Modulation Generator Loop; Current Switch; Timing Capacitor And Triangle Buffer; Hysteresis Switch - Wavetek 193 Instruction Manual

4-11
Figure 4·5. Hysteresis Switch
B
R35
-15
R39
TRIANGLE A
INPUT
R38
+15V
4.5.2.3 Hysteresis Switch
Refer to sweep/mod board schematic sheet 1 and
figure 4-5.
The hysteresis switch U3 and U13 operates as a com-
parator. When the triangle input reaches the positive
limit (
+
2.5V) the switch output toggles to - 3V. And,
when the triangle reaches the negative limit ( - 2.5V)
the switch output toggles to
+
3V. The hysteresis
switch output drives the current switch, square wave
limiter, and modulation generator sync output ampli-
fier. Figure 4-5 shows a simplified schematic and tim-
ing diagram.
4.5.2.2 Timing Capacitor and Triangle Buffer
The timing capacitor C5 and C6 together with the cur-
rents from the current sources determine the fre-
quency of the modulation generator loop. On two
ranges 3 -1 00 and 3K -100K, the current sources
directly charge and discharge the timing capacitors.
But, on the .1 - 3 and 100 - 3K ranges, resistors R46
and R47 acts as a capacitance multiplier. R46 con-
nects between the current switch and the timing
capacitor, while R47 provides a shunt to 97% of the
current, thus reducing the current at the timing
capacitor to 3% of the current from the current
sources.
The triangle buffer U12 pin 6, a unity gain non invert-
ing amplifier, follows the voltage across the timing
capacitor.
4.5.2.1 Current Switch
The current switch selects which current source will
charge or discharge the timing capacitor. The
hysteresis switch U13 pin 6 controls the current
switch.
When the hysteresis output is high
(+ 3V), diode U3
pins 4 and 9 conducts current, 02 is reverse biased,
and the positive current source charges the timing
capacitor. In addition, transistor 01 is forward biased,
thus reverse biasing diode U3 pins 3 and 4, this sinks
the negative current source to the
+
15V supply.
When the hysteresis output is low (- 3V), diode U3
pins 3 and 4 is forward biased, this reverse biases
transistor 01, the current now flows from the timing
capacitor through the diode U3 pin 3 and 4 to the
negative current source. Meanwhile, transistor 02
conducts current, which reverse biases diode U3 pins
4 and 9, thus the -15V supply sinks the current from
the positive current source.
4.5.2 Modulation Generator Loop
Refer to Sweep/Mod board schematic sheet 1. The
modulation generator loop consist of the current
switch, timing capacitors and triangle buffer, and
hysteresis switch.
increases as the other decreases; thus changing the
slope of each half cycle over a 19:1 range without
changing the frequency.