SECTION 3
CIRCUIT DESCRIPTION
Type 422 AC-DC
Change information, if any, affecting this section will be found at the rear of this manual.
Introduction
This section of the manual contains a description of the
circuitry used in the Type 422 Oscilloscope with AC-DC
Power Supply. The description begins with a discussion of
the instrument using the basic block diagram shown in Fig.
3-1. Then each circuit is described in detail using a detailed
block diagram to show the interconnections between the
stages in each major circuit and the relationship of the
front-panel controls to the individual stages.
A complete block diagram is located in the Diagrams
section at the back of this manual. This block diagram
shows the overall relationship between all of the circuits.
Complete schematics of each circuit are also given in the
Diagrams section. Refer to these diagrams throughout the
following circuit description for electrical values and rela
tionship.
BLOCK DIAGRAM
General
The following discussion is provided to aid in under
standing the overall concept of the Type 422 before the
individual circuits are discussed in detail. A basic block
diagram of the Type 422 is shown in Fig. 3-1. Only the
basic interconnections between the individual blocks are
shown on this diagram. Each block represents a major cir
cuit within the instrument. The number in each block refers
to the complete circuit diagram iocated at the back of the
manual.
Signals to be displayed on the CRT are applied to the
INPUT 1 and/or the INPUT 2 connector. The input signals
are amplified by the Channel 1 Input Amplifier and/or the
Channel 2 Input Amplifier circuit. Each Input Amplifier
circuit includes separate vertical deflection factor, input
coupling, position, gain, variable attenuation and balance
controls. A trigger-pickoff stage in the Channel 1 Input
Amplifier circuit supplies a sample of the Channel 1 input
signal to the Sweep Trigger circuit to provide triggering
from the Channel 1 signal only. The Channel 2 Input Cir
cuit contains a X10 gain stage to allow the Channel 2
deflection factors to be increased 10 times and an invert
feature to invert the Channel 2 signal as displayed on the
CRT. The output of both Input Amplifier circuits is con
nected to the Vertical Switching and Output Amplifier cir
cuit. This circuit selects the channel(s) to be displayed on
the CRT and provides the final amplification for the verti
cal signal before it is applied to the vertical deflection plates
of the CRT. A delay line is included in the Vertical Switch
ing and Output Amplifier circuit to delay the vertical signal
slightly so the horizontal circuits have enough time to start
the sweep before the input signal arrives at the vertical
deflection plates. An output signal from this circuit is con
nected to the CRT Circuit to blank out the between-
channel switching transients when in the chopped mode of
operation. A trigger-pickoff network in this circuit provides
a sample of the displayed signal(s) to the Sweep Trigger
circuit.
The Sweep Trigger circuit produces an output pulse
which initiates the sweep signal produced by the Sweep
Generator circuit. The input signal to the Sweep Trigger
circuit can be selected from either the Channel 1 Input
Amplifier circuit, the Vertical Switching and Output
Amplifier circuit, or an external signal connected to the
HORIZ IN OR TRIG IN connector. This circuit contains
level, slope, coupling, source and mode controls to select
the desired triggering. The Sweep Generator circuit
produces a linear sawtooth output signal when initiated by
the Sweep Trigger circuit. The slope (sweep rate) of the
sawtooth produced by the Sweep Generator circuit is
controlled by the TIME/DIV switch. The Sweep Generator
circuit also produces an unblanking gate signal to unblank
the CRT so the display can be presented. This gate signal is
coincident with the sawtooth produced by the Sweep
Generator. An external signal can be connected to this
circuit through the EXT BLANKING connector to blank
portions of the CRT display. A gate signal which is also
coincident with the sawtooth is available at the GATE OUT
connector. The Sweep Generator circuit also provides an
alternate trace sync pulse which is connected to the
Vertical Switching and Output Amplifier circuit. This pulse
switches the display between channels at the end of each
sweep when in the alternate mode of operation.
The sawtooth output signal from the Sweep Generator
circuit is connected to the Horizontal Amplifier circuit in
all positions of the TIME/DIV switch except the EXT
HORIZ position. In the EXT HORIZ position, external
signals connected to the HORIZ IN OR TRIG IN connector
can be applied to the Horizontal Amplifier circuit to pro
duce horizontal deflection on the CRT. The Horizontal
Amplifier circuit amplifies the applied sawtooth or external
horizontal deflection signal and produces a push-pull out
put signal to drive the horizontal deflection plates of the
3-1
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