Tektronix 453 User Manual

changed by the A TIME/DIV switch for the various sweep
rates to provide he correct holdoff time. Diagram 12 shows
a complete diagram of the A
TIME/DIV switch.
As the A SWEEP LENGTH conrol is rotated counterclock-
wise
from the FULL position, R555 place as more positive
level on the anode of D556 than is on the anode of D555
so D555 remains reverse biased. The Sweep Reset Multi-
vibrator is reset as described for FULL sweep length opera-
tion
at the point where D556 (instead of D555) is forward
biased. Since
this occurs at a more positive level on the
negative-going sawtooth, the displayed sweep
is shorter.
Thus, R555 provides a variable sweep length for the A Sweep
(from about 11 divisions in the FULL position to about four
divisions
in the fully clockwise position-not in B ENDS A
detent).
In the B ENDS A position (fully counterclockwise),
a negative-going pulse
from the B Sweep Generator circuit
is connected to the base of Q575 through D575 at the end
of the B sweep time.
If the A sweep is still running, this nega-
tive-going pulse turns Q575
on to end the A sweep also.
Since the A sweep ends immediately following the end of
the B sweep,
this position provides the maximum repetition
rate (brightest trace) for Delayed Sweep mode operation.
The HF STAB control, R551, varies the charging rate of
the Holdoff Capacitor to provide a stable display
at fast
sweep rates.
This change in holdoff allows sweep synchroni-
zation for
less display jitter at the faster sweep rates. The HF
ST AB control has little effect at slow sweep rates.
Lam,p Driver. The auto gate level from the Auto Multi-
vibrator stage in the A Trigger Generator circuit is con-
nected to the
Lamp Driver stage, QS94, through D591 and
D594. This gate level is coincident with the trigger pulse
-' generated by the A Trigger Generator circuit and is present
only when the instrument
is correctly triggered. The positive-
going auto-gate
level saturates Q594 and its collector goes
negative to about zero volts.
This applies about 12 volts
across
B596, A SWEEP TRIG'D light, and it comes on. This
light remains on as long as the auto-gate level is present.
When the auto-gate
level goes negative because the instru-
ment
is no longer triggered, D595 clamps the base level of
Q594
at about -0.5 volt and Q594 is reverse biased. The
collector of Q594 rises positive and B596 goes off.
Auto Trigger Mode Operation
Operation of the A Sweep Generator circuit in the AUTO
TRIG position of the A SWEEP MODE switch, is the same
as for the
NORM TRIG position just described when a trigger
pulse
is applied. However, when a trigger pulse is not
present, a free-running reference trace
is produced in the
AUTO TRIG mode. This occurs as follows:
The auto-gate level from the Auto Multivibrator stage in
the A Trigger Generator circuit is also connected to D592.
When the auto-gate level is positive (triggered), the current
flowing through
D592 and R593 reverse biases D593 and the
Sweep
Gate tunnel diode, D505, operates as previously
described for
NORM TRIG operation. However, when the
instrument
is not triggered, the auto-gate level drops nega-
tive and the reduction
in current through D592 and R593
allows D593 to become forward biased. Now, when the
Sweep
Reset Multivibrator stage resets at the end of the
holdoff period, the additional current
from R593-D593
flows through DSOS and is sufficient to automatically switch
®
Circuit Description-Type 453/R453
the Sweep Gate tunnel diode back into its high-voltage state.
The result is that the A Sweep Generator circuit is auto-
matically retriggered
at the end of each holdoff period
and a free-running sweep
is produced. Since the sweep free
runs at the sweep rate of the A Sweep Generator circuit (as
selected by the A TIME/DIV switch), a bright reference trace
is produced even at fast sweep rates.
Single Sweep Operation
General. Operation of the Sweep Generator in the
SINGLE SWEEP position of the A SWEEP MODE switch is
similar to operation in the other modes. However, after one
sweep has been produced, the Sweep
Reset Multivibrator
stage does not reset.
All succeeding trigger pulses are locked
out
until the RESET button is pressed.
In the SINGLE SWEEP position, the A SWEEP MODE
switch disconnects the charging current for the Holdoff
Capacitor. Now, Q575 remains
on when it is forward biased
through
D555 or D556 at the end of the sweep. With Q575
on,
D505 is held in its low-voltage state to lock out any
incoming trigger pulses.
The circuit remains in this condition
until reset by the Single-Sweep Reset Amplifier stage.
Single-Sweep Reset Amplifier. The Single-Sweep Reset
Amplifier, Q564, produces a pulse to reset the Sweep Reset
Multivibrator stage so another sweep can be produced in
the SINGLE SWEEP mode of operation. Quiescently, Q564
is biased off and the RESET switch is open. When the RESET
button is pressed, B568 ignites and the voltage at the base
of Q564 goes negative. Q564 saturates and produces a
positive-going output pulse.
This pulse has sufficient ampli-
tude to shut off Q575 and allow Q585 to conduct and enable
the Sweep Gate tunnel diode,
D505. Now the A Sweep
Generator circuit can be triggered when the next trigger
pulse
is received.
Lamp Driver. In the SINGLE SWEEP mode, the cathode of
D591 is connected to ground to block the incoming auto-gate
level.
The A SWEEP TRIG'D light, B596 is disconnected from
the collector of Q594 and the RESET light, B597, is connected
into the circuit.
The anode of D595 is also disconnected from
ground. Now the condition of Q594 is determined by the
Sweep
Reset Multivibrator stage. When Q585 is off before
the
RESET button Is pressed, the collector level of QS85 is
negative. The current through R594-D595-R587-R588 sets the
base
level of Q594 negative enough to bias it off. How-
ever, when the RESET button is pressed and Q585 turns on,
its collector goes positive. This positive level allows the
base of Q594 to go positive also and
it is biased on. The
collector of Q594 goes negative and the RESET light comes
on. Q594 and the
RESET light remain on until Q585 turns
off again
at the end of the next sweep.
B TRIGGER GENERATOR
General
The B Trigger Generator circuit is basically the same as
the A Trigger Generator circuit. Only the differences be-
tween the two circuits are discussed here. Portions of the
circuit not described in the following discussion operate in
the same manner as for the A Trigger Generator circuit
3-17

Tektronix 453 Specifications

General

Brand	Tektronix
Model	453
Category	Test Equipment
Language	English