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Fig.
4-2.
The
time
relationship
between
the
alternate-sweep
slave
pulse,
alternate-trace
sync
pulse
and
the
alternate
sweeps
of
a
Type
547
Oscilloscope.
CH
1.
This position of the MODE switch allows only the
Channel
1 signal
to pass to the output amplifier.
This is
accomplished by reverse biasing the base of Q315 from the
+225-volt supply through R311 and R314. The base of Q325
is forward biased by connecting it to ground through R324.
With Q325 conducting, a more positive voltage is devel-
oped
at the junction of R327-R328
than at the emitters of
Q354 and Q364.
This positive voltage forces D307-D308 into
conduction
and
back
biases
D306-D309.
With
D306-D309
back biased, the Channel 2 signal is blocked from the Output
Amplifier.
With
Q315
turned
off,
the
junction
of R317-R318
is suf-
ficiently more
negative than the emitters of Q354-Q364
to
reverse
bias
D302-D303.
Therefore,
with
D302-D303
not
conducting,
D301-D304
are forward
biased
and allow the
Channel
1 signal to pass to the Output Amplifier.
Fig. 4-1
shows
the condition
of the diode
gates,
in simplified
form,
when the MODE switch is set to CH 1.
ADD.
This position of the MODE
switch permits the out-
put of both channels to pass to the Output Amplifier at the
same time.
The resultant display is an algebraic addition of
the two inputs. To allow both diode gates to pass the signal
at the same time, both Q315
and Q325
must be turned off.
To do this, the MODE switch removes the voltages from the
emitters
and
bases
of Q315
and
Q325.
This
places
the
junctions
of D302-D303
and
D307-D308
sufficiently
below
Q354-Q364
emitters so the diodes are back biased.
NOTE
R323 is switched in as a dummy
load resistor that
maintains
the proper current drain on the
+10-
volt
source.
With the two channels working into the Output Amplifier
simultaneously, twice the amount of current is passed through
the diode gates.
Thus, additional current is supplied through
or
Circuit Description—Type
1A2
R352 by the MODE
switch to maintain the same output DC
level.
CH 2.
In this position of the MODE switch, only the Chan-
nel 2 signal passes to the Output Amplifier.
This is done by
reversing
the state of the switching
multivibrator from
the
condition of the CH
1 position.
That is, Q325 is turned off
and Q315 is turned on. This blocks the Channel 1 signal and
permits the Channel 2 diode gate to pass the signal to the
Output Amplifier.
CHOP.
In this position of the MODE
switch, the trigger-
ing
circuit (Q340)
is arranged
as an
astable
(free running)
blocking
oscillator.
Frequency
of the blocking
oscillator is
about 220 kHz.
The output pulse of the blocking oscillator is
coupled
to the switching multivibrator through C340 to the
steering diodes.
The only difference in the operation of the
circuits from the ALT mode of operation is that the blocking
oscillator (Q340) free runs and doesn't rely on a triggering
signal from the time base circuitry of the oscilloscope.
The
result is that the switching multivibrator rapidly switches back
and
forth between
channels
during the sweep of the time
base.
A blanking
signal
is coupled
from
the collector of
Q340
to pin 16 of the interconnecting plug.
This blanking
signal
blanks
the electron
beam
of the oscilloscope while
the switching multivibrator switches between channels.
Filament
Supplies
Filament
current for V364
is supplied from
pins 13 and
14 of the interconnecting plug. This voltage is approximately
6.3 volts at line frequency.
Filament volfage for the input
Nuvistors
is regulated
by
the supply
in the oscilloscope.
The DC filament voltage eliminates any line frequency ripple
that
might
otherwise
be
introduced
into
the cathodes
of
V133 and V233. Also, the constant filament voltage eliminates
bias shift when there is any change in line voltage.
Trigger Output
Amplifier
Q414
and
Q424
with
the associated
circuitry are feed-
back amplifiers for the signal arriving
from
R136 or R236
in the cathode of the input Nuvistor.
Variable resistor R415
simulates
the input base
impedence
of Q414 so the input
cathode
follower
of the
channel
not selected
for trigger
amplification has the same loading to ground as when it is
selected for trigger amplification.
Voltage gain for the stage
is about 2 to 2.5 for Q414 and about 5 for Q424.
The sig-
nal is AC coupled
from the collector of Q414 to the base
of Q424 to minimize the effect of DC drift, and from the col-
lector of Q424
to both the TRIG
OUT
connector and
pin
5 of the interconnecting plug to the oscilloscope.
The trigger at pin 5 is available for use as an internal trig-
ger source.
However, to make
use of this trigger the asso-
ciated
oscilloscope
must be capable
of selecting it with a
Triggering Source switch.
If the trigger cannot be selected
internally, external triggering must be used.
The trigger has
the same
polarity as the signal applied at the Input con-
nector.
Output DC level is approximately zero volts.
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