Tektronix TDS 420A, Service Manual, Page: 65 / 288

TDS 420A, TDS 430A & TDS 460A Service Manual
3- 1
Theory of Operation
This chapter describes the electrical operation of the Tektronix TDS 420A,
TDS 430A, and TDS 460A Digitizing Oscilloscopes using the major circuit
blocks or modules.
This chapter has two main parts:
H
Logic Conventions describes how logic functions are discused and
represented in this manual.
H
Module Overview
describes circuit operation from a functional
block perspective.
Logic Conventions
The digitizing oscilloscope contains many digital logic circuits. This manual
refers to these circuits with standard logic symbols and terms. Unless otherwise
stated, all logic functions are described using the positive-logic convention: the
more positive of the two logic levels is the high (1) state, and the more negative
level is the low (0) state. Signal states may also be described as “true” meaning
their active state or “false” meaning their non-active state. The specific voltages
that constitute a high or low state vary among the electronic devices.
Active-low signals are indicated by a tilde prefixed to the signal name (~RE-
SET). Signal names are either active-high, active-low, or have both active-high
and active-low states.
Module Overview
This module overview describes the basic operation of each functional circuit
block as shown in Figure 9-2 on page 9--8.
The digitizing oscilloscope is a portable, multichannel instrument. Each channel
provides a calibrated vertical scale factor. All channels can be acquired simulta-
neously.
A signal enters the oscilloscope through a probe connected to a BNC on the
A05 Attenuator board.
Attenuators. Circuitry in the attenuator selects the input coupling, the termina-
tion, and the attenuation factor. The processor system controls the attenuators.
General
Input Signal Path