Square Wave Limiter; Sine Converter; Function Switch; Modulation Control Switching - Wavetek 193 Instruction Manual

4.5.6 Modulation Control Switching
Refer to sweep/mod board schematic sheet 3. The
modulation control switching, which selects the
sweep/modulation generator operating mode, con-
4.5.5 Function Switch
Refer to Sweep/Mod board schematic sheet 1. The
function switch selects the function (waveform) of the
sweep/mod generator. Wafer SW2-A routes the
selected function, (sine, triangle or square) to SW3-D
of the Modulation Contol switching. Wafers SW2-8
and C control the current source when a pulse or
sawtooth function is selected.
amplifier, and temperature compensated ± 10V sup-
plies. The triangle from the modulation generator loop.
is reduced in amplitude by resistors R41 and R42 and
feeds the diode network U4 pins 3, 9 and 1.
The following section illustrates the operation of the
diode network when shaping a sine wave.
At the negative peak of the input triangle the diodes
U4 pins 3, 4 and 5, 6 are forward biased, which
reverse biases U4 pins 2, 5. Also, diodes U4 pins 1, 9
and 6, 8 are cut off, which forward biases U4 pins 2, 8.
When the input triangle slope increases from negative
to positive peak, diodes U4 pins 1, 9 and 6, 8 begin to
conduct, which decreases the current through U4
pins 2, 8. At the same time, diodes U4 pins 3, 4 and 5,
6 decrease conduction, which causes diode U4 pins
2, 5 to begin conduction. When the triangle reaches
the positive peak, diode U4 pins 2, 5 is forward biased
and U4 pins 2, 8 is cutoff. The current from the diode
network U4 pin 2 drives the sine buffer U6.
Shaping the negative-going slope of the sine wave is
similar to the positive-going slope. Except, when the
input triangle ramps positive to negative, diodes U4
pins 1, 9 and 6, 8 decrease conduction and diodes U4
pins 3, 4 and 5, 6 increase conduction. This, even-
tually, forward biases diode U4 pin 2, 8 and reverse
biases diode U4 pins 2, 5.
The sine buffer U6 pin 6 sums the currents supplied
by the diode network to produce a 5Vp-p sine wave
that drives the function switch.
The bias for the diode network is supplied by
temperature compensated ± 10V supplies. The
+
10V supply is amplifier, U5 pin 1, with a gain of
- 2/3, which is referenced to the - 15V supply. The
- 10V supply is amplifier U5 pin 7, with a gain of - 1,
which is referenced to the
+
10V supply. A tempera-
ture sensing device, RT1, adjusts the gain of the
+
10V supply, which gives the
±
10V supplies a
negative temperature coefficient.
4-12
4.5.4 Sine Converter
Refer to Sweep/Mod board schematic sheet 2. The
sine converter circuit converts a buffered ± 2.5V
triangle to a ± 2.5V sine waveform. The sine con-
verter uses the nonlinear characteristics of the diode
to convert a linear triangle slope to a sinusoidal curve.
The sine converter consists of a diode network, buffer
4.5.3 Square Wave Limiter
Refer to Sweep/Mod board schematic sheet 1. The
square wave limiter, which receives its input from the
hysteresis switch, limits the square wave peaks to
± 2.8V and routes the square wave to the function
switch.
When the hysteresis switch output U13 pin 6 goes
high (
+
3V), diodes CR4 and CR7 are foward biased,
and diodes CR5 and CR6 are reverse biased. This
allows current flow from the
+
15V supply, through
R29, CR4 and R27.
The diodes CR5 and CR6 are forward biased when the
hysteresis switch output is low (- 3V), also diodes
CR4 and CR7 are reverse biased. Now current flows
from circuit ground through R27, CR6, and R28 to
-15V supply.
During the triangle's positive-going ramp, the
hysteresis switch output is
+
3V. Current flows from
the
+
15V supply, through R40 and U3 pins 5 and 2 to
the triangle buffer output U12 pin 6. At the same time,
current flows from the hysteresis switch output U13
pin 6 through R38, U3 pins 6 and 8, and R37 to the
-15V supply. Diodes U3 pins 5, 6 and 2, 8 are
reverse biased. Resistor R38 provides negative feed-
back, while resistors R39 and R35 provide positive
feedback.
As the triangle reaches the positive limit
+
2.5V, the
diodes U3 pins 2, 8 and 5, 6 begin conduction. This
also decreases the current through diodes U3 pins 2,
5 and 6, 8. This causes a positive increase in the volt-
age level at U13 pin 2. As U13 pin 2 becomes slightly
more positive, with respect to pin 3, U13 pin 6 will
instantaneously toggle to - 3V.
During the triangles negative-going ramp, the current
through diodes U3 pins 2, 8 and 5, 6 increases, while
the current through diodes U3 pins 2, 5 and 6, 8
decreases and, eventually, cutoff.
As the triangle reaches the negative limit, - 2.5V, the
diodes U3 pins 6, 8 and 2, 5 begin conduction. This
decreases the current through U3 pins 5, 6 and 2, 8;
thus forcing U3 pin 2 slightly more negative, with
respect to pin 3, which causes the output pin 6 to tog-
gle to
+
3V.