Theory Of Operation; Digital Logic; Block Diagram Description - Tektronix 213 DMM Instruction Manual

Section 3-213 Service
THEORY OF OPERATION
This section of the manual contains a description of the circuitry used in the 213 DMM-Oscilloscope. The description begins
with a discussion of the instrument using the Block Diagram pullout page in the Diagrams section. Then, each circuit is des-
cribed in detail using additional detailed diagrams and the schematics in the Diagrams section. The detailed diagrams contain
the same component designations as the schematics; therefore, refer to the schematics for component electrical values and
relationships. The schematic being described is identified by its Diagrams section number (e.g.
<€))
following the first para-
graph title that begins the description of a particular schematic.
DIGITAL LOG. I C
Digital logic techniques are used to perform some functions
within this instrument. The function and operation of the
logic circuits are described using logic symbology and termi-
nology. All logic functions are described using the positive
logic convention. Positive logic is a system of notation
where the more positive of two levels is called the true, HI,
or 1 state and the more negative level is called the false, LO,
or O state. The HI-LO method of notation is used in the lo-
gic descriptions in this manual. The specific voltages which
constitute the HI or LO state may vary between individual
devices.
It should be noted that not all of the integrated circuit de-
vices in this instrument are digital logic devices. The func-
tion of non-digital devices are described individually using
detailed diagrams or other techniques to illustrate their
functions.
Table 3-1 contains the logic symbol and truth table for the
logic device used in this instrument.
BLOCK DIAGRAM DESCRIPTION
The following discussion is provided to aid in understanding
the overall concept of the 213 DMM Oscilloscope before the
individual circuits are discussed in detail. Refer to the Block
Diagram pullout page in the Diagrams section.
Signals to be displayed on the crt are applied to either
voltage probe or the mA-n input jack. Both inputs are sha-
red by the DMM and oscilloscope functions.
The signals are then coupled and attenuated (voltage inputs)
or converted to voltages (current and resistance) and ampli-
fied by the Input Buffer Amplifier. From this amplifier the
signal goes to either the DMM or Oscilloscope function.
In the DMM function, a two-pole, low-pass active filter pro-
vides ac rejection and is switch selected when measuring de.
This allows the DMM to provide a true average response to
de measurements when the source being measured contains
both a de and an ac component.
The Gm Converter rectifies bipolar ac signals into a unipolar
current to drive the RMS Converter, or A/D Converter. The
RMS Converter changes the rectified signals into a unipolar
current of equivalent rms value to drive the A/D Converter.
The A/D Converter changes the unipolar current into a
binary coded decimal (BCD) output to drive the Character
Generator. The Character Generator accepts the BCD input
and converts it to X, Y, and Z signals to drive the Output
Amplifiers and Z-Axis Amplifier which cause crt deflection
and blanking to create the readout display.
TABLE 3-1
NAND Gate Logic Chart
Input
/ Output
N/\ND gate
A device with two or more
inputs and one output. The
A
B
X
U250A, B, C, & D
;==[}-x
output of the NANO gate
LO LO
HI
is LO if and only if all of
the in puts are at the HI LO HI
HI
state.
HI
LO
HI
HI
HI LO
@
3-1