Tektronix 516 series, Инструкция по эксплуатации

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Circuit Description — Type 5 1 6
state to the other occurs very rapidly regardless of how
slowly the voltage at the input grid passes the critical level.
Thus the output of the Trigger Multivibrator is a 10-volt
square wave. The negative-going transition occurs when
the voltage at the input passes the critical level while moving
in the negative direction, and the positive-going transition
occurs when the voltage at the input passes the critical level
while moving in the positive direction. As mentioned before,
only the negative-going transistion is of significance time-
wise, and, by means of the black TRIGGER SELECTOR control
and the TRIGGERING LEVEL control, this point can be made
to coincide with virtually any point on the incoming trig­
gering signal.
The TRIG. LEVEL CENTERING adjustment, R39, varies the
level of the grid of V45B with respect to the plate of V45A.
Thus, it controls the input voltage at which the Trigger Multi­
vibrator changes states. This level is normally set at the
voltage which the plate of V24B assumes when both grids of
V24 are at ground potential. In most instruments, this is
within a volt or two of + 80 volts.
Actually, the input voltage level at which the Trigger
Multivibrator changes states on a negative-going signal is
normally slightly lower than the input voltage level at which
it changes states on a positive-going signal. The difference
between the two input voltage levels at which the two
changes in state occur is the hysteresis of the circuit. In
order to obtain maximum triggering sensitivity, the hysteresis
must be kept as small as possible. To reduce the hysteresis,
resistance is introduced between the two cathodes by means
of the TRIGGER SENSITIVITY adjustment, R47. Due to the
drop across this resistance, the cathode voltage of the non­
conducting tube is moved closer to its grid voltage, so that
the grid will not have to move as far to bring the tube into
conduction and cause the multivibrator to change states. Too
much resistance between the cathodes causes the multivibra­
tor to oscillate during transitions from one state to the other.
As will be seen in the discussion of the Time-Base Genera­
tor, not every negative trigger pulse from the Time-Base
Trigger initiates a sweep. Negative trigger pulses which ar­
rive at the Time-Base Generator during the time that a sweep
is in progress will have no effect on the circuit. It is only
after a sweep has been completed and all circuits have re­
turned to their quiescent state that the Time-Base Generator
will be retriggered by a trigger pulse from the Time-Base
Trigger.
Triggering Mode
The red TRIGGER SELECTOR control, SW10B, selects the
type, or mode, of triggering. In the DC position, the trigger­
ing signal is dc-coupled to the Trigger Input Amplifier, which
in turn is dc-coupled to the input of the Trigger Multivibrator.
R30 isolates the plate of V24B from the capacitance of the
switch; R32 isolates the grid of V45A from the switch. It
should be noted that with SW10A in the INT. position, and
SW10B and the appropriate POLARITY switch in the DC po­
sition, complete dc-coupling exists from the input of the Verti­
cal Amplifier to the Trigger Multivibrator.
In the AC position of SW10B, capacitor CIO is connected
into the input circuit. This prevents the dc component of the
triggering signal from reaching the Trigger Input Amplifier.
The Trigger Input Amplifier, however, is still dc-coupled to
the Trigger Multivibrator.
In the AUTO, position of SW10B, the Trigger Multivibrator
is converted from a bistable configuration to an astable
(free-running) configuration by the addition of feedback from
the grid of V45B to the grid of V45A through R40. In addi­
tion, the dc coupling between the Trigger Input Amplifier
and the Trigger Multivibrator is removed when the switch
is in the AUTO, position.
To understand the operation of the Trigger Multivibrator
in the free-running mode of operation, first assume that V45B
is cut off and V45A is just being driven into cutoff by the
charge on C31. The voltage at the plate of V45A starts to
rise, carrying with it the voltage at the grid of V45B. V45B
then starts to conduct, causing a negative step at its plate.
Since the two grids are coupled through R40, the grid of
V45A will start moving positively at the same time as the
grid of V45B. However, the time constant of C31 and the
resistances in the grid circuit of V45A is such that it takes
about 0.01 second for the voltage at the grid of V45A to
rise exponentially from its starting point, below cutoff, to a
point where the tube will start conducting.
When V45A does start conducting, its plate voltage will
drop, carrying with it the grid of V45B. V45B will cut off
causing a positive step at its plate. At the same time that
the grid of V45B goes negative, the grid of V45A will also
start negative. Once again, it will take about 0.01 second
for C31 to charge up sufficiently to cut V45A off. When
V45A does cut off, the cycle starts over. Thus, the Trigger
Multivibrator free runs at about 50 Hz.
During the automatic mode of operation, the total voltage
change at the grid of V45A is about 3 volts. Since the grid
of V45A is never more than 3 volts from cutoff, a triggering
signal from the Trigger Input Amplifier with a peak-to-peak
amplitude of 3 volts or more can drive the grid to cutoff at
any time and produce a trigger output. Smaller signals than
3 volts can drive the V45A grid into cutoff if they occur at
a time when the exponentially changing grid voltage is ap­
proaching the cutoff level of the tube (relatively later in the
time interval during which V45A is conducting).
Hence, in the absence of any triggering signal, the Trig­
ger Multivibrator free runs at about 50 Hz. However, since
the triggering signals are still coupled to the Trigger Multi­
vibrator through C31, virtually any triggering signal over
50 Hz in frequency and of sufficient amplitude will produce
synchronized operation of the Trigger Multivibrator. The
50-Hz free-running sweep produced in the absence of a
triggering signal provides a base line from which to make
voltage measurements and also indicates that the instrument
is adjusted to display any signal that might be applied to
the input.
The Trigger Multivibrator has a maximum switching fre­
quency of about 5 MHz. Therefore, when it is desired to
display signals above that frequency, the Time-Base Trig­
ger is bypassed by placing SW10B in the HF SYNC posi­
tion and the incoming signal is applied directly to the Time-
Base Generator. The STABILITY control is advanced to pro­
duce a free-running sweep and the signal becomes a syn­
chronizing signal, rather than a triggering signal, which
synchronizes the free-running sweep to a submultiple of its
frequency.
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