General Operating Information; Triggering Switch Logic; Triggering Modes - Tektronix 7B87 Instruction Manual

Operating Instructions—7B87
11. To trigger the single-sweep display, set the amplifier
unit input coupling to de. Triggering occurs when the
amplifier coupling switch is set to dc. The first six
milliseconds of the display is pretrigger. (Noise in the
Stored Single Sweep may be displayed at the Single
Sweep Trigger Point as a result of the coupling switch
contact closure.)
GENERAL OPERATING
INFORMATION
TRIGGERING SWITCH LOGIC
The MODE, COUPLING, and SOURCE pushbuttons of the
TRIGGERING switches are arranged in a sequence which
places the most-often used position at the top of each
series of pushbuttons. With this arrangement, a stable
display can usually be obtained by pressing the top
pushbuttons: P-P AUTO, AC, INT. When an adequate
trigger signal is applied and the LEVEL control is correctly
set, the unit is triggered as indicated by the illuminated
TRIG'D light. If the TRIG'D light is not on, the LEVEL
control is either at a setting outside the range of the
trigger signal applied to this unit from the vertical unit,
the trigger signal amplitude is inadequate, or its
frequency is below the lower frequency limit of the AC
COUPLING switch position. If the desired display is not
obtained with these buttons pushed in, other selections
must be made. Refer to the following discussions or the
instruction manuals for the associated oscilloscope
mainframe and vertical unit(s) for more information.
TRIGGERING MODES
The MODE pushbutton switches select the mode in
which the sweep is triggered.
P-P AUTO. The P-P AUTO MODE provides a triggered
display at any setting of the LEVEL control whenever an
adequate trigger signal is applied. The range of the
LEVEL control in the P-P AUTO MODE is between
approximately 10% and 90% of the peak-to-peak
amplitude of the trigger signal. The LEVEL control can be
set so that the displayed waveform starts at any point
within this range on either slope. The trigger circuits
automatically compensate for a change in trigger signal
amplitude. Therefore, if the LEVEL control is set to start
the waveform display at a certain percentage point on the
leading edge of a low-amplitude signal, it also triggers at
the same percentage point on the leading edge of a high-
amplitude signal, if the LEVEL control is not changed.
When the trigger repetition rate is outside the parameter
given in the Specification section, or when the trigger
signal is inadequate, the sweep free runs at the rate
indicated by the TIME/DIV switch to produce a bright
base-line, reference trace (TRIG'D light off). When an
adequate trigger signal is again applied, the free-running
condition ends and a triggered display is presented.
The P-P AUTO MODE is particularly useful when
observing a series of waveforms, since it is not necessary
to reset the LEVEL control for each observation. The P-P.
AUTO MODE is used for most applications because of the
1-6
ease of obtaining a triggered display. The AUTO, NORM,
and SINGLE-SWP MODE settings may be used for
special applications.
AUTO. The AUTO MODE provides a triggered display
with the correct setting of the LEVEL control whenever
an adequate trigger signal is applied (see Trigger Level
discussions). The TRIG'D light indicates when the display
is triggered.
When the trigger repetition rate is outside the frequency
range selected by the COUPLING switch or the trigger
signal is inadequate, the sweep free runs at the rate
indicated by the TIME/DIV switch (TRIG'D indicator off).
An adequate trigger signal ends the free-running
condition and a triggered display is presented. The sweep
also free runs at the rate indicated by the TIME/DIV
switch when the LEVEL control is at a setting outside the
amplitude range of the trigger signal. This type of free-
running display is useful when it is desired to measure
only the peak-to-peak amplitude of a signal without
observing the waveshape (such as bandwidth
measurements).
NORMAL. The NORM MODE provides a_ triggered
display with the correct setting of the LEVEL control
whenever an adequate trigger signal is applied. The
TRIG'D light indicates when the display is triggered.
The normal trigger mode must be used to produce
triggered displays with trigger repetition rates below
about 30 hertz. When the TRIG'D light is off, no trace is
displayed.
SINGLE-SWEEP OPERATION. The 7B87 single-sweep
function can be used with both digitizing and non-
digitizing oscilloscopes. A discussion of each follows:
Non-Digitizing Oscilloscope Single Sweep. When the
signal to be displayed is not repetitive or varies in
amplitude, waveshape, or repetition rate, a conventional
repetitive type display may produce an_ unstable
presentation, Under these circumstances, a_ stable
display can often be obtained by using the single-sweep
feature of this unit. The single-sweep mode is useful to
photgraph non-repetitive or unstable displays.
To obtain a single-sweep display of a repetitive signal,
first obtain the best possible display in the NORM MODE.
Then, without changing the other TRIGGERING controls,
press the SINGLE SWP RESET button. A single trace is
presented each time this button is pressed. Further
sweeps cannot be presented until the SINGLE SWP.
RESET button is pressed again. If the display signal is a
complex waveform composed of varying amplitude
pulses, successive single-sweep displays may not start at
the same point on the waveform. To avoid confusion due
to the crt persistance, allow the display to disappear
before pressing the SINGLE SWP RESET button again. At
fast sweep rates, it may be difficult to view the single-
sweep display. The apparent trace intensity can be
increased by reducing the ambient light level or by using
a viewing hood as recommended in the mainframe
instruction manual.