General Description - Tektronix 2235A Instruction Manual

Theory of Operation
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2235A Instruction
GENERAL DESCRIPTION
NOTE
When reading this general circuit description of
the 2235A Oscilloscope, refer to the detailed
block diagram (Figure 9-4) located in the
Diagrams section of this manual. In Figure 9-4,
the numbered diamond symbol in each major
block refers to the appropriate schematic
diagram number.
Signals to be displayed on the crt are applied to either
the CH 1 input connector or the CH 2 input connector.
These signals may be directly (DC) coupled to the
Attenuator circuit or ac (AC) coupled through an input-
coupling capacitor. The input signals may also be
disconnected from the oscilloscope circuitry and the
input attenuator grounded by setting the coupling switch
to the ground (GWD) position.
The output signal from the Attenuator circuit is applied to
the Vertical Preamplifier for further amplification.
Additionally, the Channel 2 Attenuator can invert the
Channel 2 display on the crt. Trigger Pickoff Amplifiers in
each channel supply an internal trigger signal from either
or both channels to the Internal Trigger Amplifier.
lr~put signals are selected for display by the Channel
Switching circuit under control of the front-panel Vertical
MODE switches. The output signal from the Channel
Switching circuit is applied to a Diode Gate circuit to
enable either the vertical or trigger view signal to drive
the Delay Line Driver stage. This stage converts a current
input to a voltage output and provides an impedance
match for the Delay Line. The Delay Line produces
approximately 90 ns of delay in the vertical signal. This
allows the Horizontal circuitry time to start the sweep so
the the operator can see the signal that triggered the
sweep.
Final amplification of the vertical signal is performed by
the Vertical Output Amplifier. This amplifier supplies the
signal levels necessary for vertical deflection of the elec-
tron beam in the crt. The upper frequency response of the
amplifier can be reduced by enabling the Bandwidth
Limit circuitry. For locating the position of off-screen
displays, the dynamic range of the amplifier can be
limited with the Beam Find circuitry. This circuitry also
intensifies the trace and limits horizontal deflection.
The AIB Sweep Separation circuitry supplies a dc-offset
current to the Vertical Output Amplifier which vertically
positions the B trace with respect to the A trace when Alt
Horizontal Mode is selected.
The A Trigger circuitry uses either an Internal Trigger sig-
nal, an External Trigger signal, or a Line Trigger signal
obtained from the ac power line to develop the gate sig-
nal for the A Sweep Generator. The B Trigger circuitry
uses only the Internal Trigger signal to gate the B Sweep
Generator. A P-P Auto Trigger circuit ensures that the
raqge of the A TRIGGER LEVEL control tracks the peak-
to-peak amplitude of the trigger signal when either the
P-P Auto or TV Field trigger mode is selected. This
allows triggering on most signals without needing to
adjust the A TRIGGER LEVEL control. In Norm mode, the
A TRIGGER LEVEL control must be adjusted for the
correct trigger signal level before a sweep can be gen-
erated. When the TRIG VIEW switch is activated, the
signal appearing at the input of the A Trigger circuit is
applied to the Delay Line Driver and displayed on the crt.
A TV Field sync circuit provides stable triggering on tele-
vision vertical-sync pulses. Triggering at the television
line rate is accomplished when either P-P Auto or Norm
mode is selected.
The A Sweep Logic circuit controls sweep generation
and Z-Axis unblanking for the A Sweep display. When
the A Trigger Mode switches are set to either P-P AUTO
or TV FIELD and no trigger signal is present, the Auto
Baseline circuit causes the Sweep Logic circuit to pro-
duce a sweep for reference purposes. In the NORM
setting, the Auto Baseline circuit is disabled and sweeps
are inhibited until a trigger event occurs. This is useful for
triggering on low-repetition-rate signals. The SGL SWP
setting enables only one sweep to be generated after
being reset. Following the single sweep, the A Trigger
circuit is disabled until the SGL SWP button is pressed
again.
The A Sweep Logic circuit controls the operation of the A
Miller Sweep Generator circuit. The Sweep circuit
produces a linear sweep output with a ramp time that is
controlled by the A SECIDIV switch. The sweep signal is
applied to the Horizontal Preamplifier for initial ampli-
fication and then to the Horizontal Output Amplifier to
drive the crt horizontal deflection plates.
The Horizontal Preamplifier gain is increased by afactor
of 10 when the XI0 Magnifier is used. Horizontal
positioning of the display is accomplished in the Hori-
zontal Preamplifier circuit.
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