Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
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The following is from the Daqarta Help system:



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Sound Card Trigger Toggle - Introduction

Macro: Trig

You can toggle triggered operation via this toolbar Trigger button or ALT+T. This button is also duplicated in the Trigger control dialog, which is activated by the small thin button below or by CTRL+T.

Triggering allows the 1024 samples for each display frame to be collected synchronously with the input (or output) waveform, giving a stable display in the same way that a strobe light can "freeze" the motion of a rotating wheel. Rather than manually adjusting the strobe rate to match the wheel's rotation rate, suppose the flash could be triggered whenever a mark on the rim of the wheel passed a certain point, say 9 o'clock. The wheel would then appear to be frozen in that position.

Note that a display update may be referred to as a "frame", in reference to a frame of motion picture film or a frame on a display screen. (In older data aquisition systems an update was sometimes called a "sweep", after the analogous operation on a conventional analog oscilloscope. There, the display is created by a beam of electrons that sweeps across the screen after each trigger event. Daqarta does not use that terminology to avoid confusion with frequency sweeps.)

The simplest form of triggering is Normal, where a display update is begun whenever the signal voltage passes a certain Level going in the direction specified by the Slope control.

For many signals there may be multiple suitable trigger points (say, multiple cycles of a sime wave) within the 1024 samples of each frame. Nevertheless, the next frame will not show any of these, instead triggering on some later time point. This insures that the display is kept current and does not lag behind the incoming data.

This is not the case when using triggered operation to analyze file data. Since the data is already saved, there is no concern about lagging behind. Daqarta can trigger on each and every suitable trigger point in the file, if you wish.

Normal mode triggering is fine if the signal is repetitive or otherwise contains a lot of trigger events. But if the signal level drops below the trigger threshold, there is no trigger and the display is not updated. This can be disconcerting if you are trying to understand what is going on, such as why the triggers stopped coming. On the other hand, if you toggle Trigger off you can always see the signal, but since the display is unsynchronized, even with large signals, it can be pretty hard on the eyes.

Auto Trigger attempts to provide a compromise. When there is a trigger event, the display is updated. When there has been no trigger for 1024 samples, the display is updated anyway. If you see that the display trace is free-running, you can adjust the Level control to restore the trigger operation. Auto is a good general-purpose trigger mode, and is the Daqarta default.

Auto Level is a "smarter" version of Auto that dynamically adjusts the threshold Level based upon the largest peak encountered in the prior 1024 samples. So if the signal fluctuates in level, you will get a stable trace even when it goes below the original threshold.

Normal, Auto, and Auto Level modes all rely on extracting the trigger event from the incoming signal. That's fine with a low noise level, but many real-world signals have very high noise levels, often much larger than the signal of interest. In these cases, a simple level-and-slope scheme would trigger on any noise voltage that happened to match. Waveform averaging can be used to reduce the noise, but only with a suitable trigger... one that is itself unaffected by the noise.

If you are viewing a signal that is a response to some stimulus such as a pulse or tone burst, then the stimulus makes an ideal trigger source because it is typically free of noise. The response you want to view may happen some time after the stimulus, but you can allow for that with the Delay control.

If the stimulus is generated externally to Daqarta, then you will need to use one of the input channels to provide the stimulus signal to the sound card.

However, if the stimulus is produced by the sound card itself through the Daqarta Generator, then in many cases you may be able to use the Generator output directly as the Trigger Source, and set Level and Slope based upon the clean stimulus output instead of the noisy response input. Since the source is internal, you can still have both input channels free to look at external response signals. This works as long as your sound card keeps input and output data synchronous at the chosen sample rate. (Some cards can do this only at a certain rate, such as 48000 samples per second.) You need to use the Duplex Delay option of the Auto Calibration feature to determine this for your card.

But if your card can do that, there is an option that is often even better: You can use Gen Sync mode to trigger on the generated wave directly, using the sync source selected from within the Generator. This would allow you to trigger on (say) the start of each FM modulator cycle, which you can't usually do with simple level-and-slope triggering because the trigger threshold is crossed once per carrier cycle, not per modulator cycle.

However, if your sound card can't keep input and output in sync at your chosen sample rate, you can externally connect the Generator signal from the output to an input line, and then use normal triggering. See Full-Duplex Techniques for a detailed discusion.

Note that in Single trigger mode the Trigger button changes to Trig Arm. Each time you hit Trig Arm, it stays active only until a trigger event is found; then the next frame is acquired and the display is Paused.

For very slow sampling using Decimate mode, (anything below an effective rate of about 500 Hz or so), it is usually best to run with Trigger off. This will result in a scrolling waveform to emulate a conventional chart recorder or data logger.

Macro Notes:

Trig=1 activates Trigger mode, Trig=0 turns it off, and Trig=x toggles between on and off.


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