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Electronic PRINCIPLES MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 2323 Oscillators Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Topics Covered in Chapter 23 • Theory of sinusoidal oscillation • The Wien-bridge oscillator • Other RC oscillators • The Colpitts oscillator • Other LC oscillators Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Topics Covered in Chapter 23 (Continued) • Quartz crystals • The 555 timer • Astable operation of the 555 timer • 555 circuits • The phase-locked loop • Function generator ICs Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sinusoidal oscillation requires both the correct phase and loop gain. φφφ = 0° = positive. feedback A v V . out B AVB = 1 AVB < 1 AVB > 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sinusoidal oscillators • The starting signal is thermal noise. • AVB > 1 at startup (A VB is the loop gain). • The feedback network determines B and the phase of the feedback. • Only one frequency arrives at the input as an in-phase signal (positive feedback). • Either A V or B is eventually decreased so that A VB = 1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Wien-bridge oscillator • Used for low to moderate frequencies (5 Hz to 1 MHz) • Produces an almost perfect sine wave • Nonlinear resistance is used to decrease loop gain to 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Wien-Bridge oscillator circuit .. vout 1 . fr = RL 2πππRC 2R ’ R’ Tungsten lamp Resistance of lamp C R increases until . equilibrium is reached R’ R C vout t Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Other RC oscillators • Twin-T uses an amplifier and RC circuits to produce the required loop gain and phase shift • Twin-T works well at only one frequency • Phase shift uses an amplifier and RC circuits Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Colpitts oscillator • LC oscillators are preferred for frequencies between 1 MHz to 500 MHz • One of the most widely used oscillators • Recognized by the capacitive voltage divider Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Colpitts oscillator circuit +V CC C2 C AVmin = B = 1 C1 RFC C2 R1 . vout C 3 . C1 . L . C C1 C2 2 C = R2 C1 +C 2 RE CE 1 f = r 2πππ LC Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Other LC oscillators • Armstrong uses a transformer to produce a feedback signal • Hartley uses an inductive voltage divider to produce a feedback signal • Clapp has a small series capacitor in the inductive branch of the resonant circuit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V Colpitts CB oscillator .CC C C1 C2 B = 1 C = RFC C1 +C 2 C1 +C 2 R1 . 1 C fr = . 4 . 2πππ LC C C 1 3 .. L C RL R2 2 R E . C1 +C 2 AVmin = C1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V CC Hartley oscillator L1 . L AVmin = B = 2 L2 RFC L1 R1 . .. vout . C L . 1 L = L + L R2 . 1 2 R3 L 2 1 f = r 2πππ LC Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Crystal-controlled oscillators are used when frequency stability is important. Quartz crystal Slab cut from Schematic crystal symbol Electrodes and leads Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +V Crystal oscillator .CC RFC R1 . vout . C1 . Xtal C2 R 2 . R3 CE Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Crystals • The fundamental frequency (series resonance) is controlled by the slab thickness. • Higher multiples of the fundamental are called overtones . • The electrode capacitance creates a parallel resonant frequency which is slightly higher. • Typical frequency accuracy is measured in parts per million (ppm). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The 555 timer • Contains two comparators, an RS flip- flop, and an npn transistor • Has an upper trip point ( UTP ) and a lower trip point ( LTP ) • In the monostable mode, the input triggers must fall below the LTP to start the action Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Monostable operation of the 555 timer IC +V CC T T = trigger 8 2 3 T 555 vout W 1 vout Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Astable operation of the 555 timer IC +V CC 8 3 555 vout 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 8 +V CC Discharge 7 5 k ΩΩΩ Threshold 555 6 UTP 2V CC /3 SQ Control 5 k ΩΩΩ 3 5 R Q VCC /3 LTP Out Trigger 2 5 k ΩΩΩ 1 Gnd 4 Reset Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A discrete RS flip-flop +V CC S R RC Q RC Q Q Q RB RB RS RR S R Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 555 IC configured for monostable operation +V CC 8 W = 1.1RC R . 7 C 6 W 3 2 1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Astable operation of the 555 • Produces a rectangular output whose duty cycle can be set between 50 and 100 percent • A control voltage is sometimes used to determine the frequency Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 555 IC configured for astable operation R 1 +V CC W = 0.693(R + R )C . 8 1 2 7 R1 + R 2 R2 D = 6 R1 + 2R 2 . 3 2 W C 1 T T = 0.693(R 1 + 2R 2)C Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 555 voltage-controlled oscillator R1 +V CC V -V W = -(R + R )C ln CC con 8 1 2 V - 0.5V . CC con 7 Vcon applied to pin 5 R2 6 5 3 1 . f = W + 0.693R 2C 2 C 1 +V con +V con /2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 555 circuits • Time delays • Alarms • Ramp outputs • Pulse-width modulators • Pulse-position modulators Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pulse-width modulation with the 555 timer IC The output frequency is +V CC established by the input clock. 8 4 T W R 7 3 PWM out 555 A .. Modulating 6 5 B signal in C 2 1 UTP = 2V CC /3 + v mod UTP Clock in W = -RC ln 1 - V T ( CC ) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pulse-position modulation with the 555 timer IC The leading edge of each pulse R is a function of the modulation. 1 +V CC Pulse width is variable . 8 4 7 3 PPM R 555 Space is constant . .2 out 6 A C 5 Modulating 2 1 B signal in Space = 0.693R 2C Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phase-locked loops • It is possible to phase-lock an oscillator to a signal by using a phase detector and negative feedback . • PLLs can be used to remove noise from a signal. • PLLs can be used to demodulate an FM signal. • PLLs are available as monolithic ICs. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Function generator ICs • Ability to produce sine, square, triangle, pulse, and sawtooth waveforms • By connecting external resistors and capacitors, output waveforms can be made to vary in frequency and amplitude • AM/FM, voltage-to-frequency conversion, and FSK can also be performed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display..
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