21 Filter Comparison.pdf

CHAPTER Filter Comparison 21 Decisions, decisions, decisions! With all these filters to choose from, how do you know which to use? This chapter is a head-to-head competition between filters; well select champions from each side and let them fight it out. In the first match, digital filters are pitted against analog filters to see which technology is best. In the second round, the windowed-sinc is matched against the Chebyshev to find the king of the frequency domain filters. In the final battle, the moving average fights the single pole filter for the time domain championship. Enough talk; let the competition begin! Match #1: Analog vs. Digital Filters Most digital signals originate in analog electronics. If the signal needs to be filtered, is it better to use an analog filter before digitization, or a digital filter after? We will answer this question by letting two of the best contenders deliver their blows. The goal will be to provide a low-pass filter at 1 kHz. Fighting for the analog side is a six pole Chebyshev filter with 0.5 dB (6%) ripple. As described in Chapter 3, this can be constructed with 3 op amps, 12 resistors, and 6 capacitors. In the digital corner, the windowed-sinc is warming up and ready to fight. The analog signal is digitized at a 10 kHz sampling rate, making the cutoff frequency 0.1 on the digital frequency scale. The length of the windowed-sinc will be chosen to be 129 points, providing th