Theory of Operation— 2213 Service
Internal Trigger Amplifier
The Internal Trigger Amplifier converts the differential
current input from the Trigger Pickoff circuit to a zero-
referenced, single-ended output for use by the Trigger Level
Comparator. Differential signals from the Pickoff Amplifier
circuit are connected via R421 and R422 to common-base
transistors 11421E and U421D respectively. Transistor
U421C and R428 constitute an inverting-feedback amplifier
that converts U421D collector current to a voltage at the
collector of U421C. This voltage is added in phase with the
voltage drop across R427 produced by the signal current of
U421E. The resulting sum is a single-ended voltage signal
that is applied to the base of emitter-follower U421 A. The
emitter-follower stage provides a low-output-impedance
signal source that drives both the XV Amplifier (through
R701) and emitter-follower U421B. The output signal from
U421B is applied to the Trigger Source Switching Diode
circuit at the cathode of CR440 where it is available for
selection as the triggering signal.
Trigger Source-Switching Circuit
Trigger signal selection is accomplished by using the
SOURCE switch (S440) to enable one of three triggering
signal paths (internal, external, or line) to the Trigger Level
Comparator circuit. With S440 set to INT, the inhibiting
voltage is removed from R438, causing both U421B and
diode CR440 to be biased on. The internal trigger signal is
then passed from the emitter of U421B through diode
CR440 to the Trigger Level Comparator and Auto Trigger
circuits. The SOURCE switch prevents the line and external
triggering signals from reaching the Trigger Level Com
parator by reverse biasing diodes CR444 and CR448
and also by reverse biasing Q414 and CR418 through R417.
When S440 is set to LINE, U421B and CR418 are biased
off through R438 and R417 respectively, while CR444 is
enabled by removal of the inhibiting voltage from R446.
Similarly, with S440 set to EXT, the external trigger signal
is selected by biasing off CR444 and U421B through
resistors R444 and R438 respectively and by enabling Q414
through the removal of the inhibiting voltage from R417.
External Trigger Amplifier
The External Trigger Amplifier provides a means of
triggering the instrument from an externally supplied signal
that can be applied to the EXT INPUT connector. Input
coupling to the Amplifier is selectable by the three-position
EXT COUPLING switch, S401. In the AC position, the
dc component of the external trigger signal is blocked by
coupling the signal through C402. In the DC position, all
components of the signal are coupled directly to the gate
of Q411A through an input divider composed of R404 and
R408. Resistors R402 and R403 form a voltage-divider
network that attenuates the signal by a factor of 10 when
ever S401 is set to DC-MO.
Field-effect transistors Q411A and Q411B are a matched
pair. Source-follower Q411A provides a high input
impedance for the external trigger signal. Current-source
transistor Q411B causes Q411A to operate at zero gate-to-
source bias, so the device functions with no dc offset
between the input and output signals. The output signal
from the source of Q411A drives the base of emitter
follower Q414. The emitter-follower stage lowers the
output impedance of the Trigger Amplifier and functions
as part of the Trigger Source Switching circuitry.
Auto Trigger Circuit
When either AUTO or TV FIELD triggering is selected,
the Auto Trigger circuit detects positive and negative peaks
of the input trigger signal and produces output voltages
that set the TRIGGER LEVEL control range to within
the peak-to-peak amplitude of the triggering signal. The
peak detectors are disabled when S611 is set to NORM, and
fixed voltage levels are applied to both ends of TRIGGER
LEVEL potentiometer R455.
In either AUTO or TV FIELD, the TRIGGER MODE
switch (S611) opens the Auto Disable signal line to allow
CR503 and CR504 to become reverse biased. This action
isolates the voltage divider network (composed of R525,
R527, R528, R526, and Q519) from the + inputs of
U507A and U507B. The peak detectors (composed of
Q503 for the positive peak and Q504 for the negative peak)
become enabled when the Auto Disable signal is removed.
The trigger signal is applied to the bases of Q503 and
Q504 via R444. Positive trigger signal peaks bias Q503 into
conduction, causing its emitter voltage level to rise to the
peak level of the trigger amplitude minus the base-to-
emitter voltage drop.
Capacitor C503 charges up to the positive emitter
voltage level. The charge is retained between trigger pulses
due to the long RC time constant of R505 and C503. The
comparator voltage is applied to U507A pin 3 which is a
voltage follower and level shifter that sets the voltage at one
end of the TRIGGER LEVEL potentiometer (R455).
Transistor Q507 provides the feedback path for U507A
and thermally compensates for Q503. The base-to-emitter
drop of Q507 corrects for the dc offset introduced by
Q503, and potentiometer R511 is adjusted to balance out
dc offsets introduced from the trigger circuitry.
The negative peak detector operates in the same manner
as the positive peak detector, with corresponding
components performing the identical circuit function on
the trigger-signal negative peaks.
@
3-13
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