Tektronix T922 User Manual

Circuit Description-T921/T922/T922R
VERT INPUT
(1'921) CIRCUIT
DESCRIPTION
INPUT COUPLING SWITCH
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-R4102 . 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
R4102 so that the trace remains on
screen
when
S4100 is changed to the AC position
.
VOLTS/DIV
SWITCH
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 M[2 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 R4143 through
R4147
and
three VOLTS/DIV switch contacts . Three preamplifier
gains
are selected : 1X (maximum), 2 .5X reduction, and
5X reduction .
Refer to Table 7-1 for the attenuator
and gain switching
sequence .
PREAMPLIFIER
The signal from the input attenuator
is connected to source
follower Q4122A via C4123 and R4123 .
Resistor R4122 deter-
mines the 1 Mn input resistance
and R4123 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 R4128 and R4129between +8 volts and
ground
.
Transistors U4134B
and U4134C are emitter followers . The
signal at the emitter of
U4134B follows the signal at the gate
of
04122A
. Divider network R4143 through R4147
attenuates the
signal from U4134B which drives the base of
U4134A . DC BAL,
R4132, adjusts for minimum trace
shift when switching between
adjacent positions of the
VOLTS/DIV switch .
REV
. BJUL 1977
IV
.I
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 R4306
varies
the
do voltage at the bases of 04322
and 04324 to vertically
position the trace on the crt .
Resistors R4155 and R4165 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
R4151 determines the gain of
the
preamplifier .
The VAR control, R4152, provides uncai
;~_rated
deflection
factors between VOLTS/DIV switch
settings by
attenuating
the signal to the base of U4134A
. When R4152 is
rotated clockwise, its full resistance is
in series with R4162, and
the deflection factors are calibrated .
TABLE 7-1
w
.
Attenuator
and Gain Switching Sequence
DELAY LINE
DRIVER
The delay line driver is a
push-pull feedback amplifier stage
composed of Q4322, Q4324,
04336, 04346, and associated
circuitry . A sample of the
output of Q4346 and Q4336 is fed
back
through R4336-R4328
and R4346-R4342 to the
bases
of
04322
and 04324 . Due to this
feedback, this stage forms an
inverting
operational
amplifier with a virtual ground at the
bases of 04322
and
04324 . Any current into these virtual
grounds (null points)
causes an
output voltage at the emitters of
04336 and Q4346that
is proportional to the feedback
resistance .
Components
C4333, C4334, C4344, R4333, R4334,
and R4344
provide
compensation (peaking) to correct for
delay line losses .
VOLTS/DIV
Setting
Attenuator
(signal
attenuation)
Gain Switch
(preamp gain
reduction)
2 mV
1X
1X
5
mV 1X
2 .5X
10
mV ix
5X
20 mV 10X
1X
50 mV
10X 2 .5X
.1 V
10X 5X
.2 V
100X 1X
.5 V
100X 2.5X
1 V 100X
5X
2 V 1000X
1 X
5 V
1000X 2.5X
10
V 1000X
5X

Tektronix T922 Specifications

General

Brand	Tektronix
Model	T922
Category	Test Equipment
Language	English