Analysis of the working principle of anti-interference filter

走马观花 · Published on 2020-10-23 14:51

Analysis of the working principle of anti-interference filter [Copy link]

With the rapid development of electronic technology and the rapid increase of electronic equipment, the electromagnetic environment is deteriorating day by day. A large number of electronic equipment are difficult to work normally in this electromagnetic environment. Filter is the most commonly used anti-interference method. It mainly suppresses the interference that directly enters through the circuit path. According to the frequency difference between the signal and the interference signal, filters with different performances are used to suppress the interference signal and improve modularity.

What are the anti-interference filters?

1. Digital filter

Corresponding to analog filters, digital filters are widely used in discrete systems. Its function is to process the input signal waveform or frequency using the characteristics of discrete time systems. In other words, it converts the input signal into a certain output signal, thereby achieving the purpose of changing the signal spectrum.

2. Low-pass filter

A low-pass filter is a circuit in a car amplifier that allows low-frequency signals to pass but not mid- and high-frequency signals. Its function is to filter out the mid-range and high-frequency components in the audio signal and enhance the bass component to drive the bass unit of the speaker. Since most car amplifiers are full-band amplifiers, they usually use a class AB amplification design, which has a relatively large power loss. Therefore, filtering out low-frequency signals and only driving mid- and high-frequency speakers is the best choice to save power and ensure sound quality. In addition, high-pass filters often appear in pairs with low-pass filters. No matter which one, they are all for sending a certain sound frequency to the unit where it should go.

3. Bandpass filter

An ideal filter would have a completely flat passband, i.e. no gain or attenuation within the passband, and all frequencies outside the passband are completely attenuated, with transitions outside the passband being done over a very small frequency range. In reality, there is no such thing as an ideal bandpass filter. The filter is not able to completely attenuate all frequencies outside the desired frequency range, especially if there is a range outside the desired passband that is attenuated but not isolated. This is often referred to as the roll-off of the filter, and is expressed in dB of attenuation per decade. Often, filters are designed to have as narrow a roll-off range as possible, so that the filter performs closer to the design. However, as the roll-off range becomes smaller, the passband becomes less flat - it starts to "ripple". This phenomenon is particularly noticeable at the edges of the passband, an effect known as the Gibbs phenomenon.

4. Analog filter

Analog filters are commonly used conversion devices in test systems or special instruments. For example, bandpass filters are used as frequency selection devices in spectrum analyzers; low-pass filters are used as anti-frequency mixing filters in digital signal analysis systems; high-pass filters are used in acoustic emission detectors to remove low-frequency interference noise; band-stop filters are used as notch filters in eddy current vibrometers, etc.

5. Surface acoustic wave filter

Surface acoustic wave refers to the propagation of sound waves on the surface of elastic bodies. This wave is called elastic surface acoustic wave. The propagation speed of surface acoustic wave is about 100,000 times slower than that of electromagnetic wave. Surface acoustic wave filter is a special filtering device made of piezoelectric materials such as quartz crystal and piezoelectric ceramics, using its piezoelectric effect and the physical characteristics of surface acoustic wave propagation. It is widely used in the intermediate frequency circuits of televisions and video recorders to replace LC intermediate frequency filters, greatly improving the quality of images and sounds.

6. Dielectric filter

The dielectric filter is designed and manufactured using the characteristics of dielectric ceramic materials such as low loss, high dielectric constant, small frequency temperature coefficient and thermal expansion coefficient, and ability to withstand high power. It consists of a ladder-shaped line with several long resonators connected in series or in parallel longitudinally in multiple stages.

7. Active power filter

Active power filter is a power electronic device that dynamically suppresses harmonics and compensates reactive power. It can compensate for harmonics and reactive power with varying frequencies and magnitudes, make up for the shortcomings of passive filters, and obtain better compensation characteristics than passive filters. It is an ideal harmonic compensation device.

Working principle of anti-interference filter
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The RF signal f1-f2 passes through the filter. After the filter response, the insertion loss in the passband is small, the signal is slightly smaller, and the out-of-band signal is completely suppressed after the filter response.

The function of anti-interference filter:
1. Reduce the interference voltage emission of the product to the power grid.
2. Improve the anti-interference degree of the product and block the influence of unclean power supply of the power grid on the equipment.