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Sound Card Trigger Control Features

Daqarta provides advanced Trigger options for displaying "difficult" signals. Consider the noisy tone burst shown here:

[Trigger - Noisy Tone Burst (46K image)]

A conventional slope/threshold trigger attempts to show a steady view of a signal by starting each display frame at the time the waveform crosses a specified threshold level with the selected slope.

That method works fine for a continuous pure tone, because one cycle looks exactly like the next... it doesn't matter which one the display starts with. But for a burst-type signal, it's important to set the trigger point to a particular cycle, so the display doesn't jump among them on successive updates. If one peak is significantly larger than the rest, that's usually the best target.

But even with a continuous pure tone, noise on the signal can cause the trigger threshold to be crossed early or late, causing the display to jitter from one update to the next. With a noisy burst like the one shown, the noise makes it even harder to get a stable display.

In this example, the Trigger Level is set to 2.5 mV (solid light blue horizontal line), and Slope is Positive, as shown in the Trigger Control dialog at the right. But so much noise is present that there are multiple points that meet these criteria and could thus be potential triggers.

To resolve this, Hysteresis is set to 0.5 mV, resulting in threshold limits (dotted lines) above and below the base Level setting. To be accepted as a trigger, the signal must pass through both of these in the proper order. With a positive Slope setting, it must pass through the lower first, then the upper. Looking at the image, you can see that this eliminates many of the unwanted trigger points. Most importantly, it provides a reliable trigger on the first big positive peak.

Trigger Holdoff is used to prevent false triggers on peaks other than the first. Holdoff is the minimum interval between triggers, which here is set to 24 msec since the burst repeat time in this example is known to be 25 msec. When an update starts on the first peak, all other peaks are ignored until that burst is over and the next is just about to start. The trigger system then waits for the trigger criteria to be met, which will be at the first big peak of the next burst, and the process repeats.

Since the trigger point is at the first big peak, earlier portions of the burst have already happened. Negative trigger Delay is used here to shift the display frame so the entire burst is visible. (Acquired samples are held in a circulating memory buffer, so once the trigger sample is found Daqarta merely needs to look back by the negative delay amount and start the display from that point.)

Note that noise problems can often be completely eliminated if the signal of interest is a response to a stimulus of some kind. If the stimulus itself is not noisy, then you can trigger directly on that by sending it to one input channel while you view the response on the other.

If the stimulus is created by the Daqarta Generator, you can always route the output to one of the inputs for this purpose. Better yet, if Auto Calibration is able to correct for your card's Duplex Delay (true for most cards), you can simply select the output channel as the Trigger Source, eliminating external wiring and keeping both inputs available for response signals.

Even better, for cards with correctable Duplex Delay, you can use Gen Sync mode to trigger on the generated wave directly, using the sync source selected from within the Generator itself. This allows sync to waveforms that can't be handled by any type of level-and-slope triggering. For example, suppose the stimulus has a constant level but uses FM to vary the frequency, and you want to sync to a certain phase of the modulator waveform, as shown below:

[Trigger - FM Sync (41K image)]

Since one point in every cycle of the main tone frequency (carrier) meets the slope and threshold criteria, normal triggering can't handle this. One approach (which actually will work even if your card doesn't have a stable Duplex Delay) would be to create a signal on the other output channel that has exactly the same frequency as the modulator alone, and trigger on that.

But Gen Sync essentially does that automatically, and doesn't tie up the other output. Note that the "Gen Sync" button in the Trigger Control dialog of the prior image has here changed to "L.0 FM", which means "Left Channel, Stream 0, FM Modulator Source". This is activated by toggling Gen Sync on after setting FM Sync in the FM dialog.

In this particular example the carrier frequency was set to an integer multiple of the modulator frequency, so that the live wave appeared stationary, just like this image. If you don't do this, the display will still sync to the modulator as requested, but the carrier may be in a random phase on each update. Each update would still have the "slow" part of the wave in the same place, but its phase would be constantly changing so you would typically see an "eye" pattern.



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