Logic Fundamentals; Symbols; Logic Polarity; Input/Output Tables - Tektronix 7B15 Instruction Manual

SYMBOLS
LOGIC POLARITY
The operation of circuits unique to this instrument is
described in detail in this discussion . Circuits commonly
used in the electronics industry are not described in
deta i I.
The following circuit analysis, with supporting
illustrations, gives the names of individual stages, and
shows how they are connected to form major circuits .
These illustrations show the inputs and outputs for each
circuit and the relationship of the front-panel controls to
the individual stages . The detailed circuit diagrams from
which the illustrations are derived are shown in the
diagrams foldout section.
LOGIC FUNDAMENTALS
Digital logic techniques are used to perform many
functions within this instrument . The function and
operation of the logic circuits are described using logic
symbology and terminology. This portion of the manual is
provided as an aid in the understanding of these symbols
and logic concepts, not a comprehensive discussion of
the subject.
The symbols used to describe digital circuits in this
instrument are based on ANSI standard Y32.14-1973 .
Table 3-1 provides a basic reference for the logic devices
used within this instrument . Any deviations from the
standard symbology, or devices not defined by the
standard, are described in the circuit description for the
applicable device .
NOTE
Logic symbols used on the diagrams depict
the logic function as used in this instrument,
and may differ from the manufacturer's data.
All logic functions are described using the positive logic
convention . Positive logic is a system of notation where
the more positive of two levels (HI) is called the true or 1-
state; the more negative level (LO) is called the false or 0-
state. The HI-LO method of notation is used in this
description . The specific voltages that constitute a HI or
LO state may vary between individual devices. Whenever
possible, the input and output lines are named to indicate
the function performed when at the HI (true) state.
INPUT/OUTPUT TABLES
TAILED CI
Input/Output (truth) tables are used to show the input
combinations important to a particular function, along
with the resultant output conditions . This table may be
CU IT
NON-
Theory of Operation-71315
given either for an individual device or for a complete
logic stage. For examples of input/output tables for
individual devices, see Table 3-1 .
IGITAL DEVICES
Not all of the integrated circuits in this instrument are
digital logic devices. The function of the nondigital
devices is described individually, using
operating
waveforms or other techniques to illustrate the function .
FRONT-PANEL WIRING
The Front-Panel Wiring diagram shows the
interconnections between front-panel functions (controls,
connectors, and indicators) and circuit boards within this
instrument .
TRIGGER GENE ATO R
The Trigger Generator provides a stable display by
starting the Sweep Generator (diagram 4) at a selected
point on the input waveform . The triggering point can be
varied by the LEVEL control and may be on either the
positive or negative slope of the waveform . The triggering
signal source may be from either the signal being
displayed (INT), a signal from an external source (EXT), or
a sample of the power-line voltage (LINE) .
EXTERNAL TRIGGER AMPLIFIER
The external trigger signal is connected to the Trigger
Generator through EXT TRIG IN connector J90. Push-
button switch S90 selects either 1 MO or 5052 input
impedance. The 0.2 ampere fuse protects the 50 52 load
from signal overload .
Pushing in the EXT push button of SOURCE switch (S50)
allows external trigger signals to pass from the External
Trigger Amplifier to U124 for amplification. Field-effect
transistor Q98 conducts in the DC COUPLING mode only
and 0108 is on in all coupling modes except AC LF REJ .
Components U110, Q114, and Q118 compose an
operational amplifier whose gain is approximately 1/4
determined by feedback resistors R73 and R74.
Integrated circuit U110 provides do stability. Diodes
CR74, CR76, and CR77 protect the trigger amplifier from
possible overload caused by high amplitude external
signals . A portion of the do leveling voltage from R67 is
applied to Ul10 to provide additional level range in the
EXT triggering mode . Pressing the INT button causes
Q118 to saturate which interrupts signal flow to Ul24
and also disables the Ext In amplifier of U124 .