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Time-Base Generator - Tektronix T PLUG-IN Instruction Manual

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V7350A to cutoff.
When the Schmitt Trigger is in the recurrent
configuration
just described, the Time Base
Generator
can
be
triggered
with
repetitive
signals over a wide range of frequencies
, without
the
Schmitt
Trigger
continues
to
operate
at
approximately
a 50-cycle
rate.
Thus, in the
absence of triggers, the Time-Base Generator
will
continue
to generate
a sawtooth output.
When
the
TRIGGERING MODE switch is in
the HF SYNC position, the Time-Base Trigger
is
bypassed, and the signal at the TRIGGER
INPUT connector is applied directly to the Time-
Base
Generator.
This
signal
acts
as
a
synchronizing
voltage,
superimposed
on the
holdoff
waveform,
(to
be
discussed
in
the
section
that follows).
This synchronizes the
Time-Base Generator at a sub-multiple of the
triggering-signal
frequency.
This
mode
is
useful
for
input
signals
in the range from 5
megacycles to 20 megacycles.
Tne time-Base
Trigger Produces a negative-
going
waveform
which
is coupled
to
the
Time-Base
generator.
This
waveform is
differentiated
in
the
grid
cirucit
of
V7650B
to
produce
a sharp negative-going triggering
pulse to trigger the Time-Base Generator
in the
proper time sequence. Positive-going pulses are
also produced in the differentiation process , but
they are not used in the operation of the Time-
TIME-BASE GENERATOR
The Time-Base Generator consists of three
main
circuits;,
a
Bistable
Multivibrator,
a
Miller Runup Circuit, and a Hold-Off Circuit.
The Bistable Multivibrator consists of V7650B,
V7750A and the cathode-follower V7650A. The
essential components inthe Miller Runup Circuit
are
the
Miller Tube V8110B, the Runup C.F.
V8110A,
the
On-Off
Diodes
V8020,
and
the
Timing Capacitor C160 and the Timing Resistor
R160. The Hold-Off Circuit consists of the Hold-
Off Driver V7530B , and Hold-Off Capacitor C180
and the Hold-Off Resistor R180.
In the quiescent state, V7650B of the Bistable
Multivibrator is conducting and its plate voltage
is
down.
This
cuts
off
V7750A
through
the
cathode-follower
V7650A,
the
voltage divider
R7680-R7690, and the cathode resistor R77 LO.
The
quiescent
state of the
Miller Tube is
determined by
the
dc
network between plate
and
grid
consisting
of
the neon lamp B8160,
the Runup C.F. V8110A, and the On-Off Diode
V8200.
The
purpose
of
the dc network is to
establish
a
voltage at the plate of the Miller
Tube of such a value that the tube will operate
above the knee, and thus over the linear region,
of its characteristic curve.
In the absence of signal the grid of the Miller
Tube
rests at about -3 volts.
There is about
a 1
1/2 volt drop in the Runup On-Off Diodes,
about
18.5-volts bias on the Runup C.F., and
about a 55-volt drop across the neon lamp. This
establishes a quiescent voltage of about 40 volts
at the plate of the Miller Tube.
If
the STABILITY control is now advanced,
making the
grid of V7650B more negative, a
point
will
be reached where a negative-going
triggering
pulse
from
the
Schmitt
Trigger
stage
will
cause
the
Bistable
Multivibrator
to
switch
rapidly
to
its other state,
That is
V7650B
will be cut off and V7750A will start
to conduct. (Any spiking that may occur, during
this transition period, is attenuated by the R7790-
C7790 network.) As V7750A conducts, its plate
voltage,
and
the
voltage
at
the diode plates,
drops.
As
a
result
the
diodes
are
cut
off,
which permits the grid of the Miller Tube and
the
cathode
of
the
Runup
C.F. to seek their
As there is no connection to the diodes at this
time, the grid of the Miller Tube starts negative,
since
it
is
connected to the -150-volt supply
through
the Timing Resistor R160.
The plate
of the Miller Tube then starts positive, carrying
with it the grid and cathode of the Runup C.F.,
V8110A.
This
raises
the
voltage at the top
of the
Timing Capacitor C160, which in turn
raises
the
voltage
at
the
grid
of the Miller
Tube, and prevents it from going negative. The
gain of the Miller Tube, as a class A amplifier,
is approximately 200.
This means that a 150-
volt
change in plate voltage will maintain the
grid voltage constant within three-quarters of
The
Timing
Capacitor
starts charging with
current
from
the
-150-volt
bus.
Since
the
voltage at the grid of the Miller Tube remains
essentially constant, the voltage drop across the
Timing Resistor, and hence the charging current
through it, remains essentially constant. Thus,

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