Wavetek 143 - Page 19

4.1 BASIC WAVEFORM DEVELOPMENT
The heart of the generator (the bold path in figure 4-1) is a
triangle and square wave generator. The triangle waves are
developed by capacitor charging ramps that are alternately
reversed in polarity. The polarity reversal is caused by a flip-
flop circuit, or hysteresis switch, that in turn produces the
square waves. The flip-flop changes states upon detecting
amplitude limits of the charging ramps through the triangle
amplifier.
As shown in figure 4-1, the VCG dial buffer sums the cur-
rents from the frequency dial, frequency vernier and VCG
in connector. The VCG dial buffer is an inverting amplifier
whose output voltage is used to control a positive current
source and a negative current source. For symmetrical out-
put waveforms, the currents from the two current sources
are equal and directly proportional to the voltage of the
VCG dial buffer output. The diode gate, which is controlled
by the hysteresis switch, is used to switch the positive or
the negative current to the integrating capacitor selected by
the frequency multiplier. If the positive current is switched
into the integrating capacitor, the voltage across the capaci-
tor will rise linearly to generate the triangle rise transition.
If the current is negative, the voltage across the integrating
capacitor will fall linearly to produce the fall transition.
The triangle amplifier is a unity gain amplifier whose output
is fed to the hysteresis switch. The hysteresis switch has two
voltage limit points (+1.25 aand —1.25V) at its input.
During the time the output voltage of the triangle amplifier
is rising, the output voltage of the hysteresis switch is posi-
tive, but when the output voltage of the triangle reaches
*1.25V, it triggers the hysteresis switch causing the output
to switch 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, so that the voltage across the capacitor will re-
verse, starting a linear decrease of the waveform. 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
is a square wave, the result is simultaneous generation of a
square wave and a triangle wave at the same frequency.
SECTION
CIRCUIT DESCRIPTION
HYSTERESIS
SWITCH
Figure 4-2. Basic Generator and Timing Diagram
The output frequency is determined by the magnitude of
the capacitor selected by the frequency multiplier and the
magnitude of the positive and negative current sources. Since
the current sources are linearly proportional to the control
voltage of the VCG circuit, the output frequency will also
be linearly proportional to the control voltage.
When the symmetry control is turned on, the current of the
negative current source is decreased by 19 times, and the
fall time of the triangle is 19 times longer than the rise time
of the triangle, resulting in an unsymmetrical waveform and
a division of the frequency by a factor of 10. Gradually in-
creasing the current from the negative current source and
decreasing the current from the positive current source
causes the time for the triangle to complete one cycle to
remain constant, while the symmetry of the output wave-
form is continuously varied.
The output of the hysteresis switch is fed to the sync ampli-
fier and also the square wave shaper. The square wave shaper
consists of a shaping circuit which limits the square wave
output swing to +1.25V. For positive pulse outputs, it limits
the output voltage swing from —1.25 to OV; and for negative
pulse outputs, it limits the output swing from 0 to *1.25V.
4-1