3-45
The amplitude of the output sawtooth wave is equal to 1.75 volts (4.25 volts minus 2.5 volts). The
electrical length is the same as the positive alternation of the input gate, or 500 microseconds. The prt is
700 microseconds (500 + 200) and the prf is 1/prt or 1,428 hertz.
Trapezoidal Sweep Generator
Normally, oscilloscopes and synchroscopes use ELECTROSTATIC DEFLECTION and, as the
name implies, electrostatic fields move the electron beam. The need here is for a sawtooth voltage
waveform.
Another method of electron beam deflection is ELECTROMAGNETIC DEFLECTION. Currents
through a coil produce electromagnetic fields which position the beam of electrons. The electromagnetic
system requires a sawtooth of current which increases at a linear rate. Because of the inherent
characteristics of a coil, a sawtooth voltage does not cause a linear increase of current. A linear increase
of current requires a TRAPEZOIDAL voltage waveform applied to a coil. This section discusses the
generation of a trapezoidal wave.
Figure 3-48 shows a trapezoidal wave. The wave consists of a sharp, almost instantaneous jump in
voltage followed by a linear rise to some peak value. The initial change in voltage at T0 is called a JUMP
or STEP. The jump is followed by a linear sawtooth voltage rise. The time from the jump to the peak
amplitude is the sum of the jump voltage and the sawtooth peak; where the peak value occurs is the
electrical length. The peak voltage amplitude is the sum of the jump voltage and the sawtooth peak
voltage. The waveshape can be considered a combination of a rectangular wave and a sawtooth wave.
Figure 3-48.—Trapezoidal waveform.
The inductance and resistance of a coil form a series RL circuit. The voltage drop across this
inductance and resistance must be added to obtain the voltage waveform required to produce a linear rise
in current. A linear rise of current produces a linear rise of voltage across the resistance of the coil and a
constant voltage drop across the inductance of the coil.
Assume figure 3-49, view (A), represents deflection coils. If we apply a voltage waveshape to the
circuit, which will provide a square wave across inductor L, and a sawtooth across resistor R, then a linear
current rise will result.
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