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Operator's guide
Trigger criteria
The triggering algorithm applies a simple short-term average / long-term
average calculation to the triggering stream. It works by identifying sections of
an incoming data stream when the signal amplitude increases. The purpose of
taking a short term average, rather than triggering on signal amplitude directly,
is to make it less likely that spurious spikes will trigger the device. Averaging
also introduces an element of frequency selectivity into the triggering process.
You can select which tap is tested for the trigger from the Trigger source drop-
down menu. The tap does not have to be selected for data output for you to be
able to use it here.
The next option, Trigger filter, allows you to apply a bandpass filter at this
stage (see below.)
Any or all of the channels available at the tap you have selected may be used to
determine a trigger. The next part of the window lists the channels, each with
Enable, STA, LTA and Ratio settings. The Enable boxes determine which
channels are considered for triggering. If any of the checked channels passes the
trigger condition, the trigger will activate, and will not detrigger until all of the
checked channels have fallen below their respective Ratio values.
The STA and LTA columns allow you to set the intervals over which the two
averages are calculated, in seconds. Typically, the time interval for the short
term average should be about as long as the signals you want to trigger on,
while the long term average should be taken over a much longer interval. Both
the STA and LTA values are recalculated continually, even during a trigger.
The Ratio column determines by what factor the STA and LTA must differ for
the trigger to be passed. Finding the ratio most suited to your needs is best done
by experiment. Too high a value will result in events being missed, while too
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