The filter type and design method can be selected from the pull-down menus in the Configure Multirate Filter Design dialog box where the filter factor can also be specified. The filter specifications can be entered through either the numeric controls on the right side of the tab or the magnitude response graphical interface in the configuration dialog box. The results are equivalent.

After you add the Multirate FIR Design Express VI to the block diagram, the Configure Multirate Filter Design dialog box appears, as shown in the following figure:

Using the Numerical Controls

Similar to the numerical controls for single-rate filter design, the numerical controls for multirate filter design also contain frequency ranges and ripple constraint settings. In addition to these settings, you also need to specify the sampling frequency of the input signal in the Input sampling frequency numeric control. This VI then automatically calculates the sampling frequency of the output signal based on the filter specifications you entered.

Using the Graphical Interface

The Magnitude Response graph displays the magnitude response of the designed multirate filter. The magnitude axis can be either a linear or a logarithmic scale. Remove the checkmark from the Magnitude in dB checkbox to use a linear scale, or keep the checkmark in the Magnitude in dB checkbox to use a logarithmic scale.

The Magnitude Response graph contains a set of cursors that you can use to specify the passband and stopband. Use the passband and stopband cursors to change the passband and stopband, respectively, of the multirate filter. Under the linear scale, the distance between unity and the horizontal passband cursor specifies the maximum passband ripple. The location of the vertical passband cursor indicates the passband edge frequency. The stopband cursors work the same when defining the specifications of the stopband. Under the logarithmic scale, the distance between 0 dB and the horizontal stopband cursor specifies the stopband attenuation.

Guidelines for Entering Filter Specifications

As you define a filter specification, you must adhere to a set of rules to maintain valid specifications. If you do not adhere to the following rules, the Configure Multirate Filter Design dialog box displays a message in the Tips indicator with suggestions for repositioning the cursors.

  • Keep the horizontal cursors in the range (0, 1) in a linear scale or (−inf, 0 dB) in a logarithmic scale.
  • Keep the horizontal passband cursor above the horizontal stopband cursor.
  • The Passband edge frequency value must be less than the Nyquist frequency, or you must keep the vertical passband cursor to the left of the Nyquist cursor.
  • The Stopband edge frequency value must be greater than the Passband edge frequency value, or you must keep the vertical passband cursor to the left of the stopband cursor.
  • If you remove the checkmark from the Transition band aliasing allowed checkbox to avoid aliasing in the transition band, keep the Stopband edge frequency value between the Passband edge frequency value and the Nyquist frequency, or keep the vertical stopband cursor between the vertical passband and Nyquist cursors. If you keep the checkmark in the Transition band aliasing allowed checkbox to allow aliasing in the transition band, keep the vertical stopband cursor between the vertical passband cursor and the vertical stopband limit cursor.

After you finish entering the filter specifications, click the Update Design button to apply the new specifications.