HP 120A - Circuit Descripti N; General Content

Sect, in Page 1
SECTION III
CIRCUIT DESCRIPTION
3-1 GENERAL CONTENT
This section contains a brief description of the overall
operation of the Model 120A Oscilloscope, descrip 
tions of each major section and detailed descriptions
of Schmitt triggers.
3-2 OVERALL OPERATION
The block diagram shows the basic signal circuits in
the Model 120A Oscilloscope.
a. Vertical Amplifier - The vertical amplifier re 
ceives the input signal, amplifies it, and drives
the vertical deflection plates. It provides attenuation
of the input signal, determines the vertical position
of the spot on the screen, and supplies a signal for
internal synchronization.
b. Horizontal Amplifier - The horizontal ampli 
fier receives an input signal either from the hor 
izontal input terminals or from the internal sweep
generator, amplifies the signal and drives the hor 
izontal deflection plates. The horizontal amplifier
provides attenuation of the horizontal input signal,
magnification of the internal sweep, and determines
the horizontal position of the spot on the screen.
c. Sweep Generator - The sweep generator con 
sists of a trigger generator and a sawtooth gene 
rator. The trigger generator receives the synchro 
nizing signal, either internally or exiernally, and
converts it into a pulse which initiates the action of
the sawtooth generator. The sawtooth generator will
then go through one complete cycle. The sawtooth
generator has feedback which automatically shuts
itself off upon the completion of one cycle. Another
pulse from the trigger generator will be needed be 
fore the action will start again. Thus the action of
the sweep generator is precisely controlled. The
sweep only starts when a synchronizing signal is re 
ceived and then only at the same point on the wave 
form every time.
Trigger Generator - The trigger generator con 
sists of a synchronizing circuit and a trigger gene 
rator. The synchronizing circuit receives a signal
either from the vertical amplifier for internal syn 
chronization (+ or -), from an internal 6. 3 volt
source for line frequency synchronization, or from
the horizontal input terminals for external synchro 
nization. The trigger generator converts the signal
into a fast, constant amplitude pulse for operation of
the start-stop trigger. The particular voltage which
will trigger the trigger generator is determined by the
trigger level control, when AUTOMATIC sweep is not
in use.
Sawtooth Generator - The Sawtooth generator con 
sists of a start-stop trigger, an integrator switch, a
sawtooth integrator, and a hold-off cathode follower.
The pulse from the trigger generator controls the
start-stop trigger which in turn controls the integra 
tor switch. The start-stop trigger also furnishes an
unblanking pulse through the gate inverter to turn the
trace on while the sweep is in progress.
The integrator switch controls the action of the saw 
tooth integrator. When this switch is closed the inte 
grator output is effectively held at a reference volt 
age, thereby disabling the integrator. When a sweep
signal is received this switch opens, permitting the
sawtooth integrator to commence its sweep. Sweep
speed is determined by the value of resistance and
capacitance in the grid circuit of the sawtooth inte 
grator and the voltage supplying the grid resistor.
To permit all circuits to recover after a trace, a bias
voltage is applied to the start-stop trigger by the
hold-off cathode follower, making the sawtooth gene 
rator insensitive to incoming signals during the hold-
off time.
d. Power Supplies - This oscilloscope has two low
voltage power supplies supplying + 380 volts and
-150 volts. Both of these supplies are fully regulated.
The high voltage power supply is regulated and has
an output of -2250 volts for the cathode-ray tube. The
filament supply to the input tubes of the vertical am 
plifier is regulated by transistors.