HP 120A - Horizontal Channel

Sect, in Page 3
has a potentiometer between its cathodes which con 
trols the vertical position of the pattern (VERTICAL
POSITION). The third balanced differential amplifier,
V3, a 12AZ7, is the output stage. The resistance bet 
ween cathodes of this stage provides gain stability and
improved linearity. The 5O-38Ojigf condenser across
this resistance adjusts the frequency response of this
stage. In addition, synchronization signals are coupl 
ed from the plates of this tube and fed into the sweep
generator to trigger the sweep for either INTERNAL
+ or INTERNAL - synchronization. Since the sweep
generator triggers only on the negative slope of the
signal, provision is made so that the synchronizing
signal can be taken inverted from the opposite half of
the tube for INTERNAL + synchronization.
3-4 HORIZONTAL CHANNEL
The Horizontal Amplifier consists of three parts, the
AC - DC coupling switch, the Input Attenuator, and
the Amplifier proper.
a. AC - DC Coupling Switch - In the DC position
the signals are fed directly into the grid of the
first balanced amplifier. In the AC position the sig 
nals are fed in through a . Igfd condenser.
b. Input Attenuator - The Input Attenuator consists
of a three -position switch in decade steps, . 1, 1,
10 VOLTS/CENTIMETER. The attenuator is frequen 
cy-compensated.
c. Horizontal Amplifier - The horizontal amplifier
consists of two cross-neutralized balanced dif 
ferential amplifiers in cascade. When the SWEEP
TIME HORIZONTAL SENSITIVITY switch is moved
from the sweep ranges to the horizontal sensitivity
ranges the resistance between the cathodes of input
amplifier V101, a 6DJ8, is switched to a leg having
the horizontal sensitivity VERNIER control in series
with the horiz. gain set control from the SWEEP EX 
PAND switch which has either the XI or X 5 SWEEP
EXPAND gain controls. The XI leg has the XI sweep
gain adjustment to calibrate the unmagnified (XI)
sweep while the X 5 leg has the exp. X5 gain adjust 
ment to calibrate the magnified (X 5) sweep. The
plates of the input amplifier (grids of the output am 
plifier) are connected with a 2-25/igf condenser to
adjust the frequency response. The plates of the out 
put amplifier V 102, another 12AZ7, connects to the
horizontal deflection plates through the direct connec 
tion links on the rear of the instrument
3-5 SWEEP GENERATOR
The Sweep Generator consists of a trigger generator,
a start-stop trigger, a gate inverter, an integrator
switch, a sawtooth integrator and a hold-off cathode
follower.
a. Trigger Generator - The purpose of the trigger
generator is to receive the synchronization sig 
nal and convert it into a fast, constant amplitude,
pulse for operation of the start-stop trigger. Since the
trigger generator and the start-stop trigger are forms
of Schmitt trigger circuits a discussion of them fol 
lows:
A Schmitt trigger consists of two amplifiers A and
B having de plate-to-grid coupling from amplifier
A to amplifier B and de cathode-to-cathode coupling.
In the case of the Model 120 A amplifier A is the pen 
tode and amplifier B is the triode in both the Trigger
Generator and the Start-Stop Trigger. The circuit
has two stable states: A-side conducting, B-side cut
off; B-side conducting, A-side cut off. Due to regene 
rative action the change-over from one state to the
other is very rapid, producing fast rise and decay
times from each side of the circuit, either of which
can be used for triggering subsequent circuits.
If the A-side, the input side, is conducting and the
grid voltage is driven lower than the lower hysteresis
limit the circuit will switch state rapidly. The circuit
will stay in this state until the input grid is driven
above the upper hysteresis limit. At this time the cir 
cuit will switch back to its original state. The levels
at which this switching action takes place can be ad 
justed to be close together, such as in the Trigger
Generator, or widely spaced, such as in the Start-
Stop Trigger. The de voltage applied to the input grid
will determine the state of the circuit.
To trigger the circuit, the A-side grid voltage must
cross the particular hysteresis limit which will
change the state of the circuit. If A-side is already
conducting, which is the normal case in the Model
120A, driving the grid voltage positive through its up 
per hysteresis limit will have no effect, but driving
the grid voltage negative through its lower hysteresis
limit will put A-side out of conduction, and B-side into
conduction.
The initial A-side grid bias can be placed inside or
outside the hysteresis area, thus establishing the in 
put voltage level required to change A ’ s state. In the
Trigger Generator the A-side grid bias is adjusted
with the TRIGGER LEVEL control and is placed mid 
way between the narrow upper and lower hysteresis
limits in the AUTOMATIC positiai. Narrow limits are
used so that the Trigger Generator will be sensitive
and start the synchronizing action with a small input
signal.