Tektronix CURVE-TRACER 576 Instruction Manual page 40

Operating Instructions-Type 576
The MODE switch determines whether the Collector
Supply output voltage will be a voltage sweep or a DC
voltage. When the MODE switch is set to NORM the output
is a repetitive voltage sweep varying from 0 volts to the
peak voltage set by the MAX PEAK VOLTS switch and the
VARIABLE COLLECTOR SUPPLY control. When the
MODE switch is set to DC (ANTI LOOP) or LEAKAGE
(EMITTER CURRENT) the Collector Supply output is a
DC voltage equal to the peak voltage set by the MAX PEAK
VOLTS switch and the VARIABLE COLLECTOR SUPPLY
control. This DC voltage may be either positive or negative.
The DC mode is very useful when the normal display is
exhibiting excessive looping.
Occasionally some of the characteristic curves displayed
on the CRT consist of loops rather than well defined lines
(see Fig. 2-23). This effect is known as looping and is most
noticeable at very low or very high values or current.
Looping is generally caused by stray capacitance within the
Type 576, and device capacitance. It may also be caused by
heating of the device under test. The LOOPING COMPEN-
SATION control provides complete compensation for non
heat-related looping due to the Type 576 and any standard
device adapter which may be used. In general it does not
compensate for any added capacitance introduced by the
device under test. (Control has some effect in reducing
stray capacitance in small diodes, and voltage-driven three
terminal devices.) If uncompensated looping is hindering
measurements, the MODE switch should be set to DC
(ANTI LOOP). If the collector sweep mode of operation
(MODE switch set to NORM) is desired, an imaginary line
lying inside the loop and equidistant from each side of the
loop is the best approximation of the actual characteristic
curve (see Fig. 2-23). Looping due to heating may be re-
duced by using the pulsed steps operation of the Type 576.
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Fig. 2-23. Example of a display exhibiting looping.
Interlock System
Whenever the MAX PEAK VOLTS switch is in the 75,
350 or 1500 positions, the yellow COLLECTOR SUPPLY
VOLTAGE DISABLED light comes on. This light indicates
that the Collector Supply is disabled. In order to enable the
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Collector Supply under these circumstances, the Type 576
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uses an interlock system. When the yellow light is on, the
protective box must be installed over the accessories con- , ;.
nectars (see Fig. 2-7). When the protective box is in place ....,
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and the lid closed, the yellow light turns off and the red
light turns on. The red light indicates that the Collector
Supply is enabled and that a dangerous voltage may arpear
at the Collector terminals. For further information about
the interlock system, see the Circuit Descrirtion.
Step Generator
The Step Generator provides current or voltage which
may be applied to the base or the emitter of the device
under test. The output of the Step Generator is families of
ascending steps of current or voltage (see Fig. 2-24). When
these steps together with the Collector Supply output are
applied to the device under test, families of characteristic
curves of the device are displayed on the CRT.
Fig. 2-24. Step Generator output in both polarities
The NUMBER OF STEPS switch determines the number
of steps per family and has a range of from 1 step to 1 0
steps. The AMPLITUDE switch determines the amplitude
of each step and provides both current steps and voltage
steps. The range of step amplitudes available are from 50
nA/step to 200 rnA/step for current steps and from 5
mV /step to 2 V /step for voltage steps. The STEP M U L T
.1 X button, when pressed, divides the step amplitude by
10. When voltage steps are being applied to the base of a
transistor, the base current increases very rapidly with
increasing base voltage (note Caution on front-panel). To
avoid damage to the transistor when using voltage steps,
current limiting is provided through the CURRENT LIMIT
switch.
The rate of generation of steps by the Step Generator is
determined by the RATE buttons. When the NORM RATE
button is pressed, steps are generated at a rate of 120
steps/second (assuming a 60 Hz line frequency), or one step
per cycle of the Collector Supply, POLARITY switch set to
+(NPN) or -(PNP). In this case each step occurs at the
beginning of a Collector Supply cycle. When the .5X RATE
button is pressed, the Step Generator rate is 60 steps/
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