Tektronix T922 User Manual

Circuit
Description-T921/T922/T922R
Since Channel
1
and
Channel 2 vertical input
circuits
are identical, only Channel 1 is discussed in detail . The 4100 series
circuit numbers identify the Channel 1 components and 4200 series numbers identify the Channel 2 components .
Vertical input signal is ac-coupled, dc-coupled, or grounded
by S4100 . In the DC position, the input signal is coupled directly
to the VOLTS/DIV switch attenuator . In the AC position, the input
signal passes through C4102 to the attenuator . In the GND
position, the signal path from the input connector to the
attenuator is grounded
through C4102-84102 . This provides a
ground reference
without disconnecting the signal from the input
connector . In the GND position, C4102 is charged to the average
signal level through 84102 so that the trace remains on screen
when S4100 is changed to the AC position .
The VOLTS/DIV switch selects attenuator ratio and
preamplifier gain to determine the deflection factor . The basic 1 X
deflection factor of the vertical deflection system is
2 mV/division . At this setting, no attenuators are switched
in
and
the gain
switching circuit sets the preamplifier gain to maximum .
To
provide the complete range of deflection factors indicated on
the front panel, precision attenuators are switched in and out of
the attenuator and gain switching circuit .
The attenuators are frequency
compensated
voltage dividers
which provide constant attenuation at all frequencies within the
bandwidth of the instrument . The input RC characteristics
(approximately 1 Mfg times approximately 30 pF) are maintained
for each setting of the VOLTS/DIV switch . The attenuator circuit
consists of a 10X and a 100X attenuator . 1000X is obtained when
the 10X and 100X attenuators are cascaded .
The gain switching circuit consists of 84143 through 84147
and three VOLTS/DIV switch contacts . Three preamplifier gains
are selected : 1X (maximum), 2 .5X reduction, and 5X reduction .
Refer to Table 7-2 for the attenuator and gain switching
sequence .
The signal from the input attenuator is connected to source
follower Q4122A via C4123 and 84123 . Resistor 84122 deter-
mines the 1 Mfl input
resistance and 84123 limits current drive to
the gate of Q4122A . Diode CR4123 protects the circuit from high
negative-going input signals by limiting the voltage at the gate
of
Q4122A to about -8 volts . The Q4122A gate-drain junction
provides protection from high positive-going signals by limiting
the gate voltage to about +8 volts . FET 04122B provides a
constant-current source for Q4122A . For some serial numbers,
04122 substrate (pin 8) is provided with a bias to compensate for
possible substrate leakage . This
bias is derived by the voltage
divider action of 84128 and 84129 between +8 volts and ground .
VERT INPUT (T922/T922R) CIRCUIT DESCRIPTION
REV . BJUL 1977
INPUT
COUPLING
SWITCH
VOLTS/DIV SWITCH
PREAMPLIFIER
Transistors U41346 and U4134C are emitter followers .
The
signal at the emitter of U4134B follows the
signal at the gate of
Q4122A . Divider network 84143 through 84147 attenuates the
signal from U4134B which drives the base of U4134A . DC BAL,
84132, adjusts for minimum trace shift when switching between
adjacent positions of the VOLTS/DIV switch .
Paraphase
amplifier stage, U4134A and U4134D, converts the
single-ended signal at the base of U4134A to a push-pull current
signal . These current signals are fed to the delay line driver stage
via buffer stages 04302 and 04303 . The buffer
stages isolate the
preamplifier from the delay
line driver . POSITION control 84306
varies the do voltage at the bases of
04322
and
04324 to vertically
position
the trace
on
the crt . Resistors 84155 and 84165 provide
compensation to maintain average power during voltage excur-
sions .
Capacitors C4155 and C4165 reduce Miller effect through
U4134A and U4134D .
The components connected
between U4134A and U4134D
emitters
compensate for high-frequency losses in the
preamplifier . Gain adjustment 84151 determines the gain of the
preamplifier . The VAR control, 84152, provides uncalibrated
deflection factors between VOLTS/DIV switch settings by
attenuating the signal to the base of U4134A .
When 84152 is
rotated clockwise, its full
resistance
is
in series with 84162, and
the deflection factors are calibrated .
TABLE
7-2
Attenuator and Gain Switching Sequence
VOLTS/DIV
S etting
Attenuator
(signal
attenuation)
Gain Switch
(prearnp
gain
reduction)
2 mV 1X ix
5 mV 1X 2.5X _
10 mV ix 5X
20 mV 10X
ix
50 mV
10X 2.5X
.1 V 10X 5X
.2 V 100X 1 X
.5 V 100X 2.5X
1 V 100X _ 5X
2 V 1000
X 1 X _
5 V 1000X
2 .5X
10V 1000X 5X

Tektronix T922 Specifications

General

Brand	Tektronix
Model	T922
Category	Test Equipment
Language	English