Sampling oscilloscope concepts
Trigger concepts
The oscilloscope uses the data in the waveform record to construct and display a waveform on the oscilloscope screen.
However, as the oscilloscope is constantly acquiring samples into the waveform record, each waveform record starts at a
random point of the input signal. This means that the waveform record sample values are constantly changing, which results
in a displayed waveform that is unstable or jittering. An unstable waveform cannot be accurately measured, making this
display useless except for very broad measurements (type of signal, approximate peak to peak signal amplitude).
Untriggered display
What is needed is a way to set the oscilloscope to detect, or trigger on, the same signal condition or state on the input signal
for each waveform acquisition, and store the samples in the waveform record such that the same signal condition is at
the same sample time location in the waveform record.
A
trigger
sets when the oscilloscope detects a de
fi
ned waveform condition, such as at a speci
fi
ed signal voltage level on
the rising edge of the signal. The oscilloscope uses that trigger condition to store the waveform samples in the waveform
record so that the signal sample point that meets the trigger condition is at the same location in the waveform record. When
displayed, the waveform record for each acquisition shows the waveform at the same trigger point, resulting in a stable
waveform on the screen.
Triggered display
Understanding and using triggers is probably the most important skill needed to quickly display and analyze waveforms.
A trigger condition can be as simple as when the signal passes through a speci
fi
ed voltage level when going from a low
level to a high level (positive slope). A trigger condition can also be very speci
fi
c, such as when the signal level is falling
and the width of a single pulse of the signal is less than a speci
fi
ed time period.
The trigger point in a waveform record also sets the zero time-reference point in a waveform record. By default, the trigger
point in a waveform record is in the center of the waveform record. This means that half of the record shows the signal
waveform before the trigger condition (pretrigger), and the other half shows the waveform after the trigger condition
(post-trigger).
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TBS2000 Series User Manual
