Spider DSA User’s Manual
314
sinc(SIN(X)/X) function with a window such as a Hanning, Flat Top, etc. to
produce the desired frequency effect.
Figure 179Sinc function is the Fourier transform of a square shape.
A data window FIR filter is generated by starting with an ideal “brick-wall” shaped
filter; that is a filter with vertical edges or zero transition band width as shown on
the left in Figure 179. The brick-wall filter is specified by its cutoff frequencies and
it has band-pass amplitude of 1 and stop band amplitude of zero. The problem
with the ideal brick-wall filter is that the time response oscillates forever and it
requires an infinite number of filter coefficients.
This ideal filter can be modified by applying a data window to force the time
response to decay in a finite amount of time. Of course, this degrades the ideal
shape of the brick-wall filter. It introduces ripple, increases the transition band
width and decreases the stop band attenuation but it allows the filter to be defined
by a finite number of filter coefficients. Filter performance can be modified by
using different data windowing functions that offer tradeoffs between filter order
and response time. The user must choose these settings during the filter design.
Figures below show a comparison of various data window choices for the same
filter settings. In all cases the low and high cutoff frequencies are 0.1 and 0.2
relative to the sampling frequency. The number of filter taps is 67.
To Next Page
Loading...