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



Spectrum Analyzer

Signal Generator

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LCR Meter

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Trigger Gen Sync Mode

Controls: Trigger Dialog >> Gen Sync
Macro: TrigMode=GenSync

This is a special trigger mode for use when the signal you want to observe is a response to a stimulus from the Daqarta Generator. In this mode, the effective trigger is computed based upon the Sync setting in the Generator. Trigger Level and Slope are ignored. The Trigger Source buttons are disabled, since the source is the Generator itself.

Note that if the current Generator setup specifies Gen Sync, it will be used even if the Generator is off and only the Input is active. This will result in untriggered (free-run) behavior.

Since there are many different possible Sync sources in the Generator, when the Gen Sync button is active it changes to show the specific source. A typical setting would be:

L.0 Freq

The L.0 means that the sync source is L.0 Stream, which is the Left channel, Stream 0. The Freq indicates that the source is the main Tone Freq, instead of one of the modulators: Burst, Train, AM, FM, PWM, or Sweep. If Gen Sync is active but no Sync source has been selected in the Generator dialogs, then the button shows Gen Free to indicate that the trace is free-running (no sync). This will give the same rolling effect as if the Trigger button was toggled off.

Even though the Generator "knows" the exact timing of the trigger events in the waveform it is creating, that doesn't mean you will always get a trace update for every such event in the generated signal. The reason is that the Generator allows very fine resolution in its generated waveforms, both in amplitude and in frequency. If Tone Sync has been set and the tone frequency is not synchronous with the sample rate (via Step Lines), then the start of each wave cycle will not necessarily fall exactly on a sample; one sample may be somewhat below zero, and the next may be somewhat above.

If the frame always started on the first sample above zero, the trace could jitter back and forth by nearly half a cycle in the worst case. To control the jitter, the Generator Tone Frequency dialog has an Err control just below the Tone Sync button. This sets the maximum error (decimal fraction of one sample) that will be allowed in determining the trigger; if the true zero crossing would exceed this limit, no trigger is given on that crossing, and the next cycle is awaited.

If you set Err very low to reduce jitter, many cycles may be rejected until one lands close enough to zero. If more than about a second elapses without finding an acceptable cycle, 'SYNC WAIT' is shown on the (prior) frozen trace. You may choose to wait it out, or set a larger allowable error. (This message is not shown in Single trigger mode, nor during Spectrograms.)

Gen Sync allows you to trigger on events that can't be determined by simple Level and Slope triggering. For example, suppose the Generator is producing an FM stimulus, with a 1000 Hz carrier modulated by a 123 Hz sine. Since only the frequency is being modulated, the carrier amplitude is constant. That means that there would be a level-and-slope event on every cycle of the carrier, regardless of the modulation. There would be no way to distinguish the modulator from the carrier using just level and slope.

But since Daqarta is generating the FM modulator, it can provide the needed information for triggering. This assumes that you are simply observing the stimulus waveform. In the more usual case, you want to observe the response to the stimulus. For this to work properly, the sound card must be capable of keeping the input and output data in alignment at the chosen sample rate.

Many cards can do that, but often they can do it only at one particular sample rate, such as 48000 Hz. Some can do it only at 22050 or 44100. At other rates, the input and output data drift apart over time.

Besides keeping the input and output aligned, the card must be able to start and stop both processes at the same time, or at least with a fixed lag that can be calibrated by Daqarta and is repeatable from session to session. Otherwise, even though the sync point may be stable during the session, the response waveform won't be properly aligned with the stimulus.

The Duplex Delay option in the Auto-Calibration dialog will determine the drift, if any, and also determine the fixed sync lag for your system and card.

If your card is not suitable for Gen Sync use, you can wire the output of the Generator to one input channel, assuming that you only need to observe a mono response on the other input. Then you can set the Trigger Source to the input that has the Generator signal, and use ordinary Level and Slope triggering.

Does this mean you can't sync to things like an FM modulator? Not necessarily. If your experiment only requires a mono stimulus, you can create a special stimulus on the other Generator channel, and use that to provide the trigger signal. Since signals are digitally computed, and since Left and Right outputs are always exactly synchronized, you can just set the other Generator channel to produce a square wave at the same frequency as the FM modulator.

Macro Notes:

TrigMode=GenSync or TrigMode=3 sets the mode to Gen Sync. See Macro Notes under Trigger Auto Mode for the complete list of mode names and numbers.

See also Trigger Toggle - Introduction, Trigger Control Dialog


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