Tektronix T922 User Manual

Circuit Description-T921/T922/T922R

VII . 1
TRIGGER
CIRCUIT DESCRIPTION
(For T921 SN B010130-up, T92213010170-up & T922R
all SN)
Digital logic devices are used to perform some of the functions in this instrument . LO and HI designations
are used in this
circuit description to indicate the state of the digital circuit .
HI
indicates the more
positive
of
the two levels . The specific voltages
that constitute a LO and HI logic state may vary between individual devices .
INPUT AND SWITCHING

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
The SOURCE switch, S2100, selects trigger signals from INT,

SEC/DIV settings of
.1 ms/div or slower, the sweep is triggered by
LINE, X-Y, EXT, and_ EXT .

vertical sync pulses which occur at a field rate . For SEC/DIV
10

settings of 50Ns and faster, the sweep is triggered by the TV
horizontal line signals .
SOURCE Switch
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 . Externally applied signals pass through buffer amplifier
02104-02106
.
FET 02104
is
a source follower and 02106 is a
current source . Transistor 02108 and R2107 convert the voltage
signal at the buffer output to a current for application to the
trigger input amplifier . When EXT is selected, the collector of
02108 is connected to the trigger input amplifier (U2126B) by
SOURCE switch S2100 .
_EXT
10 . The output of buffer amplifier 02104-02106 is con-
nected to R2111 . Resistor R2111 converts the buffer output
voltage to a current that is connected to the trigger input amplifier
via S2100 .
X-Y . Signals from the X (EXT) input are routed to the horizontal
amplifier via 02104, S2100, and CR2186 . The SOURCE switch
connects +8 volts to R2184, 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 .
MODE Switch
The MODE switch (S2150) selects three triggering modes :
AUTO, NORM, and TV .
AUTO .
Allows the sweep
to
free run in the absence
of
a
triggering signal . See sweep circuit description for details .
NORM
. Connects +8 volts to R2223 in the sweep generator
circuit . Allows the
sweep to run
only
when a suitable
triggering
signal is present .
REV . BJUL 1977
TRIGGER
INPUT AMPLIFIER
The trigger input amplifier consists of U2126A,
U2126B,
02124, and associated circuitry . Resistors R2127 and
R2128 set
the amplifier input at -4 volts . The
inverting configuration and
feedbackfrom the emitter of
02124 form an inverting operational
amplifier with a null
point
at
the base of U21266 . Any current into
the null point produces a voltage at the output proportional to the
feedback resistor, R2123 . 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 R2138 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 yThe 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 TV
trigger input amplifier U2126E and 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 .
TV TRIGGER CIRCUIT
The TV trigger circuit comprises U2126E, 02174, 02176,
U2156B, C, D, and associated circuitry . When the MODE
switch
is
in TV, R2161 is disconnectedfrom +8 volts, which
allows trigger
signals from the collectors of 02134 and 02136 to
pass through
the TV trigger circuit . (in AUTO
& NORM, the +8 volts applied to
R2161 biases U2126E to
saturation .)

Tektronix T922 Specifications

General

Brand	Tektronix
Model	T922
Category	Test Equipment
Language	English