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Published byConstance Rodgers Modified over 8 years ago
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MALVINO Electronic PRINCIPLES SIXTH EDITION
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Oscillators Chapter 23
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Sinusoidal oscillation requires both the correct phase and loop gain.
= 0 = positive feedback A vout B AB = 1 AB < 1 AB > 1
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Sinusoidal oscillators
The starting signal is thermal noise. AB > 1 at startup (AB 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 or B is eventually decreased so that AB = 1.
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equilibrium is reached
Wien-Bridge oscillator vout 2pRC 1 fr = RL 2R’ R’ Tungsten lamp Resistance of lamp increases until equilibrium is reached R’ C R R C vout t
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Colpitts oscillator +VCC C2 Amin = C1 C1 B = C2 RFC R1 vout C3 C1 L
RE CE 2p LC 1 fr =
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Colpitts CB oscillator
+VCC C1 C1 +C2 B = C1 C2 C1 +C2 C = RFC R1 2p LC 1 fr = C4 C1 C3 L RL C2 R2 RE C1 Amin = C1 +C2
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Hartley oscillator +VCC L1 Amin = L2 L2 B = L1 RFC R1 vout C L1
L = L1 + L2 R2 R3 L2 2p LC 1 fr =
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Crystal-controlled oscillators are used
when frequency stability is important. Quartz crystal Slab cut from crystal Schematic symbol Electrodes and leads
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Crystal oscillator +VCC RFC R1 vout C1 Xtal C2 R2 R3 CE
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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).
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Monostable operation of the 555 timer IC
+VCC T T = trigger 8 3 2 vout T 555 1 W vout
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Astable operation of the 555 timer IC
+VCC 8 3 555 vout 1
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555 +VCC 8 Discharge 7 5 kW Threshold 6 VCC S Q Control 5 kW 3 5 R Q
UTP VCC S Q Control 5 kW 3 5 R Q LTP VCC Out Trigger 2 5 kW 1 Gnd 4 Reset
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A discrete RS flip-flop
+VCC S R RC RC Q Q Q Q RB RB RS RR S R
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555 IC configured for monostable operation
+VCC 8 W = 1.1RC R 7 6 W C 3 2 VCC 1
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555 IC configured for astable operation
+VCC W = 0.693(R1 + R2)C 8 7 R1 + R2 R2 D = R1 + 2R2 6 3 2 W C 1 T T = 0.693(R1 + 2R2)C
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555 voltage-controlled oscillator
+VCC VCC - Vcon W = -(R1 + R2)C ln 8 VCC - 0.5Vcon 7 Vcon applied to pin 5 R2 6 5 3 W R2C 1 f = 2 C 1 +Vcon + Vcon
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The output frequency is established by the input clock.
Pulse-width modulation with the 555 timer IC T The output frequency is established by the input clock. +VCC 8 4 3 UTP = VCC + vmod W = -RC ln 1 - UTP VCC ( ) W R 7 PWM out 555 A Modulating signal in 6 B 5 C 2 1 Clock in
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The leading edge of each pulse is a function of the modulation.
Pulse-position modulation with the 555 timer IC The leading edge of each pulse is a function of the modulation. R1 +VCC Pulse width is variable 8 4 3 7 PPM out 555 R2 Space is constant A 6 C 5 Modulating signal in 2 1 B Space = 0.693R2C
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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.
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