Tektronix 543B, Instruction Manual

Page: 17 / 90

Circuit Description — Type 543B/RM543B
cut off its collector voltage is up and no output is devel­
oped.
The negative-going portion of the signal from the trigger
amplifier is required to drive the base of Q35 down. As
the Q35 base is driven negative, the current flow through
the transistor is restricted and the voltage at the collector
starts to rise.
The rise in voltage at the collector of Q35 corries the
base of Q45 in the positive direction.
The emitters of both transistors are coupled together, and
follow the action of the bases. With the Q45 base going in
a positive direction, and the emitter in a negative direc­
tion, Q45 starts to conduct. As Q45 starts to conduct, the
emitters of both transistors follow the action of Q45 base;
hence the emitter voltage starts to rise.
As the Q35 base goes down and its emitter goes up,
it stops conducting. As Q45 conducts, its voltage drops,
creating a negative step at the output. This transistion oc­
curs rapidly, regardless of how slowly the base falls.
When the signal applied to the base of Q35 goes in a
positive direction, the action described in the previous para­
graphs reverses itself. That is, Q35 will start to conduct
once more, while Q45 will be cut off.
In the AUTO position of the TRIGGERING MODE switch
the trigger multivibrator is converted from a bistable con­
figuration to a recurrent configuration. This is accomplished
by disconnecting +100 volts from the junction of D49 and
R38, thereby allowing C49 to charge and discharge.
In this mode of operation, the trigger multivibrator will
run in the absence of a triggering signal. For example,
assume that the base of Q35 is just being driven into cut­
off. The voltage at the collector of Q35 will rise, carrying
with if the base of Q45. As the voltage at the base of
Q45 starts to rise, Q45 starts to conduct. The falling volt­
age at the collector of Q45 is coupled to the base of Q34.
Since the voltage at the base of Q34 is falling, the col­
lector voltage is rising. This rising collector voltage of Q34
is then coupled to the base of Q35. The base of Q34 is
prevented from falling immediately by the action of C49,
which must discharge sufficiently to lower the voltage at
the base of Q34 into cutoff.
As the collector voltage of Q34 raises the base of Q35
sufficiently to bring that transistor out of cutoff, its col­
lector voltage will in turn lower. The lowering collector
voltage of Q35 is coupled through D43 to the base of Q45,
thus causing Q45 to cut off. When Q45 reaches cutoff,
the circuit has completed one cycle of an approximately
40-cycle repetition rate.
During calibration, the repetition rate for the AUTO mode
is adjusted by R47 (TRIG SENS), which comprises part of
the discharge path for C49.
Sweep Generator. The time-base generator consists of
three main circuits: the sweep gating multivibrator, the
Miller runup circuit, and the holdoff circuit.
The time-base trigger circuit furnishes the waveform which
initiates a cycle of action in the time-base generator. Square
waves from the output of the trigger multivibrator are fed
to the time-base generator where they ore differentiated
and used as trigger pulses. To explain the action of the
time-base generator assume it is in the quiescent state, just
before the arrival of a suitable trigger pulse, with V135A
conducting.
Square waves, generated by the time-base trigger cir­
cuitry, are differentiated by the C131-R131 network.
If STABILITY control R110 is advanced, the grid of V135A
will become more negative. As the grid of V135A becomes
more negative, a point is reached at which a negative-going
triggering pulse from the C131-R131 network will drive
V135A into cutoff.
As V135A is driven to cutoff, the plate voltage rises,
carrying with it the grid of cathode-follower stage V135B.
V135B, used as a cathode follower between the two halves
of the multivibrator, isolates the positive-going plate of
V135A from the capacitance of the loads requiring a posi­
tive-going pulse. This results in a faster rise of the positive­
going pulse at the plate of V135A.
The cathode of V135B is longrailed through R141 and
R143, and closely follows the action of the grid. Since the
grid of V145 has a certain shunt capacitance to ground,
Cl 41 is connected in parallel with R141 to compensate for
this capacitance.
The voltage rise at the cathode of V135B drives the grid
of V1 45 above cutoff. As VI45 begins to conduct, its plate
voltage drops rapidly. Any spiking which may occur is
attenuated by the C150-R150 network.
When V145 is conducting at the maximum determined
by circuit parameters, the sweep-gating multivibrator has
reached its other stable state and the action of the Miller
runup circuit has been initiated.
The Miller runup circuit is essentially a Class A ampli­
fier employing negative feedback. The positive-going volt­
age at the plate of the Miller tube is fed back to the grid
through runup cathode follower V173 and opposes the
attempt of rhe grid to go negative. Because the gain of
the Miller tube is high, (approximately 200) it is possible
to maintain an essentially linear rate of charge on the tim­
ing capacitor.
In the quiescent state of the time-base generator, the
voltage at the plate of the Miller tube is determined by
the voltage drop across the dc network formed by neon
lamp B167, the runup cathode follower, and the discon­
nect diodes. The purpose of this dc network is to establish
a voltage at the plate of the Miller tube of such value that
the tube will operate above the knee, and hence over the
linear region of its characteristic curve.
The grid of Miller tube V161 is returned to the —150-
volt supply through timing resistor R160. In the quiescent
state of the time-base generator the grid of the Miller tube
is held slightly negative, but well above cutoff, by the flow
of the current through D150 of the disconnect diodes. When
the disconnect diodes sfop conducting, the grid of the Mill­
er tube tends to become more negative.
As the grid of the Miller tube starts negative, the plate
becomes more positive. This positive-going excursion of
the p.'ate carries the grids of runup cathode follower V173
with it. The voltage at the grids of V173 is maintained
at a constant difference with respect to the Miller-tube plate
voltage by the voltage drop across neon bulb B167. C167
3-6