Trigger Amplifiers And Switching - Tektronix 2215A Instruction Manual

TRIGGER AMPLIFIERS AND SWITCHING

The Trigger Amplifiers, shown on Diagram 3, provide sig­
nals to the A Trigger Generator circuit from either the Verti­
cal Preamplifiers, the EXT INPUT connector, or the power
line. The A&B INT switch selects either Channel 1 or Chan­
nel 2 as the trigger source, and the A SOURCE switch se­
lects between internal, line, or external trigger sources.
Internal Trigger
Signals from the Vertical Preamplifiers drive the Internal
Trigger Amplifier with channel selection determined by the
VERTICAL and HORIZONTAL MODE switches.
Trigger pickoff from the Preamplifiers is accomplished by
Q302 and Q303 for Channel 1, and Q327 and Q328 for
Channel 2. The circuitry associated with Channel 2 is the
same as that for Channel 1 except that it does not have a
trigger offset adjustment.
Signals from the Channel 1 Preamplifier are applied to
Q302 and Q303. These emitter-follower transistors each
drive one input transistor in U310, and the collectors of the
U310 input transistors in turn supply emitter current to two
current-steering transistors. The compensation and biasing
network connected to the emitters of the input transistors in
U310 is fixed for Channel 2 but not for Channel 1. Poten­
tiometer R309 adjusts the emitter bias levels of the two in­
put transistors so that dc offsets between channels can be
matched.
The base bias voltages of one transistor in each output
differential amplifier pair is fixed by the divider network com­
posed of R321 and R322. The other base voltage is con­
trolled by the Ch 1 Trig line from the Trigger Channel
Switch. When the Ch 1 Trig signal is HI, the transistors in
each output pair with the collectors connected together are
biased on and the other transistors are off. The collector
signal currents are equal in magnitude but opposite in polar­
ity and signal cancellation occurs. If the Ch 1 Trig signal is
LO, the other transistors in each pair will be biased on and
an output signal will be developed across R314 and R315 to
drive the Internal Trigger Amplifier.
Internal trigger channels are chosen by the A&B INT
switch with the A SOURCE switch set to INT. The INT posi­
tion of S392 reverse biases CR393 and CR399 to prevent
external trigger signals or the line trigger signal from reach­
ing the A Trigger Generator. Signals from the Internal Trig­
ger Amplifier are passed to the A Trigger Generator through
forward-biased CR372.
Theory of Operation— 2215A Service
CHANNEL 1. For triggering from Channel 1, the A&B INT
switch is set to CH 1. The XY line connected to S555 will be
at ground potential and one input of U555B will be held LO
by CR556. The output of U555B will then also be LO and
the Channel 1 signal path through U310 will be enabled. The
Channel 2 signal path is disabled by the outputs of both
U555C and U565B being HI.
CHANNEL 2. For triggering from Channel 2, the A&B INT
switch is set to CH 2. One input each of U555C and U555D
will be LO and force both gate outputs LO. The LO from
U555C will enable the Channel 2 signal path through U335
and the HI outputs of U555B and U565C will disable the
Channel 1 path.
VERT MODE. When the A&B INT switch is set to VERT
MODE, trigger source selection is determined by the two
VERTICAL MODE switches. For all VERTICAL MODE
switch combinations except BOTH-CHOP, the V Mode T
line is HI. The inputs and outputs of U555B, U555C, and
U555D will all be HI, and triggering selection will then be
determined by the inputs of U565B and U565C that are con­
trolled by U540A in the Channel Switch circuit.
When Channel 1 is selected (VERTICAL MODE switch
set to CH 1), the input to U565C will be HI. The gate output
will be LO and the Channel 1 signal will be selected. The LO
from the other output of U540A is applied to U565B and
causes the Ch 2 Trig line to go HI and the Channel 2 Trigger
signal is disabled.
When Channel 2 is selected (VERTICAL MODE switch
set to CH 2), the outputs of U540A, U565B, and U565C will
be the reverse of the states described for Channel 1 selec­
tion. The Channel 2 signal will be selected and the Channel
1 Trigger signal disabled.
When selecting ALT VERTICAL MODE, the inputs of
U565B and U565C will toggle with each sweep. The outputs
of the two gates will also toggle and the Trigger signal
source will alternate with the displayed channel.
In the ADD VERTICAL MODE position, both inputs to
U565B and U565C will be HI and the gate outputs will be
LO. Both Channel 1 and Channel 2 signal paths will be en­
abled and their output current will be summed at the inputs
of the Internal Trigger Amplifier to produce the internal trig­
ger signal.
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