Wavetek 180 - Circuit Description; Voltage Controlled Generator (Vcg); Frequency Control

4
SECTION
CIRCUIT DESCRIPTION
4.1 VOLTAGE CONTROLLED GENERATOR (VCG)
As shown in figure 4-1, the VCG summing amplifier sums
the currents from the frequency dial, sweep generator,
crystal control and VCG input connector. The VCG sum
ming amplifier is an inverting amplifier whose output
current is used to control a positive current source and a
negative current source. The currents from the two current
sources are equal and opposite polarity and the magnitudes
are directly proportional to the current of the VCG sum
ming amplifier output. The diode gate, which is controlled
by the hysteresis switch. Is used to switch the positive
current or the negative current to the integrating capacitor
selected by the frequency multiplier. If the positive current
is switched into the capacitor, the voltage across the capacitor
will increase linearly to generate the positive slope of the
triangle wave. If the current is negative, the voltage across
the capacitor will decrease linearly to produce the negative
slope.
0,
HYSTERESIS
SWITCH
BUFFER^
The triangle buffer amplifier is a unity gain amplifier whose
output is fed to the hysteresis switch as well as to the sine
converter. The hysteresis switch has two voltage limit points
(+1.25V and —1.25V). (See figure 4-2.)
During the time the output voltage of the triangle buffer
amplifier is increasing, the output voltage of the hysteresis
switch is positive, but when the output voltage of the
triangle amplifier reaches +1.25V, it triggers the hysteresis
switch causing the switch output to become negative. Once
the control voltage into the diode gate becomes negative, it
will switch the positive current out and switch the negative
current in to the integrating capacitor, starting a linear
decrease of the voltage across the capacitor. When the
decreasing voltage reaches -1.25V, the output of the hys
teresis switch will switch back to positive, reversing the
process. This action generates the triangle waveform as
shown in figure 4-2. Since the output of the hysteresis
switch isa square wave, the result is simultaneous generation
of a square wave and triangle wave at the same frequency.
4.2 FREQUENCY CONTROL
The output frequency is determined by the magnitude of
the integrating capacitor selected by the frequency multiplier
and the magnitude of the positive and negative current
sources (figure 4-1). Since the current magnitudes are linearly
proportional to the sum of the VCG current, the output
frequency will also be linearly proportional to the current
sum.
ni-
+1.25V
-1.25 V
wv
^0.91
SQUARE
WAVE
TRIANGLE
B WAVE
Figure 4-2. Simplified Timing Diagram
Figure 4-3. Current Divider
4-1