FIR Windowed Filter

C/C++ Equivalent: NISCOPE_VAL_WINDOWED_FIR_FILTER

FIR filters are well suited for applications that require phase information, because the phase response is always linear. This linearity prevents distortion of time-domain filtered data. The transient response of an FIR filter is a fixed number of points, filter taps –1, which are eliminated in the filtered waveform returned from this VI.

Windowed FIR filters use the cutoff frequency as part of the design process, but the cutoff is specified before the windowing operation. For Windowed FIR filters of sufficient taps (more than 10 for a simple lowpass), the cutoff frequency has a magnitude near 0.5 (or 6 dB down). This magnitude varies slightly with the order and the window type.

The number of taps in the filter (that is, the number of coefficients in the filter) is given by the Filter Taps property or the NISCOPE_ATTR_MEAS_FILTER_TAPS attribute. This number must be odd for highpass and bandstop filters. The window for the coefficients is set with the FIR Window property or the NISCOPE_ATTR_MEAS_FIR_FILTER_WINDOW attribute.