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MALVINO

SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. Oscillators. Chapter 23. Sinusoidal oscillation requires both the correct phase and loop gain.  = 0 = positive feedback. v out. A. B. AB = 1. AB < 1. AB > 1. Sinusoidal oscillators. The starting signal is thermal noise.

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MALVINO

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  1. SIXTH EDITION MALVINO Electronic PRINCIPLES

  2. Oscillators Chapter 23

  3. Sinusoidal oscillation requires both the correct phase and loop gain.  = 0 = positive feedback vout A B AB = 1 AB < 1 AB > 1

  4. 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.

  5. Resistance of lamp increases until equilibrium is reached 1 R’ fr = 2pRC Wien-Bridge oscillator vout RL 2R’ R’ Tungsten lamp C R C R vout t

  6. 1 fr = 2p LC C1 B = C2 C1 C2 C2 C = C1 +C2 Amin = C1 Colpitts oscillator +VCC RFC R1 vout C3 C1 L C2 R2 RE CE

  7. C1 +C2 1 Amin = C1 fr = C1 B = 2p LC C1 +C2 C1 C2 C = C1 +C2 Colpitts CB oscillator +VCC RFC R1 C4 C1 C3 L RL C2 R2 RE

  8. 1 fr = 2p LC L2 B = L1 L1 Amin = L2 Hartley oscillator +VCC RFC R1 vout C L1 L = L1 + L2 R2 R3 L2

  9. Crystal-controlled oscillators are used when frequency stability is important. Quartz crystal Slab cut from crystal Schematic symbol Electrodes and leads

  10. Crystal oscillator +VCC RFC R1 vout C1 Xtal C2 R2 R3 CE

  11. 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).

  12. W Monostable operation of the 555 timer IC +VCC T T = trigger 8 3 2 vout 555 T 1 vout

  13. Astable operation of the 555 timer IC +VCC 8 3 vout 555 1

  14. +VCC 8 UTP  VCC  VCC LTP Discharge 7 555 5 kW Threshold 6 S Q Control 5 kW 3 5 R Q Out Trigger 2 5 kW 1 Gnd 4 Reset

  15. Q Q A discrete RS flip-flop +VCC S R RC RC Q Q RB RB RS RR S R

  16. 555 IC configured for monostable operation +VCC 8 W = 1.1RC R 7 W 6 C 3 2  VCC 1

  17. 555 IC configured for astable operation R1 +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

  18. 1 f = W + 0.693R2C 555 voltage-controlled oscillator R1 +VCC VCC - Vcon W = -(R1 + R2)C ln 8 VCC - 0.5Vcon 7 Vcon applied to pin 5 R2 6 3 5 2 C 1 +Vcon + Vcon

  19. The output frequency is established by the input clock. W UTP =  VCC + vmod ( ) UTP 1 - VCC W = -RC ln T T Pulse-width modulation with the 555 timer IC +VCC 8 4 3 R 7 PWM out 555 A Modulating signal in 6 B 5 C 2 1 Clock in

  20. 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

  21. 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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