Chapter 21 Oscillators. © Goodheart-Willcox Co., Inc.Permission granted to reproduce for...

Chapter 21

Oscillators

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.

Objectives

• Explain what occurs during an oscillation cycle.

• Identify various oscillators.

• Discuss and compare the Armstrong oscillator and the Hartley oscillator.

• Outline the operation of the crystal oscillator and the power oscillator.


Basic Oscillators

• Made up of:– Wave-producing circuit– Amplifier– Feedback circuit

• Cycle of voltage amplitude

• Cycle passes in one period


Sustaining Oscillation

• Regenerative feedback, or positive feedback

• Enough feedback voltage amplitude to replace energy lost


Armstrong Oscillators


Step One

• Current flows from B- to B+


Step Two

• First half-cycle of oscillation


Hartley Oscillators


Hartley Oscillators (Cont.)

• Used in radio receivers and transmitters

• More stable than Armstrong oscillators

• Radio frequency choke (RFC)

• Series fed oscillators

• Shunt fed oscillators


Shunt Fed Oscillators

• No dc path through coil L1 for emitter-collector current


Review

What is an oscillator?

Electronic circuit that generates an ac signal at a desired frequency


Review

What is a complete set of events in a repeated series called?

A cycle


Review

What differentiates a Hartley oscillator from other oscillators?

The tapped coil


Review

What are the two types of Hartley oscillators?

Series fed oscillators and shunt fed oscillators


Other Oscillators

• Colpitts oscillators

• Crystal controlled oscillators

• Power oscillators

• Operation amplifier oscillators


Colpitts Oscillators

• Similar to Hartley oscillators

• Split stator capacitor• Ganged capacitor


Crystal Controlled Oscillators

• Radio communications, broadcasting stations

• Piezoelectric effect


Crystal Controlled Oscillators (Cont.)

• Crystals control transmitter frequency

• Crystal ovens• Crystal added to

feedback circuit


Crystal Controlled Oscillators (Cont.)

• Crystal used instead of inductor in the tank circuit of Colpitts oscillator

• Pierce oscillators


Additional Uses for Crystals

• Two crystals producing two different frequencies for measuring temperature

• Timing devices• Signal generators


Power Oscillators

• Collector load of each transistor is primary of transformer

• Imbalance in conductivity starts oscillation


Op Amp Oscillators

• Crystal used in feedback circuit

• NAND gate starts op amp output


Wien Bridge Oscillators

• Use positive and negative feedback

• Balance of feedback in circuit crucial


Radio Detection and Ranging (Radar)

• Existence of electromagnetic waves that travel at the speed of light

• Advances occurred prior to World War II

• Microwaves are transmitted out of radar dish in energy pulses

• When waves strike reflective material, some is reflected back

• Types of radar– Pulsar radar– Continuous-wave radar– Doppler radar– FM radar

• Some planes are “invisible” to radar– Stealth bombers– Stealth fighters


Review

What is the distortion in a crystal caused by a voltage applied to the surface of a crystal?

Piezoelectric effect


Review

What is a Pierce oscillator?

A Colpitts oscillator that uses a crystal in place of the inductor in the tank circuit


For Discussion

• Compare and contrast an electronic oscillator and a clock pendulum.

• Explain the operation of a Hartley oscillator.

• Why are crystal oscillators used in many commercial transmitters?

Chapter 21 Oscillators. © Goodheart-Willcox Co., Inc.Permission granted to reproduce for...

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Transcript of Chapter 21 Oscillators. © Goodheart-Willcox Co., Inc.Permission granted to reproduce for...