Wavetek 193 Instruction Manual page 34

4.4.11 Square Shaper
The square shaper schematic is located on sheet 6. In
square function, CR20 pulls U4 pin 13 low, enabling
the hysteresis switch input (HYS) at U4 pin 12. A low
at U4 pin 12 causes a low at U4 pin 15 and a high at
pin 9. 022 turns on while 023 is turned off, producing
a
+
1.2 volt high at the bases of the current switch
control transistors 024 and 025. Transistor 025 is on,
reverse biasing CR23. Transistor 024 is off allowing
positive current to flow through R147, CR22, R154
and into the preamplifier node via R152.
When HYS toggles high, 023 turns on forcing the
bases of 024 and 025 to -1.2 volts. 024 turns on
and 025 turns off, allowing negative current to flow
through R157, CR23, R154 and the the amplifier node
via R152.
R152 and R154 form a current divider to obtain a 2mA
full scale current into the preamplifier. Overshoot
caused by diode capacitance is reduced by R153 and
C73. The output of the square shaper is disabled in all
4.4.10 Sync
Refer to schematic sheet 2. The SYNC OUT amplifier
is driven from the Signal at U6 pin 10 in the triangle
and sine functions, and from U6 pin 7 when the func-
tion switch is in the square function. These two inputs
are wire ORed at U6 pin 13.
In the triangle and sine functions, SYNC SEl allows
R23 to pull CR4 high causing a low at U6 pin 2. This
enables the signal from the zero crosslnq detector
output (U10 pin 15), and disables the hysteresis
switch input at U6 pin 7. When the positive going ramp
crosses 0 volts at the zero crossing detector input
U10 pin 13, U10 pin 15 and U6 pin 10 go high. This
causes a low at U6 pins 14 and 13. U6 pin 9 goes low
and pin 15 goes high turning on 05 and turning off 06.
This results in a high at SYNC OUT. As the triangle at
U10 pin 13 crosses 0 volts in a negative direction, pin
15 goes low, causing 06 to be turned on, producing a
low at SYNC OUT. Therefore the SYNC OUT always
toggles when the triangle crosses 0 volts.
When the square wave function is selected, CR4 pulls
U6 pins 4 and 6 low. U6 pins 2 and 11 now go high,
disabling the zero crossing detector input from pin 10,
and enabling the square wave input from U6 pin 7. U5
pin 3 now drives the SYNC OUT connector in a similar
manner as U10 pin 15. The SYNC OUT is in phase
with the square wave output.
R26, a 49.90 resistor sets the 500 output impedance.
The RUN signal is used to hold the sync output low
during quiescent periods.
4-8
referenced to pin 12, causing pin 15 to also go high.
When either U12 pin 3 or pin 15 go high, U4 pin 3 goes
low because these outputs are wire ORed to U4 pin 7.
U4 pin 3 is connected to pins 4 and 10. Because pin
10 was previously high, U4 pin 14 was low causing a
low at U4 pin 5. The trigger pulse low at U4 pin 4
causes a 10 ns ECl high pulse at U4 pin 2. At the
same time, U4 pin 14 goes high and after the time
delay set by R126 and C62, U4 pin 5 also goes high.
This causes U4 pin 2 to return low.
In the gate mode CR14 holds U4 pin 11 high, forcing
pin 14 low. The length of the control pulse at U4 pin 2
is now equal to the period during which U4 pin 7 is
held high. In the continuous mode, U4 pin 2 is held
high by CR16 regardless of any input trigger signals.
4.4.9 Trigger Baseline
Refer to sheet 5. In the trigger mode, with no trigger
inputs, U5 pin 12 is held low. On the next positive go-
ing triangle, the trigger reset (TRIG RST)signal at U5
pin 11 causes U5 pin 14 to go high. This turns 018 off
and 017 on, which turns off 019. The 019 emitter
voltage is pulled down by the negative current
i
sources 020 and 021, causing CR19 to conduct.
Because the anode is at ground and CR18 is matched
to CR19, the voltage at the anode of CR18 is also zero.
This causes the triangle on the positive going ramp to
stop at exactly zero volts. When a trigger signal oc-
curs, U5 pin 12 goes high for about 10ns, causing pin
15 to also go high. This turns on 018 and turns off
017, which turns on 019, causing the emitter to rise
to about 1.7 volts. This reverse biases CR18 and CR19
causing the generator to run for exactly one cycle. In
the gate mode, U5 pin 12 is held high for the duration
of the input signal causing the generator to run for this
interval plus the time required to complete a partial
cycle.
In the trigger or gated mode, quiescent state, positive
charging current I flows in CR18. As the VCG current
is varied, I also varies, causing the voltage across
CR18 to vary. To prevent this from causing a baseline
shift, current (I) must also flow in the reference diode
CR19. A negative current source (ITRGBl) is con-
nected to the bases of 020 and Q21. Negative current
( - I) flows through the collector of 020 and R133.
Because of the configuration of 020 and 021, and
because R133 and R134 are both 1kO, an equal
amount of current -I also flows through the collector
of 021 and R134, causing - 21to flow at the junction
of R133 and R134. Half of this current (-I) flows
through CR19, while the remaining current flows
through CR18.Therefore, the anode of CR18 is held at
zero volts regardless of the VCG summing node
current.