Trigger Coupling; Trigger Source - Tektronix 7B80 Instruction Manual

Operating
Instructions-7B80
The normal trigger mode must be used to produce triggered
displays with trigger repetition rates below about 30 hertz.
When the TRIG'D light i s off, no trace is displayed.
SINGLE SWEEP. When the signal to be displayed
is
not re-
petitive 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 i s also useful t o
photograph 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 present-
ed each time this button is pressed. Further sweeps cannot
be presented until the SINGLE SWP RESET button is pres-
sed again. I f the displayed signal is a complex waveform
composed of varying amplitude pulses, successive single-
sweep displays may not start a t the same point on the wave-
form. To avoid confusion due to the crt persistence, allow
the display t o disappear before pressing the SINGLE SWP
RESET button again. A t fast sweep rates, it may be diffi-
cult 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.
When using the single-sweep mode to photograph wave-
forms, the graticule may have t o be photographed
separately in the normal manner to prevent over exposing
the film. Be sure the camera system is well protected
against stray light, or operate the system in a darkened
room. For repetitive waveforms, press the SINGLE SWP
RESET button only once for each waveform unless the sig-
nal is completely symmetrical. Otherwise, multiple wave-
forms may appear on the film. For random signals, the lens
can be left open until the signal triggers the unit. Further
information on photographic techniques is given in the
appropriate camera instruction manual.
Trigger Coupling
The TRIGGERING COUPLING push buttons select the
method in which the trigger signal
is
connected t o the trig-
ger circuits. Each position permits selection or rejection of
some frequency components of the signal which triggers
the sweep.
AC. AC COUPLING blocks the dc component of the trig-
ger signal. Signals with low-frequency components below
about 30 hertz are attenuated. In general, AC COUPLING
can be used for most applications. However, i f the signal
contains unwanted frequency components or i f the sweep i s
t o be triggered a t a low repetition rate or dc level, one of
the other COUPLING switch positions will provide a better
display.
AC LF REJ. AC LF REJ COUPLING rejects dc, and atten-
uates low-frequency trigger signals below about 30 kilo-
hertz. Therefore, the sweep i s triggered only by the higher-
frequency components of the trigger signal. This position is
particularly useful for providing stable triggering i f the trig-
ger signal contains line-frequency components. Also, the
AC LF REJ position provides the best alternate-mode verti-
cal displays a t fast sweep rates when comparing two or
more unrelated signals.
AC HF REJ. AC HF REJ COUPLING passes
all
low-
frequency signals between about 30 hertz and 50 kilohertz.
Dc i s rejected and signals outside the above range are atten-
uated. When triggering from complex waveforms, this posi-
tion is useful t o provide a stable display of the low-
frequency components.
DC. DC COUPLING can be used t o provide stable trigger-
ing from low-frequency signals which would be attenuated
in the other COUPLING switch positions. DC COLlPLlNG
can be used t o trigger the sweep when the trigger signal
reaches a dc level set by the LEVEL control. When using
internal triggering, the setting of the vertical unit position
control affects the triggering point.
Trigger Source
The TRIGGERING SOURCE push buttons select the
source of the trigger signal which i s connected t o the trigger
circuits.
INTERNAL. The INT position connects the trigger signal
from the vertical plug-in unit. Further selection of the in-
ternal trigger signal may be provided by the vertical plug-in
unit or by the mainframe; see the instruction manuals for
these instruments for more information. For most applica-
tions, the internal source can be used. However, some
applications require special triggering which cannot be ob-
tained in the INT position. In such cases, the LlNE or EXT
positions of the SOURCE switches must be used.
LINE. The LlNE position connects a sample of the power-
line voltage from the mainframe to the trigger circuit. Line
triggering
i s
useful when the input signal is time-related
(multiple or submultiple) t o the line frequency. It is also
useful for providing a stable display of a line-frequency
component in a complex waveform.
Scans by ArtekMedia O 2006