Tektronix 323, Инструкция по эксплуатации

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Circuit Description— Type 323
Trigger Multivibrator
Non-automatic Operation. The Trigger Multivibrator is
a Schmitt Trigger circuit when operated in the non-automatic
mode. When Q253 is conducting, the current through R260
and that through the voltage divider connected to the base
of Q263 create a combination of voltages which prevents
Q263 from conducting. When the output voltage from the
comparator decreases, the Q253 emitter voltage decreases
and the collector voltage increases. Q263 is thereby per­
mitted to go into conduction. Q263 emitter voltage rises
and Q253 cuts off. The sudden increase of Q253 collector
voltage is coupled through R256 and C256, aiding Q263
conduction. The current increase through R262 creates a neg­
ative step which is differentiated and applied to the Sweep
Gentralor circuit to develop a negative trigger which in­
itiates a horizontal sweep. When the Q253 base voltage
returns sufficiently positive, Q253 goes back into conduc­
tion, cutting Q263 off. The circuit is then ready for another
cycle.
Automatic Operation. When the TRIGGER control is
switched to either the - f or — AUTO position, the follow­
ing circuit changes are made: wafer IF inserts C250 in the
Comparator Amplifier output signal path, causing the Q253
DC base voltage to be determined by R251, R252 and R253;
wafer 2R inserts C221 in the triggering signal path, placing
the Q231 base at ground potential; wafer 1R connects the
base of Q239 to ground, simultaneously inserting one of the
Trig Auto Caps into the base and collector circuit of Q263.
The Trig Auto Cap value is dependent upon the position
of the TIME/DIV switch.
In AUTO TRIGGER mode, the Schmitt trigger circuit be­
comes a free-running multivibrator which will synchronize
to any triggering signal having a frequency greater than
the multivibrator repetition rate. In the absence of trigger­
ing signals from Q231, operation occurs as follows: Assume
that Q253 is conducting, Q263 is cut off, and that C270
has no charge on it. Circuit design causes the junction of
R257 and R258 to go slightly positive, charging C270. The
voltage at the base of Q263 increases in proportion to the
voltage "ramp" at the C270-R257-R258 junction, until Q263
is turned on. This increases the voltage at the emitter, turn­
ing Q253 off. The rise of Q253 collector voltage is coupled
through C256 and R256, aiding Q263 conduction. The re­
sulting current lowers the voltage at the collector of Q263,
sending a negative gate to C301 in the Sweep Generator.
The negative gate voltage is also coupled through R264,
causing C270 to discharge. As C270 discharges, its negative­
going voltage ramp is coupled through R257, decreasing the
Q263 base voltage. The Q263 emitter voltage follows the
base, carrying the Q253 emitter with it. Q253 conducts
when its emitter becomes sufficiently negative. The resulting
change of Q253 collector voltage is coupled through R256,
cutting Q263 off. The cycle then repeats itself.
If a signal is coupled in through C250 during AUTO oper­
ation, it will either combine with the voltage ramp to cause
switching action, or it will override the ramp and cause
switching action by itself.
Consider the AUTO condition existing when Q263 is cut
off. A positive-going ramp occurs at the base of Q263. If
a negative signal simultaneously appears at the base of
Q253, it will be coupled to the emitter, lowering the Q263
emitter voltage. The positive ramp at the base and the neg­
ative signal at the emitter combine their effects to increase
the emitter-base forward bias, placing Q263 into conduc­
tion. In similar fashion, when Q253 is turned off, a positive
signal at its base will work in conjunction with the negative
ramp at its emitter to turn Q253 on. It should be noted
that if both the ramp and the input signal are required to
produce switching action, the switching rate will not be much
greater than the AUTO frequency, although it will be syn­
chronized to the signal frequency or a sub-multiple of it.
This situation occurs when trigger inputs are less than those
specified under Trigger Sensitivity at the beginning of this
manual.
If the signal in from the comparator has a higher fre­
quency than the AUTO multivibrator, and has sufficient am­
plitude to override the ramp voltage, its effect alone will
cause the previously explained switching action, creating
negative gates at the frequency of the input signal.
Smaller Trig Auto Caps are substituted when the VOLTS/
DIV switch is changed from the .5 to .2 mS positions and
from the 50 to 20 /tS positions, thus increasing the AUTO
repetition rate of the multivibrator. Changing the AUTO
repetition rate keeps the sweep intensity relatively constant
despite changes in sweep rate.
SWEEP GENERATOR <§>
General
The basic purpose of the Sweep Generator is to provide
a linear sawtooth voltage to the Horizontal Amplifier. It
also controls the minimum time between sweeps and pro­
vides unblanking to the cathode-ray tube electron beam dur­
ing sweep time. When EXT HORIZ operation is selected, the
Sweep Generator stops generating sweep voltages and
provides continuous unblanking to the cathode-ray tube.
The unblanked state can be interrupted by application of
an external blanking signal.
Block Diagram Description
Refer to the block diagram contained on the Sweep Gen­
erator schematic diagram page. Triggers from the Trigger
Generator are received through C301 and applied to the
Sweep Gate circuit, which then develops a negative gate.
As a result, the Disconnect Diode stops conducting. This
allows the Miller Circuit to create a linear sawtooth volt­
age which is sent to the Horizontal Amplifier. When the
sawtooth has sufficient amplitude to provide full horizontal
trace deflection, feedback current through the SWEEP
LENGTH potentiometer is sufficient to reset the Sweep Gate
circuit. The disconnect diode then conducts and the sweep
voltage rapidly decreases to its initial value, causing retrace
to occur.
The sweep must start at the same DC quiescent voltage
level for each sweep, or horizontal jitter will appear. The
same signal that causes retrace is therefore sent to the
Holdoff Circuit to block triggers from the Sweep Gate until
the Sweep Generator circuit has stabilized. The holdoff
time is controlled by capacitors which are selected by the
various positions of the TIME/DIV switch.
The electron beam is allowed to strike the face of the
cathode-ray tube only during sweep time by connecting the
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