Normal Trigger Mode Operation - Tektronix 7B70 Time Base Instruction Manual

Circuit Description-7B7O (SN B205000 and Up)
Normal Trigger Mode Operation
Disconnect Amplifier. The Disconnect Amplifier, Q526,
is quiescently conducting current through R523 and the
Timing Resistor, R530. The negative-going step from the
Sweep Gate Multivibrator is applied to the base of the input
emitter-follower, Q524. Q524 couples the negative step to
the emitter of Disconnect Amplifier Q526, turning i t off.
Sweep Start Amplifier. Q592A
sets
the quiescent voltage
of the sawtooth. The collector current of Q592A; flowing
through R523, establishes the correct starting point of the
sawtooth output signal. L593-R596 prevent oscillations
a t
the faster sweep rates. They are paralleled with R592 a t the
slower sweep rates. CR594 and CR592 prevent base-emitter
breakdown of Q592A and Q592B.
The timing current through Timing Resistor R530 now
starts to charge Timing Capacitor C530, enabling the Time-
The sawtooth signal i s applied through R539 to the base
Base Generator.
of Q592B. This positive-going signal causes 0594 to turn
on, pulling down .the collector voltage of Q592B and Q594.
The Disconnect Amplifier is a low-leakage transistor to
improve timing at low sweep speeds and high temperatures.
The negative-going voltage at the collector of Q592 is
~t
the end of the sweep, 0526 turns on, discharging the
applied through current-limiting diode CR581, to the base
Timing Capacitor. CR525 provides isolation between the
of Q582 to charge the holdoff capacitor.
Disconnect Amplifier and ,the input emitter-follower during
holdoff time.
Sawtooth Sweep Generator. The basic sweep generator
circuit is a Miller Integrator circuit.' The current flow
through the Disconnect Amplifier is interrupted by the sig-
nal from the Sweep Gate Stage. The 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 a s listed on the front panel.
As the Timing Capacitor begins to charge toward the
voltage applied to the Tirrling Resistor, the gate of Q532A.
which is tied to the junction of R530-C530, goes negative
by the same amount. This decreases the current through
Q532A. forcing more current through Q532B. This increase
in Q532B drain current flowing through R534 gives a
negative-going voltage a t the base of 0533. This causes the
collector voltage of Q533 to pull up. The positive side of
the Timing Capacitor, being tied to the collector of (2534,
i s also pulled up. This action results in a positive-going
voltage being applied across the Timing Capacitor, main-
taining a constant charging current and giving a linear rate
of rise to the sawtooth output signal. The sawtooth output
signal continues to rise in this manner until the Sweep Gate
Multivibrator i s switched to turn on the Disconnect
Amplifier.
Sweep Stop Comparator. The Sweep Stop Comparator
is used to terminate .the sawtooth sweep when the sweep
has reached a pre-determined level. The circuit components
are chosen to set the reference level for sweep termination
at approximately 10.5 volts at the base of Q408. The
positive-going sawtooth applied to the base of 0408 in-
creases the current through R408-R409. The base of Q406,
which is tied to the junction of R408-R409, rises an
amount corresponding to the sawtooth amplitude at the
base of Q408. This action continues until the emitter of
Q406 becomes positive enough to forward bias CR406.
When CR406 turns on, the positive rise at the base of Q436
switches the reset multivibrator. The voltage at the collec-
tor of Q436 goes negative and this negative-going step
applied to the sweep gate circuit ends the sweep.
Holdoff Capacitor and Amplifier. (3582, which is turned
on during sweep time, charges the holdoff capacitor. When
the sweep ends, (3582 turns off. Now the holdoff capacitor
starts to discharge toward -50 volts through R582 and at
some sweep rates, through R580. When the top of C580
discharges negative enough to forward-bias CR574 and
CR575, 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
Q582. CR581 limits the "on" base current of (2582.
VR576 and CR576 limit the negative voltage on holdoff
capacitor C580.
The sawtooth output from the collector of 0534 is con-
nected to the Horizontal Preamp, and through R539 to the
Sweep Start Amplifier, Sweep Stop Comparator, and
The HF STAB control, R146B, through R583, varies the
through amplifier Q546 to the Indicator Oscillosco~e-
discharge rate of the holdoff capacitor to provide a stable
CR528-CR529 Protects against gate-source bt~akdown of
display at fast sweep rates. This change in holdoff allows
Q532A.
sweep synchronization for less display jitter at the faster
sweep rates. The HF STAB circuit has little effect at slow
'MILLMAN & TAUB
pp 540-548.
sweep rates.
Scans by ArtekMedia O 2006