Trigger Circuits<Y; Trigger Preamp - Tektronix 7D11 DIGITAL DELAY Service Instructions Manual

Theory of Operation-7D11 Service
THEORY OF OPERATION
The following theory of operation discussion is refer-
enced to the schematic diagrams in the diagram section of
this manuaL Each main topic heading is followed by the
number of the schematic to which it applies.
TRIGGER CIRCUITS0
The trigger circuit consists of two main sections, the
trigger pream p and the trigger generator.
Trigger Preamp
The trigger preamp serves to select trigger source and
coupling for the trigger generator. This circuit may be
considered as consisting of the following four elements:
Trigger Source Switching, U60; External Trigger Preamp or
external input amplifier, 032, 037, and 041; Balanced-to-
Single-Ended Converter, 071, 075, and 078; and Trigger
Coupling, 082, 084, and 086.
Trigger Source Switching. U60 receives internal trigger
inputs at pins 2 and 15 and external trigger signals at pin 7.
U60 determines which input signal is selected by means of a
digital signal (voltage level) at pin 4. A LO on pin 4
activates pins 2 and 15 for internal triggering, while a HI on
pin 4 switches U60 to activate pins 7 and 10 for external
triggering.
To further examine U60, assume that pin 4 is low,
activating pins 2 and 15 for internal triggering. This input is
a relatively high impedance differential configuration. Pin
15 receives the positive-going trigger signal and pin 2 is the
negativeijoing input. The inputs are biased at the center of
their dynamic range and signal limiting in the trigger
pickoff circuitry (in the indicator oscilloscope) ensures that
the inputs will not be driven into cutoff or saturation. R55
and R57 terminate the internal trigger signal from the
indicator oscilloscope. The analog current source for
internal triggering is through pins 1 and 16.
The switch output current appears at pins 12 and 13. A
positive-going signal at pin 15 will cause an increase in
current into pin 13 and out through pin 16, R66, and R69.
Simultaneously, the negative-going signal at pin 2 causes a
decrease in current into pin 12 and out through pin 1, R68,
and R69. The net result is that the total current through
pins 12 and 13 and through R69 remains constant.
External Trigger Preamp. This circuit includes 032,037
and 041. The SOU RCE switch (S5) at the input selects
internal, external, or line signals for triggering. The external
trigger signal may be attenuated to one-tenth amplitude by
2-2
selecting EXT 710. R6 and R7 (paralleled by R30) form a
10: 1 attenuator.
The input impedance for the trigger input is one
megohm, consisting primarily of R 12 and R30. This resistor
pair also causes a 2X attenuation of the input signal as seen
at the gate of 032 A and B. C24 serves to compensate the
input stage and Cl0 compensates the lOX attenuator.
CR27 and CR28 protect 032 from excessive input signal
by clamping the gate if the signal at the input connector
exceeds approximately plus or minus 2.5 volts. The signal
at the source of 032 is coupled through emitter follower
037 to the base of 041. 041 is another emitter follower,
which drives U60. The signal at pin 7 of U60 is terminated
in approximately 50 ohms by R46 to preserve the
high-frequency characteristics.
R49 sets the DC level at pin 10 of U60, which is the
negative side of the external trigger differential input. This
serves to match the DC balance of the external trigger input
of U60 to that of the internal trigger input.
Balanced-To-Single-Ended Converter. 071, 078, and
075 convert the balanced (push-pull) output of U60 to a
single-ended signal at the emitter of 075.
The trigger signal through U60 causes a decrease in
current into pin 12 from R77 and R78 and an increase in
current into pin 13 from R71. This would normally cause
the voltage at pin 12 to swing in a positive direction, while
pin 13 goes in a negative direction. However, the current
through R77 and R78 actually increases due to the
feedback via R79 and 078, causing the voltage at pin 12 to
swing negative along with pin 13. 078 is connected as a
diode and is enclosed in the same heat-sink with 071,
provid i ng good DC stab i I ity .
Trigger Coupling. When DC coupling is selected by the
front panel COUPLING switch, 086 is turned on by the
+15 volt supply through R92, S95, and R86 to its base. The
triggering signal is then coupled through R80 and 086 to
the base of 0100.
084 is turned on when AC coupling is selected. The
triggering signal then passes through 084 and C87 to the
base of 0100. For AC LF REJ coupling, 084 is off and the
triggering signal is coupled through C88 and C87 to
attenuate low frequency signals.
For AC H F REJ coupling, both 084 and 082 are turned
on. The high-frequency components are coupled through
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