Daqarta for DOS Contents
TRIGGER SOURCE Main (Virtual Source mode only):In this mode, the input to the trigger logic is taken from the main Wave signal after it has been scaled to the indicated signal level but BEFORE any noise, FM, or AM is applied. So, for example, even with AM active the triggered trace will always start at the same phase of the carrier, regardless of how large or small its instantaneous amplitude becomes due to the modulation. The AM envelope will just roll through the trace area, so you will probably want to use Pause or better yet Single Sweep to see the actual trigger point.
Since the trigger source is taken after the main wave level scaling, if you set the absolute value of the trigger Level higher than the main wave level and Norm mode is active, no trigger will be detected. The prior trace will remain on the screen and you will see the TRIGGER WAIT message in the upper right portion of the trace.
Also, since the trigger source is taken before the addition of any added noise, the triggering will be much more stable than a real-world noisy signal, where the noise would cause trigger jitter by forcing the trigger level to be reached earlier or later than the desired point on the wave.
TRIGGER SOURCE AM (Virtual Source mode only):Here only the AM modulator is used as the trigger signal, so its envelope will appear frozen on the screen while the main carrier rolls by beneath it. Note that the instantaneous modulator value is zero at the level where the carrier is at its original maximum, and goes above and below that point. The most-negative level of the modulator is where the modulated carrier has minimum amplitude... with 100% AM, setting the trigger Level to negative full-scale will give a trace that starts at the nulls of the modulation. The trigger is taken from the AM modulator before the AM Pct scaling, so AM Pct will not affect the phase of the trigger point relative to the envelope.
TRIGGER SOURCE FM (Virtual Source mode only):Similar to Source AM, but here only the FM modulator is used as the trigger signal. The trigger is taken before the delta-F Hz scaling, so that has no effect on the trigger phase. See the Frequency Modulation and FM Triggering sections of the Virtual Source Menu for more discussion.
TRIGGER SOURCE Intern (Board mode only):This is just like the Internal triggering option on conventional oscilloscopes. The trace starts when the signal voltage crosses a value specified by the trigger Level control, in the Slope direction. The default Level of 0 will probably be fine for viewing signal sources having good signal-to-noise ratio, especially if they are simple repetitive waveforms like sine waves.
Real-world response signals, however, often have a lot of noise, and typical waveforms may consist of periods of low-level waves with a few higher-level peaks. A good example to consider is the typical heart monitor, where there is no signal of interest between beats... only noise. In this case you can't set the Level to 0, since noise could trigger the trace at random time points. Instead, you must locate one peak that is reliably higher than any instantaneous noise peak, and set the Level to trigger on that alone.
CAUTION: Do not connect any electrical equipment to a living subject without proper signal isolation techniques. A lethal shock could result.
For a heart monitor, it happens that there is a convenient peak known as the "R wave", but there are many signals that are not so cooperative. If a signal has multiple peaks for each response, the trigger point needs to unambiguously select the largest one... otherwise, you will sometimes trigger on one peak, and sometimes on another. You can always use Trigger Delay to align the time axis of the trace if the trigger point isn't at the beginning of the part you want to view.
Due to its sensitivity to noise, Internal triggering is not very useful if you want to use waveform averaging to reduce noise, unless the noise is already much smaller than the peak you are triggering on. Remember that the noise is usually present throughout the waveform, so that even if your trigger Level is high, it will be jiggled up and down by the noise. This will cause the trace to sometimes start earlier or later than the specified instant, and may cause an average to smear the details of the desired waveform. Higher noise levels thus cause greater smearing in an average when Internal triggering is used.
TRIGGER SOURCE Extern (Board mode only):This is only available in Sequential mode, not RTime. The driver looks at a digital input bit specified in the Board control menu, and triggers the sweep when that bit makes the transition indicated by Slope: Low-to-high for Pos, and high-to-low for Neg. The trigger Level control is not available.
External triggering is usually more stable than Internal level triggering, since it does not depend upon the response reaching a certain level. Noise or response variations thus have no effect on triggering action.
However, this mode does require that you have a TTL-type digital signal available to trigger from. This is often the case when you are looking at the response to an externally generated stimulus. If the stimulus is initiated by a pulse generator, for example, you may be able to get a trigger signal from that. Many laboratory-type oscillators have a TTL output that is exactly synchronized with the main output sine, triangle, or square wave.
If your board does not have digital inputs (most sound cards do not), Daqarta may still allow external triggering by using a printer port specified via a parameter on the ADC line of the configuration file. See the Help system for your board via CTRL-H with the Board menu active, or via the ALT-H installed module list, to find the proper parameter setting and pins to use.
If there is no digital input capability, Daqarta will not allow Extern to be selected.
TRIGGER SOURCE Stim (Board mode with StGen active only):(See also Pulse source, following.)
With the Stim source, the trigger is initiated by Daqarta at the same time the stimulus is generated for the experiment. There is thus "perfect" trigger stability. Trigger Delay may be used to view events before or after the start of the stimulus. The trigger Mode and Level controls are always blanked when Stim is active.
Of course, to use this mode Daqarta must generate the stimulus. If a Stimulus Generator module is present in your DQA.CFG file, the CTRL-StGen option (CTRL-G) will be available. This will allow you to specify complex stimuli on up to two analog output channels and 8 digital output lines. You may also specify stimuli that alternate on successive acquistion sweeps.
In Sequential mode, the Stim source is always selected if the Stimulus Generator is on, and the Slope controls vanish along with Mode and Level. The trigger Cycle control is available to determine the stimulus repetition rate and hence sweep repetition rate.
The only exception to this is if the special Pulse option is active in the STIM3A Sync menu (old STIM3 Misc menu), in which case the trigger source will be Pulse even though the Stimulus Generator is active.
In RTime mode, Source can be set to Stim when the Stimulus Generator module is active, otherwise the cursor will bypass the Source option and it will be fixed at Intern. When Stim is set all other menu items vanish except Delay. Repetition rate is controlled strictly by the length of the stimulus cycle set via the Stimulus Generator control menu.
If there is no Stimulus Generator active, you will not be able to select the Stim source.
TRIGGER SOURCE Pulse (Board mode only):There is also a very simple "stimulus generator" available even without the StGen option: In Sequential mode only, most boards will allow Daqarta to generate a high or low digital pulse output, depending on the trigger Slope polarity, for the duration of each sweep.
If your board has no built-in digital outputs (most sound cards don't), you may still be able to use a printer port output to provide the pulse. See your board's Control Help system to find the proper way to do this.
If the trigger Slope is Alt, the pulse will go high at the start of one sweep, and low at the start of the next. This pulse may be used to trigger an external stimulus generator, or may be modified by simple circuits or used directly. This makes a good "step function" for transient analyses.
The trigger Cycle control is available to control the timing of the Pulse sweeps, just as for StGen operation.
Even if the Stimulus Generator module is available, there may be good reasons to use the Pulse option instead. When the StGen module produces digital outputs, it reads stimulus data from a buffer and outputs one byte for each input sample acquired. This output happens even if there is no change from the last output. If your board or system has improper shielding or layout, the "strobe" for each output sample can be picked up as a low-level "glitch" signal on the input line. With the Pulse option, this contamination is confined to the trigger sample only. Since the response of a real-world system will be delayed from the stimulus, it should escape the contamination.
The Pulse option can also be used together with the STIM3A StGen system, with DAC analog outputs produced by the stimulus generator module and the normal DigOut option inactive. To do this you need to go to the Stimulus Gen Misc menu page with CTRL-Pg and set the Pulse option to On. Now the main Trig menu will show Source Pulse and the Slope options will work as for a simple Pulse output.
The Pulse option can also be activated while using a sound card's built-in synthesizer to generate stimuli. Unlike STIM3A, using the synth does not affect the Trigger Source options that are available. However, if the synth is set for Burst or Gap stimulus generation, the Pulse output (from the LPT printer port, since the sound card lacks TTL outputs) will only be active during the Burst or Gap portion of the sweep, not for the entire sweep as when the synth is Off or On.
The Pulse source option will not be available unless your board supports it, either directly or via a printer port.
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