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Electronic Instruments and Measurements: UNIT-4 Signal Generator

A signal generator is a device that generates signals of various frequencies and amplitudes. It produces stable, distortion-free signals with controllable amplitude and frequency values. Common types include standard, audio, radio-frequency, function, pulse, and sweep signal generators. An oscillator is a circuit that generates an AC output signal without an external input signal. It uses positive feedback from an amplifier to sustain oscillations that satisfy the Barkhausen criteria of a loop gain greater than 1 and a net phase shift of 0 or 360 degrees. Common oscillator types are the Wien bridge and phase-shift audio oscillators, which employ RC feedback networks.

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50 views29 pages

Electronic Instruments and Measurements: UNIT-4 Signal Generator

A signal generator is a device that generates signals of various frequencies and amplitudes. It produces stable, distortion-free signals with controllable amplitude and frequency values. Common types include standard, audio, radio-frequency, function, pulse, and sweep signal generators. An oscillator is a circuit that generates an AC output signal without an external input signal. It uses positive feedback from an amplifier to sustain oscillations that satisfy the Barkhausen criteria of a loop gain greater than 1 and a net phase shift of 0 or 360 degrees. Common oscillator types are the Wien bridge and phase-shift audio oscillators, which employ RC feedback networks.

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Tenda Tiy
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© © All Rights Reserved
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Electronic Instruments and

Measurements

UNIT-4 Signal Generator

Dr. Girraj Sharma


Associate Professor, Deptt. Of ECE
JECRC
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
Introduction to Signal Generator
Cont’d

• Types of signal generator:


• Standard Signal Generator
• Audio Signal Generator
• Radio-Frequency Signal Generator
• Function Generator
• Pulse Generator
• Sweep Generator
• Random Noise Generator
OSCILLATOR
• 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 CONT’D
• 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.
REQUIREMENTS FOR
OSCILLATION
• 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: A Vo
Af  
1  A Vs

Af = gain with feedback


A = open-loop gain
β = feedback factor, Vi/Vo
REQUIREMENTS FOR
OSCILLATION CONT’D
• Gain for amplifier with positive feedback:

Input + Vi = Vs - Vf Output
Vs A Vo
Signal Signal
-

Vf = βVo
B

Feedback
Signal

Closed-loop system consisting of amplifier with feedback


POSITIVE FEEDBACK AMPLIFIER
OSCILLATOR
• 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 CONT’D
• If a positive-feedback circuit has a loop gain that satisfies two
conditions:

Barkhausen
Criteria

Loop Gain, Aβ ≥ 1 Net Phase Shift = 0 or 360 degree


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.
TYPES OF OSCILLATOR
• 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

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