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For Trigger modes other than Gen Sync (namely Auto, Auto Level, or Normal), a trigger occurs whenever the input signal reaches or crosses the threshold set by Level while going in the direction specified by Slope. (This assumes Trigger Hysteresis is zero.)
The default Level units are percent of full-scale, indicated by '%' on the small button to the right of the Level control. You can toggle this button to change the units to Volts, or to User Units, if active. Note that the units and scaling will be those of the Trigger Source channel. These may be different than currently shown on the Y axis. To get the Y axis to match, you can toggle higher-priority display channels off.
The default trigger level of zero is a general purpose value, since most input signals pass through zero. But you will often get better results by setting a larger value, positive or negative, depending upon your signal. If each waveform cycle has multiple zero crossings, a level set at zero could trigger on any one of them. Similarly, if the input spends a lot of time near zero, setting the trigger level there will result in spurious triggers from noise which goes back and forth through zero.
For example, suppose you are looking at the response of a system to a recurring impulse... say a hammer striking a bell, or a plucked guitar string. The response will have large oscillations that decay toward zero. You would set the trigger level just below the highest peak in order to avoid triggering on lesser peaks or random background noise between impulses.
However, for high frequencies (say, over 10 kHz), Level should be near zero for best results. This is because the trigger logic must find at least one sample prior to the trigger level to establish the selected slope, before it can accept a valid trigger. At high frequencies, there are fewer samples per cycle; if Level is set too high (assuming positive Slope), the first sample can be just below it but the very next sample may be beyond the peak and actually be below the Level value.
Note that Hysteresis has a major effect on trigger operation, by causing the effective Level setting to be increased or decreased depending on signal direction. It is especially useful for noisy signals, particularly when using the Frequency Counter.
You can adjust Level by entering or scrolling the control value, but it may be more convenient to drag the solid horizontal line that shows the Level setting directly on the waveform display: Hold the SHIFT key down, press the left mouse button, and drag the line up or down as desired. (The dotted lines, if any, represent a non-zero Hysteresis setting and can be ALT-dragged.)
Note that Hysteresis is always positive, and that the sum with the absolute value of Level is limited to 100%. So if Hysteresis is at 10%, for example, then Level is limited to +/-90%.
If you are also using Screen Zero to affect the apparent position of the waveform, please note that the Trigger Level value applies to the raw data, prior to any Zero offset. You may wish to toggle Screen Zero off (ALT+Z) while making Level adjustments.
The % button determines the units that are used when setting Trigger Level, and also Hysteresis. TrigLevUnit=0 sets the default % units, TrigLevUnit=1 sets Volts (or User Units, if active), and TrigLevUnit=x toggles between them. If you want to set Level in User Units, be sure to give a UserUnits=1 command as well.
If % units are active, TrigLevel=500m sets Trigger Level to 0.500%. Otherwise, it sets Trigger Level to 0.500 Volts or User Units.
Alternatively, TrigLevel=>1 increments Trigger Level by 1% and TrigLevel=>-1 decrements by 1% if the units are %. Otherwise, these increment or decrement by the minimum step size equivalent to 0.00305% in the chosen units.
When reading, X=TrigLevel sets variable X to the Trigger Level value, in the selected units. Alternatively, X=TrigLevel?% returns percent regardless of the selected units. This allows unit-independent macro behavior, such as for Array Frequency Counting or Edge/Event/Trigger Detection.
Caution: Be mindful of the interaction between Level and Hysteresis when setting these values via macros. If you want to set Level to 95% but Hysteresis is currently at 10%, the resultant Level will only be 90%.
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