![]() ![]() The maximum tolerated variation of signal power in the pass-band (here from DC to fPB) is called the ripple of the filter. Click image to enlarge.īut perhaps an attenuation of 3 dB is already too much for your application. Take your calculator and enter 10 × log(0.5), you will get –3.01, which everybody rounds to –3 dB.įIGURE 2: A filter (low-pass in this case) is specified by its cutoff frequency f3dB, its ripple in the pass-band, and its rejection in the stopband. Aren’t you fluent with decibels? A decibel is one tenth of a Bel, and a Bel is the base-10 logarithm of the ratio of two powers. This means that the losses of the filter will be 3 dB at that frequency. By definition, this is the frequency at which the filter attenuates the power of the signal by 50%. ![]() The first parameter is the filter cut-off frequency, of course. Want to specify a filter? Figure 2 illustrates this on a low-pass filter. For example, a 50- or 60-Hz notch filter is included in virtually every weight scale to remove EMC perturbations from the surrounding power lines. Lastly, a band stop filter, often called a notch filter, does the opposite, and it attenuates a selected range of frequencies. For example, any radio frequency receiver is a band-pass filter, providing attenuation of all signals except for frequencies close to its preset frequency. Band-pass filters are a combination of both, and they attenuate all frequencies below or above a given range. Click image to enlarge.Ĭonversely, a high-pass filter attenuates the low frequencies, and could in particular remove any DC component of a signal. Each one attenuates a specific frequency range. It is perfect for removing high-frequency noise on a signal coming from a sensor.įIGURE 1: Four classic types of frequency filters. A low-pass filter lets the low frequencies pass through, but attenuates high-frequency signals. Figure 1 depicts the most classic filter types. By definition, a filter is a circuit that attenuates some signals more than others, depending on their frequency. I promise, no Laplace transforms or poles or zeros, just electronics. I will try to help you to specify a filter, understand the main filter variants, and efficiently use some great computer-aided design tools. My goal for this article is more pragmatic. I bet a textbook about filters full of math would bore you, right? Well, relax. Filters are definitively useful, simple, and even fun. Unfortunately, filter design, or even their use, is often perceived as a difficult task close to black magic. So analog filters must be in the bag of tricks of any designer. This month, I’ll focus on filters-more precisely, analog frequency-selective filters, which are used in audio devices, as well as for noise reduction, antialiasing before digitizing a signal, separation of frequency-multiplexed signals, frequency response correction, and so on. Op-amps shine in plenty of applications-in particular, to build active filters. I spoke about operational amplifiers (op-amps) in my last few columns. The following article by Robert Lacoste appears in Circuit Cellar 307, 2016. ![]()
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