Filters are devices that permit certain frequencies of a signal to pass through while interfering with other frequencies. Filters are commonly used in electronic circuits, in sound recording and reproduction, in television broadcasting, in optical communication, and in many other applications.

A high pass filter lets high frequencies pass through and blocks the low frequencies. And vice versa for low pass filter. A high pass filter is applied to signals which have low frequencies and are less abrupt as compared to signals that have higher frequencies. On the other hand, low pass filter is used to filter signals with high frequencies. While high pass and low pass filter enable signals to pass through, band pass filter will block signals whose frequencies fall outside the set range.

A low-pass filter is a filter that allows low-frequency signals to pass but cuts off high-frequency signals. A high-pass filter, on the other hand, passes high-frequency signals but cuts off the low-frequency signals. A band-pass filter allows a certain range of frequencies to pass through, while blocking frequencies outside of this range. A band-pass filter is a combination of a low-pass and a high-pass filter.

A Butterworth filter is designed to have the flattest amplitude response possible, with a cutoff frequency and Q value specified by the engineer. The response is linear. The cutoff frequency is the cutoff point between the passband and the stopband . The Q value determines the width of the transition from passband to stopband. With higher Q values, the gain is reduced in the passband while the subsequent stopband is narrower than with lower Q values. A Butterworth filter has the advantage of a flat amplitude response, which is desirable in many engineering applications.

The Butterworth filter plays a significant role in many branches of electrical engineering and is used extensively in telecommunications. The function of Butterworth filters is often illustrated with a plot of their frequency response. The frequency response is represented by a Bode plot, which is a graph with one axis representing frequencies (usually from zero to a few hundred Hertz) and the other axis representing the amplitude response of the filter of interest. The amplitude response is usually normalized so that it is a percentage of the case when there is no filter. The normalized amplitude vs. frequency is also called a transfer function. A Butterworth filter has a characteristic shape to its frequency response. A Butterworth filter has a frequency response that is a real and positive function of frequency. In other words, the phase angle is zero for all frequencies. That means that the frequencies are passed through the system without distortion. A Butterworth filter has a single frequency response as shown in the plot. It is a flat curve with no overshoot or undershoot. The plot of a Bode plot is a straight line.