In the field of electronic technology, we often hear the word "filter". Especially when we need to complete signal transmission, filters play a very important role! As the name suggests, filters are used to filter waves. In the field of electronics, waves describe the fluctuations of various physical quantities (voltage, current) over time, forming a time function. This time function contains a lot of information, which can be called a signal. This signal may be distorted by the environment during transmission, so that our information is buried in these noises. The filter is like a magical signal filter that allows electrical signals in a specific frequency range to pass through, while blocking signals of other frequencies and restoring the original information. In the fields of communication, audio processing, image processing, power systems, etc., filters play a huge role. They can filter interference and make signals cleaner.
- Types of filters
Filters can be classified into analog filters and digital filters according to the signal processing method.
Analog filters: filters that use analog electronic components such as resistors, capacitors, and inductors to achieve filtering, and mainly process continuous-time analog signals.
There are two types of analog filters. One is a passive filter, which is composed of components such as resistors, inductors, and capacitors that are not very "lively". The structure is simple and cheap, but the performance is easily affected by component parameters. The other is an active filter, which is composed of components such as operational amplifiers and passive components. The advantage is that the gain is high and the performance is relatively stable.
Digital filter: It is usually implemented through the DSP algorithm on the digital processor, and it mainly processes discrete-time digital signals. Digital filters can accurately control their performance through digital signal processing algorithms, and can also be easily adjusted through programming. Digital filters are divided into finite impulse response (FIR) filters and infinite impulse response (IIR) filters.
According to frequency characteristics, they can be divided into low-pass, high-pass, band-pass, and band-stop filters.
Low-Pass Filter (LPF): Like a fence, it only allows low-frequency signals to pass through and blocks high-frequency signals. In audio processing, it can remove high-frequency noise; in power systems, it can make DC voltage smoother.
High-Pass Filter (HPF): Just the opposite of the low-pass filter, it only allows high-frequency signals to pass through, and low-frequency signals cannot pass through. High-pass filters are often used to remove low-frequency interference, such as DC offset in audio systems.
Band-Pass Filter (BPF): Only signals within a specific frequency range are allowed to pass through, and others are not allowed. In communication systems, band-pass filters are widely used to select signals of specific frequencies for transmission or reception.
Band-Stop Filter (BSF): Specially blocks signals within a specific frequency range and allows signals of other frequencies to pass through. It can be used to remove interference of specific frequencies, such as harmonics in power supplies.
All-Pass Filter (APF): Like a channel that is "fair" to all frequencies. It allows signals of all frequencies to pass through, and after these signals pass through, their size (amplitude) will basically not change. The all-pass filter is mainly used to change the phase of the signal. The phase is like the order of a group of people in a queue. After the signal passes through the all-pass filter, although the size remains unchanged, the queue order (phase) may change. In some sound processing or signal transmission situations, changing the phase can play a role in adjusting the sound space or correcting the signal.
According to the filter structure, it can be divided into Butterworth filter, Chebyshev filter, elliptic filter, and Bessel filter.
Butterworth filter: Its amplitude response is particularly flat, and the frequency response in the passband and stopband is very uniform. Butterworth filter is particularly suitable for low-frequency applications. If the signal amplitude requirement is high, Butterworth filter is very suitable.
Chebyshev filter: It has wave-like equiripple characteristics in the passband or stopband. In order to obtain a steeper transition band, the flatness of the passband or stopband can be sacrificed. If the transition band is strictly required, Chebyshev filter is a good choice.
Elliptic filter: The transition band is the steepest, but there are some ripples in the frequency response in the passband and stopband. If the requirements for both the transition band and the filter size are very high, then choose elliptic filter.
Bessel filter: In addition to changing the amplitude of the input signal that depends on the frequency, the Bessel filter also introduces a delay to it. The delay causes the frequency-based phase shift to produce non-sinusoidal signal distortion. It is very suitable for situations where the waveform needs to be maintained.
- Application of filters
① Communication system
In wireless communication, filters are used to select signals of specific frequencies for reception and transmission, which can improve communication quality and anti-interference ability. In wired communication, filters can remove noise and interference in the signal to ensure the transmission quality of the signal.
② Audio processing system
In the audio system, low-pass filters, high-pass filters and band-pass filters can adjust the frequency characteristics of the audio signal to make the sound quality better. In audio noise reduction, filters can remove background noise and improve the signal-to-noise ratio of audio signals.
③ Image processing system
In image enhancement, filters can remove noise in the image and enhance the details and edges of the image. In image compression, filters can perform frequency decomposition on the image, remove high-frequency components, and reduce the amount of data.
④ Power system
Power filters can remove harmonics in the power system and improve the quality of power. In DC power supply, low-pass filters can make the DC voltage smoother and remove ripples.