Tektronix 2236, Instruction Manual

Page: 47 / 350

Theory of Operation— 2236 Service
In the X-Y Mode of operation, the Channel 1 signal from
the Internal Trigger circuitry passes through the X-Y Ampli­
fier to the Horizontal Preamplifier. In this operating mode,
the Channel 1 Internal Trigger signal supplies the horizontal
deflection to the crt, and the Miller Sweep circuit is disabled
to inhibit sweep generation.
The Alternate B Sweep circuitry controls the Alt and B
Horizontal mode displays and includes the B Miller Sweep
Generator and B Sweep Logic circuitry. In addition to pro­
viding the B Sweep sawtooth waveform, signals are gener­
ated which control the display switching between the A and
B displays.
The intensity levels of both the A and B Sweeps are set
by the front-panel A and B INTENSITY controls. These con­
trols, along with signals from the A and B Sweep Logic
circuits, determine the drive level to the Z-Axis Amplifier.
The Z-Axis drive from both the A Sweep Logic circuit and
the Alternate B Sweep circuit is applied to the Z-Axis Ampli­
fier. The output signal from the Z-Axis Amplifier circuit sets
the crt intensity. When using Chop Vertical Mode, a
blanking signal from the Chop Oscillator circuit blanks the
crt display while switching between the vertical channels.
The Dc Restorer circuit applies the output voltage of the
Z-Axis Amplifier between the cathode and grid of the crt.
High dc potentials on these elements prohibit direct coupling
to the crt.
The Power Supply provides the necessary operating
voltages for the instrument. Operating potentials are ob­
tained from a circuit composed of the Preregulator, Inverter
and Transformer, and Rectifiers and Filters. The
Preregulator produces approximately + 43 V dc from the ac
power line which is used to drive the 20-kHz Inverter stage.
The transformer secondary windings provide various ac lev­
els that are rectified and filtered to produce the operating
voltages. A high-voltage multiplier circuit produces the ac­
celerating, focus, and cathode potentials required by the crt.
A front-panel PROBE ADJUST output is provided for use
in adjusting probe compensation. The voltage at the
PROBE ADJUST connector is a negative-going square
wave that has a peak-to-peak amplitude of approximately
0.5 V and a repetition rate of approximately 1 kHz.
CTM
The CTM (Counter-Timer-Multimeter) section of the 2236
Oscilloscope utilizes input signals from the three front-panel
BNC connectors, the DMM leads, or the temperature probe
to calculate and display CTM parameter results. Measure­
ments which are a function of time use additional control
signals from the oscilloscope.
The central processor unit (CPU) circuitry monitors the
front-panel switches, drives the display circuitry, and per­
forms calculations on data from the counter. Through read
ports, the CPU accesses counter data and oscilloscope
control signals. Write ports are used to transmit controlling
signals to the DMM.
The Counter section utilizes two counters, the oscillo­
scope and CPU control signals, and the crystal oscillator
frequency to produce data for CPU calculations. One part
produces a count proportional to an input signal and the
other part totalizes the number of cycles of the phase-
locked loop frequency for that same time period.
Input switching circuitry in the DMM front end selects the
input signal source for the particular function chosen by the
operator. This input signal is processed into a dc voltage
which, along with a precision reference voltage and a
ground reference, is applied to a voltage-to-frequency con­
verter. The converter then produces a proportional fre­
quency output for use by the Counter section as a data
source. The processing and converter circuitry are all under
microprocessor control. DMM circuitry is powered by a
floating power supply to provide circuitry isolation and to
allow nonground-referenced measurements.
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