Trigger Coupling - Tektronix 7B51 Instruction Manual

Trigger Coupling
The pushbuttons located below the COUPLING title
(see Fig. 2-4) select the method in which the trigger signal is
connected to the trigger circuits. The selected coupling is
indicated by a lighted pushbutton. Each position permits
selection or rejection of the frequency components of the
trigger signal which trigger the sweep. Fig. 2-6 graphically
illustrates the band of frequencies covered by each position
of the COUPLING switch.
AC. In the AC position of the COUPLING switch, the
DC component of the trigger signal is blocked. Signals with
low-frequency components below about 30 hertz are atten-
uated. In general, AC COUPLING can be used for most
applications. However, if the signal contains unwanted fre-
quency components or if the sweep is to be triggered at a
low repetition rate or DC level, one of the remaining COU·
PLiNG switch positions will provide a better display.
The triggering point in the AC position depends upon
the average voltage level of the trigger signal. If the trigger
signal occurs randomly, the average voltage level will vary,
causing the triggering point 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.
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Operating Instructions-Type 7851
AC LF REJ. In the AC LF REJ position, DC is rejected
and low-frequency trigger signals below about 30 kilohertz
are attenuated. Therefore, the sweep is triggered only by
the higher-frequency components of the trigger signal. This
position is particularly useful for providing stable triggering
if the trigger signal contains line-frequency components.
Also, the AC LF REJ position provides the best alternate-
mode vertical displays at fast sweep rates when comparing
two or more unrelated signals (with dual-trace vertical or
slaved triggering operation for dual-vertical unit operation).
AC HF REJ. The AC HF REJ position passes all low-
frequency signals between about 30 hertz and 50 kilohertz.
DC is rejected and signals outside the above range are atten-
uated. When triggering from complex waveforms, this posi-
tion is useful for providing stable display of the low-
frequency compondnts.
DC. DC COUPLING can be used to provide stable trig-
gering with low-frequency signals which would be atten-
uated in the other modes, or with 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 LEVEL/SLOPE control When using internal triggering,
the setting of the vertical unit position controls affects the
DC triggering point.
30 Hz 30kHz 50 kHz
Fig.2-6. Frequency range of each COUPLING switch position.
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