Tektronix 114, Instruction Manual

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SECTION 3
CIRCUIT DESCRIPTION
Introduction
This section contains the theory of operation of the various
circuits in the Type 114. The text is supplemented by two
block diagrams inserted in the text and by schematics in
Section 5. The reader should follow the circuits on the dia ­
grams as they are presented in the text.
CAUTION
There is no fixed chassis ground for the circuit
board used in the Type 114. A common negative
point is used instead. This permits changing the
polarity of the output by grounding the appro­
priate side of the power supply. For this reason,
always connect probe grounds to the main chassis
rather than to a point on the circuit board.
Period Generator
The Period Generator (see Fig. 3-1) supplies the trigger
pulse which activates the Width Generator. The trigger pulse
is generated internally, or is derived from an externally gen­
erated triggering signal, depending upon the setting of the
PERIOD switch.
Internal Operation. When the Type 114 is operating in
the internally triggered modes, the operation of the Period
Generator is as follows:
Transistors Q115 and Q125 in conjunction with the ap ­
propriate RC timing combination form a free running oscil ­
lator. At the start of a cycle of operation, Q 115 is biased off
and Q125 is biased slightly on. The charge on timing ca­
pacitor C 1 15 has been removed by the preceding cycle and
now starts charging toward a common point voltage at an RC
rate. As the timing capacitor charges, the voltage across it
reaches a point where it turns on diode D 1 14 and transistor
Q 1 15. At this instant the circuit becomes regenerative with
Q115 turning Q125 on hard, which in turn biases Q1 15 into
heavy conduction. The heavy conduction of Q 1 15 removes
the charge accumulated on timing capacitor C115 and ends
the cycle.
At the instant Q1 25 is turned on hard by Q 115, the steep
wave-front is coupled through R131 and C1 31 to the base of
Q 134, and thence to pulse transformer T131. The pulse out ­
put of T131 is in the order of 20 nanoseconds in width.
External Trigger Operation. When the Type 114 is op ­
erating in the externally triggered mode, the Pulse Generator
functions as a pulse shaper. Period switch SW120 discon­
nects the base of Q 1 15 from the collector of Q1 25 and re ­
connects it to the external trigger input circuit. SW120 also
connects the base of Q1 25 to +25 volts through R121 and
disconnects the timing capacitor from the circuit. Under
these conditions none of the transistors in the Period Gen ­
erator are conducting. A positive pulse of 2 to 20 volts in
amplitude and having a risetime of 1 microsecond or less
is required at the EXTERNAL TRIGGER—INPUT connector
in order to make the transistors conduct and deliver the
proper pulse to the Width Generator.
When a pulse having the proper amplitude and risetime is
applied to the EXTERNAL TRIGGER — INPUT connector,
Q 1 15 is biased into conduction and in turn biases Q1 25
and Q134 into conduction. The resulting pulse at the pri­
mary of T131 has a risetime of about 10 nanoseconds.
Width Generator
The Width Generator receives the trigger pulses from
T131 and generates pulses of the desired width. The out­
put of the width generator is applied to the Trigger Out
Circuit and to the Output Amplifier. Except when operat ­
ing in the square wave mode, the Width Generator
operates as a monostable multivibrator; in the square wave
mode the Width Generator functions as a bistable.
Normal Operation. In normal operation transistors
Q205 and Q215 form a bistable network whose output
drives ramp transistor Q224, trigger output transistor Q234,
and the output amplifier. Q205 is the normally “on" tran­
sistor. The arrival of a negative trigger pulse from T131
cuts off Q205. Q215 now conducts and cuts off Q224,
permitting the voltage at the collector of Q224 to start
charging ramp capacitor C195. When the selected ramp
capacitor reaches the desired voltage, D 1 93 is forward
biased and transistor Q 1 94 is biased into conduction. The
conduction of Q1 94 increases the conduction of Q1 84 and
thereby biases Q205 back into conduction. The conduction
of Q205 ends the output pulse. Turning on Q205 turns
off Q215 and turns on Q224. The conduction of Q224 dis­
charges the ramp capacitor.
When the ramp capacitor is discharged down to about
0.3 volt, Q174 turns off and Q164 is turned on. Transistors
Q1 64 and Q1 74, diodes D1 36 and D 161, and pulse trans ­
former T131 function as a pulse steering circuit. With Q1 64
on, the pulse from the Period Generator via T131 triggers
the width monostable circuit on, generating an output pulse.
As soon as the ramp capacitor reaches approximately 0.3
volt, Q174 conducts and turns Q164 off. Turning off Q164
reverse biases D 1 61, while turning on Q1 74 biases D1 36
near zero. If the Period Generator produces a pulse while
these conditions exist, that is, whenever the ramp capacitor
has a charge above approximately 0.3 volt, the pulse is
steered to the WIDTH > PERIOD light circuit. This mono­
stable circuit is actuated by the pulse from T131, turning
off Q145 and turning Q155 on, lighting the WIDTH >
PERIOD light. Turning on Q1 55 causes C1 55 to discharge
through T131 and R136. When C155 is discharged, D141
and Q 145 conduct and reset the circuit, making it ready
for another pulse.
Square Wave Mode. When the Type 114 is operated
in the Square Wave Mode (see Fig. 3-2), the Width Gen­
erator operates as a bistable and requires two trigger pulses
from the Period Generator for each cycle of operation.
Consequently, the RC network in the Period Generator is
3-1