Tektronix T PLUG-IN Instruction Manual page 14

TIME-BASE TRIGGER
Trigger Input Amplifier
The function of the Time-Base Trigger is to
develop the triggering pulse which initiates a
cycle of action inthe Time-Base Generator. The
signal from which the triggering pulse is
produced may emanate from either of two
sources, as determined by the setting of the
TRIGGER SLOPE switch (black knob), SW7010.
When the switch is in the +EXT. or -EXT.
positions, an external signal is employed in the
development of the triggering pulse. In the
at the power line frequency is used to develop
the triggering pulse. To obtain a stable display
In addition to selecting the source of the
triggering signal, switch SW7010 (TRIGGER
SLOPE) also selects the polarity of the slope
on which triggering will occur.
The Trigger-Input Amplifier V7040 is a
polarity-inverting, cathode-coupled amplifier.
The output is taken from the plate of V7040A,
but the grid of either stage may be connected
to the input signal source. When the TRIGGER
SLOPE knob is in the -EXT. or -LINE position
the grid of V7040B is connected to the input
signal source. The grid of V7040A is connected
to a dc bias source, adjustable by means of the
TRIGGERING LEVEL control. This bias voltage
establishes the voltage present at the plate
of V7040A under no-signal conditions. When the
TRIGGER SLOPE knob is in the +EXT. or
+LINE position the grid of V7040A is connected
to the input-signal source, and the grid of
V7040B is connected to the bias voltage source.
CIRCUIT
DESCRIPTION
The voltage at the grid of V7040B and the
voltage at the plate of V7040A are in phase
with each other; that is, they both go through
ac zero in the same direction at the same time.
By this arrangement, V7040B acts as acathode-
follower, having a gain of approximately .5,
and the signal voltage developed across the
cathode resistor becomes the input signal for
When the switch is moved to either the
+EXT, or +LINE position, the grid of V7040A
is connected to the input-signal source. With
this configuration, the voltage at the plate of
V7040A will be 180 degrees out of phase with
the input-signal voltage. Thus, depending on the
setting of the TRIGGER SLOPE switch ( + or
-), the plate of V7040A swing may be in phase,
or 180 degrees out of phase with the input-
The Schmitt Trigger consists of V7350, A and
B, connected as a dc-coupled multivibrator.
In the quiescent state, ready to receive asignal
V7350A is conducting and its plate is down.
This holds the grid of V7350B below cut-off
since the circuits are dc-coupled by the voltage
divider R7440, R7460 and R7470. With V7350B
cut off its plate voltage is up and no output
A negative-going signal is required at the
grid of V7350A to drive the Schmitt Trigger
into its other state, at which time a triggering
pulse is produced. The signal coupled to the
grid of V7350A may contain both positive-and
negative-going components.
The negative-going portion of the signal will
drive the grid of V7350A in the negative direction
and the cathodes of both tubes will follow the
grid down. As the grid of V7350A is driven
negative the current flow through the tube is
AA Circuit Description - Type T 3-1