Multirate FIR Design Express VI

Digital Filter Design VIs and Functions

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Multirate FIR Design Express VI

Owning Palette: Multirate Filter Design VIs

Installed With: Digital Filter Design Toolkit

Creates a finite impulse response (FIR) multirate filter.

Details  

Dialog Box Options
Block Diagram Outputs
 Place on the block diagram  Find on the Functions palette

Dialog Box Options

ParameterDescription
Magnitude ResponseContains the plot of the magnitude response. You can drag the cursors in the plot to change the specifications. The color you specify in Passband represents the passband response and the passband edge frequency. The color you specify in Stopband represents the stopband attenuation and the stopband edge frequency. The color you specify in Stopband limit represents the stopband limit. The color you specify in Nyquist represents half the sampling frequency, also known as the Nyquist frequency.
  • Magnitude in dB—Specifies whether this Express VI uses decibels or a linear scale to express the magnitude response. If you place a checkmark in the checkbox, this Express VI converts a linear magnitude response to decibels. This Express VI uses decibels by default.
  • Nyquist—Specifies the color of the line in the magnitude plot that represents the Nyquist frequency. The default is green.
  • Passband—Specifies the color of the lines in the magnitude plot that represent the passband response and the passband edge frequency. The default is blue.
  • Stopband limit—Specifies the color of the line in the magnitude plot that represents the maximum stopband edge frequency. The default is yellow.
  • Stopband—Specifies the color of the lines in the magnitude plot that represent the stopband attenuation and the stopband edge frequency. The default is red.
Flow DiagramReturns the flow diagram of the multirate filter you specify in Filter type.
Filter CoefficientsReturns the coefficients of the multirate filter according to the settings you specify on the Floating-Point Design page.
  • Filter order—Displays the order of the multirate filter you design. The value of Filter order+1 equals the number of coefficients.
Main SettingsContains the following options:
  • Design method—Specifies the method that this Express VI uses to design the multirate filter. The valid values include Kaiser Window, Dolph-Chebyshev Window, and Equi-Ripple FIR. The default is Equi-Ripple FIR.
  • Filter type—Specifies the type of multirate filter that this VI creates. The valid values include No Rate Change, Interpolation, Decimation, and Rational Resampling. The default is Decimation.
  • Filter factor—Specifies the sampling frequency conversion factor of the multirate filter. Contains the following options:
    • L—Specifies the interpolation factor for an interpolation or rational resampling filter. This input is available only if you set Filter type to Interpolation or Rational Resampling. The value of this input must be an integer greater than or equal to 2. The default is 1.
    • M—Specifies the decimation factor for a decimation or rational resampling filter. This input is available only if you set Filter type to Decimation or Rational Resampling. The value of this input must be an integer greater than or equal to 2. The default is 2.
Filter SpecificationsContains the following options:
  • Input sampling frequency—Specifies the sampling frequency, in hertz, of an input signal for the multirate filter. This input must contain a value greater than zero. The default is 1k Hz.
  • Passband edge frequency—Specifies the passband edge frequency of the multirate filter. The default is 200 Hz.
  • Stopband edge frequency—Specifies the stopband edge frequency of the multirate filter. The default is 250 Hz.
  • Output sampling frequency—Returns the sampling frequency of the output signal for the multirate filter.
  • Passband ripple—Specifies the passband ripple of the multirate filter in a unit that the Magnitude in dB option determines. If you place a checkmark in the Magnitude in dB checkbox, Passband ripple must be greater than zero. The default is 0.1 dB. If you remove the checkmark from the Magnitude in dB checkbox, the valid range of Passband ripple is (0, 1). The default then is 0.011.
  • Stopband attenuation—Specifies the stopband attenuation of the multirate filter in a unit that the Magnitude in dB option determines. If you place a checkmark in the Magnitude in dB checkbox, Stopband attenuation must be greater than zero. The default is 60 dB. If you remove the checkmark from the Magnitude in dB checkbox, the valid range of Stopband attenuation is (0, 1). The default then is 0.011.
Transition band aliasing allowedSpecifies if you allow frequency aliasing in the transition band.
Quantization SettingsContains the following options:
  • Quantize filter—Specifies if you want to use this Express VI to quantize the floating-point multirate filter you design. If you enter the multirate filter specifications, you can place a checkmark in the Quantize filter checkbox after you click the Update Design button.
  • Coefficients word length—Specifies the word length, in number of bits, that this Express VI uses to represent the filter coefficients. This option is available only if you place a checkmark in the Quantize filter checkbox. The valid range is [1, 32]. The default is 16.
  • Gain word length—Specifies the word length, in number of bits, that this Express VI uses to represent the multirate filter gain if the gain processing occurs on an NI Reconfigurable I/O (RIO) target. This option is available only if you place a checkmark in the Quantize filter checkbox and select On Target from the Gain processing pull-down menu. The valid range is [1, 32]. The default is 16.
  • Coefficients scale type—Specifies how this Express VI scales the multirate filter coefficients. This option is available only if you place a checkmark in the Quantize filter checkbox. The valid values include No Norm, Time Domain-1 Norm, Time Domain-2 Norm, and Time Domain-Inf Norm. The default is Time Domain-1 Norm.
  • Scale by power of 2—Specifies whether this Express VI scales the multirate filter with the original norm value or the smallest power of 2 value that is greater than the norm value. This option is available only if you place a checkmark in the Quantize filter checkbox. If you place a checkmark in the Scale by power of 2 checkbox, this Express VI scales the multirate filter with the smallest power of 2 value. If you remove the checkmark from the Scale by power of 2 checkbox, this Express VI scales the multirate filter with the original norm value.
  • Gain processing—Specifies whether the gain processing occurs on a host machine or an NI-RIO target. This option is available only if you place a checkmark in the Quantize filter checkbox.
  • Postprocessing filter gain—Displays the value of the postprocessing gain. Manually multiply the fixed-point output signal by the postprocessing gain to obtain the floating-point output signal or automatically use the DFD FXP MRate Postprocessing VI to handle the postprocessing gain.
TipsDisplays tips and error messages that help you use this Express VI to design a multirate filter.
Update DesignUpdates the floating-point multirate filter design with the specifications you entered. When you click the Update Design button, you enable the Quantize filter option on the Fixed-Point Quantization page.

Block Diagram Outputs

ParameterDescription
error outDescribes the error status that this VI or function produces.
multirate filter outReturns the new multirate filter.
postprocessing filter gainReturns the value of the postprocessing gain. Manually multiply the fixed-point output signal by the postprocessing gain to obtain the floating-point output signal or automatically use DFD FXP MRate Postprocessing VI to handle the postprocessing gain.

Multirate FIR Design Details

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 place a 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.
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