Tektronix T921 Instruction Manual page 152

Circuit Description-T921/T922/T922R
TRIGGER CIRCUIT DESCRIPTION
(For T921 Below SN B010130 and T922 Below SN B010170)
Digital logic devices are used to perform some of the functions in this instrument . LO and HI designations are used in this
circuit desciption to indicate the state of the digital circuit . HI indicates the more positive of the two levers . The specific voltages
which constitute a LO and HI logic state may vary between individual devices .
SOURCE Switch
The SOURCE switch, S2100, selects trigger signals from INT,
LINE, X-Y, EXT, and_
EXT .
10
INT. Signal from the trigger pickoff circuit in the vertical
amplifier is connected to the trigger input amplifier U2126B .
LINE . A sample of the line voltage, obtained from the power
transformer, is connected to the trigger input amplifier U2126B .
EXT. External trigger signals applied to J2110 external trigger
(X) input are connected to the trigger input amplifier, U2126B, via
input buffer amplifier 02114-02116 and coupling capacitor
C2123 . Diode CR2116 protects 02114 from large negative-going
trigger signals . (The 02114 gate-drain junction provides protec-
tion from large positive-going signals by limiting the 02114 gate
voltage to about +8 volts .) Transistor 02116 provides a relatively
constant current source for 02114 . Resistor R2111 sets the
external trigger sensitivity and converts the voltage signal to a
current signal .
EXT
MODE Switch
REV. B JUL 1977
INPUT AND SWITCHING NORM . Connects +8 volts to R2223 in the sweep generator
circuit. Allows the sweep to run only when a suitable triggering
signal is present .
10 . Resistors R2119 and 82120, with R2111, form a current
divider to attenuate the external trigger signal by a factor of 10 .
Diodes CR2115 and CR2186 form a current switch that prevents
external trigger signals from coupling into the horizontal
amplifier in EXT and _EXT .
10
X-Y. Signals from the X (EXT) input are routed to the horizontal
amplifier via 02114, S2110, and CR2186 . The SOURCE switch
connects +8 volts to 82184, forward biasing CR2186 and
providing current to the horizontal amplifier to center the crt
display . Also, +8 volts is applied to CR2182 and R2134 to switch
the sweep into the NORM mode and to disable the trigger circuit .
The MODE switch S2150 selects three triggering modes :
AUTO, NORM, and TV .
AUTO . Allows the sweep to free run in the absence of
triggering signal . See sweep circuit description for details .
TV . The trigger signal is applied to the TV sync separator
circuit (U2126E, 02174, 02176) and the normal trigger generator
circuit is disabled . In this mode, the SLOPE switch is used to
supply the sync separator with a signal of the proper polarity . For
SEC/DIVsettings of .1 ms/divor slower, the sweep is triggered by
vertical sync pulses which occur at a field rate . For SEC/DIV
settings of 50 Ns and faster, the sweep is triggered by the TV
horizontal line signals .
TRIGGER INPUT AMPLIFIER
The trigger input, amplifier consists of U2126A, U2126B,
02124, and associated circuitry . Resistors R2127 and 82128 set
the amplifier input at -4 volts . The inverting configuration and
feedback from the emitter of 02124 form an inverting operational
amplifier with a null point at the base of U2126B . Any current into
the null point produces a voltage at the output proportional to the
feedback resistor 82123 . Diode CR2124 prevents the emitter of
02124 from going below ground and reversing the voltage across
C2132 .
TRIGGER LEVEL COMPARATOR
Differential amplifier 02134-02136 functions as a comparator .
The LEVEL control 82138 selects the point on the waveform that
starts a sweep . Capacitor C2132 couples the trigger signal to the
comparator . As the trigger signal at the base of 02134 passes
through the same voltage level as the base of 02136 (set by
LEVEL control), the signal at the collector of U2126D passes
through the threshold (about 1 volt) of Schmitt trigger U2156A-
U2156B producing a logic trigger signal . The trigger signals at the
collectors of 02134 and 02136 are of opposite polarity . This
allows the SLOPE switch S2140 to invert the signals applied to the
TV trigger input amplifier U2126E and the normal trigger input
amplifier U2126D, C . When the SLOPE switch is in the +OUT
position, the output at the collector of U2126D is in phase with the
trigger source signal . Transistors U2126D and U2126C convert
the current signal from the collector of 02134 or 02136 to a
voltage signal for triggering the Schmitt trigger.