Tektronix 585 - Page 14

Operating Instructions — Type 585
Fig. 2-6. The triggering signal source is selected by the TRIGGERING SOURCE switch.
Selecting the Triggering Signal
1. To trigger either the Time Base A or B sweep from
the waveform being observed, set the appropriate TRIGGER 
ING SOURCE knob to INT.
1. To trigger the sweep from the power-line wave (as
in the case when observing a waveform which has a time
relationship to the power-line frequency), set the TRIGGER 
ING SOURCE knob to LINE.
3. To trigger the sweep from some external waveform
(one having a time relationship to the waveform being
observed), connect the source of the triggering signal to
the TRIGGER INPUT connector and set the TRIGGERING
SOURCE knob to EXT., DC or AC.
When the frequency of the external triggering signal
is below about 15cps, the TRIGGERING SOURCE switch
must be placed in the EXT. DC position to obtain stable
triggering. Below 15cps, the coupling circuit in the EXT.
AC position attenuates the input signal, thereby making it
difficult or impossible to obtain reliable operation. When
in doubt, the proper setting of the TRIGGERING SOURCE
switch can be chosen by observing which position produces
the best triggering. Refer to Fig. 2-6 for a pictorial pre 
sentation of the various triggering source options.
Selecting the Triggering Slope
The horizontal sweep can be triggered on either the
rising ( + slope) or falling ( — slope) portion of the triggering
waveform. This is determined by the position of the TRIG 
GER SLOPE switch. When the switch is in the + position,
the sweep is triggered on the rising portion of the trigger 
ing waveform; when the TRIGGER SLOPE switch is in the
— position, the sweep is triggered on the falling portion
of the waveform (see Figure 2-8).
In many applications the triggering slope is not important
since triggering on either slope will provide a display which
is suitable to the application. However, in other cases,
such as pulse risetime or falltime measurements, the trigger 
ing slope is very important and must be selected properly if
you are to obtain a useful display. To accurately measure
the risetime of a pulse it is necessary to trigger the sweep on
the rising portion of the waveform. Also, to observe the
fall of a pulse it is necessary to trigger the sweep on the
falling portion of the waveform. In either case, selection
of the wrong triggering slope will make it difficult or im
possible to observe the portion of the waveform to be
checked.
When displaying sine waves above 50 megacycles the
Time Base A trigger circuit may produce a stable display
on only one slope. Be sure to try both the + and — posi 
tions of the A TRIGGER SLOPE switch for the best triggering.
2-6