Tektronix CURVE-TRACER 576 Instruction Manual page 58

Circuit Description-Type 576
the Collector Supply and ground. 350 V and 1500 V LOOPING
COMP adjustment C339 has been added between the trans-
former center tap and the junction of C343 and C341, for
bridge operation of the Collector Supply to compensate for
unbalanced operation of the transformer.
Interlock
The Type 576 has an interlock system designed to protect
the user of the instrument from potentially dangerous voltages
which may appear at the Collector terminals of the Standard
Test Fixture. The interlock system is shown on the Collector
Supply schematic in Section 8.
Coil K323 enables or disables the Collector Supply output
through K323-B, enabling it when the coil is energized. The coil
is always energized when the MAX PEAK VOLTS switch is set
to 15. When this switch is set to the 75, 350 or 1500 positions,
one side of the coil is opened and the Collector Supply is
diabled. The yellow COLLECTOR SUPPLY VOLTAGE DIS-
ABLED light is turned on through K323-A. In order to enable the
Collector Supply under these conditions, the Protective Box
must be put in place on the Standard Text Fixture and the lid
closed. With the lid closed, High Voltage Interlock switch
SW360 is closed and
+
12 5 volts is applied through the red
DANGEROUS VOLTAGE light, B360, to coil K323, thus en-
abling the Collector Supply. With the coil now activated, the
COLLECTOR SUPPLY VOLTAGE DISABLED light is turned off.
The COLLECTOR SUPPLY VOLTAGE DISABLED light may
also be turned on if thermal cutout TK346 becomes open.
TK346 opens whenever the internal heat in the instrument be-
comes hot enough to damage the collector supply or the
readout.
Step Generator
The purpose of the step generator is to present a discrete
level of current or voltage to the base or emitter (or equivalent
terminals) of the device under test for each sweep, or change
of direction of sweep, of the collector supply. These discrete
levels are generated in the form of ascending steps which
have a calibrated current or voltage separation.
The step generator circuit consists of four major sections:
the clock, the counter, the digital-to-analog converter, and the
pulsed steps operation section. The clock circuit produces
negative-going clock pulses which determine the rate and
phase, with respect to the collector supply, of the Step Gen-
erator output. The counter circuit counts these clock pulses
and transforms each count into a digital code which controls
3-4
the digital-to-analog converter. The digital-to-analog converter
transforms the digital code into analog current which is
summed at a current summing node and transmitted to the
step amplifier. The pulsed steps operation circuit provides a
variation of the Step Generator output where short duration
pulsed steps rather than normal steps are generated.
Logic. The clock circuit, the counter circuit and a portion of
the digital-to-analog circuit are digital circuits which make use
of transistors and integrated circuits in digital configurations.
The most convenient method of describing and understanding
digital circuitry is through a logic description rather than a de-
tailed circuit description. In order to make this description un-
derstandable by a wider range of readers, a simplified logic
description, using high and low rather than true and false, has
been utilized. A knowledge of basic logic symbols and truth
tables will help in understanding this description.
Simplified schematics of each of these circuits are shown in
Figs. 3-5, 3-6 and 3-7. Pertinent information such as internal
logic diagrams, truth tables, timing charts and descriptions of
operation are given in Fig. 8-1 at the beginning of the Diagrams
section, for all the logic devices used in the Step Generator
circuit. Logic level information for these logic devices is shown
in blue on the Step Generator schematic. Familiarity with the
logic symbols and related truth tables of these logic devices
will greatly aid in understanding the following description.
Clock. Sine waves produced at line frequency by trans-
former T701 provide the timing source for the clock (see the
Step Generator schematic). Transformer T701, steering diodes
D1-D2 and D10-D11, and trigger generators U3A-U3B and U3C-
U3D operate together to produce low level pulses at the inputs
of U22A. Using U3A-U3B as an example, each time the trans-
former voltage at the anode of D1 crosses zero going negative,
D1 will turn off and D2 will turn on. When D2 is conducting, the
voltage at the pin 1 input of U3A is held at a low voltage level.
Since the other input to U3A, pin 2, is held at a high voltage
level by voltage divider R4-R5, this low causes a high to appear
at the output of U3A (see Fig. 8-1 at the beginning of the
Diagrams section for truth table of inverted input OR gate).
This high is inverted by U3B and the resulting low is applied to
the pin 1 input of U22A. This low output produced by the trig-
ger generator continues until C5 charges to a high voltage level
as determined by divider R4-R5. When the voltage at D1
crosses through zero going positive, D1 turns on and D2 turns
off. With D2 off, both inputs to U3A are high, the output goes
low and the output of U3B goes high. This is the quiescent
state of the trigger generator. Trigger generator U3D-U3C op-
erates the same as U3B-U3A except that the additional input at
pin 9 of U3C allows the trigger generator to be inhibited when a
low is applied to it.
REV MAR 1985
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