3-5
beam to move in both a vertical and horizontal (diagonal) direction at the same time. This then is how a
sawtooth wave is made to appear on an oscilloscope. You should refer to NEETS, Module 6, Electronic
Emission, Tubes, and Power Supplies, Chapter 2, for a review of oscilloscopes.
Trapezoidal Wave
A TRAPEZOIDAL wave looks like a sawtooth wave on top of a square or rectangular wave, as
shown in figure 3-1, view (E). The leading edge of a trapezoidal wave is called the JUMP voltage. The
next portion of the wave is the linear rise or SLOPE. The trailing edge is called the FALL or DECAY. A
trapezoidal wave is used to furnish deflection current in the electromagnetic cathode ray tube and is found
in television and radar display systems. Electromagnetic cathode ray tubes use coils for the deflection
system, and a linear rise in current is required for an accurate horizontal display. The square or
rectangular wave portion provides the jump voltage for a linear rise in current through the resistance of
the coil. This will be explained further in a discussion of the trapezoidal sweep generator.
Triggers
A trigger is a very narrow pulse, as shown in figure 3-1, view (F). Trigger pulses are normally used
to turn other circuits on or off.
WAVEFORM GENERATOR
Nonsinusoidal oscillators generate complex waveforms such as those just discussed. Because the
outputs of these oscillators are generally characterized by a sudden change, or relaxation, these oscillators
are often called RELAXATION OSCILLATORS. The pulse repetition rate of these oscillators is usually
governed by the charge and discharge timing of a capacitor in series with a resistor. However, some
oscillators contain inductors that, along with circuit resistance, affect the output frequency. These RC and
LC networks within oscillator circuits are used for frequency determination. Within this category of
relaxation oscillators are MULTIVIBRATORS, BLOCKING OSCILLATORS, and SAWTOOTH- and
TRAPEZOIDAL-WAVE GENERATORS.
Many electronic circuits are not in an "on" condition all of the time. In computers, for example,
waveforms must be turned on and off for specific lengths of time. The time intervals vary from tenths of
microseconds to several thousand microseconds. Square and rectangular waveforms are normally used to
turn such circuits on and off because the sharp leading and trailing edges make them ideal for timing
purposes.
MULTIVIBRATORS
The type of circuit most often used to generate square or rectangular waves is the multivibrator. A
multivibrator, as shown in figure 3-3, is basically two amplifier circuits arranged with regenerative
feedback. One of the amplifiers is conducting while the other is cut off.
