How to deal with the electromagnetic interference problem of wireless temperature measurement?

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Electromagnetic compatibility (EMC) refers to "the performance of a device, equipment or system, which can make it work normally in its own environment and at the same time will not cause strong electromagnetic interference to any other equipment in this environment (IEEEC63.12- 1987)."

How to deal with the electromagnetic interference problem of wireless temperature measurement?

Electromagnetic compatibility (EMC) refers to "the performance of a device, equipment or system, which can make it work normally in its own environment and at the same time will not cause strong electromagnetic interference to any other equipment in this environment (IEEEC63.12- 1987)." For wireless transceivers, the use of discontinuous spectrum can partially achieve EMC performance, but many related examples also show that EMC cannot always be achieved. For example, there are high-frequency interference between notebook computers and test equipment, between printers and desktop computers, and between mobile phones and medical equipment. We call this interference electromagnetic interference (EMI). Sources of EMC problems

All electrical appliances and electronic equipment will have intermittent or continuous voltage and current changes when they work, and sometimes the rate of change is quite fast, which will cause electromagnetic energy to be generated in different frequencies or between a frequency band, and the corresponding circuit will emit this energy Into the surrounding environment.

There are two ways for EMI to leave or enter a circuit: radiation and conduction. Signal radiation leaks out through slits, slots, openings or other gaps in the shell; while signal conduction leaves the shell by coupling to power, signal and control lines, radiating freely in open spaces, thereby causing interference.

A lot of EMI suppression is achieved by the combination of shell shielding and slot shielding. Most of the time, the following simple principles can help achieve EMI shielding: reduce interference from the source; isolate the interference generating circuit by shielding, filtering or grounding, and Enhance the anti-interference ability of sensitive circuits, etc. EMI suppression, isolation, and low sensitivity should be the goals of all circuit designers, and these properties should be completed early in the design phase.

For design engineers, the use of shielding materials is an effective way to reduce EMI. Nowadays, a variety of shell shielding materials have been widely used, from metal cans, thin metal sheets and foil tapes to spray coatings and plating (such as conductive paint and zinc wire spraying, etc.) on conductive fabrics or tapes. Whether it is metal or plastic coated with a conductive layer, once the designer has determined that it is the housing material, he can begin to select the gasket. Metal shielding efficiency

Shielding efficiency (SE) can be used to evaluate the suitability of the shielding cover, the unit is decibel, and the calculation formula is SEdB=A+R+B

where A: absorption loss (dB) R: reflection loss (dB) B: correction factor (dB) (applicable to the case where there are multiple reflections in a thin shield)

A simple shield will reduce the intensity of the electromagnetic field generated to one tenth of the original, that is, SE is equal to 20dB; and in some cases, it may be required to reduce the field strength to one ten thousandth of the original, that is, SE must be equal to 100dB.

Absorption loss refers to the amount of energy loss when electromagnetic waves pass through the shield. The calculation formula for absorption loss is AdB=1.314(f×σ×μ)1/2


where f: frequency (MHz) μ: copper permeability σ: copper conductivity t: shield thickness

"The size of the reflection loss (near field) depends on the nature of the electromagnetic wave source and the distance from the wave source. For rod-shaped or linear transmitting antennas, the closer to the wave source, the higher the wave resistance, and then decreases as the distance from the wave source increases, but the plane wave resistance does not change (constantly 377).

On the contrary, if the wave source is a small coil, the magnetic field will dominate at this time. The closer to the wave source, the lower the wave resistance. The wave resistance increases as the distance from the wave source increases, but when the distance exceeds one-sixth of the wavelength, the wave resistance no longer changes and remains constant at 377. The reflection loss varies with the ratio of wave resistance to shielding impedance, so it depends not only on the type of wave, but also on the distance between the shield and the wave source. This situation applies to small shielded equipment. The near-field reflection loss can be calculated as follows

R(electricity)dB=321.8-(20×lgr)-(30×lgf)-[10×lg(μ/σ)]R(magnetic)dB=14.6+(20×lgr)+(10×lgf)+ [10×lg(μ/σ)] where r: the distance between the wave source and the shield.

The last term of the SE calculation formula is the correction factor B, and its calculation formula is B=20lg[-exp(-2t/σ)]

This formula is only applicable to the case of a near magnetic field environment and the absorption loss is less than 10dB. Since the absorption efficiency of the shield is not high, the internal re-reflection will increase the energy passing through the other side of the shielding layer, so the correction factor is a negative number, which indicates the decrease of the shielding efficiency. EMI suppression strategy

Only materials with high magnetic permeability such as metal and iron can achieve high shielding efficiency at very low frequencies. The permeability of these materials will decrease as the frequency increases. In addition, if the initial magnetic field is strong, the permeability will also decrease, and the use of mechanical methods to make the shield into a prescribed shape will also decrease the permeability. In summary, the selection of high-permeability materials for shielding is very complicated, and it is usually necessary to seek solutions from EMI shielding material suppliers and related consulting agencies.

In the high frequency electric field, a thin layer of metal as the shell or lining material can achieve a good shielding effect, but the condition is that the shielding must be continuous and completely cover the sensitive parts without gaps or gaps (forming a Faraday cage). However, in practice, it is impossible to manufacture a shielding cover without seams and gaps. Since the shielding cover has to be divided into multiple parts for production, there will be gaps that need to be joined. In addition, it is usually necessary to perforate the shielding cover. In order to glue the connection with add-on cards or assembly components. The difficulty in designing a shield is that pores will inevitably occur during the manufacturing process, and these pores will also be used during the operation of the equipment. Manufacturing, panel wiring, vents, external monitoring windows, and panel adhesive components need to be punched in the shielding cover, which greatly reduces the shielding performance. Although trenches and gaps are unavoidable, it is beneficial to carefully consider the trench length related to the circuit's operating frequency and wavelength in shielding design. The wavelength of electromagnetic waves at any frequency is: wavelength (λ) = speed of light (C) / frequency (Hz)

When the length of the slot is half the wavelength (cutoff frequency), the RF wave begins to decay at a rate of 20dB/10 times (1/10 cutoff frequency) or 6dB/8 times (1/2 cutoff frequency). Generally, the higher the RF emission frequency, the more severe the attenuation, because its wavelength is shorter. When it comes to

When the highest frequency is reached, any harmonics that may appear must be considered, but in fact only the first and second harmonics need to be considered.

Once the frequency and intensity of the RF radiation in the shield are known, the maximum allowable gaps and grooves of the shield can be calculated. For example, if the radiation of 1GHz (wavelength is 300mm) needs to be attenuated by 26dB, the 150mm gap will begin to attenuate, so when there is a gap less than 150mm, the 1GHz radiation will be attenuated. So for 1GHz frequency, if 20dB attenuation is required, the gap should be less than 15mm (1/10 of 150mm), if 26dB attenuation is required, the gap should be less than 7.5mm (above 1/2 of 15mm), and 32dB attenuation is required It should be less than 3.75mm (above 1/2 of 7.5mm). A suitable conductive gasket can be used to limit the size of the gap to a specified size to achieve this attenuation effect. Difficulties in shielding design

Because the seam will cause the conduction rate of the shield to decrease, the shielding efficiency will also be reduced. It should be noted that the attenuation of radiation below the cut-off frequency only depends on the length-to-diameter ratio of the slot. For example, when the length-to-diameter ratio is 3, an attenuation of 100dB can be obtained. When perforations are needed, the waveguide characteristics of the small holes on the thick shield can be used; another way to achieve a higher length-to-diameter ratio is to attach a small metal shield, such as a gasket of a suitable size. The above principles and their promotion in the case of multiple slits constitute the basis for the design of the porous shielding cover.

Porous thin shielding layer: There are many examples of porous, such as ventilation holes on thin metal sheets, etc. When the spacing of the holes is close, careful consideration must be given to the design. The following is the calculation formula for shielding efficiency under such circumstances SE=[20lg(fc/o/σ)]-10lgn where fc/o: cut-off frequency n: number of holes

Note that this formula is only applicable to the case where the hole spacing is smaller than the hole diameter, and can also be used to calculate the relative shielding efficiency of the metal woven mesh. Joints and joints: Electric welding, brazing or soldering is a common method for permanent fixation between sheets. The metal surface of the joint must be cleaned so that the joint can be completely filled with conductive metal. It is not recommended to use screws or rivets for fixing, because the low-resistance contact state of the joint between the fasteners is not easy to maintain for a long time.

The role of the conductive gasket is to reduce the slots, holes or gaps in the joints or joints, so that RF radiation will not be emitted. EMI gasket is a conductive medium used to fill the gaps in the shield and provide continuous low-impedance contacts. Generally, EMI gaskets can provide a flexible connection between two conductors, allowing the current on one conductor to pass to the other conductor.

The selection of the sealed EMI gasket can refer to the following performance parameters:? Shielding efficiency in a specific frequency range? Adhesion method and sealing strength? Current compatibility with the outer cover and corrosion resistance to the external environment. ? Operating temperature range? Cost. Most commercial gaskets have sufficient shielding performance to make the equipment meet EMC standards. The key is to properly design the gasket in the shield.

Gasket system: An important factor that needs to be considered is compression. Compression can produce a higher electrical conductivity between the gasket and the gasket. Poor conductivity between the gasket and the gasket will reduce the shielding efficiency. In addition, if there is one missing piece of the joint, a slot will appear to form a slot antenna, and its radiation wavelength is about 4 times smaller than the length of the slot.

To ensure the continuity, the surface of the gasket must be smooth and clean and must be treated to have good conductivity. These surfaces must be covered before joining; in addition, the shielding gasket material has continuous good adhesion to this gasket. It is also very important. The compressible properties of the conductive gasket can compensate for any irregularities in the gasket.

All gaskets have an effective working minimum contact resistance. Designers can increase the compression of the gaskets to reduce the contact resistance of multiple gaskets. Of course, this will increase the sealing strength and make the shield more curved. Most liners work better when compressed to 30% to 70% of their original thickness. Therefore, within the recommended minimum contact surface range, the pressure between two opposing concave points should be sufficient to ensure good electrical conductivity between the gasket and the gasket.

On the other hand, the pressure on the liner should not be so high that the liner is in an abnormally compressed state, because this will cause the liner to fail to contact and may cause electromagnetic leakage. The requirement of separation from the gasket is very important to control the gasket compression within the manufacturer's recommended range. This design needs to ensure that the gasket has sufficient stiffness to avoid large bending between gasket fasteners. In some cases, additional fasteners may be required to prevent bending of the housing structure.

Compressibility is also an important characteristic of rotating joints, such as in doors or boards. If the gasket is easy to compress, the shielding performance will decrease with each rotation of the door. At this time, the gasket needs a higher compression force to achieve the same shielding performance as the new gasket. In most cases this is unlikely to be possible, so a long-term EMI solution is required.

If the shielding cover or gasket is made of plastic coated with a conductive layer, adding an EMI gasket will not cause too many problems, but the designer must consider that many gaskets will wear on the conductive surface, usually metal gaskets The surface of the coating is more prone to wear. Over time, this wear will reduce the shielding efficiency of the gasket joints and cause trouble for the manufacturers behind. If the shielding cover or gasket structure is metal, a gasket can be added to cover the surface of the gasket before spraying the polishing material, and only conductive film and tape can be used. If tape is used on both sides of the joint gasket, mechanical fasteners can be used to fasten the EMI gasket, such as a "C-type" gasket with plastic rivets or pressure sensitive adhesive (PSA). The gasket is glued to one side of the gasket to complete the EMI shielding. Pads and accessories

There are many shielding and gasket products currently available, including beryllium-copper joints, metal mesh cables (with or without elastic core), metal mesh and directional wires embedded in rubber, conductive rubber, and polyurethane foam gaskets with metal plating Wait. Most shielding material manufacturers can provide estimates of SE that can be achieved by various gaskets, but remember that SE is a relative value and also depends on the pores, gasket size, gasket compression ratio, and material composition. The pads come in a variety of shapes and can be used in a variety of specific applications, including wear, sliding, and hinged applications. At present, many pads have glue or have fixing devices on the pads, such as squeeze inserts, pin inserts or barb devices.

Among all kinds of cushions, the coated foam cushion is one of the latest and most widely used products on the market. This type of gasket can be made into a variety of shapes, with a thickness greater than 0.5mm, and can also be reduced in thickness to meet UL combustion and environmental sealing standards. There is another new type of gasket, the environmental/EMI hybrid gasket, which eliminates the need to use a separate sealing material, thereby reducing the cost and complexity of the shield. The outer coating of these liners is UV-stable, resistant to moisture, wind, and cleaning solvents, while the inner coating is metallized and has high conductivity. Another recent innovation is the installation of a plastic clip on the EMI gasket. Compared with the traditional pressed metal gasket, it is lighter in weight, shorter in assembly time, and lower in cost, making it more attractive in the market.