces a rectangular output whose duty cycle can be set between 50 and 100 percent. The capacitor charges between VCC/3 and 2VCC/3. When a control voltage is used, it changes UTP to Vcon. This control voltage determines the frequency. SEC. 23-9 555 CIRCUITS The 555 timer can be used to create time delays, alarms, and ramp outputs. It can also be used to build a pulse-width modulator by applying a modulating signal to the control input and a train of negative-going triggers to the trigger input. The 555 time can also be used to build a pulse-position modulator by applying a modulating signal to the control input when the timer is in the astable mode. SEC. 23-10 THE PHASE-LOCKED LOOP A PLL contains a phase detector, a dc amplifier, a low-pass filter, and a VCO. The phase detector produces a control voltage that is proportional to the phase difference between its two input signals. The amplified and filtered control voltage then changes the frequency of the VCO as needed to lock on to the input signal. SEC. 23-11 FUNCTION GENERATOR ICS Function generator ICs have the ability to produce sine, square, triangle, pulse, and sawtooth waveforms. By connecting external resistors and capacitors, the output waveforms can be made to vary in frequency and amplitude. Special functions including AM/FM generation, voltage-to-frequency conversion, and frequency-shift keying can also be performed by these ICs. |