Tektronix 323, Инструкция по эксплуатации

Страница: 43 / 170

Circuit Description— Type 323
effect, simultaneously changing the base voltages of Q133,
Q137, Q163 and Q173.
Miscellaneous Circuit Components. Cl 02, C l60, C l07
and C l70 insure that the ratio of feedback impedance to
input impedance remains constant regardless of frequency,
to avoid high frequency gain changes which would other­
wise be introduced by stray capacitance. C l62, R162, C l72
and R172 provide a shunt impedance across Q133 and
Q137 to aid in the development of high frequency drive
signals to Q160 and Q170.
TRIGGER GENERATOR 3
General
The function of the Trigger Generator is to develop trig­
gers to initiate horizontal sweeps. In EXT HORIZ mode,
part of the Trigger Generator circuit processes external
horizontal signals for application to the Horizontal Ampli­
fier.
Block Diagram Description
Refer to the block diagram contained on the Trigger
Generator schematic diagram page. Signals from the Ver­
tical Output Amplifier or the EXT TRIG OR HORIZ INPUT
are applied to a protection circuit and then to an FET
Source Follower circuit. Trigger signals pass through the
TIME/DIV switch to a Comparator Amplifier. When the
input signal reaches the voltage level determined by the
(TRIGGER) Level control, the voltage out of the Comparator
Amplifier causes the Trigger Multivibrator to generate a
trigger. The TRIGGER control can be used to select the
direction of voltage change (+ or — Slope) which actually
causes triggers to occur.
When AUTO operation is selected by the TRIGGER con­
trol, the Trigger Multivibrator free-runs at one of 3 fre­
quencies (approximately 30 Hz, 300 Hz, 3 kHz) as determined
by the TIME/DIV control. Triggers occur more often at high­
er sweep rates to maintain a relatively constant trace bright­
ness regardless of sweep rate. A triggering signal whose
frequency is higher than that of the multivibrator will over­
ride the automatic operation and synchronize the multivi­
brator (and therefore the sweep) to the signal frequency.
In EXT HORIZ mode, external horizontal signals pass
through the Source Follower circuit and are routed to the
Horizontal Amplifier by contacts of the TIME/DIV switch.
Protection and Source Follower Circuits
Refer to the Trigger Generator schematic (and to the Tim­
ing Switch schematic as necessary). Circuit operation dur­
ing non-automatic internal triggering will be discussed first.
The output signal from the upper half of the Vertical Ampli­
fier is coupled through the C201-R201 attenuator circuit to
a contact of the Trig/Horiz Coupling switch. With the switch
in either internal position, the signal is applied to C209. The
AC component is developed across R210 and applied
through R212-C212 to the junction of D213, D214 and Q215.
Under normal conditions, only leakage current flows in the
Q215 gate circuit,
so no signal loss occurs across R212.
D213, D214, C212 and R212 provide overload protection for
Q215 during EXT TRIG OR HORIZ operation and have no
effect upon the internal vertical signal applied to Q215.
Source-follower action (comparable to cathode-follower ac­
tion) provide the signal to a contact of the TIME/DIV switch.
In all except EXT HORIZ position, the output of the source-
follower is sent through or around C221 and C223 to the
base of Q231. With the Trig/Horiz Coupling switch
in INT
TRIG AC LF REJ, signals below approximately 30 kHz are
attenuated, to avoid interfering with higher frequency trig­
gering operation. During INT TRIG AC operation, C223 is
bypassed and triggers are generated in response to signal
frequencies as low as 20 Hz.
External triggering can be selected by placing the Trig/
Horiz Coupling switch to either the EXT TRIG OR HORIZ
— AC or DC position. DC triggering is possible only when
the switch is at DC and the TRIGGER control is not at AUTO.
At that time, C209, C221 and C223 are bypassed to per­
mit the EXT DC potential to reach Q231.
The 10X position of the (EXT TRIG OR HORIZ) ATTEN
switch provides a frequency-compensated voltage divider
which increases the EXT TRIG OR HORIZ INPUT operating
range by a factor of 10, without appreciably changing cir­
cuit input impedance.
When the TIME/DIV switch is set at the EXT HORIZ posi­
tion, the output of Source Follower Q215 is disconnected
from the trigger generating circuitry and is routed through
switch contacts to the Horizontal Amplifier. Horizontal de­
flection of the beam then occurs in response to external
horizontal input signals (Trig/Horiz Coupling switch in either
EXT TRIG OR HORIZ position) and the horizontal POSITION
control. R218 permits setting of the voltage at the source
of Q215 so that no beam position shift occurs when rotat­
ing the (EXT HORIZ) VAR control.
Comparator Am plifier
The Comparator Amplifier (Q231, Q239 and associated
resistors) quiescently has the Q231 base referenced to ground
through R230. The Q239 base is set to some voltage level
determined by R246 (TRIGGER LEVEL), R242 and R244. If
R246 is set to a point midway between the center tap and
either side, 0 V will be applied and the two transistors will
be conducting equal current. The collector voltages will be
approximately equal under those circumstances. A signal
input to the Comparator Amplifier will generate an in-phase
signal at the Q239 collector and an inverted signal at the
Q231 collector. Both outputs are made available to con­
tacts of the (TRIGGER SLOPE) switch.
Triggering action occurs when the selected collector varies
approximately —0.1 V from a balanced output condition.
If the TRIGGER LEVEL potentiometer is offset from 0 V, sig­
nals at the Q231 base must compensate for the offset be­
fore causing trigger action. Through use of the TRIGGER
control, both TRIGGER LEVEL and TRIGGER SLOPE can be
manipulated to select any point along the rising or falling
slope of a signal to cause trigger action. If two separate
signals of different amplitudes are presented simultaneously
to the comparator, R246 (TRIGGER LEVEL) can be set to a
point where only the larger of the two signals can cause
triggering action, thereby causing the sweep to be triggered
at the frequency of the larger signal.
3-8