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

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,