Application and development of filters in anti-EMI

With the continuous progress of science and technology and the gradual improvement of human living standards, the number of household appliances, mobile and personal portable electronic devices has increased, so the mutual influence and interference between electronic devices has become increasingly serious and complicated. IBM analyzed computer power failures and believed that nearly 90% of the failures were caused by electromagnetic interference ( EMI ); in 1994, relevant departments of China conducted a survey on the wireless interference performance of different models of cars produced by 147 companies, and only 1/4 of the cars met the standards. Electromagnetic interference also threatens human health and safety. Biologists at Heidelberg University found that even weak electromagnetic radiation can invade the brain through the eyes and sow the seeds of cancer in people, so they called on people to be vigilant against "electromagnetic smoke". It can be seen that the harm caused by EMI is no less than tangible pollution.

----Nowadays, how to reduce or even eliminate the EMI of electronic equipment has become a common concern of the global electronic information industry and consumers. The world has established the International Special Committee on Radio Interference (CISPR) for this purpose. The United States, Germany, and Japan have also established special organizations FCC, VDE, and the Electromagnetic Environment Engineering Research Institute. They have higher and higher requirements for the anti-EMI performance of electronic instruments and measurement equipment, and stipulate that products that have not been strictly inspected shall not be shipped or put on the market. As China's economy has entered the international arena, the problem of electromagnetic interference (also known as electromagnetic noise) has been put before us.

----Using the ferrite-ceramic multilayer co-firing technology developed in recent years, several ferrite multilayer inductors and ceramic multilayer capacitors can be integrated together and co-wound to form a chip-type electromagnetic interference filter with sharp cutoff characteristics. Selecting an appropriate filter according to the circuit characteristics can achieve a very satisfactory anti-EMI effect, which is particularly suitable for suppressing the noise generated by digital electronic equipment and high-speed data buses . EMI filters can be divided into four types according to the components: inductors, capacitors, LC composite filters and capacitive variable resistors; according to the different materials of the components and the production process, they can be divided into ceramic type, quartz crystal type, dielectric type and surface acoustic wave type EMI filters; according to the specific use, they can be divided into two categories: EMI filters for power lines and signal lines. The following table lists several representative EMI filters and their characteristics.

----LC composite ceramic filters are a type of multifunctional products composed of ferrite inductors L and ceramic capacitors C in different combinations. Their center frequency, passband, amplitude, delay and other characteristics can be flexibly designed according to circuit requirements. They have a wide range of applications and are widely used in EMI suppression of audio and video appliances, communications and office automation equipment. However, their frequencies were relatively low and their volumes were relatively large at first. After entering the 1990s, due to the development of ceramic laminate printing technology, foreign countries have successively developed LC composite chip laminate ceramic products such as 254MHz and 380MHz bandpass filters for cordless phones, 700-950MHz filters for portable phones, 430MHz low-pass filters for small-function walkie-talkies, and 1.9GHz bandpass filters for digital cordless phones. Their built-in shielding layer has strong anti-interference and can adapt to the current high-frequency demand of 200MHz-1.5GHz without adjustment. The future development trend is further high frequency (above 2GHz), miniaturization, componentization (built-in delay lines, multilayer resonators, amplifiers and mixers integrated with filters, etc.) and low cost.

----Monolithic crystal filter (MCF) is a bandpass or bandstop filter composed of several pairs of metal electrodes on the surface of a quartz substrate. It uses the energy trap theory of the piezoelectric effect to select the geometric size of the electrode vibrator, the return frequency and the spacing between the electrode vibrators to control the acoustic coupling of the ultrasonic wave, thereby achieving the purpose of filtering. Its characteristics are very steep frequency selectivity, low loss, good stability, and high stopband attenuation. It is now widely used in mobile communication equipment and is an indispensable primary intermediate frequency filter. It plays an important role in improving the sensitivity and anti-interference ability of the whole machine. The practical level of foreign MCF products is a center frequency of several MHz to 150MHz, a bandwidth of 0.001 to 0.1%, a channel spacing of 12.5 to 25kHz, a minimum package size of 8×8×3.2mm, and a weight of only 0.4g . The development direction of MCF is to develop new piezoelectric materials, expand bandwidth, reduce in-band delay fluctuations, increase out-of-band attenuation, expand and improve intermediate frequency points and linearity, and further miniaturize and chip the size of the packaged monolithic chip.

----The EMI dielectric filter designed and manufactured by utilizing the characteristics of dielectric ceramic materials such as low loss, high dielectric constant, small frequency temperature coefficient and thermal expansion coefficient, and high power tolerance is composed of a ladder-shaped line with multiple stages of 1/4 wavelength resonators connected in series or in parallel. Its notable features are low insertion loss, good power resistance, and narrow bandwidth. It is particularly suitable for level-coupling filtering of portable phones, car phones, radio stations, cordless phones, and integrated transceiver duplexers. The most produced abroad is a series of EMI dielectric filters in the range of 800 to 1000MHz. Its technical indicators are insertion loss of 2 to 3dB, ripple less than 1dB, voltage standing wave ratio of about 1.5, out-of-band suppression within the specified frequency band can reach more than 20dB, and some special requirements can reach about 60dB. The filtering characteristics are excellent. Its development direction is to use new dielectric materials and other resonance modes to design and manufacture new miniaturized, chip-type low insertion loss, high attenuation, and high-performance EMI dielectric filters.

----Surface acoustic wave (SAW) filters were once widely used for sub-intermediate frequency filtering in the frequency band above 20MHz. It plays a good role in suppressing high-order harmonics, image information, transmission leakage signals and various parasitic interference of electronic equipment. Its characteristics are flexible design, analog/digital dual use, excellent group delay time deviation and frequency selection characteristics, and small input and output impedance errors. In recent years, SAW filters have been made into chips, weighing about 0.2g; in addition, due to the use of new crystal materials and the latest fine processing technology, the upper limit frequency of SAW devices has been increased to 2.5GHz, which will inevitably promote SAW filters to be more widely used in the field of anti-EMI. Low insertion loss technology and chip packaging are the research and development directions of this type of EMI filter at present and in the future.