How to Filter Data Using the App

Filtering Time-Domain Data in the App

The System Identification app lets you filter time-domain data using a fifth-order Butterworth filter by enhancing or selecting specific passbands.

To create a filtered data set:

Import time-domain data into the System Identification app, as described in Represent Data.
Drag the data set you want to filter to the Working Data area.
Select <--Preprocess > Filter. By default, this selection shows a periodogram of the input and output spectra (see the etfe reference page).
Note
To display smoothed spectral estimates instead of the periodogram, select Options > Spectral analysis. This spectral estimate is computed using spa and your previous settings in the Spectral Model dialog box. To change these settings, select <--Estimate > Spectral model in the System Identification app, and specify new model settings.
If your data contains multiple input/output channels, in the Channel menu, select the channel pair you want to view. Although you view only one channel pair at a time, the filter applies to all input/output channels in this data set.
Select the data of interest using one of the following ways:
- Graphically — Draw a rectangle with the mouse on either the input-signal or the output-signal plot to select the desired frequency interval. Your selection is displayed on both plots regardless of the plot on which you draw the rectangle. The Range field is updated to match the selected region. If you need to clear your selection, right-click the plot.
- Specify the Range — Edit the beginning and the end frequency values.
  For example:
  8.5 20.0 (rad/s).
  Tip
  To change the frequency units from rad/s to Hz, select Style > Frequency (Hz). To change the frequency units from Hz to rad/s, select Style > Frequency (rad/s).
In the Range is list, select one of the following:
- Pass band — Allows data in the selected frequency range.
- Stop band — Excludes data in the selected frequency range.
Click Filter to preview the filtered results. If you are satisfied, go to step 8. Otherwise, return to step 5.
In the Data name field, enter the name of the data set containing the selected data.
Click Insert to save the selection as a new data set and add it to the Data Board.
To select another range, repeat steps 5 to 9.

Filtering Frequency-Domain or Frequency-Response Data in the App

For frequency-domain and frequency-response data, filtering is equivalent to selecting specific data ranges.

To select a range of data in frequency-domain or frequency-response data:

Import data into the System Identification app, as described in Represent Data.
Drag the data set you want you want to filter to the Working Data area.
Select <--Preprocess > Select range. This selection displays one of the following plots:
- Frequency-domain data — Plot shows the absolute of the squares of the input and output spectra.
- Frequency-response data — Top axes show the frequency response magnitude equivalent to the ratio of the output to the input, and the bottom axes show the ratio of the input signal to itself, which has the value of 1 at all frequencies.
If your data contains multiple input/output channels, in the Channel menu, select the channel pair you want to view. Although you view only one channel pair at a time, the filter applies to all input/output channels in this data set.
Select the data of interest using one of the following ways:
- Graphically — Draw a rectangle with the mouse on either the input-signal or the output-signal plot to select the desired frequency interval. Your selection is displayed on both plots regardless of the plot on which you draw the rectangle. The Range field is updated to match the selected region.
  If you need to clear your selection, right-click the plot.
- Specify the Range — Edit the beginning and the end frequency values.
  For example:
  8.5 20.0 (rad/s).
  Tip
  If you need to change the frequency units from rad/s to Hz, select Style > Frequency (Hz). To change the frequency units from Hz to rad/s, select Style > Frequency (rad/s).
In the Range is list, select one of the following:
- Pass band — Allows data in the selected frequency range.
- Stop band — Excludes data in the selected frequency range.
In the Data name field, enter the name of the data set containing the selected data.
Click Insert. This action saves the selection as a new data set and adds it to the Data Board.
To select another range, repeat steps 5 to 8.

Related Examples

How to Filter Data at the Command Line

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Filtering Data