Industrial Electronics

Video: Introduction to Analog Filters

12 December 2016

I am introducing a presentation video related to the general topic of analog filters. The presentation is a general overview of analog filters without the rigor of mathematical equations or design methods. This first video includes the following concepts:


-Signal representation, including time domain and frequency domain

-Ideal filters

-Practical filters

-Filter types

-Frequency domain characteristics of Butterworth, Chebyshev, elliptic and Bessel-type filters

-Filter realizations, passive filters and active filters


Filters are electrical circuits designed to remove, attenuate, or alter the characteristics of electrical signals. In particular, these devices reduce the magnitude and the phase of unwanted signals with certain frequencies. For example, noise (normally at a frequency of 60 Hz in electronic circuits) is always present, and it is desirable to suppress the noise from the system. We can achieve this by passing the system signal (voltage and noise) through a filter. If the filter is designed to suppress or attenuate the magnitude of the noise, the output of the filter will contain only (or mostly) the system signal. For another example, consider a typical radio receiver (the radio in your car); by tuning to a particular radio station, you are selecting one signal while attenuating the signals of the other radio stations. This process is accomplished by means of a filter.

Several categories of filters exist, but the main distinction is between analog and digital filters. Analog filters are designed to attenuate signals in analog systems, while digital filters attenuate digital signals in digital systems. These notes will concentrate on the study of analog filters, leaving digital filters for another occasion. The study of filters entails the use of complex mathematical techniques such as z-transform, Laplace Transform, convolution, recursion, and others that will be discussed in later articles. This module presents filter behavior without engaging the reader with advanced mathematics and complex techniques.

Types of Analog Filters

Filters are broadly classified according to the type of frequencies that the filter is able to suppress or attenuate. In this regard, there are four main categories:

Low-pass filter (LPF). This type of filter attenuates or suppresses signals with frequencies above a particular frequency called the cutoff or critical frequency ( fc). For example, an LPF with a cutoff frequency of 40 Hz can eliminate noise with a frequency of 60 Hz.

High-pass filter (HPF). This is a filter that suppresses or attenuates signals with frequencies lower than a particular frequency—also called the cutoff or critical frequency. For instance, an HPF with a cutoff frequency of 100 Hz can be used to suppress the unwanted DC voltage in amplifier systems, if so desired.

Band-pass filter (BPF). A filter that attenuates or suppresses signals with frequencies outside a band of frequencies. This is the general type of filter used when tuning radio or TV signals.

Band-reject (BRF) or notch filter. A filter that attenuates or suppresses signals with a range of frequencies. For instance, we can use such a filter to reject signals with frequencies between 50 Hz and 150 Hz.

Response Characteristics

The frequency response of any filter (LPF, HPF, BPF, BRF) can be designed by properly selecting the circuit components. The characteristics of filters are defined by the shape of the frequency response curve; the most important response shapes are named after a researcher who studied the particular filters. The following are types of filters: Butterworth, Chebyshev (type I and type II), Elliptic (or Cauer), and Bessel, to mention the most important. These filter types are named after the British researcher Stephen Butterworth, the Russian mathematician Pafnuty Chebyshev, the German scientist Wilhelm Cauer, and the German mathematician Friedrich Bessel, respectively. Each one of these filter types has a particular advantage in certain applications. The following figure shows the characteristics of four low-pass filters, each one of three-poles and with a cutoff frequency of 10. Note the different shapes represented by the frequency responses.

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