Tektronix T921 Instruction Manual page 106

Options-T92 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, act as
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
U4364B go 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