Tektronix 422, Instruction Manual

Page: 45 / 213

Circuit Description-Type 422 AC-DC
from the X9 Gain Stage (nine times amplification) is alge­
braically added to the normal signal through the Paraphase
A m plifier stage (one times amplification) to provide an out­
put signal at the collectors of Q184 and Q194 which is
amplified 10 times. Since the signal connected to the X9
Gain Stage is AC coupled through Cl 51, the DC level o f the
applied signal is not preserved. Therefore, DC level measure
ments cannot be made from the CRT display when using
this feature.
Paraphase A m p lifie r
The basic Channel 2 Paraphase A m plifier configuration
and operation is the same as fo r Channel 1 when the X I 0
GAIN AC switch is pushed in. However, the INVERT
switch SW195 is a feature o f the Channel 2 circuit. This
switch allows the displayed signal from Channel 2 to be
inverted when displayed on the CRT.
V E R T IC A L SW ITCHING A N D OUTPUT A M P L IF IE R
General
The Vertical Switching and O utput A m p lifie r circuit
determines whether the Channel 1 and/or Channel 2 signal
is displayed on the CRT. This circuit also provides the final
amplification fo r the vertical deflection signal before it is
applied to the CRT. Fig. 3-4 shows a detailed block diagram
of the Vertical Switching and O utput A m plifier circuit. A
schematic o f this circuit is shown on diagram 4 at the back
of the manual.
Diode Gate
The Diode Gates, consisting of fo ur diodes each, can be
thought o f as switches which allow either o f the Input
A m p lifie r output signals to be connected to the vertical
Output Am plifier. D201 through D204 control the Channel
1 output and D205 through D208 control the Channel 2
output. These diodes are in turn controlled by the Switch­
ing M ultivibrator stage fo r dual-trace displays, or by Mode
switch SW260 fo r single trace displays.
CH 1. In the CH 1 position o f the Mode switch, +12
volts is applied to the junction o f D206-D207 in the Chan­
nel 2 Diode Gate through R273 (see simplified diagram in
Fig. 3-5). This forward biases D206-D207 and reverse biases
D205-D208 to block the Channel 2 signal so it cannot pass
to the Delay-Line Driver stage. A t the same time in the
Channel 1 Diode Gate, D202-D203 are connected to —12
volts through R265. This voltage holds D202-D203 reverse
biased while D201-D204 are forward biased. Therefore, the
Channel 1 signal can pass to the Delay-Line Driver stage.
CH 2. In the CH 2 position o f the Mode switch, the
above conditions are reversed. D202-D203 are connected to
+12 volts through R263 and D206-D207 are connected to
—12 volts through R275. The Channel 1 Diode Gate blocks
the signal and the Channel 2 Diode Gate allows the signal to
pass.
Sw itching M u ltivib ra to r
ALT. In this mode o f operation the Switching M ultivi­
brator operates as a bistable m ultivibrator3. In the A L T
position o f the Mode switch, +12 volts is connected to the
collector of Alternate Trace Switching A m p lifie r stage
Q264, through R260. Q264 is normally o ff and the current
through R260 passes to the ''o n " Switching M ultivibrator
transistor through either D264 or D274. For example, if
Q265 is conducting, current is supplied to Q265 through
D264. The current flo w through collector resistor R265
produces a more positive voltage at the collector o f Q265
which is connected to D202-D203 in the Channel 1 Diode
Gate. This forward biases D202-D203, and the Channel 1
Diode Gate is blocked as it is fo r Channel 2 only operation.
A t the same time, Q275 is reverse biased and its collector
d ro p s n e g a tive to th e v o lta g e set b y d iv id e r
R267-R279-R275. This negative voltage reverse biases
D206-D207 in the Channel 2 Diode Gate and allows the
Channel 2 signal to pass through the Diode Gate to the
Delay-Line Driver stage.
The alternate trace sync pulse is applied to Q264
through R261 and C261. The positive-going sync pulse at
the base o f Q264 momentarily turns Q264 on to conduct
the R260 current away from the Switching M ultivibrator
stage. This turns o ff both Q265 and Q275. When Q264
turns o ff again after the alternate-sync pulse, the charge on
C269 and C279 determines whether Q265 or Q275 con­
ducts. For example, w ith Q265 conducting, the collector of
Q275 drops more negative than the collector o f Q265. This
produces a greater charge on C279 than on C269. This
charge is stored by C269 and C279 while Q264 is on. When
the current is again applied to the Switching M ultivibrator
stage after the alternate-sync pulse ends, the greater charge
on C279 holds the base of Q265 more positive than the
base o f Q275. Therefore, Q275 conducts first and its collec­
to r rises positive to hold Q265 reverse biased. The m ultivi­
brator has switched and the conditions described previously
are reversed; now the Channel 2 Diode Gate is reverse
biased and the Channel 1 signal passes through the Channel
1 Diode Gate.
The Reference Feedback stage, Q283, provides common­
mode voltage feedback from the Delay-Line Driver stage to
allow the diode gates to be switched w ith a minimum amp­
litude switching signal. The emitter level o f Q283 is con­
nected to the collectors o f the Switching M ultivibrator tran­
sistors through D281-R281 and D282-R282. The collector
level of the "o n " Switching M ultivibrator transistor is more
positive than the " o f f " transistor and either D281 or D282
is forward biased. This clamps the anodes o f the forward-
biased shunt diodes o f the applicable Diode Gate about 0.5
volts more negative than the emitter level o f Q283. The
level at the emitter o f Q283 follows the average voltage
level at the emitters of the Delay-Line Driver transistors.
This configuration clamps the forward-biased shunt diodes
near their switching level so they can be switched quickly
w ith a minimum amplitude switching signal. It also main­
tains about the same current through the Diode Gate shunt
diodes so they can be switched w ith a minimum amplitude
switching signal, regardless o f the deflection signal at the
cathodes of the shunt diodes.
3Millman and Taub, pp. 362-389.
3-7