Block Diagram; Detailed Circuit Description - Tektronix 7A19 Instruction Manual

Introduction
CIRCUIT
This section of the manual contains a description of the
circuitry used in the 7A19 amplifier. The description begins
with a discussion of the instrument using the block diagram
shown in the Diagrams section . Then, each circuit is
described in detail using block diagrams to show the
interconnections between stages in each major circuit and
the relationship of the front panel controls to the individual
stages .
A complete schematic of the circuits are given in the
Diagrams section . Refer to this schematic throughout the
following circuit description for electrical values and
relationship .
BLOCK DIAGRAM
The following discussion is provided to aid in under-
standing the overall concept of the 7A19 before the
individual circuits are discussed in detail . Only the basic
interconnections between the blocks are shown on the
block diagram (see Diagrams section) . Each block repre-
sents a major circuit in the instrument .
Signals to be displayed on the CRT are applied to the
INPUT connector. The signal passes through the Input
Coupling Switch, where the appropriate coupling is
selected, to the Attenuators . The VOLTS/DIV switch
selects the correct amount of attenuation .
Transit time for the 7A19 is standardized by the Fixed
Delay Line . An optional Variable Delay Line can be
ordered that allows the transit time of the plug-in to be
varied up to 1 ns .
Signals from the Delay Line are connected to the Signal
Line . The Signal Line represents a 50
S2
line that is
connected to Signal Amplifier, Trigger Amplifier, Limiter,
and is terminated in the Signal Line Terminater . Input
impedance is effectively adjusted, therefore, by the Input
Current control .
Overvoltage protection is provided by the Limiter
circuitry and the fuse located in the Input Coupler.
The Signal Amplifier and Trigger Amplifier circuits are
identical . They provide positioning and gain switching for
the 10 mV and 20 mV ranges . GAIN calibration and
temperature compensation is provided by the Gain Current
Amplifier.
The Signal Amplifier and Trigger Amplifier is connected
to the Polarity circuit where the oscilloscope is provided
with +UP or inverted signal and trigger outputs. Outputs are
connected to the oscilloscope via the interface connectors .
DETAILED CIRCUIT DESCRIPTION
Input Coupling
Attenuator
Delay Lines
Section 3---7A19
Input signals connected to the INPUT connector can be
AC-coupled, DC-coupled, or internally disconnected . S5 is a
cam-type switch ; a contact-closure chart showing the
operation is given on the schematic in the Diagrams section .
When the Input Coupling Switch is in the DC position, the
input signal is coupled directly to the Attenuator . The DC
GND position disconnects the input signal from the
Attenuators and connects it to ground through two 100 2
resistors in parallel . Paralleling the 100
52
resistors reduces
the inductance associated with the resistors so that the
combination will remain 50 92 for all frequencies within the
limits of this plug-in . Connected in a similar manner and for
the same reasons are the input fuses F6 and F7 . In the AC
position, the input signal passes through capacitor C5 . The
capacitor prevents the DC component of the signal from
passing to the Attenuator . AC GND position disconnects
the input signal from the Attenuators and connects it to
ground through C5 and the two 100 2 resistors. 'This
represents the same load to the signal source . The PRE-
CHG position limits the instantaneous current caused by
connecting a large signal to the INPUT connector by
inserting a resistor in series with C5 . Since this resistor is
4.95 k2 and the input to the Attenuator is 50 2, the
PRE-CHG position will act as a X100 attenuator .
The effective overall deflection factor of the 7A19 is
determined by the setting of the VOLTS/DIV switch, S135
and the Step Attenuator, A10 . Gain switching for the
10 mV and 20 mV positions occur in the Signal and Trigger
Amplifiers. The signal in the 10 mV and 20 mV position is
passed through the Step Attenuator, unattenuated . Pre-
cision microwave attenuators, located in the Step Attenu-
ator, are switched in at all other VOLTS/DIV positions.
Unless Variable Delay Option 4 is requested, all 7A19's
are equipped with a Fixed Delay Line consisting of a
precision length of 50
S2
cable . The Fixed Delay Line is
inserted in series with the Signal Line so that the transit
times of all 7A19 plug-in's can be closely regulated . The
optional Variable Delay Line, when requested, replaces the
Fixed Delay Line .