Circuit Description—Type 422 AC-DC
CRT. This circuit also contains a 10X magnifier to increase
the sweep rate ten times in any TIME/DIV switch position
or to provide a ten times reduction in external horizontal
deflection factor for external horizontal operation.
The CRT Circuit provides the voltages and contains the
controls necessary for the operation of the cathode-ray
tube (CRT). The AC-DC Power Selector circuit provides
power to the DC-DC Regulator circuit. This power is
selected from an external AC or DC voltage connected to
this instrument or from internal rechargeable batteries. This
circuit also contains a battery charging circuit to recharge
the internal batteries. The DC-DC Regulator circuit
provides the low-voltage power necessary for the operation
of this instrument. The voltages produced by the DC-DC
Regulator circuit are connected to the Calibrator and
Regulators circuit through the interconnecting plug between
the indicator and power supply. The Calibrator and
Regulators circuit distributes the voltage to all of the
circuits in this instrument and also produces several stable
voltages from the unregulated supplies. The Calibrator and
Regulators circuit also produces a square wave output with
accurate amplitude which can be used to check the gain of
the vertical system or to compensate attenuator probes. In
the CALIBRATE 4 DIVISIONS position of the Channel 1
and Channel 2 VOLTS/DIV switches, the square wave
signal is internally connected to the Input Amplifier
circuits. If the gain of the Input Amplifier circuits is
correctly set, a display exactly four divisions in amplitude is
presented. The square wave signal is also available at the
front-panel CALIBRATOR jack.
CIRCUIT OPERATION
.General
This section provides a detailed description of the
electrical operation and relationship of the circuits in the
Type 422 with AC-DC Power Supply. The theory of
operation for circuits unique to this instrument are
described in detail in this discussion. Circuits which are
commonly used in the electronics industry are not
described in detail. Instead, references are given to
textbooks or other source material which describe the
complete operation of these circuits.
The following circuit analysis is written around the
detailed block diagrams which are given for each major cir
cuit. These detailed block diagrams give the names of the
individual stages within the major circuits and show how
they are connected together to form the major circuit. The
block diagrams also show the inputs and outputs for each
circuit and the relationship of the front-panel controls to
the individual stages. The names assigned to the individual
stages of these block diagrams are used throughout the
following discussion. The circuit diagrams from which the
detailed block diagrams are derived are shown in the Dia
grams section.
CHANNEL 1 INPUT AMPLIFIER
General
Input signals for vertical deflection on the CRT can be
connected to the INPUT 1 connector. The Channel 1 Input
Amplifier circuit provides control of vertical deflection
factor, input coupling, vertical position, vertical gain,
balance and variable deflection factor. It also contains a
stage to provide a sample of the Channel 1 input signal to
the Sweep Trigger circuit for internal triggering from the
Channel 1 signal only. Fig. 3-2 shows a detailed block
diagram of the Channel 1 Input Amplifier circuit. A
schematic of this circuit is shown on diagram 1 at the back
of the manual.
Input Coupling
Input signals applied to the INPUT 1 connector can be
AC-coupled, DC-coupled or internally disconnected. When
Input Coupling switch SW1 is in the DC position, the input
signal is directly coupled to the Input Attenuator stage. In
the AC position, the input signal passes through blocking
capacitor C l. This capacitor prevents the DC component of
the signal from passing to the amplifier. The GND position
opens the signal path and the input to the amplifier is con
nected to ground. This provides a ground reference without
the need to disconnect the applied signal from the INPUT 1
connector.
Input Attenuator
The effective overall Channel 1 deflection factor of the
Type 422 is controlled by the Channel 1 VOLTS/DIV
switch. In all positions of the Channel 1 VOLTS/DIV
switch except .01 and .02, the basic deflection factor of the
vertical deflection system is 0.05 volt per division of CRT
deflection. To increase this basic deflection factor to the
values indicated on the front panel, precision attenuators
are switched into the circuit. In the .01 and .02 positions,
input attenuation is not used. Instead, the gain of the Feed
back Amplifier stage is changed to decrease the deflection
factor.
For Channel 1 VOLTS/DIV switch positions above .05,
the attenuators are switched into the circuit singly or in
pairs to produce the vertical deflection factor indicated on
the fro n t panel. These attenuators are frequency-
compensated voltage dividers. For DC and low-frequency
signals they are primarily resistance dividers and the voltage
attenuation is determined by the resistance ratio in the cir
cuit. The reactance of the capacitors in the circuit is so high
at low frequencies that their effect in the circuit is negligi
ble. However at higher frequencies, the reactance of the
capacitors decreases and the attenuator becomes primarily a
capacitance voltage divider.
In addition to providing constant attenuation at all fre
quencies within the bandwidth of the instrument, the Input
Attenuators are designed to maintain the same input RC
characteristics (one megohm X 33 picofarads) for each set-
M
3-3
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