Operating Instructions—434
remaining TRIG G ER COUPLING switch positions w ill
provide a better display.
The triggering po in t in the AC position depends upon
the average voltage level o f the trigger signals. If the trigger
signals occur at random, the average voltage level w ill vary,
causing the triggering poin t to vary also. This shift of the
triggering point may be enough so it is impossible to main
tain a stable display. In such cases, use DC coupling.
LF REJ. In the LF REJ position, DC is rejected and
low-frequency signals below about 30 kilohertz are attenu
ated. Therefore, the sweep is triggered only by the higher-
frequency components of the trigger signal. This position is
particularly useful fo r providing stable triggering if the
trigger signal contains line-frequency components. Also, in
the A LT position of the Vertical Mode switch the LF REJ
position provides the best display at fast sweep rates when
comparing two unrelated signals (TRIGGER COUPLING
switch set to COMP).
HF REJ. The HF REJ position passes all low-frequency
signals between DC and about 50 kilohertz. Signals outside
the above range are attenuated. When triggering from
complex waveforms, this position is useful for providing
stable display of the low-frequency components.
DC. DC coupling can be used to provide stable trigger
ing w ith low-frequency signals which w ould be attenuated
in the AC position, or w ith low repetition rate signals. It
can also be used to trigger the sweep when the trigger signal
reaches a DC level selected by the setting of the LEV EL
control. When using internal triggering, the setting o f the
Channel 1 and 2 POSITION controls affect the DC
triggering p oint of a display when the TR IG G ER SOURCE
switch is in the COMP position.
DC trigger coupling should not be used in the A LT dual
trace mode if the TRIGG ER SOURCE switch is set to
COMP. If used, the sweep w ill trigger on the DC level of
one trace and then either lock o ut completely or free run
on the other trace. Correct DC triggering for this mode can
be obtained w ith the TRIG G ER SOURCE switch set to CH
1.
W ith both the LF REJ and HF REJ buttons pushed,
only those signals between approximately 30 kiloh ertz and
50 kilohertz w ill be passed to the Trigger Generator w ith
ou t being attenuated.
Trigger Slope
The TRIGG ER SLOPE switch determines whether the
trigger circuit responds to the positive-going or negative
going po rtion of the trigger signal. When the SLOPE switch
is in the positive-going (+) position, the display starts w ith
the positive-going portion o f the w aveform ; in the negative
going (—) position, the display starts w ith the negative-
going po rtion of the waveform (see Fig. 2-6). When several
cycles of a signal appear in the display, the setting of the
SLOPE switch is often unim portant. However, if only a
certain po rtion of a cycle is to be displayed, the setting of
the SLOPE switch is im portant to provide a display w hich
starts on the desired slope o f the input signal.
Trigger Level
The TR IGG ER LE V E L control determines the voltage
level on the triggering waveform at which the sweep is
triggered. When the LE VE L control is set in the + region,
the trigger circu it responds at a more positive poin t on the
trigger signal. In the — region, the trigger circu it responds at
a more negative point on the trigger signal. Fig. 2-6 illus
trates this effect w ith different settings of the SLOPE
sw itch.
To set the LE V EL control, first select the TRIG G ER
source, coupling, and slope. Then, set the LEV E L control
fu lly counterclockwise and rotate it clockwise un til the
display starts at the desired point.
Sweep Mode
AUTO . The AUTO position o f the Sweep MODE switch
provides a triggered display when the LEVE L control is
correctly, set (see Trigger Level in this section) and an
adequate trigger signal is available.
When the trigger repetition rate is less than about 10
hertz, or in the absence of an adequate trigger signal, the
Sweep Generator free runs at the sweep rate selected by the
T IM E /D IV switch to produce a reference trace. When an
adequate trigger signal is again applied, the free-running
condition ends and the Sweep Generator is triggered to
produce a stable display (w ith correct LEVE L control
setting).
NORM. Operation in the NORM position of the Sweep
MODE switch is the same as in the AUTO position when a
trigger signal is applied. However, when no trigger signal is
present, the Sweep Generator remains o ff and there is no
display.
Use the NORM mode to display signals w ith repetition
rates below about 10 hertz. This mode provides an indica
tion o f an adequate trigger signal as well as the correctness
of trigger control settings, since there is no display w ith o u t
proper triggering.
2-10
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