The filter is a circuit that is used to restructure, change, and block any undesired frequencies. Passive filters are often made up of resistors and capacitors in low frequency (100 kHz) applications. As a result, it's known as a passive RC filter. Passive filters can also be implemented as resistor-inductor-capacitor combinations for high frequency (> 100 kHz) transmissions. As a result, these circuits are referred to as passive RLC circuits. Low-pass filters, high-pass filters, and band-pass filters are the most common filter designs. Low-pass filters are discussed in this article.

Ⅰ. What is a Low Pass Filter

A low-pass filter (LPF) is a filter that allows low-frequency signals to pass while attenuating high-frequency sounds. The frequency response of a low-pass filter is mostly determined by the low-pass filter's design. Each form of LPF is described in detail below.

Ⅱ. The Classification of Low Pass Filter

Electronic circuits (such as hiss filters used in audio equipment), digital algorithms for smoothing data, sound barriers, image blurring, and other applications are all examples of low-pass filters. Both tools have been approved. Short-term swings are eliminated but long-term development trends are preserved, resulting in a smooth signal. Low-pass filters come in a variety of shapes and sizes, with Butterworth and Chebyshev filters being the most popular. Each form of LPF is described in detail below.

1. First-order low-pass filter

The integrator is the basic building element of a first-order LPF, as shown in the diagram.

First-order low-pass filter

The low-pass filter's transfer function is that the output reduces inversely proportional to the frequency (attenuation). The output is half when the frequency is twice (-6 dB for each doubling of the frequency).

2. Second-order low-pass filter

The following figure shows a second-order low-pass filter.

Second-order low-pass filter

If the frequency is doubled, the output reduces (attenuates) inversely proportional to the square of the frequency. (- Each frequency doubling is 12 dB).

3. Low-pass filter using operational amplifier

The operational amplifier's feedback loop can be combined with the fundamental elements of the filter, resulting in a high-performance LPF that can be simply manufactured using only the required components other than the inductor.

4. First-order active LPF circuit using operational amplifier

Below is a diagram of a single-pole active low-pass filter. Capacitors are used across the feedback resistor in low-pass filter circuits that use operational amplifiers. The circuit works better when the frequency is increased to raise the feedback level and the capacitor's reactive impedance is reduced.

First-order low-pass filter using operational amplifier

This filter can be calculated by processing the frequency at which the capacitor's reactance equals the resistance. This can be calculated using the formula below.

Xc stands for capacitive reactance in ohms; is a standard letter with the value of 3.412; f stands for frequency (unit-Hz); and C stands for capacitance (Units-Farads).

The in-band gain of these circuits may be easily estimated by removing the influence of the capacitor.

This sort of filter is known as a first-order or single-pole low-pass filter because it helps reduce high-frequency gain and provides -6 dB of attenuation for each octave.

5. Second-order active LPT circuit using operational amplifier

A large range of filters with variable gain levels and attenuation models can be built using operational amplifiers.

Second-order active LPF circuit using operational amplifier

Among them：

When choosing a value, make sure that the resistance value is in the range of 10 kiloohms to 100 kiloohms.

6. Low-pass filter calculator

A Bode plot is used to calculate the crossover frequency and depict the associated spectrum for the RC low-pass filter circuit.

For example, if we know the resistance and capacitance values in the circuit, we can compute the low-pass filter transfer function using the formula below.

Calculate the frequency value and capacitance value of a given resistance:

LPF waveform

Ⅲ. How does Low Pass Filter Work?

The low-pass filter works on the simple premise of using capacitors to pass high frequencies while blocking low frequencies and inductors to pass low frequencies while blocking high frequencies. Use capacitive absorption and inductance to block the passage of the high frequency that needs to be switched off. Use the properties of high capacitance and low resistance to pass the needed low frequency. A low-pass filter works on this basis.

Ⅳ. The Applications of Low Pass Filter

Applications of low-pass filters include:

●In telephone systems, low-pass filters are employed to convert audio frequencies in speakers into speech frequency band signals that are band-limited.

●LPF is used to remove "noise" from the circuit by filtering high-frequency signals. Most of the high-frequency signals are deleted as the signal travels through the filter.

●In image processing, the low-pass filter is used to enhance the image.

●Audio apps are sometimes to blame for these filters.

●Low-pass filters are employed in RC circuits, and RC low-pass filters are RC circuits.

●The LPF serves as the RC circuit's integrator.

●When the interpolator is utilized in a multi-rate DSP, the LPF is used as an anti-imaging filter. When the decimator is employed, the filter is used as an anti-aliasing filter as well.

●For the signal from the human body's medical device, a low-pass filter is utilized, and the test frequency utilizing the electrode is lower. As a result, these signals can pass through the LPF to filter out some undesired background noise.

●In a phase-locked loop, these filters are employed to translate the duty cycle amplitude and phase detection.

●In AM radio, LPF is utilized to convert AM modulated intermediate frequency impulses into audio signals in diode detectors.