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4.1
FUNCTION GENERATOR
As shown in figure 4-1, the VCG summing amplifier sums
the currents from the frequency dial and VCG input con-
nector in function generator mode or from the low pass
filter output and the .frequency-to-voltage (F/V) converter
output in synthesizer mode. The low pass filter is part of
the generator frequency .phase lock loop which provides a
feedback current that corrects generator frequency to be
exactly that of the synthesizer loop output. (Phase lock
loop operation is described in paragraph 4.5.) The F/V input
is also from the synthesizer loop and provides a gross cor-
rection to increase the response time of the generator to
changes in synthesizer frequency programming.
The VCG summing amplifier is a noriinverting buffer 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 summing 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 posi-
tive 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.
The triangle buffer amplifier is a unity gain amplifier whose
output is fed to the hysteresis switch as well as to the sine
SECTION
4
CIRCUIT DESCRIPTION
generates the triangle waveform as shown in figure 4-2. Since
the output of the hysteresis switch is a square wave, the re-
sult is simultaneous generation of a square wave and triangle
wave at the same frequency.
+1.25V- - -
-1.25V
I
I
+2V
1
-2V
HYSTERESIS
converter. The hysteresis switch has two voltage limit points
Figure 4-2. Generator Loop and Waveforms
(+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 cur-
rent 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 hysteresis switch
will switch back to positive, reversing the process. This action
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.
By using current division, the magnitude of the capacitor is
effectively increased, allowing the generation of lower fre-
4-1
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