Tektronix 7B70 Time Base Instruction Manual page 19

Operating Instructions-7B70 (SN B205000 and Up)
vided by the associated vertical unit or indicator oscillo-
scope; see the instruction manuals for these instruments for
information. For most applications, the INT-SOURCE posi-
tion can be used. However, some applications require
special triggering which cannot be obtained in the INT-
SOURCE position. I n such cases LINE- or EXT-SOURCE
must be used.
LINE. The LINE position of the SOURCE switch con-
nects a sample of the power-line voltage, to which the indi-
cator oscilloscope is connected, t o the trigger circuit. Line
triggering is useful when the input signal is time-related
(m~.~ltiple or sub-multiple) to the line frequency. It is also
useful for providing a stable display of a line-frequency
component in a complex waveform.
EXT. An external signal connected to the EXT TRlG
I N connector can be used to trigger the sweep in the EXT-
SOURCE position. The external signal must be time-related
to the displayed waveform for a stable display. A n external
trigger signal can be used to provide a triggered display
when the internal signal i s too low in amplitude for correct
triggering or contains signal components on which it is not
desired to trigger. It is also useful when signal tracing in
amplifiers, phase-shift networks, wave-shaping circuits, etc.
The signal from a single point in the circuit under test can
be connected t o the EXT TRlG I N connector through a
probe or cable. The sweep is then triggered by the same
signal at all times and allows amplitude, time relationship or
waveshape changes of signals at various points in the circuit
to be examined without resetting the TRIGGERING con-
trols.
EXT
+
10. Operation in the EXT
+
10 position is the
same as described for EXT except that the external signal is
attenuated 10 times. Attenuation of high-amplitude exter-
nal trigger signals i s desirable t o broaden the range of the
LEVELISLOPE control.
Trigger Slope
The LEVELISLOPE control determines whether the
trigger circuit responds on the positive-going or negative-
going portion of the trigger signal. When the indicator line
on the outer ring of the LEVELISLOPE control is to the
left of center, the display starts on the positive-going por-
tion of the waveform (notice positive-going waveform t o
left of control. To the right of center, the display starts on
t h e negative-going portion of the waveform (notice
negative-going waveform). The effect of the LEVEL1
SLOPE control on the displayed waveform is shown in Fig.
2-7. When several cycles of a signal appear in the display,
the selection of the trigger slope i s often unimportant.
However, if only a certain portion of a cycle is t o be dis-
played, correct setting of the LEVELISLOPE control is
important to provide a display which starts on the desired
slope of the input signal.
Trigger Level
The LEVELISLOPE control determines the voltage level
on the trigger signal at which the display is triggered, as well
as selecting the trigger slope. The center knob of the
LEVELISLOPE control provides about a 3:1 reduction in
relation t o the outer indicator ring, to allow precise level
selection. This control can also be turned throughout the
complete 360' rotation to allow continuous triggering over
the selected slope. This is particularly useful in the P-P
AUTO MODE. When .the LEVELISLOPE con.trol is set to
the "0" on either the positive-going or negative-going wave-
form, the sweep is triggered near the zero-volt level of the
trigger signal. As the LEVELISLOPE control is rotated
away from the "O", the displayed waveform starts at a
point corresponding to the position of the indicator line on
the associated slope waveform (within amplitude range of
trigger signal except in P-P AUTO MODE which provides
triggered display throughout entire slope). For example if
the LEVELISLOPE control is turned clockwise from the
"0" on the positive-going slope, the displayed waveform
starts at a more positive level. Fig. 2-7 illustrates the opera-
tion of the LEVELISLOPE control at different levels and
slopes of a displayed waveform.
Before setting the triggering level, select the TRIG-
GERING SLOPE, MODE, COUPLING and SOURCE. Then
adjust the LEVELISLOPE control so the displayed wave-
form starts at the desired point. The triggering slope can be
changed at any time by rotating the LEVELISLOPE con-
trol to the corresponding point on the other slope wave-
form.
Horizontal Sweep Rate
The T I MEIDIV switch provides 26 calibrated sweep
rates ranging from 0.02 microsecond1division to five
secondsldivision. The VARIABLE TIMEIDIV control must
be in the calibrated position and the MAGNIFIER at X I
for the indicated sweep rate. The VARIABLE TIMEIDIV
control incorporates a two-position switch to determine if
the sweep rate is calibrated or uncalibrated. When the
VARIABLE knob is in the inward position, the VARI-
ABLE TIMEIDIV control is inoperative and the sweep rate
is calibrated. When pressed and released, the VARIABLE
knob moves outward to activate the VARIABLE TIME/
DIV control for uncalibrated sweep rates. A calibrated
sweep rate can be obtained at any position of the VARI-
ABLE TIMEIDIV control by pressing in the VARIABLE
knob. This feature is useflil when a specific uncalibrated
sweep rate has been obtained and
it
is desired t o switch
between this uncalibrated sweep rate and a calibrated sweep
rate. Switching from uncalibrated to calibrated operation or
vice versa does not affect the setting of the VARIABLE
TI MEIDIV control. The VARIABLE TI ME/DIV control
allows the sweep rate in each TIMEIDIV switch position to
be reduced t o at least the sweep rate of the next adjacent
switch position. This provides an overall uncalibrated vari-
Scans by Artekhledia O 2006