Filter Design Toolbox

To Transform Lowpass FIR Filters

To transform the passband of your FIR filter, use the Transformations option on the menu bar:

1. Design or import your FIR filter into FDATool.
2. Select Transformations-->FIR from the menu bar.

FDATool present two transformation options -- Lowpass to Lowpass and Lowpass to Highpass.

3. Perform one of the following procedures to transform your lowpass FIR filter to a new passband form.
• To transform your lowpass filter to another lowpass filter, select Lowpass to Lowpass.

The following Confirmation Transformation dialog opens to tell you about the transformation result. Click OK to continue and transform your filter. Clicking Cancel stops the transformation, leaving your filter unchanged.



After you click OK, FDATool transforms your filter and updates the Current Filter Information to show you that your filter has been transformed. In the filter information, the Source is Transformed.

For example, the figure shown here includes the magnitude response curves for two filter. The original filter is a lowpass filter with rolloff between 0.2 and 0.25. The transformed filter is a lowpass filter with rolloff region between 0.8 and 0.85.



• To transform your lowpass filter to a highpass filter, select Lowpass to Highpass.

When you select Lowpass to Highpass, FDATool returns the dialog shown here. More information about the Select Transform... dialog follows the figure.

To demonstrate the effects of selecting Narrowband Highpass or Wideband Highpass, the next figure presents the magnitude response curves for a source lowpass filter after it is transformed to both narrow- and wideband highpass filters. For comparison, the response of the original filter appears as well.

For the narrowband case, the transformation algorithm essentially reverses the magnitude response, like reflecting the curve around the y-axis, then translating the curve to the right until the origin lies at 1 on the x-axis. After reflecting and translating, the passband at high frequencies is the reverse of the passband of the original filter at low frequencies with the same rolloff and ripple characteristics.

Transforming Filters

To Transform Lowpass IIR Filters