Circuit Description; Basic Waveform Development - Wavetek 145 Instruction Manual

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 voltag~ across the capaci-
tor will rise linearly to generate the triangle rise transition.
If the current is negative, the voltage across the integrating
capacitor wilt faH 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 and -1.25V) at its input.
During the trme the output voltage of the triangle amplifier
rs rising, the output voltage of the hystere-sis switch is posi-
tive, bot 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 wil I 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
4
CIRCUIT DESCRIPTION
HYSTERESIS
SWITCH
+ 1 . 2 5 ~
A,B
-1.25
c
+
2
•
5
n
n
1
-2.s
l LJ LJ
B
Figure 4-2. Basic Generator and Timing Di~gram
The output frequency is determined by the magnitude of the
capacitor selected by the frequency multiplier and the magni-
tude of the positive and negative current sources. Since the
current sources are linearily proportional to the control voltage
of the VCG circuit, the output frequency will also be linearily
proportional to the control voltage.
The output of the hysteresis switch is fed to the sync amplifier
and also the 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 voltage swing from Oto+ 1.25V. The PULSE
or PULSE from the auxiliary board are bipolar and processed
as the square wave.
The triangle wave from the triangle amplifier is coupled through
a buffer amplifier and made available to the function selector
switch. The buffer amplifier provides a low impedance to drive
the sine converter circuit. The sine converter, using the
nonlinear characteristics of its diodes, converts the triangle
wave into a sine wave.
The square wave from the sync amplifier, processed through a
one-shot and the sync out buffer, is externally available at the
sync out connector. The sync pulse, then, is a TTL level pulse
output of the generator frequency.
4-1