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Topic 2

Oscillators

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Sudahkah anda mengisi COURSE

ENTRANCE SURVEY? Hanya 4

orang sahaja yang mengisinyasehingga 16/1/2013

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COURSE LEARNING OUTCOMES

(CLO)

CLO1. explain correctly the principles of electronic

circuits by using block diagram or circuit diagram (C4)CLO2. conduct the construction of electronic circuits

application during practical works based on the theory

and principle operation of the circuits. (P4)

CLO3. deliver an oral presentation to display good

communication skills. (A2)

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LEARNING OUTCOMES

Upon completion of this topic students should be able to:

Draw block diagram of an oscillator2.1

Explain requirements of oscillator circuit2.2

2.3

2.4

2.5

Construct the oscillator circuit

Explain applications each oscillator

Compare the performance of the oscillator

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Oscillator

Oscillators are devices that can

convert the DC voltage to AC voltage

without any external source at a

particular frequency.

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Basic oscillator block diagram

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http://localhost/var/www/apps/conversion/tmp/Oscilator.FLV

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Oscillator

Oscillator is divided into two classes

depending on the waveform produced as

follows:

i. Harmonic oscillator - the sine wave.ii. Non-harmonic oscillator - in terms of the

fourth wave, triangle wave, etc..

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Oscillator

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Oscillator circuit

All the basic oscillator circuit consists of

three parts:

i. Amplifier

ii. Feedback

iii. Frequency Generation Circuit

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Block diagram of oscillator

circuit

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Question

Draw the block diagram of an oscillator

and name each block

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Answer

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Amplifier

Generally, an amplifier or simply amp is a

device for increasing the power of a signal.

For electronic amplifier, the input "signal"

is usually a voltage or a current.

In this case oscillator requires DC power

supply to produce output. That means this

DC power supply is used as an input of

the amplifier

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Feedback

Feedback is a condition where part of the

output signal supplied to the input.

Since the oscillator has no input signal, thefeedback signal is the input signal to the

amplifier in the oscillator circuit.

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Feedback

There are two principles of feedback,

positive feedback and negative feedback.

Oscillators using the principle of positive

feedback.

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Concept feedback

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Feedback

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Figure below shows the basic block diagram of a feedback

system that are commonly used:

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Feedback

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A di ill d f i i h

http://en.wikipedia.org/wiki/Audio_oscillatorhttp://en.wikipedia.org/wiki/Audio_oscillator

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Frequency generation circuit

Generally, the frequency of the oscillator

circuit generation in consumer goods is

divided into two categories:

i.Audio frequency oscillator whichgenerates: Oscillator Network RC

(resistance-capacitor). It is to produce low

and medium frequency signal.Example is the oscillator RC type oscillator

phase shift and Wein bridge Oscillator.

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An audio oscillatorproduces frequencies in the

audiorange, about 16 Hz to 20 kHz

A l f ill t (LFO) i l t i ill t th t t f b l

http://en.wikipedia.org/wiki/Audio_oscillatorhttp://en.wikipedia.org/wiki/Audio_frequencyhttp://en.wikipedia.org/wiki/Audio_frequencyhttp://en.wikipedia.org/wiki/Audio_oscillatorhttp://en.wikipedia.org/wiki/Low_frequency_oscillationhttp://en.wikipedia.org/wiki/Low_frequency_oscillationhttp://en.wikipedia.org/wiki/Low_frequency_oscillationhttp://en.wikipedia.org/wiki/Low_frequency_oscillation

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Frequency generation circuit

ii. Radio frequency generating oscillator:

Oscillator Networks LC(inductor-

capacitor). It is also known as tank circuits.

It is to produce a high frequency signal (>1MHz), and usually it produces a stable

frequency.

LC oscillator is an example ofArmstrongOscillator, Colpitts, Hartley and crystal.

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An RF oscillator produces signals in the radio frequency(RF) range of about 100 kHz to

100 GHz

A low-frequency oscillator(LFO) is an electronic oscillator that generates a frequency below

20Hz. This term is typically used in the field of audio synthesizers, to distinguish it from an

audio frequency oscillator.

http://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Low_frequency_oscillationhttp://en.wikipedia.org/wiki/Synthesizershttp://en.wikipedia.org/wiki/Synthesizershttp://en.wikipedia.org/wiki/Low_frequency_oscillationhttp://en.wikipedia.org/wiki/Low_frequency_oscillationhttp://en.wikipedia.org/wiki/Low_frequency_oscillationhttp://en.wikipedia.org/wiki/Radio_frequency

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RC Oscillator

Combination of resistors and capacitors.

Operate in low frequency. Two types of

RC oscillator:-

Phase shift oscillator

Wein bridge oscillator

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http://localhost/var/www/apps/conversion/tmp/scratch_1//upload.wikimedia.org/wikipedia/commons/c/c5/RC_phase_shift_oscillator.svg

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LC Oscillator

Armstrong Oscillator

Crystal Oscillator Hartley Oscillator

Colpitts Oscillator

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LC Oscillator

Combination of inductor and

capacitor.

Operate in high frequency

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Armstrong Oscillator

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Using a combination of a transformer to step the phase of 180.

FREQUENCY-DETERMINING DEVICEFEEDBACK NETWORK

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Operation

Resistors R1, R2 and R3 is usage

to provide bias voltage to the

transistor. Capacitors C1 and C2are used to block an alternating

signal. This configuration will

provide 180phase difference ofoutput signal.

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Operation

The transformer Tr is selected to produce

a phase shift of 180 to get the feedback

voltage in phase with the input transistor.

Usually the ratio between the coil windings

L1 with L2 coil arranged.

So that the product of the gain, A, with the

feedback factor, , is a (| A| = 1).

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The frequency of oscillation

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Colpitts Oscillator

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Using a separate capacitor for resonant circuit.

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Operation

The amplifier will provide 180 phase

difference of the output signal. LC circuit in

the feedback loop will produce a phaseshift of 180. So the feedback voltage will

be in phase with the input voltage on the

transistor.

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The frequency of oscillation

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Example

A Colpitts Oscillatorcircuit having two capacitors of 10pF and

100pF respectively are connected in parallel with an inductor of10mH.

Determine the frequency of oscillations of the circuit.

The frequency of oscillations for a Colpitts Oscillator is given as:

The circuit consists of two capacitors in series, so the total capacitance is given as:

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The inductor is of 10mH then the frequency of oscillation is:

Then the frequency of oscillations for the Oscillator is 527.8kHz

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Hartley Oscillator

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R1

R2

RE

RFC

C1

LP

VCC

vout

R1

R2

RE

RFC

C1

L1

VCC

vout

L2

Hartley oscillator.

1 2

1

1

2

T

r

T

L L L

fL C

1

1

2r

P

fL C

Using a tappedinductor for the

resonant circuit

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Operation

Transistors and resistors R1, R2, R3 and

R4 is a combination of amplifier circuit

(amplifier manufacturers Common Voltage

Divider bias technique). Inductance L1 andL2 are used to intercept an alternating

signal to the ground

36Intercept - memintas

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Operation

The amplifier will provide 180 phase

differenceof the output signal. LC circuit in

the feedback loop will produce a phase

shift of 180. So the feedback voltage willbe in phase with the input voltage on the

transistor.

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The frequency of oscillation

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The Hartley oscillator was extensively used on allbroadcast bandsincluding the

FM 88-108 MHz band.

http://en.wikipedia.org/wiki/Broadcast_bandhttp://en.wikipedia.org/wiki/Broadcast_band

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Example

A Hartley Oscillator circuit having two individual inductors of 0.5mH each, are

designed to resonate in parallel with a variable capacitor that can be varied from 100pF to

500pF. Determine the upper and lower frequencies of oscillation and also the Hartleyoscillators bandwidth.

The frequency of oscillations for a Hartley Oscillator is given as:

The circuit consists of two inductive coils in series, so the total inductance is given as:

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Upper Frequency

Lower Frequency

Oscillator Bandwidth

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Question

With reference to Figure below,

i. Name the type of oscillator

ii. Calculate the oscillation frequency of the circuit.

iii. Name the components that affect the oscillation

frequency

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Answer

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Crystal oscillator

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Crystal oscillator

The most stable oscillator and precisely

using a piezoelectric crystal in the

feedback circuit. When an alternating

voltage applied to these crystals, themechanical vibrations and these vibrations

occur with the natural resonance

frequency depends on the thickness of thecrystal. To obtain a high frequency crystal

to be thinner.

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Crystal oscillator

Electrical properties of crystals can be

expressed with resonant circuit in Figure

below. Inductance Lh represents the

electrical equivalent of the mass of thecrystal. Capacitance, Ch represents the

elasticity of crystals, Rh represents the

frictional resistance in the crystal structure.Cm represents the capacitance of the

capacitor crystal loaded containers.

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Represents the capacitance of the capacitor

Cm crystal loaded containers.

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The frequency of oscillation

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Crystal oscillator

Wide frequency range (depending on the

size of the crystal) are often used in the

emitter circuit

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Example

A series resonant crystal has the following values after

being cut, R = 1k,C = 0.05pF and L = 3H. Calculate the

fundamental frequency of oscillations of the crystal.

The frequency of oscillations for Crystal Oscillators is given

as:

Then the fundamental frequency of oscillations for the crystal is

given as

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Phase shift oscillator

Phase shift oscillator comprising an

amplifier and feedback network with three

RC circuit.

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Phase shift oscillator

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Operation

The output signal from the amplifier

different phase 180o with the input signal.

To generate a positive feedback signal,

the output signal phase should be shiftedby 180o to be in phase with the input. RC

network to produce a phase shift of 180o

RC network where each one will transferphase by 90o.

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If all the resistors, Rand the capacitors, Cin the phase shift

network are equal in value, then the frequency of

oscillations produced by the RC oscillator is given as:

Where:

is the Output Frequency in Hertz

R is the Resistance in Ohms

C is the Capacitance in Farads

N is the number of RCstages. (in our example N = 3)

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Example

Determine the frequency of oscillations of a RCOscillator circuit having 3-stages each with a resistor

and capacitor of equal values. R = 10kand C = 500pF

The frequency of oscillations for a RC Oscillator is givenas:

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The circuit is a 3-stage oscillator which consists of three

10k resistors and three 500pF capacitors thereforethe frequency of oscillation is given as:

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THE END

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