Electromagnetic compatibility design of an electronic device on a small satellite platform

Key points: For a certain electronic equipment on the small satellite platform, the electromagnetic compatibility (EMC) design principles and specific methods are given, including structural design, shielding design, filter design and PCB board design. EMC tests show that the equipment The electromagnetic compatibility design is reasonable, which has a high reference value for the electromagnetic compatibility design of spaceborne electronic equipment.

With the increasing development of electronic technology applications, electronic equipment is becoming more and more complex, and the electromagnetic environment is becoming increasingly harsh, which affects the normal operation and performance of electronic equipment and systems. An electronic product with good performance must consider electromagnetic compatibility issues. It must not have electromagnetic radiation that interferes with the normal operation of other electronic devices, but must also have low sensitivity and be able to resist specified electromagnetic interference.

Spacecraft electronic equipment has very strict restrictions on volume, mass and power consumption, and the pursuit of high performance products is also constantly increasing. A certain electronic equipment is adapted to the needs of miniaturization and integration of small satellite platforms. It combines multiple functional lines. There are many interference sources and sensitive devices inside the equipment, many signal transmission lines, and a small space. It is easy to cause interference with each other. If the interference effect is serious, it will cause system failure and may even cause serious malfunctions, so electromagnetic compatibility is an important indicator of this equipment.

1 Electromagnetic compatibility and electromagnetic interference

Electromagnetic compatibility (EMC) refers to the ability of electronic equipment to work harmoniously and effectively in the expected electromagnetic environment. Its purpose is to enable electronic equipment to suppress various external interferences and reduce its own electromagnetic interference to other electronic equipment. Electromagnetic interference (EMI) can be understood as an electromagnetic phenomenon that damages useful signals. The main sources of interference include interference formed within the electronic device and interference caused by external coupling to the electronic device.

Electromagnetic compatibility mainly solves the problem of electromagnetic interference between electronic and electrical equipment or systems. There must be three factors to constitute electromagnetic interference, namely the interference source, the interfered object (sensitive equipment) and the coupling path between the two. The basic model of electromagnetic interference is the series connection of these three factors,

Electromagnetic compatibility design of an electronic device on a small satellite platform
For electromagnetic compatibility problems to occur within a system or equipment, the above three factors must exist at the same time. When solving electromagnetic compatibility problems, we must start from these three factors and eliminate one of them to solve the problem. For newly developed electronic equipment, electromagnetic compatibility should be considered from the beginning of the design stage and electromagnetic compatibility design should be carried out. Taking electromagnetic compatibility into consideration during the design stage will save manpower and material resources far more than taking measures to meet electromagnetic compatibility standards after production. Therefore, electronic equipment must consider electromagnetic compatibility issues during the product design stage.

2 Introduction to a certain spaceborne electronic equipment

A certain electronic device on the small satellite platform is one of the main controllers of the satellite, as shown in Figure 2.

Electromagnetic compatibility design of an electronic device on a small satellite platform
With the processor control unit as the center, the periphery includes DC/DC modules, serial communication modules, thruster drive modules, magnetic torquer drive modules, power supply and distribution modules and other units. In the equipment, there are not only strong interference source signals such as DC/DC, component power supply and distribution, and thruster drive, but also small A/D and D/A input and output signals. There are also high-speed pulse input signals, as well as high-speed signals inside the control unit. clock signal. Therefore, the electromagnetic environment inside the equipment is very complex, and electromagnetic compatibility has become an important design content of the equipment. The quality of the results directly affects the performance of the product.

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3 Electromagnetic compatibility design

(1) Structure and ground wire design

This spaceborne electronic equipment adopts a cable-free chassis structure. Compared with the traditional cable structure chassis, the cable-free chassis can greatly reduce electromagnetic interference. This is because 80% of EMC problems are caused by cables. Cables are efficient electromagnetic wave receiving antennas and radiation The antenna is also a good channel for interference conduction. As shown in Figure 3, the equipment adopts a dual bus board structure. The internal bus board realizes signal transmission between the functional boards inside the equipment. The internal electrical connector is transferred through the “large core number” between the internal bus board and the external bus board. ” and the “soldering pin type in-line printed board electrical connector” on the external bus board realize the transmission between internal signals and external signals of the device.

Electromagnetic compatibility design of an electronic device on a small satellite platform
When designing the chassis structure, separate the circuit boards where strong and weak signals are located for a reasonable layout. Circuit boards that are prone to electromagnetic interference are placed on the upper and lower sides of the chassis so that the generated interference signals can be released through the chassis shell. Digital and other sensitive signal processing circuits The board is placed in the middle of the chassis. The secondary power module is the main source of interference. It is composed of two circuit boards: primary and backup. The primary and secondary circuit boards are used during normal operation, so the secondary board is placed on the outermost side of the chassis. The backup secondary power board usually does not work. , becoming a barrier to prevent interference signals generated by the main and secondary power boards from radiating into the chassis.

The ground wires inside the equipment include primary ground, digital ground and analog ground, of which digital ground and analog ground are secondary grounds. During the design, the digital ground and analog ground are routed separately, and a single point connection is made within the device. The primary ground is strictly isolated from the digital ground and analog ground to prevent a single machine ground fault from bringing fatal hazards to the primary power supply and affecting the normal operation of the entire satellite. . The internal and external bus boards are the input and output channels for strong and weak signals of the equipment. In order to contain and reduce the interference between these signals, the strong and weak signals on the internal and external bus boards are divided into regions, and the corresponding ground layers are divided at the same time. Since the ground layer is of equal potential everywhere , will not produce common-mode resistive coupling, nor will it form a loop current through the ground wire to produce an antenna effect, so that electromagnetic interference can enter the ground wire through the shortest path and disappear.

(2) Filtering and shielding

The function of filtering is to pass signals within a specified frequency range and suppress signals at other frequencies. It is one of the most commonly used means to reduce conducted interference and radiated interference, and is especially effective in suppressing transient interference. The filtering design of this onboard electronic equipment is mainly to filter the DC/DC power module and filter the line decoupling capacitor.

The input filter circuit of the DC/DC module is mainly composed of a one-stage filter and a two-stage LC filter circuit. It adopts differential mode and common mode combined filtering measures to effectively prevent noise interference from the power bus and at the same time block the DC/DC power supply. The switching noise generated by itself is fed back to the internal resistance of the primary bus, causing public nuisance.

When designing a printed circuit board, decoupling capacitors are added to the circuit to meet the power supply stability and cleanliness requirements for digital circuit operation. The charging and discharging effect of the decoupling capacitor makes the power supply voltage obtained by the integrated chip relatively stable and reduces the phenomenon of voltage oscillation. The integrated chip can absorb or release current on its own decoupling capacitor nearby without having to pass the power line from a distant power supply. Get current from it. Therefore, it will not affect the speed of the integrated chip. At the same time, the decoupling capacitors provide nearby high-strength channels for the transient changing current of the integrated chip, thereby greatly reducing the outward radiated noise, and there is no common impedance between them, so suppressing common impedance coupling.

When designing, add a 0.01 to 0.1 μF decoupling capacitor between the power supply and ground pin of each integrated chip to effectively remove high-frequency noise in the signal line. Connect a set of 5 μF capacitors and a set of 0.01 μF capacitors to the power input terminal of each circuit board to filter out high-frequency interference and low-frequency noise on the power line.

Shielding is a measure that uses the shielding body to absorb and emit interfering electromagnetic waves to prevent or weaken the transmission of electromagnetic energy. It can effectively prevent electromagnetic waves from propagating from one space to another and is mainly used to suppress radiation interference. The shield uses a low-impedance material as a shell to surround the components that need to be isolated. The isolated components can be interference sources or sensitive equipment that prevents interference. Connecting the shield to the ground can greatly reduce interference coupling.

As the main source of interference inside the equipment, the secondary power supply module is installed in a sealed and shielded metal cavity during design. The shell and the chassis are closely connected and connected to the ground, thereby isolating the external radiation interference of the internal interference signal of the secondary power supply. , and can also well improve the anti-interference ability of the secondary power supply. In order to increase the shielding effect of the chassis, a slot structure is designed between each panel of the cabinet to increase the overlap size at the joints of each panel. The overlap amount of the two connecting surfaces at the joints is related to the shielding effectiveness. Increasing the overlap amount is equivalent to increasing the The depth of the gap. In addition, increase the number of fixing screws at the joints of the chassis shell as much as possible and reduce the spacing between screws, so that the gap length response is reduced and the shielding efficiency is improved. The surface of the chassis is treated with conductive anodization, and each printed board component in the chassis is also treated with conductive anodization to keep all connection surfaces conductive, and is ultimately connected to the system case ground through the connection address on the box.

(3) PCB layout and wiring design

Electromagnetic interference problems in printed circuit boards include common impedance coupling and crosstalk, radiation generated by high-frequency current-carrying wires, induction of high-frequency radiation by printed lines, etc. Among them, high-frequency radiation is the most serious problem. This is because the impedance of power lines, ground lines and signal lines increases with the increase of frequency, so it is easier to cause interference through public impedance coupling. At the same time, the increase of frequency causes parasitic capacitance between lines. The capacitive reactance is reduced, so crosstalk is more likely to occur.

Within the equipment, improper layout or wiring is the primary cause of interference. Most interference occurs between analog and digital mixed layout networks or improperly routed printed lines. Therefore, correct layout and wiring are the key to reliable equipment. One of the basic guarantees of operation.

This onboard electronic equipment contains low-level analog circuits and digital logic circuits. The two are laid out separately during PCB design, which shortens the routing path of the high-frequency current of the digital circuit on the printed board, helping to reduce the wiring cost. Crosstalk, common impedance coupling and radiated emissions within the board. One of the first factors to consider in the layout of components is electrical performance. Components that are closely connected should be placed together as much as possible, and high-speed lines should be kept as short as possible. Power signal and small signal devices are separated, which reduces electromagnetic interference between components.

In digital circuit design, what cannot be ignored is the parasitic inductance, capacitance and admittance existing on devices, wires, printed lines and plugs. For this reason, the following measures should be taken during wiring:

1) Try to leave a large gap between all parallel signal lines to reduce crosstalk. If there are two signal lines that are close to each other, it is best to run a ground wire between the two lines to provide shielding. When designing a signal transmission line, avoid sharp turns to prevent reflection and ringing caused by sudden changes in the characteristic impedance of the transmission line. Try to design a uniform arc line with a certain size.

2) If the printed board is equipped with high-current devices, such as relays, their ground wires should be routed separately to reduce noise on the ground wire. Try to avoid changing wiring layers for clock signals and high-speed signals, and use less vias to reduce the harm caused by parasitic capacitance and parasitic inductance of vias.

3) The power plane is close to the ground plane and arranged below the ground plane. In this way, the capacitance between the two metal plates can be used as the smoothing capacitance of the power supply. At the same time, the ground plane also plays a shielding role in the radiation current distributed on the power supply plane.

4) Pay special attention to the size of wire loops in the circuit where current flows, because these loops are equivalent to small antennas at work, radiating into space anytime and anywhere.

4 Experimental verification

Electromagnetic compatibility testing includes electromagnetic interference emissions (EMI) and electromagnetic susceptibility (EMS) measurements. According to the test requirements of the national military standard GJB151A-97 “Electromagnetic Emission and Sensitivity Requirements for Military Equipment and Subsystems”, this spaceborne electronic equipment has carried out the following EMC test items: RE102: 10 kHz ~ 18 GHz electric field radiated emission; RS103: 10 kHz ~40 GHz electric field radiation sensitivity; CE102: 10 kHz ~ 10 MHz power line conducted emissions; CS101: 25 Hz ~ 50 kHz power line conducted sensitivity; CS114: 10 kHz ~ 200 MHz cable bundle injection conducted sensitivity; CS115: Cable bundle injection pulse excitation conducted sensitivity; CS116: 10 kHz ~ 100 MHz cable and power line damped sinusoidal transient conducted sensitivity. Among them, RE102 and CE102 belong to the measurement of EMI (electromagnetic interference), and RS103, CS101, CS114, CS115, and CS116 belong to the measurement of EMS (electromagnetic sensitivity).

Electromagnetic compatibility design of an electronic device on a small satellite platform
The above 7 EMC test items were tested on the equipment, and all 7 test items were qualified. The conduction sensitivity and radiated emission test data of the product met the requirements of CJB151A-9 (RE102 test curve is shown in Figure 4), proving that this electronic The electromagnetic compatibility design of the equipment is effective and the performance meets the requirements.

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5 Conclusion

Electromagnetic compatibility issues play a vital role in the performance of spacecraft electronic products. This article takes a variety of technical and effective measures in design and implementation based on the electromagnetic environment characteristics of an electronic device on a small satellite platform and combined with actual engineering experience. , to optimize the design effect, pass the relevant electromagnetic compatibility test and detection, and achieve twice the result with half the effort. This has a good reference value for the design of similar electronic products.

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