signal generators i.e Oscillators

1. Unit-2 Signal Generators
Prepare by : Tejas Prajapati
Reference Books:
1. Ramakant Gayakwad: Op-amp and Linear Integrated Circuits, Pearson Education.
2. Kalsi H S: Electronic Instrumentation, Tata McGraw-Hill Education.
3. Jigar Shah: Electronics Devices and Circuits-II (Solid State), Atul Prakashan,
Ahmadabad

2. Introduction

An oscillators is a device which generates an
alternating signal.

Means AC output signal without requiring any
externally applied input signals.

The oscillators converts DC energy to AC energy at
very high frequency.

The function of an oscillator is similar to that of an
amplifier circuit.

3. Difference between amplifier and
oscillator
Amplifier Oscillator
Amplifier
AC I/p
Signal
AC O/p
Signal
DC
Power
Amplifier
AC O/p
Signal
DC
Power
1. Negative feed back is used.
2. In an amplifier circuit the frequency, waveform,
and magnitude of AC power generated is
controlled by AC signal voltage applied at the
input
3. Output frequency depend on active as wall as
passive components used.
4. Phase shift between input and output is
generally 180o
or π.
1. Positive feed back is used.
2. In an oscillator circuit the frequency, waveform,
and magnitude of AC power generated is
controlled by circuit it self.
3. Output frequency depend on only passive
components used.
4. Phase shift between input and output is
generally 0o
or 360 or multiple integer of 2π.
(i.e 2π, 4π, 6π ...)

4. Applications of Oscillators

They are widely used in many electronic devices.
Common examples of signals generated by
oscillators include signals broadcast by radio and
television transmitters, clock signals that regulate
computers and quartz clocks, and the sounds
produced by electronic beepers and video games.

5. Classification of oscillators
(or types of oscillators )
1. According to the nature of generated waveform:
a) Sinusoidal or harmonic oscillators: Generates sine wave of definite
frequency
b) Relaxation or non-sinusoidal oscillators: Generates square wave,
triangular wave, pulse and saw-tooth wave etc.
2. According to frequency generated signal:
a) AF (audio frequency) oscillators: signal generated in range of 20Hz to
20kHz.
b) RF(radio frequency) oscillators: signal generated in range of 20kHz to
30MHz.
c) VHF(very high frequency) oscillators: signal generated in range of
300MHz to 3GHz.
d) Microwave oscillators: signal generated in above 3GHz.

6. 3. According to components used:
a) Tuned or LC oscillators: Use LC tune circuits
- used for generating RF signals
- e.g Hartley oscillator, Colpitts oscillator, Tuned collector oscillator
b) RC oscillators: Use RC phase shift network
- used for generating AF signals
- e.g. RC phase shift oscillator, Wien bridge oscillator.
c) Crystal oscillators: Crystal is used in place of LC circuits
- work on Piezo elelctric effect
- used for generating RF signals
d) Negative resistance oscillators: Used the negative resistance characteristics
of certain devices e.g. Tunnel diode, UJT, Gun Diode.
-used for generating VHF and microwave signals

7. Principle of oscillation or Barkhausen
Criterion

Feed back is must be positive

9. 
From above equation requirement of oscillation:
1)The magnitude of the loop gain Av
B must be at least
1 and
2) Total phase shift of the loop gain Av
B must be 0o
or
360 or multiple integer of 2π. (i.e 2π, 4π, 6π …)
Above two statements are known as Barkhausen
Criterion.

10. Phase shift oscillator

13. Example

14. Wien Bridge Oscillator

16. Example

17. Square Wave Generator

21. Example

22. Triangular Wave Generator
Method-1

24. Method-2

28. Example

29. Function Generator

30. 
A function generator produces different wave forms of adjustable
frequency.

Common outputs are sine, square, triangular and saw-tooth
waves.

The frequency may be adjusted from a fraction of Hz to several
kHz.

Block diagram shown in fig.

Usually the frequency is controlled by varying the capacitor in
LC or RC circuit.

In instrument the frequency is controlled by varying of
magnitude of current which drives the integrator.

The frequency controlled voltage regulates two current source
supplies constant current to integrator whose output voltage
increases linearly with time , the according to the equation of the
output signal voltage,

31. eout
=-1/C0
∫t
idt

An increase and decrease in the current increase and
decrease the slope of the output voltage and hence
controls the frequency.

The voltage comparator multi-vibrator changes states at
per-determined maximum level of the integrator output
voltage. This change cut-offs the upper current supply
and switches the lower current supply .

The lower current source supplies a reverse current to
the integrator, so that its output decrease linearly with
time. When the output reaches a per-determined
minimum level, the voltage comparator multi-vibrator
changes state and switch on the upper current source.

32. 
The output of the integrator is triangular waveform
whose frequency is determined by the magnitude of
the current supplied by the constant current sorces.

The comparator output is square wave voltage of the
same frequency.

The resistance diode network alters the slope of the
triangular wave as its amplitude change and
produces a sine wave.

33. Sweep Generator

34. 
It provides a sinusoidal output voltage whose
frequency varies smoothly and continuously over an
entire frequency band.

The process of frequency modulation may be
accomplished electronically.

It is done by using the modulating voltage to vary the
reactance of the oscillator tank circuit components.

Block diagram shown in fig.

The frequency sweeper provides a variable
modulating voltage which cause the capacitance of
the master oscillator to vary.

A representative sweep rate could be of the order of
20 sweeps/ sec.

35. 
A manual control allows independent adjustment of the
oscillator resonant frequency.

The frequency sweeper provides varying sweep voltage
for synchronization to derive the horizontal deflection
plates of the CRO. Thus the amplitude of the response of
the test device will be locked and displayed on the
screen.

To identify a frequency interval, a maker generator
provides half sinusoidal waveform at any frequency with
in a sweep range.

The maker voltage can be added to the sweep voltage of
the CRO during alternate cycles of the sweep voltage,
and appears superimposed on the response curve.

36. 
The automatic level control circuit is closed loop
feedback system which monitor the RF level some
point in measurement system.

This circuit holds the power delivered to the load to
test circuit constant and independent of frequency and
important changes.

A constant power level prevents any source mismatch
and also provides a constant readout calibration with
frequency.

37. The end