ten to it from the Microprocessor.
The
preamp also has
provisions for
VAR
gain, vertical positioning,
and
a trigger
signal pickoff.
The
Channel 1 vertical input signal is applied to pin A
of
Channel
1 Preamplifier
U100
. Control data from the pro-
cessor
is
clocked into the internal control register via pin
22
(CD)
by
the clock signal applied to pin 23 (
CC
).
The
data sets the device to
have
an
input-to-output gain ratio
of
2,
4,
or
10,
depending
on
the VOL TS/DIV control set-
ting.
Two analog control voltages set
by
DACs modify the
differential output signal at pi
ns
9
and
10. The front-panel
Channel
1 POSITION control supplies a position signal to
U100
pin
17
(via MUX U2530
and
sample-and-hold U2430
and
C2432) that vertically positions the Channel 1 display
on
the CRT. A
DC
Bal signal is applied to
pin
2 of U100
from MUX U170
via
the sample-and-hold circuit composed
of U160A
and
C177
. This
DC
BAL signal is a
de
offset-null
level
that
is
determined during the automatic
DC
Bal
pro-
cedure.
The
offset value is stored as a calibration constant
in
RAM
and
is recalled at regular intervals to set the
DC
Bal
level,
holding the Preamplifier
in
a
de
balanced condi-
tion.
The
Channel 1 VOL TS/DIV
VAR
control is monitored
by
the Microprocessor during the front-panel scanning rou-
tine.
When
the processor has determined where the
VOL TS/DIV
VAR
control is positioned, it causes DAC
U2101
(diagram
2)
to produce a corresponding control
level
and
routes it to the
VAR
gain sample-and-hold circuit
composed of U160D, C179,
and
associated components.
The control voltage at the output of U160D
(pin
14) sets
the variable gain of the Preamplifier.
A pickoff amplifier internal to U100 conditions the
trigger signal
and
provides the proper signal
level
at
pin
15
to drive the A/B Trigger Generator (U500, diagram
5).
The
pickoff point for
the
trigger signal is prior to the addition of
the vertical position offset,
so
the position of the signal
on
the crt has
no
effect
on
the trigger operation. However,
the pickoff point
is
after the
DC
Bal
and
Variable gain sig-
nals have
been
added to the signal so both of these func-
tions
will affect trigger operation.
Common-mode signals
are
rejected from the trigger sig-
nal
by
the circuitry composed of operation amplifier U450B
and
associated components. The inverting input of U450B
(pin
6)
is
connected to the common-mode point between
APO+
(pin
12)
and
TPO-
(pin
15)
of U100. Any
common-mode signals present are inverted
and
applied to
a common-mode point between
R451
and
R453 to cancel
the signals from the differential output. A filter network
composed of
LR
180
and
the built-in circuit board capaci-
tor
(5
.6
pF)
reduces
trigger
noise
susceptibility
.
Trigger
sig-
Theory
of
Operation-2445A/2455A Service
nals for options
are
obtained from J100.
The Channel 1 input signal used to provide the horizon-
tal deflection for the
X-Y
displays is obtained from U100
pin
11
.
The
components between pin
11
and
the Horizon-
tal Output Amplifier provide phase compensation
of
the
signal. During instrument calibration, the delay produced
by
C115, C116,
L115,
R115, and variable capacitor C118
is matched to the 78-ns delay of the vertical delay line
(DL100, diagram
6).
Channel 2 Preamplifier
Operation of
Channel
2 Preamplifier U200
is
nearly
identical to that of the Channel 1 Preamplifier just
described.
The
exceptions are that the output polarity of
the Channel 2 signal
may
be
either normal or inverted
and
that the signal obtained from the
BPO
+ output
(pin
11)
is
conditioned differently for a different purpose than
in
the
Channel
1 Preamplifier circuitry.
Inverting the
Channel
2 signal for the
CH
2 INVERT
feature
is
accomplished
by
biasing
on
different amplifiers.
The control data clocked into the internal control register
from
pin
22 sets
up
the necessary switching.
The
Channel 2 BPO + signal at U200
pin
11
provides
an
accurate representation
of
the Channel 2 signal at the
rear-panel
CH
2
OUT
connector.
Channel 3 and Channel 4 Preamplifier
The
functions provided by the Channel 3
and
Channel 4
Preamplifier are similar to those provided
by
the Channel 1
and
Channel
2 Preamplifiers. The single-ended
CH
3
and
CH
4 input signals are converted to differential signals,
and
vertical gain
and
vertical positioning are
added
to the
output signals. Trigger pickoff signals
are
generated for
both channels
and
are routed to the Trigger hybrid.
Channel 3
and
Channel 4 gains
may
be
either 0.1 volt
per division or 0.5 volt per division. The logic levels of con-
trol bits applied to U300 pin 30 (GA3)
and
pin
31
(GA4)
from Auxiliary Control Register U140 sets the gain of the
Channel 3
and
Channel 4 preamplifiers respectively. Verti-
cal
positioning of the Channel 3
and
Channel 4 signals
on
the crt
is
controlled
by
the voltage levels applied to
pin
29
(POS3)
and
pin
32
(POS4) from the front-panel
CH
3
and
CH
4 POSITION potentiometers
(via
MUX
U2530
and
sample-and-hold amplifiers U2430C and C2333
and
U2430D
and
C2332).
De
offsets
in
the output signal due to any tracking
differences
between
the
+5-V
and
the
-5-V
supply
to
3-15
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