Transcript Lecture1

Signal Generators
Term 8
INTRODUCTION:
A signal source is a vital component at a test setup, •
whether at the end of a production line, on the
service bench, or in the research laboratory. Signal
sources have a variety of applications including
checking stage gain, frequency response, an
alignment in receivers and in a wide range of other
electronics equipment.
Signal sources provide a variety of waveforms for •
testing electronic circuits, usually at low power. The
various waveforms are generated by several different
kinds of instruments, which range in complexity from
simple fixed frequency sine-wave oscillators to highly
sophisticated instruments such as might be used in
testing complex communications equipment.
The difference between Oscillators and
Generators:
• Although terminology is not universal. the 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.
• Although we speak of oscillators as "generating" a
signal, it should be emphasized that no energy is
created; it is simply converted from a do source into an
ac energy at some specific frequency.
REQUIREMENTS FOR OSCILLATION
• Basically, an oscillator is an amplifier with positive
equation for an amplifier with positive feedback
•
Af = A
(1-1)
1+A β
where
Af = gain with feedback
A = open-loop gain
β = feedback factor Vi/Vo
• Feedback is provided by the feedback, or phase-shift,
network shown in Fig. 1-1. The output signal of the
amplifier is fed back to the amplifier's input terminal
through the phase-shift network, undergoing there a
180° shift in phase.
Fig. 1-1 Closed – loop system consisting of amplifier with feed back
• The amplifier also causes a 180° phase shift
Therefore, the net phase shift in the circuit is
zero, which is one condition that must be
satisfied for sustained oscillation. A second
requirement for oscillation is that the closedloop gain, which is the product A , must be
equal to. or greater than, unity. These two
conditions are known as the Barkhausen
criteria.
• It is not necessary to supply an input signal to
initiate oscillation. A noise voltage or a
transient is sufficient to start the process. It is
usually desirable for the closed-loop gain to be
greater than unity to ensure strong
oscillations. However. if the gain is too high,
the circuit operates at saturation and the
output waveform will not be sinusoidal.
• The phase-shift network consists of a
combination of components with resistance and
capacitance (RC) or inductance and capacitance
(LC). A combination of inductance and
capacitance is usually called a tuned circuit or a
tank circuit.
Oscillators that employ an RC combination have a
distinct advantage over LC-type oscillators at low
frequencies since the physical size of the required
inductors is prohibitive.