Guralp Systems CMG-5TD User Manual
Page 21
Operation
advisable to use an STA/LTA trigger directly from broadband data. The
Bandpass Filter button allows the user to select from a set of standard
bandpass filters from a pull-down menu (a full list of options is given later in
the STA/LTA chapter). The chosen filter will be applied to the streams from
the triggering components before they are tested for the trigger condition.
The corner frequencies of the pass band of the filter are determined by the
Nyquist frequency, which is given by the sampling rate of the triggering data.
The three filter options have pass bands between 10% and 90%, between 20%
and 90% and between 50% and 90% of the data’s Nyquist frequency,
respectively.
Trigger criteria: Trigger criteria for the STA/LTA and Level triggers function
may be set in the Trigger Setup window, accessed by clicking on the Trigger
button near the bottom of the Output Control window.
The three tick boxes down the left side of the windows (Z, N, E) allows the
user to choose the channels (for the specified tap) which will be tested for a
trigger condition.
STA/LTA parameters: The user sets the parameters by clicking on them.
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 system declares a trigger when any one of the triggering components
exceeds this value. The trigger ratio is continuously recalculated for all
components and the system will only cancel the trigger condition when all
the components selected for triggering have fallen below their respective ratio
values.
The user can also specify the pre-trigger and post-trigger data intervals. These
values determine the minimum length of data that will be saved prior to the
trigger condition, and how much data will be saved after the trigger condition
has lapsed. Triggered streams will always start and end on integer seconds.
If the box “Common Values” is ticked, a trigger parameter entered for one
component will be used for all selected components. (BANDPASS, STA, LTA,
RATIO, PRE-TRIG and POST-TRIG).
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Issue D - April 2013