Tektronix 453 User Manual

Circuit Description-Type 453/R453
Input Stage
The trigger signal from the A COUPLING switch is con-
nected to the Input Stage through the network
C440-R438-
R439-R440-R441. R438-R439 provide the input resistance for
this stage. The voltage-divider action of R438-R439 allows
about 98% of
DC or low frequency signals applied to R438
to be available at the junction of R438 and R439. C440
along with the stray capacitance in the circuit forms an AC
divider which maintains about this same voltage division
for high-frequency signals.
R440 limits the current drive to
the
gate of FET Q443. Diode D441 protects the circuit by
clamping the gate of Q443 at about -12.5 volts if a high-
amplitude negative signal
is applied to the EXT TRIG INPUT
connector. Over-voltage protection for high-amplitude posi-
tive signals
is provided by the forward conduction of FET
Q443.
Q443
is connected as a source follower to provide a high
input impedance and a low output impedance.
As a result,
this stage provides isolation between the A Trigger Gen-
erator circuit and the trigger signal source.
The output signal
from Q443 is connected to the Slope Comparator stage
through emitter follower Q453. Diodes
D449 and D459 pro-
vide protection for the Slope Comparator stage transistors,
Q454 and Q464.
Slope Comparator
Q454 and Q464 are connected as a difference amplifier
(comparator)
5
to provide selection of the slope and level at
which the sweep is triggered. The reference voltage for the
comparator
is provided by the A LEVEL control, R460, and
the A Trigger
Level Center adjustment, R462. The A Trigger
Level Center adjustment sets the level at the base of Q464
so the display is triggered at the zero-volt DC level of the
incoming trigger signal when the A
LEVEL control is centered.
The A LEVEL control varies the base level of Q464 to select
the point
on the trigger signal where triggering occurs.
R458 establishes the emitter current of Q454 and Q464.
The transistor with the most positive base controls conduc-
tion of the comparator.
For example, assume that the trigger
signal
from the Input Stage is positive going and Q454 is
forward biased. The increased current flow through R458
produces a larger voltage drop and the emitters of both
Q454 and Q464 go more positive. A more positive voltage
at
the emitter of Q464 reverse biases this transistor, since its
base is held at the voltage set by the A LEVEL control, and
its collector current decreases. At the same time, Q454 is for-
ward biased and its collector current increases. Notice that
the signal currents
at the collectors of Q454 and Q464 are
opposite
in phase. The sweep can be triggered from either
the negative-going or positive-going slope of the input trig-
ger signal by producing the trigger pulse
from either the
signal
at the collector of Q464 for - slope operation or
the signal
at the collector of Q454 for + slope operation.
This selection is made by the SLOPE switch, SW455.
When the A
LEVEL control is set to 0 (midrange), the base
of Q464
is at about one volt positive which corresponds to
a zero-volt level
at the input to this circuit (with correct cali-
bration).
The base-emitter drop of Q464 sets the common
emitter
level of Q454-Q464 to about +0.3 volts. Since the
5Phillip Cutler, "Semiconductor Circuit Analysis", McGraw-Hill, New
York,
pp. 365-372.
3-12
base of Q454 must be about 0.65 volts more positive than
the emitter before
it can conduct, the comparator switches
around the zero-volt level of the trigger signal (zero-volt
level on the trigger signal corresponds to
about one volt
positive
at this point). As the A LEVEL control is turned
clockwise
toward +, the voltage at the base of Q464
becomes more positive.
This increases the current flow
through
R458 to produce a more positive voltage on the
emitters of both Q454 and Q464. Now the trigger signal
must rise more positive before Q454 is biased on. The result-
ant
CRT display starts at a more positive point on the dis-
played signal. When the A
LEVEL control is in the - region,
the effect
is the opposite to produce a resultant CRT display
which starts
at a more negative point on the trigger signal.
The slope of the input signal which triggers the A sweep
is determined by the A SLOPE switch, SW455. When the A
SLOPE switch is set to the - position, the collector of Q454
is connected to the + 12-volt supply through D456 and
R467. The anode of D466 is grounded and it is reverse
biased. Now the collector current of Q464 must flow through
D465, R459, the parallel combination D475 and R468-R469-
L469 and R467 to the + 12-volt supply (see Fig. 3-9). Since
the output pulse
from the A Trigger Generator circuit is
derived from the negative-going portion of the signal applied
to the Trigger
TD stage, the sweep is triggered on the nega-
tive-going portion of the input trigger signal (signal applied
to Trigger
TD stage is in phase with the input signal for -
slope triggering). When the A
SLOPE switch is set to +, con-
ditions
are reversed (see Fig. 3-10). Q464 is connected to the
+ 12-volt supply through
D466 and R467. The anode of
D456 is grounded to divert the collector current of Q454
through the Trigger
TD stage. The signal applied to the
Trigger
TD stage is now 180° out of phase with the input
trigger signal
so the sweep is triggered on the positive-going
portion of the input signal.
Trigger TD
The Trigger TD stage shapes the output of the Slope Com-
parator to provide a trigger pulse with a fast leading edge.
Tunnel diode D475
6
is quiescently biased so it operates in
its low-voltage state. The current from one of the transistors
in the Slope Comparator stage is diverted through the Trig-
ger TD stage by the A SLOPE switch. As this current increases
due
to a change in the trigger signal, tunnel diode D475
switches to its high-voltage state. L469 opposes the sudden
change
in current which allows more current to pass through
D475 and switch it more quickly. As the current flow stabi-
lizes, L469 again conducts the major part of the current.
However, the current through
D475 remains high enough to
hold it in its high-voltage state. The circuit remains in this
condition until the current from the Slope Comparator stage
decreases due to a change
in the trigger signal applied to
the input.
Then, the current through D475 decreases and it
reverts to its low-voltage state.
Pulse Amplifier
The trigger signal from the Trigger TD stage is connected
to the base of the
Pulse Amplifier, Q473, through R472. The
trigger pulse at this point is basically a negative-going pulse
with a fast rise. The width of the pulse depends upon the
6
Millman and Taub, pp. 452-455.
®

Tektronix 453 Specifications

General

Brand	Tektronix
Model	453
Category	Test Equipment
Language	English