Tektronix T921 User Manual

Options-T 92 1 /T922/T922R
Introduction
CIRCUIT DESCRIPTION
The following paragraphs describe circuitry unique to
Option 1 . Refer to the Circuit Description section of this
manual for a description of those portions of the
circuitry
that are unchanged by Option
1 .
Note that
numbering of
the terminals and sections of
several integrated circuits in
Option
1 differs from their counterparts in the standard
instrument .
Refer to the Option 1 schematic diagrams at the back of
the Diagrams section of this manual for circuitry discuss-
ed in this 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
which constitute
LO
and HI
logic states,
may vary between individual devices .
Preamplifier
The buffer stages 04302
and
04303
isolate the channel
1 preamplifier from the switching diodes and from the
delay line driver .
The channel 2 preamplifier is isolated from the
switching
diodes and
the
delay
line driver by transistors
04310 and 04311 or transistors 04312 and 04313 .
For a normal display (DIFF button not depressed), the
channel 2 signal is passed through 04312 and 0431'3 to
the switching diodes . Vertical Mode switch S4370 con-
nects approximately +4 .4 V from R4302, R4303, and
R4313 through R4312 to the
bases of
04312 and 04313 . At
the same time,
+8
V
is
connected
to
04310 and 04311
through R4311 and R4310 . This biases off 04310 and
04311 .
For
a
DIFF
display
(DIFF
button depressed),
the
channel 2 signal passes through 04310 and 04311 to the
switching diodes . S4370 connects +4 .4 V from R4302,
R4303, R4311, and R4310 to the bases of 04310 and
04311 . This biases on 04310 and 04311 . At the same time *
+8 V is connected to the bases of 04312 and 04313
through R4313 and R4312 . This biases off 04312 and
04313 . The signal that was normally supplied to CR4314,
through 04312, is now supplied to CR4315 through
04310 . The signal normally supplied to CR4315, through
04313, is now supplied to CR4314 through 04311 . The
resulting display is the difference between the signals
applied
to
the
CH
1 and
CH
2 inputs
(CH
1
minus CH
2) .
Option 1
Page
2
Vertical Switching
The vertical
switching circuit determines whether
CH
1,
CH
2,
or
both CH 1 and CH 2 are connected to
the
delay line driver stage . This circuit is
controlled by the
Vertical Mode switch .
In the DUAL TRACE alternate or
chopped modes,
both channels are alternately displayed
on a time shared
basis . In the DIFF mode, the channel 2
signal is inverted and both channels are
connected to the
delay line driver stage at the same
time .
The diode gates,
consisting of four diodes each, actas
switches that allow
either or both of the vertical
preamplifier
signals to be coupled to the delay line driver .
Diodes
CR4302,CR4303,CR4304, and CR4305 control
the
channel 1 output and CR4312, CR4313,
CR4314, and
CR4315 control the channel 2 output
.
These diodes
are
controlled by U4364B, U4360
respectively, and by the
Vertical Mode switch, S4370 .
CH 1 Mode : When the Vertical Mode switch is in the
CH 1 position, pin 11 of U4360D is held LO causing
pin 13
of U4364B to go HI . At the same time pins
2 and 3 of
U4360A are H I causing
pin
10 of
U4364B to go LO . A LO on
pin 10 and a
HI
on
pin 13 of U436413 causes pin 9 to go HI
and
pin 8
to go
LO . A HI at pin 9 (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 channel
1 signal
to
pass to the delay line driver .
When pin 9 is HI, pin 8 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
CR4314
and CR4315 are reverse biased .
This
prevents
the channel
2 signal from passing to the delay
line driver .
CH 2 Mode : In the
channel 2 mode, the above
conditions are reversed, passing
the channel 2 signal and
blocking the channel 1 signal .
DIFF Mode : In the DIFF mode, pin 11 of
U4360D and
pins 2 and 3 of U4360A
are HI, causing pins 10 and 13 of
U4364B
to go
low . This in turn makes pins 8 and 9 of
U4364Bgo HI . Diodes CR4302, CR4303, CR4312, and
CR4313 are no longer reverse biased, and CR4304,
CR4305,CR4314, and CR4315 are also forward biased .
This allows both channel 1 and channel 2
signals
to
pass
on to the delay line driver stage .
The
-8
V supplied via
S4370 to R4320 and
R4321 provides sufficient current to
keep both diode gates turned on
without altering the do
levels of the delay line driver .
DUAL TRACE Mode :
In the DUAL TRACE mode,
channel 1 and channel 2 are alternately
connected to the

~
delay
line
driver
. There are two dual trace modes : chopped
and alternate . These modes are determined by the

Tektronix T921 Specifications

General

Brand	Tektronix
Model	T921
Category	Test Equipment
Language	English