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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.
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Basic Oscillators
• Made up of:– Wave-producing circuit– Amplifier– Feedback circuit
• Cycle of voltage amplitude
• Cycle passes in one period
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Sustaining Oscillation
• Regenerative feedback, or positive feedback
• Enough feedback voltage amplitude to replace energy lost
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Armstrong Oscillators
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Step One
• Current flows from B- to B+
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Step Two
• First half-cycle of oscillation
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Hartley Oscillators
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Hartley Oscillators (Cont.)
• Used in radio receivers and transmitters
• More stable than Armstrong oscillators
• Radio frequency choke (RFC)
• Series fed oscillators
• Shunt fed oscillators
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Shunt Fed Oscillators
• No dc path through coil L1 for emitter-collector current
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What is an oscillator?
Electronic circuit that generates an ac signal at a desired frequency
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What is a complete set of events in a repeated series called?
A cycle
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What differentiates a Hartley oscillator from other oscillators?
The tapped coil
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What are the two types of Hartley oscillators?
Series fed oscillators and shunt fed oscillators
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Other Oscillators
• Colpitts oscillators
• Crystal controlled oscillators
• Power oscillators
• Operation amplifier oscillators
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Colpitts Oscillators
• Similar to Hartley oscillators
• Split stator capacitor• Ganged capacitor
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Crystal Controlled Oscillators
• Radio communications, broadcasting stations
• Piezoelectric effect
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Crystal Controlled Oscillators (Cont.)
• Crystals control transmitter frequency
• Crystal ovens• Crystal added to
feedback circuit
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Crystal Controlled Oscillators (Cont.)
• Crystal used instead of inductor in the tank circuit of Colpitts oscillator
• Pierce oscillators
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Additional Uses for Crystals
• Two crystals producing two different frequencies for measuring temperature
• Timing devices• Signal generators
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Power Oscillators
• Collector load of each transistor is primary of transformer
• Imbalance in conductivity starts oscillation
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Op Amp Oscillators
• Crystal used in feedback circuit
• NAND gate starts op amp output
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Wien Bridge Oscillators
• Use positive and negative feedback
• Balance of feedback in circuit crucial
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
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
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What is the distortion in a crystal caused by a voltage applied to the surface of a crystal?
Piezoelectric effect
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What is a Pierce oscillator?
A Colpitts oscillator that uses a crystal in place of the inductor in the tank circuit
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
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?