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Triggering

When triggering is enabled, the processor will test for a valid trigger condition before performing an FFT.  

Trigger Mode: Controls the behavior of the analyzer after a trigger event occurs:
  • Rearm and wait for next trigger - process the current trigger event and wait for the next event.  This is the default action.
  • Stop the analyzer - process the current trigger event and then stop the analyzer.  
  • Continue running or recording - process the current trigger event and then run/record without waiting for any additional trigger events to occur.

    Trigger Source:  Analog channel - tests the specified analog input channel for the presence of the specified trigger conditions.  External Trigger TTL - waits for the occurence of a TTL signal (+5.0 V) on the dedicated external trgigger input channel of the DT-9800 hardware (not all modules provide an external trigger channel).

    Trigger Channel: specifies the analog input channel to test for the specified trigger condition.  

    Trigger Type: sets the type of trigger event to test for.  Level triggering - a value meeting or exceeding the threshold.  Edge triggering - a difference in level between any two sucessive samples that exceeds the threshold.  Edge triggering is most useful with pulse type signals.

    Trigger Polarity: controls whether the trigger threshold is a positive or negative value.

    Threshold: Sets the trigger threshold. The threshold units correspond to the current calibration units. 

    Trigger Delay: adjusts the trigger position within the FFT block. 

    Trigger Delay/Type: can be used to adjust the acquisition window within the FFT block.  Pre-delay will move the trigger point to the right, post-delay to the left.  If the delay is set to zero, the trigger point is positioned at the start of the FFT data block.  The delay value can be specified in milliseconds or as a percent of the FFT size.

    Prompt User to Accept/Reject Trigger Waveform:
    causes the analyzer to display a message box asking the user to accept or reject the current trigger event.  The current trigger event will be shown in the time series plot for preview.  If accepted the spectral data will be computed and averaged with previous data.  If rejected, the trigger event will be discarded and the analyzer will continue waiting for an other trigger condition.  

    This is very useful for impact testing.  It allows the user to preview the trigger waveform to verify that the impact hammer did not bounce.  You can also use the Run Control options to set the number of FFT's to perform before stopping.  Only accpeted triggers will result in an FFT being performed.

    Pressing Cancel on the message box will stop the analyzer.

    Note
    When triggering is enabled, the analyzer may appear not to work if the signal stays below the threshold.  Check the status bar along the bottom edge of the program for the current trigger status.