Data AcQuisition And Real-Time AnalysisScope - Spectrum - Spectrogram - Signal Generator
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Spectrum Average Add and Average Subtract
Once a spectrum average (other than Exponential or Continuous Peak) has reached the requested number of Frames, it enters Pause mode. If you simply hit Average again, a fresh average starts and the old average is discarded. Instead, you can use Average Add or Subtract to continue from where you left off.
If you hit Avg Add (CTRL+ALT+A) the existing average will simply be extended by the requested number of frames. For example, if the original average was 128 frames and you hit Avg Add without changing the request value, another 128 frames will be added. The frames counter will count up from 128 to 256 as the extended average proceeds, and the final result will be as if you had requested 256 frames in the first place.
On the other hand, if you instead hit Avg Sub (CTRL+SHIFT+A), the second 128 frames will be subtracted from the average. In other words, the spectra will be added with inverted polarity, which will tend to reduce the accumulated spectrum. The frames counter will still count up from 128 to 256, but any constant features of the signal will average down toward zero. Note, however, that displayed spectra are magnitudes that show only the absolute values of the components at each frequency. So if you go too far with Avg Sub, the spectrum may appear to rise again.
You can start a normal-polarity initial average with Avg Add or the usual Average toggle. If you start the initial average with Avg Sub the averaged spectrum will be inverted, but that fact won't be obvious due to the magnitude-only display.
Although the spectrum traces only show absolute values, the cursor readouts will show signed values. However, in Y-log (power spectrum) mode the absolute value is taken before the logarithm used for dB conversion, so negative Avg Sub components can not be detected there. (A negative dB value indicates a value below that channel's full-scale range value.)
You might want to use Avg Add in cases where the result from the original average is not quite clear enough, and you don't want to waste the original frames. This might be the case where you have a limited time to collect data before conditions change, or where averaging is inherently slow because of some limit on how fast you can induce responses from the system being measured.
One use for Avg Sub is to remove background noise or signals. You take the initial average with the desired signal present, then use Avg Sub with only the background present. This method is typically not as effective as synchronous waveform averaging, but it has the advantage that no synchrony (Trigger) is required.
If your experiment involves a response to a stimulus that the Daqarta Generator is creating, then you can use Gen Sync for the waveform average. Or, if a separate sync signal is available, you can put it on another channel and trigger on that. But suppose you are interested in some signal whose presence you may or may not be able to control. You can take an initial average when the signal is present, then use Avg Sub when the signal is absent to reduce the background.
SpavgAdd= and SpavgSub= can be used to start a new average or to extend an average that has completed. Either command will abort if given while an average is running or Paused prior to completion, since the controls are disabled.
Any value given with the command is ignored. In particular, you can not extend an average by a specified value.
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