NEETS Module 9 − Introduction to Wave− Generation and Wave−Shaping
and Direct Current
||Alternating Current and Transformers
||Circuit Protection, Control, and Measurement
||Electrical Conductors, Wiring Techniques,
and Schematic Reading
||Generators and Motors
||Electronic Emission, Tubes, and Power Supplies
||Solid-State Devices and Power Supplies
||Wave-Generation and Wave-Shaping Circuits
||Wave Propagation, Transmission Lines, and
||Introduction to Number Systems and Logic Circuits
||- Introduction to Microelectronics
||Principles of Synchros, Servos, and Gyros
||Introduction to Test Equipment
||Radio-Frequency Communications Principles
||The Technician's Handbook, Master Glossary
||Test Methods and Practices
||Introduction to Digital Computers
||Introduction to Fiber Optics
|Note: Navy Electricity and Electronics Training
Series (NEETS) content is U.S. Navy property in the public domain.
Upon completion of this chapter you will be able to:
1. List the two broad classifications of oscillators (wave generators).
2. Identify the three frequency-determining devices for sine-wave
3. Describe the differences between series-fed and shunt-fed oscillators.
4. Explain how the crystal is equivalent to the series and parallel
5. Identify the Armstrong oscillator.
6. Identify the Hartley oscillator.
7. Identify the Colpitts oscillator.
8. Identify the resistive-capacitive oscillator.
9. Determine the frequency of a resistive-capacitive oscillator.
10. Explain the operation of a pulsed oscillator.
11. Determine how many cycles are present in the output of a pulsed
12. Explain how frequency multiplication takes place.
WAVE GENERATORS play a prominent role in the field of electronics. They generate
signals from a few hertz to several gigahertz (109 hertz). Modern
wave generators use many different circuits and generate such outputs as SINUSOIDAL,
SQUARE, RECTANGULAR, SAWTOOTH, and TRAPEZOIDAL waveshapes. These waveshapes serve
many useful purposes in the electronic circuits you will be studying. For example,
they are used extensively throughout the television receiver to reproduce both picture
One type of wave generator is known as an Oscillator. An oscillator can be regarded
as an amplifier which provides its own input signal. Oscillators are classified
according to the waveshapes they produce and the requirements needed for them to
Classification of Oscillators (GENERATORS)
Wave generators can be classified into two broad categories according to their
output waveshapes, SINUSOIDAL and NONSINUSOIDAL.
A sinusoidal oscillator produces a sine-wave output signal. Ideally, the output
signal is of constant amplitude with no variation in frequency. Actually, something
less than this is usually obtained. The degree to which the ideal is approached
depends upon such factors as class of amplifier operation, amplifier characteristics,
frequency stability, and amplitude stability.
Sine-wave generators produce signals ranging from low audio frequencies to ultrahigh
radio and microwave frequencies. Many low-frequency generators use resistors and
capacitors to form their frequency-determining networks and are referred to as RC
Oscillators. They are widely used in the audio-frequency range.
Another type of sine-wave generator uses inductors and capacitors for its frequency-determining
network. This type is known as the LC Oscillator. LC oscillators, which use tank
circuits, are commonly used for the higher radio frequencies. They are not suitable
for use as extremely low-frequency oscillators because the inductors and capacitors
would be large in size, heavy, and costly to manufacture.
a third type of sine-wave generator is the Crystal-Controlled Oscillator.
The crystal- controlled oscillator provides excellent frequency stability and is used from the middle of the audio range through the radio frequency range.
Nonsinusoidal oscillators generate complex waveforms, such as square, rectangular,
trigger, sawtooth, or trapezoidal. Because their outputs are generally characterized
by a sudden change, or relaxation, they are often referred to as RELAXATION Oscillators.
The signal frequency of these oscillators is usually governed by the charge or discharge
time of a capacitor in series with a resistor. Some types, however, contain inductors
that affect the output frequency. Thus, like sinusoidal oscillators, both RC and
LC networks are used for determining the frequency of oscillation. Within this category
of nonsinusoidal oscillators are MULTIVIBRATORS, BlockING Oscillators, SAWTOOTH
GENERATORS, and TRAPEZOIDAL GENERATORS.
The Basic Oscillator
An oscillator can be thought of as an amplifier that provides itself (through
feedback) with an input signal. By definition, it is a nonrotating device for producing
alternating current, the output frequency of which is determined by the characteristics
of the device. The primary purpose of an oscillator is to generate a given waveform
at a constant peak amplitude and specific frequency and to maintain this waveform
within certain limits of amplitude and frequency.
An oscillator must provide amplification. Amplification of signal power occurs
from input to output. In an oscillator, a portion of the output is fed back to sustain
the input, as shown in figure 2-1. Enough power must be fed back to the input circuit
for the oscillator to drive itself as does a signal generator. To cause the oscillator
to be self-driven, the feedback signal must also be
Figure 2-1. - Basic oscillator block diagram.
Regenerative (positive). Regenerative signals must have enough power to compensate
for circuit losses and to maintain oscillations.
Since a practical oscillator must oscillate at a predetermined frequency, a Frequency-
Determining DEVICE (FDD), sometimes referred to as a Frequency-Determining Network
(FDN), is needed. This device acts as a filter, allowing only the desired frequency
to pass. Without a frequency-determining device, the stage will oscillate in a random
manner, and a constant frequency will not be maintained.
Before discussing oscillators further, let's review the requirements for an oscillator.
First, amplification is required to provide the necessary gain for the signal. Second,
sufficient regenerative feedback is required to sustain oscillations. Third, a frequency-determining
device is needed to maintain the desired output frequency.
The basic oscillator requirements, in addition to the application, determine
the type of oscillator to be used. Let's consider some factors that account for
the complexity and unique characteristics of oscillators.
Virtually every piece of equipment that uses an oscillator has two stability
requirements, Amplitude STABILITY and Frequency STABILITY. Amplitude stability refers
to the ability of the oscillator to maintain a constant amplitude in the output
waveform. The more constant the amplitude of the output waveform, the better the
amplitude stability. Frequency stability refers to the ability of the oscillator
to maintain its operating frequency. The less the oscillator varies from its operating
frequency, the better the frequency stability.
A constant frequency and amplitude can be achieved by taking extreme care to
prevent variations in Load, BIAS, and COMPONENT Characteristics. Load variations
can greatly affect the amplitude and frequency stability of the output of an oscillator.
Therefore, maintaining the load as constant as possible is necessary to ensure a
As you should know from your study of transistor biasing, bias variations affect
the operating point of the transistor. These variations may alter the amplification
capabilities of the oscillator circuits as well. a well-regulated power supply and
a bias-stabilizing circuit are required to ensure a constant, uniform signal output.
As a result of changing temperature and humidity conditions, the value or characteristics
of components such as capacitors, resistors, and transistors can change. The changes
in these components also cause changes in amplitude and frequency.
Output power is another consideration in the use of oscillators. Generally speaking,
high power is obtained at some sacrifice to stability. When both requirements are
to be met, a low-power, stable oscillator can be followed by a higher-power BUFFER
Amplifier. The buffer provides isolation between the oscillator and the load to
prevent changes in the load from affecting the oscillator.
If the oscillator stage must develop high power, efficiency becomes important.
Many oscillators use class C bias to increase efficiency. Other types of oscillators
may use class a bias when a high efficiency is not required but distortion must
be kept at a minimum. Other classes of bias may also be used with certain oscillators.
RC networks, LC tanks, and crystals may appear in sine-wave oscillator circuits.
An amplifier can be made into a sine-wave oscillator by providing regenerative feedback
through an RC network.
Figure 2-2, view (A), shows the block diagram of an amplifier with an RC network
through which regenerative feedback is provided. The RC network also acts as the
frequency-determining device. View (B) shows a vector analysis of the signal E at
various points in the circuit.
Figure 2-2A. - RC oscillator. Amplifier WITH and RC Feedback Network.
Figure 2-2B. - RC oscillator. VECTOR ANALYSIs
To analyze the operation of the circuit in view (A), assume that the amplifier
is a common-emitter configuration. The signal on the collector (M) is 180 degrees
out of phase with the signal (input) on the base (R). For the circuit to produce
regenerative feedback, the RC network must provide a 180-degree phase shift of the
collector signal. When power is applied to the circuit, a noise voltage (noise contains
many different frequencies) will appear on the collector. This noise signal is represented
by vector LM in view (B). As the signal couples through C1 and across R1 (view (A)),
a phase shift occurs. The voltage across R1 (ER1), represented by vector
LN, has been shifted in phase (about 60 degrees) and reduced in amplitude. The signal
at point N (view (A)) is then coupled to the next RC section (R2 and C2). Using
the same size resistor and capacitor as before will cause another 60-degree phase
shift to take place. The signal at point P is the voltage across R2, represented
by vector LP. Now the signal at point P has been shifted about 120 degrees and its
amplitude is reduced still further. The same actions occur for the last section
(R3 and C3). This signal experiences another 60-degree phase shift and has further
amplitude reduction. The signal at point R (ER3) has been shifted 180 degrees and
is represented by vector LR.
Notice that point R is the input to the base of the common-emitter amplifier.
Also, vector LR shows that the signal on the base is regenerative (aiding the circuit
operation). This meets the regenerative feedback requirement. An exact 60-degree
phase shift per stage is not required, but the sum of the three phase shifts must
equal 180 degrees.
For a given RC network, only one frequency of the initial noise signal will be
shifted exactly 180 degrees. In other words, the network is frequency selective.
Therefore, the RC network is the frequency- determining device since the lengths
of the vectors and their phase relationships depend on frequency. The frequency
of oscillations is governed by the values of resistance and capacitance in these
sections. Variable resistors and capacitors may be used to provide tuning in the
feedback network to allow for minor variations in phase shift. For an RC phase-shift
oscillator, the amplifier is biased for class a operation to minimize distortion
of the wave or signal.
Some sine-wave oscillators use resonant circuits consisting of inductance and
capacitance. For example, recall the tank circuit in which a resonant circuit stores
energy alternately in the inductor and capacitor, producing a sine wave. You studied
this action of the tank circuit in chapter 1.
If there were absolutely no internal resistances in a tank circuit, oscillations
would continue indefinitely, as shown in figure 2-3, view (A). Each resonant circuit
does, however, contain some resistance which dissipates power. This power loss causes
the amplitude to decrease, as shown in views (B) and (C). The reduction of amplitude
in an oscillator circuit is referred to as DAMPING. Damping is caused by both tank
and load resistances. The larger the tank resistance, the greater the amount of
damping. Loading the tank causes the same effect as increasing the internal resistance
of the tank. The effect of this damping can be overcome by applying regenerative
Figure 2-3A. - Effects of damping.
Figure 2-3B. - Effects of damping.
Figure 2-3C. - Effects of damping.
Figure 2-4 shows a block diagram of a typical LC oscillator. Notice that the
oscillator contains the three basic requirements for sustained oscillations: amplification,
a frequency-determining device, and regenerative feedback.
Figure 2-4. - LC oscillator.
The amplifier supplies energy to begin what is known as the FLYWHEEL EFFECT.
The flywheel effect is the maintenance of oscillations in a circuit in the intervals
between pulses of excitation energy. Recall that in chapter 1 the tank circuit alternately
stored energy in the inductor and capacitor. The LC network provides initial oscillations.
a portion of the output of the LC network is then returned to the input of the amplifier
through the regenerative-feedback network to sustain the oscillations.
When a tank circuit is used to develop oscillations in an oscillator, the output
frequency of the oscillator is primarily the resonant frequency of the tank circuit
and can be found by the formula:
Another frequency-determining device is the Crystal. The crystal may be used
with a tank circuit, or it may perform alone. Crystals exhibit a characteristic
known as the PIEZOELECTRIC EFFECT. The piezoelectric effect is the property of a
crystal by which mechanical forces produce electrical charges and, conversely, electrical
charges produce mechanical forces. This effect is a form of oscillation similar
to the flywheel effect of a tank circuit.
The piezoelectric effect can be seen in a number of crystal substances. The most
important of these are the minerals quartz and Rochelle salt. Although quartz does
not exhibit the piezoelectric effect to the degree that Rochelle salt does, quartz
is used for frequency control in oscillators because of its greater mechanical strength.
Another mineral, tourmaline, is physically strong like quartz; but because it is
more expensive, it is not used extensively as an FDD. This discussion will deal
only with the quartz crystal.
The crystals used in oscillator circuits are thin sheets, or wafers, cut from
natural or synthetic quartz and ground to a specific thickness to obtain the desired
resonant frequency. The crystals are mounted in holders, which support them physically
and provide electrodes by which voltage is applied. The holder must allow the crystals
freedom for vibration. There are many different types of holders. One type is shown
in figure 2-5.
Figure 2-5. - Crystal holder.
The frequency for which a crystal is ground is referred to as the NATURAL RESONANT
Frequency of the crystal. Voltage applied to the crystal produces mechanical vibrations
which, in turn, produce an output voltage at the natural resonant frequency of the
crystal. a vibrating crystal can be represented by an equivalent electrical circuit
composed of capacitance, inductance, and resistance.
Figure 2-6, view (A), illustrates the symbol of a crystal; view (B) shows an
equivalent circuit for the crystal. View (C) shows an equivalent circuit for the
crystal and the holder; C1 represents the capacitance between the metal plates of
Figure 2-6A. - Crystal symbol and equivalent circuits. SYMBOL
Figure 2-6B. - Crystal symbol and equivalent circuits. EQUIVALENT Circuit.
Figure 2-6C. - Crystal symbol and equivalent circuits. HOLDER ADDED
The Q (discussed in chapter 1) of a crystal is many times greater than that of
an LC tank circuit. The high Q is present because the resistance in the crystal
is extremely small. Commercially produced crystals range in Q from 5,000 to 30,000.
The high Q causes the frequency stability to be much greater than that
of an ordinary LC tank circuit. This is the reason a crystal is used in many
sine-wave generator circuits.
Q-1. What are the two classifications of wave generators according
to their output waveshapes?
Q-2. What are the three networks used for frequency-determining devices?
Q-3. What is another name for nonsinusoidal oscillators?
Q-4. What is a nonrotating device that produces alternating current?
Q-5. What are the three requirements necessary for oscillations to
exist in a circuit?
SOLID-STATE LC Oscillators
As you have just studied, a basic oscillator can be broken down into three main
sections: a frequency-determining device, an amplifier, and a feedback circuit.
The frequency-determining device in an LC oscillator is usually an LC tank circuit.
Although the tank circuit is normally found in the input
circuit of an oscillator (both electron tube and transistor), it sometimes appears
in the output circuit. The differences in magnitude of plate and collector currents
and shunting impedances are considerations in the designed locations of such tank
circuits. In both solid-state and electron tube circuits, oscillations take
place in the tuned circuit. Both the electron tube and the transistor function
primarily as electrical valves that amplify and automatically deliver to the input
circuit the proper amount of energy to sustain oscillations. In both tube and transistor
oscillators, the feedback circuit couples energy of the proper amount and of the
correct phase from the output to the input circuit to sustain oscillations.
Let's review what you have studied up to this point concerning feedback. Feedback
is the process of transferring energy from a high-level point in a system to a low-level
point in a system. This means transferring energy from the output of an amplifier
back to its input. If the output feedback signal opposes the input signal, the signal
is Degenerative or Negative Feedback. However, if the feedback aids the input signal,
the feedback is Regenerative or Positive Feedback. Regenerative or
positive feedback is one of the requirements to sustain oscillations in an oscillator.
This feedback can be applied in any of several ways to produce a practical oscillator
TYPES of Feedback
Chapter 1 described the resonant or tank circuit and how a sinusoidal signal
is generated by the action of an inductor and a capacitor. The feedback signal is
coupled from this circuit by either of two means. The first method is to take some
of the energy from the inductor. This can be done by any one of the three ways shown
in figure 2-7, views (A), (B), and (C). When an oscillator uses a TICKLER COIL,
as shown in view (A), it is referred to as an ARMSTRONG Oscillator. When an oscillator uses a tapped coil (view (B)) or a split coil (view (C)), it is referred to as a
HARTLEY Oscillator. The second method of coupling the feedback signal is to use
two capacitors in the tank circuit and tap the feedback signal between them. This
is shown in view (D). An oscillator using this method is referred to as a COLPITTS
Oscillator. Each of these particular oscillators is named after the person who
originally designed them.
Figure 2-7. - Feedback signals.
Configuration of Oscillators
Any of the three basic amplifier configurations (common collector, common base,
or common emitter) described in NEETS, Module 7, Introduction to Solid-State Devices
and Power Supplies, Chapter
2, may be used for the oscillator circuit. However, certain considerations in
the application of the circuit, such as the operating frequency and output power
required, usually determine which of the three configurations is to be used. The
three basic configurations are shown in figure 2-8, views (A), (B), and
Figure 2-8A. - Basic configurations. Common-Collector Configuration
Figure 2-8B. - Basic configurations. Common-Base Configuration.
Figure 2-8C. - Basic configurations. Common-Emitter Configuration.
Since there is no phase reversal between the input and output circuits of a common-collector
configuration, the feedback network does not need to provide a phase shift. However,
since the voltage gain is less than unity and the power gain is low, the common-collector
configuration is very seldom used in oscillator circuits.
The power gain and voltage gain of the common-base configuration are high enough
to give satisfactory operation in an oscillator circuit. The wide range between
the input resistance and the output resistance make impedance matching slightly
harder to achieve in the common-base circuit than in the common-emitter circuit.
An advantage of the common-base configuration is that it exhibits better high- frequency
response than does the common-emitter configuration.
The common-emitter configuration has high power gain and is used in low-frequency
applications. For the energy which is fed back from the output to be in phase with
the energy at the input, the feedback network of a common-emitter oscillator must
provide a phase shift of approximately 180 degrees. An