175A - HP Archive

Section IV
Paragraphs 4-34 to 4-41
INPUT
A. CIRCUIT
m
OUTPUT
B. COMPOSITE CURVE LO - M - 643
Figure 4-5. Trigger Generator Simplified Circuit
and Composite Characteristic
change in voltage at the input to integratorV106 dur-
ing the sweep time is only about one volt. Voltage
across the sweep resistor, then, changes by about
1%, and the current through the resistor changes by
the same amount. The current through the sweep
resistor is the charging current for the sweep capaci-
tor; therefore, the voltage across the sweep capacitor
changes quite linearly with time, and the sweep sig-
nal is a nearly linear voltage ramp. The SWEEP
TIME switch changes the value of sweep resistor or
capacitor to change the sweep time. The sweep out-
put is applied to the horizontal amplifier through the
horizontal plug-in.
4-34. An attenuated sweep signal is applied to the
input of gate generator V103/V104A through hold-off
cathode follower V105B and section B of bias control
cathode follower V109. This signal drives the input
of the gate generator up to the upper hysteresis limit
and causes the gate generator to switch back to its
pre-sweep state. The gate generator then ends the
the gates, blanking the crt and forward biasing switch
4-4
I
V
Model 175A
diode CR106. The switch diode returns the input of
integrator V106 to its pre-sweep level, discharging
the sweep capacitor.
4-35. During a sweep time, hold-off cathode follower
V105B charges a hold-off capacitor. After the sweep
ends, this capacitor lets the input to gate generator
V103/V104A down slowly enough to prevent that cir-
cuit from being triggered again until the remaining
sweep circuits have recovered completely. The
SWEEP TIME switch changes the size of the hold-off
capacitor with sweep time.
4-36. Clamp V104B ensures that each sweep starts
from the same voltage level, about -50 volts.
4-37. The SWEEP MODE control R172 (figure 5-22)
determines the no-signal bias at the input to gate gen-
erator V103/V104A by setting the bias on the A sec-
tion of bias control cathode follower V109. With the
control set to PRESET or in the TRIGGER portion of
its adjustable range, the gate generator bias cannot
drop below its lower hysteresis limit unless the trig-
ger generator provides a trigger. With the control
set in the FREE RUN portion of its adjustable range,
the gate bias is allowed to drop below its lower hys-
teresis limit. Thus as the hold-off capacitor dis-
charges, it lets the gate generator bias all the way
down to the lower hysteresis limit, and another sweep
starts automatically.
4- 38. SINGLE - SWE EP OPERATION.
4-39. The SWEEP OCCURRENCE switch (on the panel
of the horizontal plug-in unit) selects normal or
single-sweep operation. Normal operation is dis-
cussed above. For single-sweep operation, the
SWEEP OCCURRENCE switch converts V109 into a
Schmitt circuit. As the sweep signal from hold-off
cathode follower V105B rises to end the gate from
the gate generator, the sweep signal also switches
the Schmitt circuit of V109 so that V109B conducts
and V109A is cut off. The B section of V109 then
holds the input to gate generator V103/V104A high
enough so that triggers from the trigger generator
cannot actuate the gate generator, and the sweep
generating circuits are effectively disabled. A pos-
itive signal applied to V109A switches the Schmitt
circuit of V109 so that V109A conducts and V109B is
cut off. The A section of V109 then sets the input to
the gate generator according to the setting of the
SWEEP MODE control, and the sweep generating cir-
cuits are effectively armed. The switching signal
for V109A can be an external signal applied to the
ARMING INPUT connector or an internal signal ob-
tained by switching SWEEP MODE control out of its
PRESET position.
4-40. SCHMITT TRIGGER CIRCUIT.
4-41. The Schmitt trigger circuit is a form of bi-
stable multivibrator used where fast-rising signals
are required. Figure 4-6 shows a simplified Schmitt
trigger circuit and input and output waveforms. If
initially the input voltage is such that V1 is cut off,
V2 conducts. As the input voltage becomes more pos-
itive, it will eventually reach a predetermined level
(a) at which the circuit changes state; V1 conducts
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