Tektronix 7B51 Instruction Manual page 36

The Disconnect Amplifier is a low leakage transistor to
improve timing at low sweep speeds and high temperatures.
At the end of the sweep, 0526 turns on, discharging the
Timing Capacitor. CR526 provides isolation between the
Disconnect Amplifier and the input emitter-follower during
holdoff time.
Sawtooth Sweep Generator. The basic sweep generator
circuit is a Miller Integrator Circuit.
l
The current flow
through the Disconnect Amplifier is interrupted by the sig-
nal from the Sweep Gate Stage.
Timing Capacitor C530 now begins to charge through
Timing Resistor R530. The Timing Resistor and Timing
Capacitor are selected by the TIME/DIV switch (a front
panel control) to provide sweep rates as listed on the front
panel.
As the Timing Capacitor begins to charge toward the
voltage applied to the Timing Resistor, the gate of 0532A
(which is tied to the junction of R530-C530l, goes negative
by the same amount. This decreases the current through
0532A, forcing more current through 0532B. This increase
in 0532B drain current flowing through R534 gives a
negative-going voltage at the base of 0534. This causes its
collector voltage to pull up. The positive side of the Timing
Capacitor, being tied to the collector of 0534, is also pulled
up. This action results in a positive-going voltage being ap-
plied across the Timing Capacitor, maintaining a constant
charging current and giving a linear rate of rise to the saw-
tooth output signal. The sawtooth output signal continues
to rise in this manner until the Sweep Gate Multivibrator is
switched to turn on the Disconnect Amplifier.
The sawtooth output from the collector of 0534 is con-
nected to the Horizontal Preamp, the Delay Pickoff circuit,
and through R539 to the Sweep Start Amplifier, Sweep
Stop Comparator, and through amplifier 0546 to the Indi-
cator Oscilloscope. CR534 protects against reverse emitter-
base breakdown of 0534. CR529 protects against gate-
source breakdown of 0532A.
Sweep Start Amplifier. Delay start adjustment R590 ad-
justs the base voltage of 0592A to set the quiescent voltage
of the sawtooth. The collector current of 0592A, flowing
through R523, establishes the correct starting point of the
sawtooth output signal. L593-R593 prevent oscillations at
the faster sweep rates. They are shorted out at the slower
sweep rates. CR596 A and B prevent base-emitter break-
down of 0592 A and B.
The sawtooth output signal is applied through R539 to
the base of 0592B. This positive-going signal causes 0592B
to turn on harder, pulling down its collector voltage.
1
MI LLMAN & T AUB, pp 540-548
Circuit Description-Type 7B51
CR597 and CR598 switch the current in R596 to 0592B to
make a more definite step. The negative-going voltage at the
collector of 0592B is applied through current-limiting
diode CR581 to the base of 0582 to charge the holdoff
capacitor.
Sweep Stop Comparator. The Sweep Stop Comparator
is used to terminate the sawtooth sweep when the sweep
has reached a predetermined length. Ouiescently 0406 is
biased off by voltage divider, R404-R405. These resistors
set the reference level for sweep termination at approxi-
mately +10.5 volts. When the sawtooth sweep rises to about
+10.5 volts, 0408 is turned off. 0406 is turned on and the
positive-going rise at the base of 0436 switches the reset
multivibrator. The voltage at the collector of 0436 goes
negative and this negative gate applied to the sweep gate cir-
cuit ends the sweep.
Holdoff Capacitor and Amplifier. 0582, which is turned
on during sweep time, charges the holdoff capacitor. When
the sweep ends, 0582 turns off. Now the holdoff capacitor
starts to discharge toward' -50 volts through R582, and at
some sweep speeds, through R580. When the top of C580
discharges enough in the negative direction to forward-bias
CR574 and CR579, the holdoff capacitor voltage is applied
to the base of 0554. This drives 0554 to cutoff, switching
the holdoff multivibrator to end holdoff time. Voltage
limiting diodes CR580 and CR582 protect the base-emitter
junction of 0582. CR581 limits the "on" base current of
0582. Capacitor C582 and the current through R581 deter-
mine the rate of charge of the holdoff capacitor, C580.
VR576 and CR578 limit the negative voltage on the hold-
off capacitor, C580.
The HF STAB control R260B through R583 varies the
discharge rate of the holdoff capacitor to provide a stable
display at fast sweep rates. This change in holdoff allows
sweep synchronization ·for less display jitter at the faster
sweep rates. The H F STAB circuit has little effect at slow
sweep rates.
Sweep Holdoff Multivibrator. 0554-0564 are connec-
ted in a bistable Schmitt Trigger configuration. Ouiescently,
0554 is off and 0564 is on. A positive pulse from the
sweep reset multivibrator switches the holdoff multivibra-
tor to its second stable state, 0554 on and 0564 off.
The sweep holdoff multivibrator is returned to its first
stable state when the holdoff capacitor has charged negative
enough to drive 0554 to cutoff. The fast switching action
of the sweep holdoff multivibrator produces sharp leading
and trailing edges for the sweep holdoff gate. The output of
the multivibrator, coupled through emitter-follower 0566,
goes to the indicator oscilloscope and to the holdoff discon·
nect diode.
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