Tektronix T921 User Manual

Circuit Description-T921/T922/T922R
VERT
SWITCHING (T922/T922R) CIRCUIT DESCRIPTION
Digital logic devices
are used to perform some of the functions in this instrument . LO and HI
designations are used in this
circuit
description to indicate the state of the digital circuit . HI indicates the more positive of
the two levels . The specific voltages
that consitute a LO and HI logic state, may vary between individual
devices
.
PREAMP
BUFFER

In the CH 2 mode, the above conditions
are reversed . Diodes
CR4312 and CR4313 are reverse biased, passing the CH 2 signal
and blocking the CH 1 signal .
The buffer stages 04302 and 04303 isolate the
preamplifier
from the delay line driver .
DELAY LINE DRIVER
The delay line driver is a push-pull feedback amplifier stage
composed of 04322, 04324, 04336, 04346, and associated
circuitry
. A sample
of
the output of 04346 and 04336 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
04346 that
is proportional to the feedback
resistance
.
Components
C4333,
C4334,C4344, R4333, R4334, and R4344
provide compensation (peaking) to correct for delay line losses .
The delay line, DL4339, provides approximately 200 ns delay
in the vertical signal . This allows the sweep generator circuit time
to initiate a sweep before the vertical signal reaches the crt
vertical deflection plates .
The vertical switching circuit determines whether CH 1 orCH 2
is connected to the vertical output amplifier . In the alternate or
chopped mode, both channels are alternately displayed on a time
shared basis .
The diode gates, consisting of four diodes each, act as
switches that allow either of the vertical preamplifier signals to be
coupled to the delay line driver . Diodes CR4302, CR4303,
CR4304, and CR4305 control the CH 1
output
; CR4312,CR4313,
CR4314, and CR4315 control the CH
2 output . These diodes are
controlled by flip-flop U4364A,which in turn is controlled by
Vertical Mode switch, S4370 .
When the Vertical Mode switch is in the CH 1 position, pin 4 of
U4364A is held LO, causing pin 5 to go
HI . A HI
at pin 5 (a voltage
higher than at
the bases
of
04322 and 04324) reverse biases
CR4302 and CR4303 and forward biases CR4304 and CR4305 .
This allows the CH 1 signal to pass to the delay line driver . When
pin 5 is HI, pin 6 is LO, causing the cathodes of CR4312 and
CR4313 to be connected to a voltage much lower than on the
bases of 04322 and 04324 . Diodes CR4312 and CR4313 are now
forward biased and diodes
CR4314
and CR4315 are
reverse
biased, preventing the CH 2 signal
from
passing to the delay
line
driver
.
REV . B JUL 1977
DELAY LINE
VERTICAL SWITCHING
VI . 1
In the DUAL TRACE
Vertical Mode, CH 1 and CH 2 are
alternately connected to
the delay line driver . There are two dual
trace modes : chopped
and alternate . These modes are deter-
mined by the SEC/DIV switch setting . Chopped mode
is obtained
for sweep speeds of 1 ms and slower and alternate for
sweep
speeds of 0 .5 ms and faster .
In the chopped mode pin 2 of U4376A is ungrounded, allowing
the multivibrator, U4376A and U4376D, to free run at about
250 kHz . The output at pin 8 of U4376C serves as a clock pulse
for
U4364A,which in turn switches the diode gates at
the 250 kHz
rate . The clock pulse is also fed to U4364B,which provides
an
output pulse to the Z Axis amplifier to blank out the
transition
between CH 1 and CH 2 traces . If pin 13 of
U4364B goes LO, the
output pin 9 is set LO, causing
pin
6
of U4376B to go HI . This
causes
pin 13
of
U4364A to go HI after being delayed by C4386
charging through R4386 .
The clock pulse applied to pin 11 of U4364B causes pin 9 to go
HI, which in turn, after passing through the inverter and after
some delay, sets pin 13 LO again . This causes pin 9 to go LO
again . The positive-going voltage pulse (whose width is
deter-
mined by R4386 and C4386) is converted to current by R4385-
R4384 and sent to the Z Axis Amplifier to
blank switching
transients .
In the alternate mode, pin 2 of U4376A is grounded, preventing
multivibrator operation, thus keeping pin 10 of U4376C
HI . At the
end of each sweep, the base of 04372 receives a current pulse,
driving it into saturation . The resulting negative-going pulse at
the collector is fed through C4372 to pin 9 of U4376C causing pin
8 to go HI . This in turn switches U4364A to pass either CH 1 or CH
2 to the delay line driver at the end of each sweep . Pin 12 of
U4364B is grounded through the SEC/DIV switch and prevents an
output at pin 9 .
The Vertical
Mode
switch also selects the appropriate internal
triggering source for CH 1 and CH 2 . With the Vertical Mode
switch set to CH 1 and DUAL TRACE,CR4364 is forward biased
and the signal from the CH 1 trigger pickoff goes to the sweep
circuit . I n these modes, CR4363 is connected to the -8 volts, thus
reverse biasing CR4366, preventing the CH 2
trigger signal from
'entering
the trigger input amplifier . With the Vertical Modeswitch
set to CH 2, CR4366 becomes
forward biased while CR4364 is
reverse biased because CR4362 is now connected to -8 volts .

Tektronix T921 Specifications

General

Brand	Tektronix
Model	T921
Category	Test Equipment
Language	English