Daqarta for DOS Contents
TRIGGERING INTRODUCTION:Triggering allows the N samples for each trace to be collected synchronously with the input 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.
The simplest form of triggering is Internal, where a sweep is begun whenever the signal voltage passes a certain Level going in the direction specified by the Slope control. This works well when the signal being viewed has 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.
External triggering can be used if the input signal is a response to some stimulus you provide. The idea here is that the trigger comes from the stimulus, instead of from the response. You will need to provide a synchronous TTL digital version of that stimulus, such is available on most laboratory oscillators.
If all you need is a simple digital level stimulus, perhaps to start some external source or to use directly as a "click", then the trigger Pulse mode can provide it.
Special triggering modes are also provided for Virtual Source operation to allow synchronization to AM or FM modulators as well as the Main carrier wave.
TRIGGER (Trig) TOGGLE:The unshifted T-key toggles between Triggered and Free-run modes. In Triggered mode, the Trig option is highlighted and the trigger status on the title line will show TRIG. Triggering will use the either the Auto or Norm method, as selected by the Mode item in the trigger control menu.
In Free-run, the highlight is gone and the status shows FREE. The trigger Mode also shows Free, and is highlighted to indicate that it can't be accessed to change it until you toggle back to Trig operation.
You will probably want to run in Triggered mode while viewing repetitive waveforms, since for most sample rates they would otherwise roll by too fast to see. However, Free-run is often useful in conjunction with RTime mode when running at very low sample rates, where Triggered operation might involve long waits between screen updates: Free-run RTime mode will allow overlap operation, and at slow sample rates the trace will update on nearly every sample. Waveforms will seem to scroll slowly across the screen from right to left, with each new sample appearing on the right end of the trace. This allows you to see changes immediately.
You may thus see a discontinuity at the change or unPause point of the waveform, which will scroll across the screen. This does not necessarily represent a real discontinuity in the input signal. When you Pause and unPause the trace, for example, it is only the display of the signal which is discontinuous.
The discontinuity may or may not be obvious when viewing waveform data, but you must take special care with spectra: The discontinuity can make a large change in the spectrum, which will not dissipate until all samples for a complete new trace are acquired.
Overlap sampling is also commonly used for Spectrograms even at higher sample rates, which again requires RTime Free-run mode.
The Trig option is disabled such that the current state can't be changed when the trace is Paused, so that the trigger status always shows the conditions under which the visible trace was acquired. The exception to this is when you are in DDisk Read mode, where manual motion through the file is allowed during the Pause state. Here the trigger status indicates the state that will be in effect when you unPause to go into automatic replay operation.
Trig is disabled to prevent changes during True averages, since the final result would be misleadingly identified. There is no similar limitation for Exponential mode, since operation is continuous until manually Paused. If you toggle Trig just before Pausing an Exponential average, you are on your own in trying to interpret the results. Similarly, spectrum Peak mode does not disable Trig, whether the Sweeps parameter is set to a specific number or to Continuous.
The Trig option is also disabled in Sequential mode when the Stim or Pulse trigger sources are active, since by the nature of sequential stimulus generation the output is synchronous with the input. RTime mode allows you to select Internal triggering even during stimulus generation, if you need it.
TRIGGER CONTROL MENU:CTRL-T brings up a Trigger control menu appropriate for the current state of operations. The Virtual Source trigger menu has no Cycle option, and has different Source options than the Board version. These are separate menus: Changes to one don't affect the other. There are also changes in the Board trigger menu depending on RTime mode, Source selection, and the state of the Stimulus Generator.
TRIGGER MODE:Toggle between Auto and Norm by hitting Enter with the cursor at this item. This option is always present in the Virtual Source trigger menu, but if Board is active it is only present if Source is Intern or Extern, not Stim or Pulse.
If you use the T-key to toggle Trig off, Mode shows Free and is highlighted as a reminder that cursor access to this menu item is blocked until you return to Trig operation.
TRIGGER MODE Auto:This is similar to the auto-trigger option on a conventional oscilloscope, if you are already familiar with that. The trigger logic waits for the indicated signal Slope polarity, then waits for the signal to cross the indicated trigger Level. In contrast to Norm mode, however, the level actually used is adjusted after each sweep so that it is the same proportion to the signal peak as Level is to full-scale.
Suppose, for example, that Level is set to 500 mV on a 1 Volt input range, or half of full scale. Then the actual trigger level will be at half of the zero-to-peak amplitude of the signal, and it will self-adjust as the input level changes... if the signal level is only 42 mV, then the effective trigger level will be 21 mV. This allows a stable trace under changing input conditions.
The Auto-trigger logic scans N samples waiting for a valid trigger, based upon the current level set by the previous sweep. If a trigger is not found, the next N samples are processed anyway and used to compute the trigger level for the next sweep. The waveform display may thus "hiccup" for one sweep when the level drops suddenly.
A powerful advantage of Auto mode is that you thus ALWAYS get a more-or-less ongoing sweep display. This allows you to see what is going on, even when things are not happening the way you assumed when you set the original trigger Level.
TRIGGER MODE Norm:Normal mode, just like that mode on an oscilloscope, will wait forever for the signal to cross the indicated trigger Level with the indicated Slope. However, unlike many scopes, it will show TRIGGER WAIT at the upper right of the (prior) trace if the wait is more than two times N samples. To break out of a wait for a trigger that doesn't arrive, you can either adjust your experiment, or change the trigger parameters. You could punch Auto to view the current signal, then set a new trigger Level based upon that.
You can combine this event-waiting ability with the Key Macro system to create complex sequences of operations that are initiated when the trigger event arrives. The Applications section gives suggestions for sound-triggered recording, sound-triggered spectrograms, and Oother event-triggered processes.
Since Daqarta can't predict the time between triggers in Norm mode, the spectrogram time axis may not be accurate. In RTime mode an attempt is made to estimate the spectrogram time by measuring a few sweeps, so if the triggers are very far apart it may take a long time for the axes to appear when you first go to Sgram mode. In Sequential mode the axes will appear immediately, but they will be the same as for Auto mode. With infrequent triggers the X-axis may thus greatly underestimate the Norm time.
TRIGGER MODE Free:Free-run means that there is no triggering... the next sweep starts immediately after the last one, and waveforms roll by unsynchronized with the start of the trace area. This mode is not actually selected by any operation on this Mode item... it is the result of toggling the Trig option off with the T-key. Free is then shown highlighted in the menu, which is a reminder that cursor access to the Mode item is denied until you toggle Trig back on.
The main use of Free-run mode is with Spectrum or Spectrogram modes, where the effects are not so hard on the eyes as is trying to view an unsynchronized waveform. In RTime mode, Free-run allows overlap processing, if your system is fast enough relative to the sample rate.
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