Tektronix 503, Instruction Manual

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Circuit Description— Type 503
is in AUTO, feedback is added to the circuit converting it
to a free-running 50-Hz multivibrator. In the AUTO mode,
the circuit recognizes trigger pulses that occur at a rate
greater than 50 Hz. For these signals, it w ill function just
as if the LEVEL control were set up for a stable display.
The operation of the circuit is as follows: assume that
V45A is conducting and is ready to receive a triggering
signal. When a negative-going signal from the plate of
V24B arrives the grid of V45A is driven downward. Since
the tube is conducting, the cathode is also driven down­
ward, until a point is reached where the lowered cathode
voltage permits current flow to start in V45B. At the same
time, V45A begins to cut off. As V45A cuts off, a positive
step appears at the plate of V45A which is coupled to the
grid of V45B, causing V45B cathode to move more posi­
tive, cutting off V45A. The voltage level at V45A grid when
this switching occurs is known as the lower hysteresis point
of the Schmitt multivibrator. A negative step is produced
at the plate of V45B when the multivibrator switches. This
negative step is differentiated by C43 and R44 and used
to initiate a sweep.
When the trigger signal goes positive, the reverse action
takes place. As the grid of V45A goes positive, a point is
reached where V45A begins to conduct and the current
through V45B begins to decrease, causing a negative step
at the plate of V45A. This negative step is coupled to the
grid of V45B, driving it toward cutoff. As the grid of V45A
goes more positive, and the cathode follows, the cathode of
V45B also goes more positive, reinforcing V45B cutoff. When
this switching occurs, V45A grid is at a voltage level known
as the upper hysteresis point of the Schmitt multivibrator.
A positive step is produced at the plate of V45B when the
multivibrator switches. This positive pulse has no function
and is bypassed by diode D44 to AC ground in the -f-250
volt power supply.
Automatic Triggering
NOTE
The action o f the LEVEL control in AUTO position,
w hich causes a free running Trigger M u ltiv i­
b ra to r, should not be confused w ith the action o f
the LEVEL co n tro l turned clockwise to the FREE RUN
position. In the clockwise position, the FREE RUN
switch permits the sweep g e n e ra to r to fre e run.
This o p e ra tio n is e xp la in e d under the ope ra tio n o f
the Sweep G enerator.
The AUTO position of the switch couples the grid of
V45B to the grid circuit of V45A through R40, and removes
the DC coupling between the Trigger Input Am plifier and
the Trigger Multivibrator. The action of the multivibrator
in this mode is similar to that described in the bistable
mode, with the exception that a feedback network (C31 and
R40) has been added.
AUTO operation can be described as follows: assume
that V45A is conducting and V45B is cut off. In this state,
V45B grid is negative with respect to V45A grid. Thus C31
will attempt to charge to the voltage level of V45B grid
through R40. As C3I charges to this level, it moves V45A
grid negative to the lower hysteresis point, causing the
multivibrator to change states. (The action was described
in tire early pa rf o f (lie Trigger M u ltiv ib ra to r Jiscussion.)
In this new state, V45B grid is more positive than V45A
grid. C31 will change its charge and move V45A grid to­
ward the new more positive level on V45B grid. As V45A
grid moves positive, it passes through the upper hysteresis
point, causing the multivibrator to again switch states. This
switching occurs at a rate of approximately 50 Hz. Hence,
in the absence of a triggering signal, the multivibrator
provides a trigger to the sweep generator a a rate of ap ­
proximately 50 Hz. The sweep produced by this trigger
provides a base line from which to make voltage measure­
ments, and also indicates that the instrument is adjusted to
display any signal that might be applied to the instrument.
SWEEP GENERATOR
The Sweep Generator, upon receipt of a trigger pulse
from the Sweep Trigger, produces a linearly rising (saw­
tooth) voltage which is applied through the Horizontal
Am plifier to the CRT deflection plates. This causes the spot
to move from left to right on the CRT screen and form the
sweep. The amplitude o f the sawtooth voltage is about
100 volts. Its rate of rise is controlled by the values of the
timing capacitor and timing resistor switched into the circuit
by the SWEEP TIME/CM switch on the front panel.
The Sweep Generator consists of three main circuits; the
Sweep-Gating Multivibrator, the M iller Runup Circuit, and the
Hold-off Circuit. The Sweep-Gating M ultivibrator consists of
V135A, V135B, and V145A. The essential components of the
M iller Runup Circuit are: the Miller Runup Tube, V160A;
the Runup Cathode Follower, V160B; the Disconnect Diodes,
V152A and D152 (V152A and V152B SN 101-4229); the
Timing Capacitor, Cl 60; and the Timing Resistor, R160. The
H old-O ff Circuit consists o f the H old-O ff Diode, V152B;
the H old-O ff Cathode Follower, V145B; the H old-O ff Resis­
tor, R181; and the H old-O ff Capacitor, Cl 60 (the H old-O ff
Circuit makes use of some of the same timing capacitors as
the Miller Runup Circuit).
In the quiescent state, that is, when no sweep is being
generated, V135A is conducting the V145A is cut off. The
plate of V145A is at about —2.5 volts with respect to ground.
The Disconnect Diodes are conducting and hold both sides of
the Timing Capacitor at about —2.5 volts. W ith its cathode
grounded and its grid at —2.5 volts, V160A is conducting
heavily and its plate is at about + 3 0 volts.
Sweep Generation
A negative trigger pulse, arriving at the grid of V135A
from the Sweep Trigger circuit, causes the Sweep-Gating
M ultivibrator to switch rapidly to its other state. That is,
V135A cuts off and V145A conducts. As V145A conducts,
its plate voltage goes down, cutting off the Disconnect
Diodes. When the Disconnect Diodes cut off, the plates of
the Timing Capacitor are no longer held at —2.5 V, and
the Timing Capacitor starts to charge toward the instan­
taneous potential difference between the — 100-volt supply
and potential on the cathode of V160B. However, as the
lower plate of the Timing Capacitor starts to move in a
negative direction, it takes the grid of V160A with it. This
produces a positive swing at the plate o f V160A which is
coupled, through B167 and V160B, to the upper plate of the
Timing Capacitor. This increases the voltage to which the
Timing Capacitor is frying to charge. The effect is to
straighten out the charging curve by increasing the charging
volfa ge w ifli eacli increment o
(
cliarge on l i e capacitor.
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