CHAPTER 3

OSCILOSCOPE AND

SIGNAL CONDITIONING

Introduction to Signal Generator

Oscillator

Requirement for Oscillation

Positive Feedback Amplifier

Oscillator

Radio Frequency Oscillator

OUTLINE

Introduction to Signal Generator

Device that generate signal of various frequency and

amplitude.

Common and vital equipment in any electronic

laboratory.

Signal generator characteristics:

Stable and distortion-free signal

Controllable amplitude values

Controllable frequency values

DC offset

Types of signal generator:

Standard Signal Generator

Audio Signal Generator

Radio-Frequency Signal Generator

Function Generator

Pulse Generator

Sweep Generator

Random Noise Generator

Introduction to Signal Generator Cont’d

Oscillator is a circuit that generates an ac output signal

without requiring any externally applied output signal.

The only input power to an oscillator is the d.c power

supply.

Term ‘Oscillator’ is generally used for an instrument that

provides only a sinusoidal output signal, and the term

‘generator’ is applied to an instrument that provides

several output waveforms, including sine wave, square

wave, triangular wave and pulse trains as well as

amplitude modulation of the output signal.

OSCILLATOR

ADVANTAGES:

An oscillators is a non-rotating device. Consequently,

there is little wear and tear and hence longer life.

Due to the absence of moving parts, the operation of

an oscillator is quite silent.

An oscillator can produce waves from 20 Hz to

extremely high frequencies 100 MHz

The frequency of oscillations can be easily changed

when desired.

It has good frequency stability.

It has very high efficiency.

OSCILLATOR CONT’D

Basically, an oscillator is an amplifier with positive

feedback.

The signal regenerate and sustain itself.

The gain equation for an amplifier with positive

feedback is:

Where:

REQUIREMENTS FOR OSCILLATION

s

of

V

V

A

AA

1

Af = gain with feedback

A = open-loop gain

β = feedback factor, Vi/Vo

Gain for amplifier with positive feedback:

REQUIREMENTS FOR OSCILLATION

CONT’D

A

B

Vs+

-

VoOutput

Signal

Input

Signal

Feedback

Signal

Vf = βVo

Vi = Vs - Vf

Closed-loop system consisting of amplifier with feedback

POSITIVE FEEDBACK AMPLIFIER

OSCILLATOR CONT’D

A transistor amplifier with proper positive feedback can

act as an oscillator. You must remember that a positive

feedback amplifier is the one that produces a feedback

voltage, Vf that is in phase with the original input signal.

A phase shift of 180° is produced by the amplifier and a further phase shift of 180° is introduced by feedback network.

The signal get shifted by 360°.

This signal is then feedback to the input. The feedback voltage is in phase with the input signal.

The circuit is producing oscillations in the output.

However, this circuit has an input signal. But, then this is inconsistent with our definition of an oscillator which states that an oscillator is a circuit that produces oscillations without any external signal source.

POSITIVE FEEDBACK AMPLIFIER

OSCILLATOR

If we open the switch S in the figure, we will get the circuit as shown:

It means that the input signal is removed. However, the feedback voltage is still applied to the input signal. The amplifier will respond to this signal in the same way that it did to the input signal and that is the feedback voltage will be amplified and sent to the output.

The function of the feedback network is to send a portion of the output back to the input. Therefore, the amplifier receives another input cycle and another output cycle is produced. This process will continue so long as the amplifier is turned on.

Therefore, the amplifier will produce sinusoidal output with no external signal source.

POSITIVE FEEDBACK AMPLIFIER

OSCILLATOR

If a negative-feedback circuit has a loop gain that

satisfies two conditions:

POSITIVE FEEDBACK AMPLIFIER

OSCILLATOR CONT’D

Loop Gain, Aβ ≥ 1 Net Phase Shift = 0 @ Phase Aβ = 0

Barkhausen

Criteria

Oscillator categorized in two types:

i) Audio Oscillators

ii) Radio Frequency Oscillators

There are two common types of Audio Oscillators:

i)Wien Bridge Oscillator

ii) Phase-Shift Oscillator

Both of which employ RC feedback network

The Wien Bridge offers some very attractive features,

including a straightforward design, a relatively pure

sine-wave output and very stable frequency

TYPES OF OSCILLATOR

There are of many types of radio frequency, but

the famous is HARTLEY OSCILLATOR.

Radio frequency must satisfy the same basic

criteria for oscillation, that is Barkhausen

criteria.

The phase-shift network for RF oscillators is an

inductance-capacitance (LC) network.

This LC combination which generally referred to

as a tank circuit, acts as a filter to pass the

desired oscillating frequency and block all other

frequencies.

RADIO FREQUENCY OSCILLATOR

RADIO FREQUENCY OSCILLATOR

CONT’D

15

Hartley oscillator

LCf

2

1

Frequency of oscillation,

2

1

L

L

Feedback factor, β

L = L1 + L2

Solve the equation for inverting

Amplifier

1

21

L

LA

i

f

R

RA

Barkhausen Criteria, Aβ ≥ 1 to

sustain oscillation

EXAMPLE 1

Determine the frequency of oscillation and the minimum value of

Rf to sustain oscillation for the Hartley oscillator shown in figure:

R

Ri=15k

Rf

L1=10µH L2=270µHC=0.001µF

+Vcc

+Vcc

-

+

SOLUTION

EXAMPLE 2

Determine the minimum value of Rf in figure below to

sustain oscillation if L2=125µH

R

Ri=10kΩ

Rf

L1=15µH L2C=0.015µF

+Vcc

+Vcc

-

+

SOLUTION

EXAMPLE 3

Determine the value of L2 in the circuit below if the

frequency of oscillation is to be 100kHz

R

Ri=10kΩ

Rf

L1=15µH L2C=0.015µF

+Vcc

+Vcc

-

+

SOLUTION

END OF CHAPTER 3