In the past 20 years, the compatibility of military electronic equipment for electromagnetic working environment has been paid more and more attention. EMC is not only an important indicator for evaluating the environmental adaptability of military equipment, but also for some military electronic equipment, electromagnetic compatibility is the most important position in all environmental requirements. This is because after the electronic degree of modern military equipment has been greatly improved, the power spectrum and frequency spectrum of military electronic equipment continue to extend to the high-end and low-end directions. The installation of military electronic equipment on various platforms of sea, land and air The density has also increased significantly, resulting in more and more problems of electromagnetic interference (EMI) between electronic devices. Therefore, it has become the consensus of all parties involved in equipment design, production and use to require military electronic equipment to have the specified electromagnetic compatibility capability.
In order to evaluate the EMC performance of military electronic equipment, almost all military electronic equipment must pass the electromagnetic compatibility test specified by the national military standard. Therefore, in recent years, the test standards and compliance technologies for electromagnetic compatibility of military electronic equipment have received unprecedented attention.
Different from the assessment requirements of other environmental conditions, the "electromagnetic compatibility" test not only assesses the adaptability of the equipment to the electromagnetic environment, but also whether the existence of the equipment will create an electromagnetic environment that is not conducive to the normal operation of other equipment. Therefore, the electromagnetic compatibility test is a bidirectional test, and the equipment under test (EUT) must meet the standards both in terms of withstanding external electromagnetic interference and not generating external electromagnetic interference to be considered qualified. And because electromagnetic signals can have effects through circuit conduction and space radiation, in order to make military electronic equipment meet the standards in electromagnetic compatibility tests, measures must be coordinated in both the electronic and electrical systems and mechanical structural systems of the equipment. These factors determine that the electromagnetic compatibility test is more complicated than other routine environmental tests, and it is not easy to reach the standard.
For personnel engaged in the electromagnetic compatibility design and testing of military electronic equipment, in addition to mastering the professional knowledge related to the equipment and the essential basic knowledge of electromagnetics, electronics, electrical engineering, as well as related materials science and structural design In addition to the knowledge of the electromagnetic compatibility test, you must also be familiar with the military standards related to electromagnetic compatibility testing, and understand the physical meaning of each test and the requirements for the test in as much detail as possible.
Around GJB151A. The main provisions of the 97 Standard ¨, the author combines the practice of electromagnetic compatibility design and testing of military electronic equipment on various installation platforms of sea, land and air for more than ten years, aiming at the main assessments of military standard electromagnetic compatibility testing. requirements, and provide some practical techniques and experience that are conducive to making the pilot project meet the standards.
1 Introduction to GJB151A-97 Standard
GJB151A. The 97 standard is called "Electromagnetic Emission and Sensitivity Requirements for Military Equipment and Subsystems". , which specifies the EMC requirements that military equipment must meet. The standard was approved by the National Defense Science, Technology and Industry Committee, issued on May 23, 1997, and implemented on December 1, 1997. Another standard closely related to this standard and issued and implemented at the same time is the GJB152A-97 standard [2] "Electromagnetic Emission and Sensitivity Measurement of Military Equipment and Subsystems", which specifies the measurement methods of various test indicators in the GJB151A-97 standard. .
The predecessor of the GJB151A-97 standard is the GJB151A-86 standard released in 1986. The new version of the standard revised the old standard with reference to foreign military standards (mainly the US military standard MIL), and made more stringent requirements for some indicators.
According to the GJB151A-97 standard, the EMC test of military electronic equipment includes the following 19 items:
· CE101 25 Hz to 10 kHz power line conducted emissions
· CE102 10 kHz～10 MHz power line conducted emission
· CE106 10 kHz～40 GHz Antenna Terminal Conducted Emission
CE107 power line spikes (time domain) conducted emissions
· CS101 25 Hz ~ 50 kHz power line conduction sensitivity.
· CS103 15 kHz～10 GHz Antenna Terminal Intermodulation Conduction Susceptibility
· CS104 25 Hz to 20 GHz Antenna Terminal Unwanted Signal Suppression Conducted Sensitivity
· CS105 25 Hz～20 GHz Antenna Terminal Intermodulation Conduction Sensitivity
CS106 Power Line Spike Conduction Sensitivity
· CS109 50 Hz～100 kHz case current conduction sensitivity
· CS114 10 kHz to 400 MHz cable bundle injection conduction sensitivity
CS1 15 cable bundle injection impulse excitation conduction sensitivity
CS116 10 kHz to 100 MHz Cable and Power Line Damped Sinusoidal Transient Conduction Susceptibility
· REIO1 25 Hz～100 kHz magnetic field radiation emission
· RE102 10 kHz～18 GHz electric field radiated emission
· RE103 10 kHz to 40 GHz antenna harmonic and spurious output radiated emissions
· RSIO1 25 Hz～100 kHz magnetic field radiation sensitivity
RS103 10 kHz 40 GHz electric field radiation sensitivity
RS105 Transient Electromagnetic Field Radiation Sensitivity
Not all of the above 19 EMC tests are mandatory for various military installation platforms. The so-called military installation platforms are divided into nine categories: surface ships, submarines, army aircraft (including route support equipment), naval aircraft, air force aircraft, space systems (including launch vehicles), army ground, naval ground, and air force ground. In the GJB151A-97 standard, each test item specifies the applicability of each platform.
For military electronic equipment that requires EMC test, usually among all test items, CE102, CSIO1, CS114, RE102, RS103 are the most important must-do items. For equipment loaded on ships and aircraft, some items of CE101, CS115, CS116, RE101, and RS101 are often required. Together with the aforementioned 5 items, the total required test items are between 7 and 9 items. The remaining items shall be determined by the ordering unit according to the relevant specifications or not.
2 EMC characteristics and design countermeasures of military electronic equipment
Compared with general non-military electronic equipment or non-electronic military equipment, the electromagnetic compatibility of military electronic equipment has the following characteristics.
1. The installation density is high. Due to tactical and technical considerations, the installation of military electronic equipment is very compact, and a large number of military electronic equipment with different functions are concentrated in a small space, which makes the problem of electromagnetic interference between equipment particularly prominent.
2. Strong and weak signals coexist. Almost all types of military electronic equipment must simultaneously process multiple signals with different amplitudes. Strong signals cause interference to external equipment, while weak signals are extremely sensitive to external interference.
3. Wide spectrum distribution. Military electronic equipment makes full use of frequency resources, occupying various frequency bands from DC to microwave. Some devices, such as radar, work in pulse mode, covering a wide frequency range, causing strong interference to peripheral devices.
4. Shared power and ground. A large number of military electronic devices on various installation platforms often share power supply, backup power supply, and common ground wire, so that the mutual interference caused by power coupling and ground wire coupling cannot be ignored.
5. There is little room for manoeuvre in the electromechanical structure of the equipment. Military electronic equipment has a sturdy structure, and the internal redundant space of the equipment is small. If the equipment is strengthened by EMC in the later stage of design, it will often conflict with the original mechanical structure or electrical layout of the equipment. At this time, it is difficult to take into account all aspects of tactical and technical performance indicators.
Due to the above characteristics, the EMC design of military electronic equipment is more complex and difficult than general electronic equipment, and it is more difficult to meet the electromagnetic compatibility test. To design in line with GJB151A. 97 electromagnetic compatibility standard military electronic equipment, first of all to follow the general EMC design principles, and then strengthen EMC measures on this basis, with particular attention to power supply, chassis shielding, circuit design, grounding quality.
2.1 The relationship between power supply and EMC
In GJB151A. In the 97 standard, the five items CE101, CE102, CE107, CS101 and CS106 are directly related to the power supply, the three items CS114, CS115 and CS116 are related to the power supply cable, and the other items of radiation emission and sensitivity are indirectly related to the power supply. Therefore, it can be said that the first step in the EMC design of military electronic equipment is the EMC design of the equipment power supply.
2.1.1 Main Countermeasures for Power EMC Design
(1) Electromagnetic shielding and power line filtering at the power input end. As soon as the power cord enters the chassis, it must be directly connected to the power filter, or a power filter whose input end doubles as a power socket is used. The installation of the power filter is very particular, the output line of the filter should be far away from the input line, and the metal casing should be grounded in a large area. If you bundle the power cables going in and out of the filter, the filter is pretty much useless.
(2) Use an isolation transformer. If AC power is used, it is best to use an isolation transformer when cost and installation conditions permit. The simplest isolation transformer is a power transformer with a shielded isolation layer between the primary and secondary stages. This transformer can play a variety of roles such as safety protection, voltage transformation, isolation of ground wire circulation, and improvement of common mode interference suppression capabilities. Complementary to power filters.
(3) Reasonable design of secondary power supply. There are two types of secondary power supply for equipment: switching power supply and linear power supply. Although the switching power supply has a certain ability to suppress external interference, the external radiated and conducted emissions of many switching power supplies are too large, so that in the EMC test, the sensitivity item can pass but the emission item cannot be passed. Therefore, in a low-power circuit, if you can use a switching power supply, try not to use it, and choose a linear regulator to avoid external interference.
(4) The overall shielding of the power supply. In view of the importance of the power supply part in the EMC performance of electronic equipment, it is also possible to shield the power supply part as a whole in another space isolated from other parts inside the shielded chassis to form an overall shielding of the power supply.
2.2 Chassis electromagnetic shielding
Chassis electromagnetic shielding is the most basic and most effective way to prevent electromagnetic radiation in space. In GJB151A. In the 97 standard, the five items RE101, RE102, RS101, RS103, and RS105 are directly related to the shielding of the chassis, and the other items related to cables are also indirectly related to the shielding of the chassis, because the cables are going in and out through the chassis.
2.2.1 Several principles for designing shielded enclosures
(1) Ensure the electrical continuity of the shielding layer. Both theoretical analysis and EMC test have proved that the slender gap on the electromagnetic shielding body will greatly reduce the shielding effect. Therefore, all external gaps in the chassis structure must have continuous and good conductive contact. For small holes whose diameter is smaller than the thickness of the shielded chassis, generally there is no need to worry about affecting the EMC effect.
(2) Properly handle various openings of the chassis. The opening of the chassis is mainly used to install switches, buttons, indicator lights and displays. When the opening is large, if it is difficult to take shielding measures in front of the installed device, a shielding layer should also be installed behind the device (rear shielding method), and the wires passing through the shielding layer should be filtered.
(3) Correctly select and install the chassis connectors to solve the problem of cable shielding. Improper handling of cables entering and leaving the chassis can weaken or even lose the shielding effectiveness of the chassis. Therefore, an external shield can be added to the external cable connected to the socket of the chassis, and the external shield of the cable must maintain electrical continuity with the shield of the chassis. The sockets installed on the chassis should use shielded connectors that meet military standards. The contact surface of the socket installed on the chassis must not have any insulating material such as paint film or plastic coating.
(4) It is best to use natural air cooling for the heat dissipation of the chassis, allowing some small cooling holes. If you want to install a cooling fan, you need to install a cut-off waveguide shielding ventilation plate outside the fan.
2.3 EMC countermeasures in circuit design
The basic principles of circuit EMC design have been covered in many literatures, and only a few practical details are mentioned here.
1) Application of multilayer printed circuit boards and surface mount components. The EMC characteristics of more than 4-layer printed circuit boards with power supply layer and ground layer are better than ordinary single-sided and double-sided printed circuit boards, and multi-layer boards should be used as much as possible in circuit design. The equivalent electromagnetic radiation area of ​​surface mount components is significantly smaller than that of plug-in components, which has better EMC performance. Therefore, the combination of multi-layer circuit boards and surface mount components should be the first choice for the design of printed circuit boards that meet the requirements of the GJBI51A-97 standard.
2) Selection of signal sensor and design of sensor signal amplifier. Sensors are generally installed outside the main chassis of the equipment, so the electromagnetic shielding measures taken on the main chassis do not cover the sensors. And because the signal from the sensor is very weak, the sensor is often the weak link in the electronic equipment that is most vulnerable to external electromagnetic interference, especially when doing RS101 and RS103 tests.
There are single-ended and differential input sense amplifiers. In theory, an ideal balanced input differential amplifier has a strong ability to suppress common-mode interference signals, so this input method should generally be used. However, when the interference signal is large to a certain extent (for example, the maximum interference field strength can reach 200 V/m during the RS103 test), it may cause the working range of the active differential amplifier to deviate from the linear region, making the common mode suppression ineffective. The results of the actual test also show that under the condition of strict shielding and good grounding, the anti-interference ability of the single-ended input sense amplifier is sometimes better. Therefore, which input amplifying circuit to choose depends on the actual situation.
3) Strengthen the high frequency bypass of active devices. According to GJB151A. When the 97 standard is used to test the RS103 project, this situation sometimes occurs: when the interference signal is a constant amplitude wave, the output signal is not disturbed; when the interference signal is an amplitude modulated wave, there is interference in the output signal. After analysis, it may be that after the AM wave interference signal enters the circuit, high-frequency detection is generated due to the nonlinear response of the active device, thereby causing interference. To prevent this, strengthening high-frequency bypassing of active devices can do something.
2.4 Pay attention to grounding quality
In the three aspects of power supply, shielding and circuit design, high attention must be paid to grounding and grounding quality issues. The grounding quality is first reflected in the correct grounding, that is, choosing the correct grounding point and grounding method; secondly, the grounding must be reliable, the grounding area should be large, the grounding wire should be thick and short, and the grounding bolts should be installed and tightened to reduce the grounding resistance. .
To sum up, when EMC design of military electronic equipment, the design focus is power supply, shielding, and circuit in turn, and the design consideration of grounding runs through these three aspects from beginning to end.
3 Compliance technologies for various military standard EMC tests
The physical essence of electromagnetic interference is the interaction of electromagnetic fields. In theory, any question about electromagnetic fields can be solved precisely by solving Maxwell's equations. However, most military electronic equipment is composed of a large number of structural parts and electronic components, and the spatial distribution of electromagnetic fields is very complex. Therefore, it is impossible to obtain accurate enough boundary conditions consistent with the real environment when solving Maxwell's equations. It is well known that the solutions of mathematical and physical equations are strongly dependent on boundary conditions. As long as the boundary conditions assumed in the theoretical calculation differ slightly from the actual distribution, the calculated results may become meaningless. Under such circumstances, practical experience still plays a very important role in EMC design of military electronic equipment.
In GJB151A. Of the 19 EMC tests listed in the 97 standard, 5 are related to antennas. If the tested device is not a wireless communication device, these 5 items generally do not need to be done. The two tests, CS109 and RS105, are usually less done. The remaining 12 tests can be divided into 4 categories according to their nature: conducted emission test, conduction sensitivity test, radiation emission test, and radiation sensitivity test. The following introduces some design criteria and experience that have been proven effective in practice for the purpose of testing compliance with these four types of electromagnetic compatibility test items.
3.1 Conducted emission test
Conducted emission tests include CE101, CE102, and CE107. The first 2 items belong to the conventional conducted emission test of the power line, both of which test the signal transmitted from the EUT to the power line, the difference is that the tested conducted emission frequency bands are different; These three conducted emission tests are aimed at the interference of the EUT power supply to the environment, and the requirements must be below the specified value to prevent any one device from interfering with other devices through the shared power supply.
The EUT's power line conducted emission signal has two sources: the functional circuit from the EUT and the power circuit from the EUT. The main means of blocking the conducted emission of EMI signals in the power supply circuit are isolation and filtering. If the EUT is powered by AC, the simplest isolation method is to use a power transformer with a shielded isolation layer, which has a strong isolation function for low-frequency EMI.
In the case of DC power supply, in order to achieve the purpose of isolation, it is necessary to use a DC/DC converter whose output and input are not in common ground. However, the DC/DC converter adopts pulse width modulation technology, which itself is a source of interference. Therefore, the selection is very important, and the DC/DC converter with low EMI should be selected as much as possible.
A filter at the power inlet is essential. Because the filter is a symmetrical passive circuit structure, it can play the role of bidirectional isolation and filtering, which can not only prevent external interference from entering the EUT, but also prevent internal interference from being transmitted to the outside. However, the power filter is mainly used to filter out the interference of the high frequency band, and is basically ineffective for the interference of the low frequency band.
The output filtering of the power supply circuit is also important. For power-type electronic devices, when the load power changes, the power supply changes, which in turn causes the power supply on the external line to fluctuate. If the frequency of this fluctuation exceeds 25 Hz and the amplitude is too large, the CE101 will not meet the standard. A large-capacity filter capacitor is connected in parallel at the output end of the power supply circuit, and the power supply fluctuation can be smoothed by using the energy storage function of the capacitor. As long as the frequency of power fluctuation is reduced to below 25 Hz, the lower limit of the CE101 test frequency can be avoided to make the test meet the standard. For signal-type electronic equipment, the energy of the interference signal transmitted by the front-end circuit through the power supply is mainly concentrated in the high frequency band, and a small-capacity filter capacitor with excellent high-frequency performance should be used. And because the AC equivalent impedance of the output end of the regulated power supply is very low, the filtering effect of a simple parallel capacitor is not obvious, so it is necessary to combine the method of series inductance to improve the high-frequency impedance and enhance the bypass effect of the filter capacitor to filter out high frequency. frequency interference.
By adopting these methods, the parameters of the anti-jamming devices used can be selected with reference to the power and operating frequency of the EUT, so that the CE101 and CE102 test items can meet the standards.
The CE107 project tests the conducted emission interference of power line spikes. Various types of spike interference signals may be generated when electronic equipment is working, but considering the strength of the conducted power and the impact on the shared power supply, the power switch of the EUT is a major source of spike interference. For example, in a multi-device EMC joint test of a project, it was found that every time a device was turned on and off, it would cause another nearby device to crash. The inspection results found that the previous device failed the CE107 test, affecting adjacent devices. There are many ways to make CE107 meet the standard. You can connect the peak interference absorption circuit in parallel with the power switch, or change the power supply of the equipment from cold start to hot start, or replace the mechanical switch with a non-contact switch, or reduce the time when the switch is turned on/off. The rate of current rise/fall, etc.
3.2 Conduction sensitivity test
Conducted sensitivity tests include CS101, CS106, CS114, CS115, and CSI16. The first 2 items test the power cord of the EUT, and the last 3 items test all cables (including the power cord) connected to the EUT. This type of test tests the sensitivity of the EUT to the external interference transmitted through the cable. It is required that the EUT is not sensitive to the interference and can maintain normal operation when the specified external interference is transmitted.
The two tests CS101 and CS106 require that the EUT can work normally under the action of the conducted interference signal from the power line. The measures for isolation and filtering of the power supply in Section 3.1 are also applicable here. In general, however, conducted susceptibility testing is more difficult to meet than conducted emission testing. This is because in the conducted emission test, the signal under test comes from the device, and the device may not necessarily have interference conducted emission, or even if there is, the conducted emission of the interference signal according to its function and purpose. Amplitude and frequency also do not necessarily fall within the range being measured. For example, when there is only a low-frequency small-signal circuit inside the device under test, the conducted emission test is easier to pass. When conducting the conducted sensitivity test, the interference signal comes from the outside, and the EUT must defend against external interference in the entire frequency band. To deal with this kind of interference, it is not enough to rely solely on the power supply filter. For the low frequency band, the required filter capacitor capacity is very large, and the general power filter cannot use such a large capacitor capacity. Because the filter capacitor of the power filter is connected across the power line and the ground plane, an excessively large filter capacitor will cause the bypassed interference current to be coupled to other devices on the same ground plane through the common ground wire, which will cause new electromagnetic interference. This is especially true for equipment mounted on ships. So in GJB151A. In the 97 standard, there is a limit on the upper limit of the grounding filter capacitor capacity at the power input end, which should generally be less than 0.1μF.
It is necessary to block the EMI from the power line and cannot use a large-capacity filter capacitor. At this time, a device that can absorb and attenuate EMI can be selected. Magnetic rings and magnetic columns are such devices. The use of suitable magnetic components at the power input can effectively absorb EMI energy. These magnetic components have many varieties and specifications. Under the premise of meeting the applicable frequency requirements, the varieties with high magnetic permeability can generally be selected, but it is necessary to avoid the magnetic saturation during use and the failure of the anti-interference performance. Put a pair of input power lines side by side on the magnetic ring for several turns, or pass through the magnetic column together, so that the magnetic fields generated by the power supply current can cancel each other out, avoid magnetic saturation, and suppress common mode interference. I have had such experience in EMC testing. When CS101 almost failed to meet the standard, string a magnetic ring on the power cord, and often get immediate results.
For conduction sensitivity test items CS114, CS115, and CS116, the interference frequency ranges from 10 kHz to hundreds of MHz, and anti-interference measures that combine high-frequency filtering and low-frequency electromagnetic attenuation can be used. Now there are commercial EMI three-terminal filters on the market, which comprehensively use magnetic beads, inductors and high-frequency capacitors to form a T-type or double-T-type filter network, which has a good inhibitory effect on high-frequency interference. These three-terminal filters are small in size and can be connected in series with each wire going in and out of the device. Where the cable is routed into the equipment chassis, a connector lined with magnetic material can be used. This type of connector is lined with high-frequency magnetics except for the metal contact couples of the pins and jacks, which is equivalent to a magnetic ring on each wire, which can absorb high-frequency at the cable access device. interference.
In terms of circuit design, if the input signal of the circuit adopts a balanced differential method, the signal cable connected to the EUT should be a twisted pair cable, and an appropriate laying distance should be selected so that the main energy of the common mode interference signal is mutually in the input circuit. offset.
3.3 Radiation emission test
The radiation emission test includes the magnetic field radiation emission project RE101 and the electric field radiation emission project RE102. The most important test item is the electric field radiation emission RE102. The frequency range of the test is 10 kHz to 18 GHz. The radiated emission signal must be lower than the specified value to be judged as test compliance.
For a specific device under test, the actual radiated emission frequency cannot cover the entire frequency range mentioned above, and the radiated emission energy is often concentrated in certain frequency points or frequency bands. In most cases, the radiated emission at the low-frequency end of the EUT often comes from the switching power supply, and the radiated emission at the high-frequency end mainly comes from the fundamental and higher harmonics of the oscillator in the circuit.
The radiation emission of a switching power supply is closely related to the quality of the power supply. A high-quality switching power supply not only has high efficiency, but also has less stray radiation. Therefore, when choosing a switching power supply, you must choose a power supply that meets the requirements of the military standard. For example, the military standard power supply of Ericsson and Vicor companies has the characteristics of low radiation.
The pulse frequency of the DC/DC converter in the switching power supply is a very important parameter. This frequency is generally between tens of kHz and hundreds of kHz, and some use MHz-level conversion frequency. If there are some frequency bands that are the most difficult to meet the standard during the RE102 test, sometimes switching power supplies with different frequencies can be used to avoid these frequency bands during the test.
Experience has shown that in addition to the direct external radiation emission of the switching power supply, the pulse width modulation (PWM) signal of the power supply circuit may also have a parasitic modulation effect on the adjacent circuits in the equipment, especially the high-frequency circuits, so that at frequencies far away from the operating frequency of the switching power supply. Radiated interference occurs at the point. This kind of interference is difficult to predict in advance, and even if it occurs, it is difficult to think that it is caused by the switching power supply. Shielding the switching power supply separately inside the chassis can greatly suppress this interference.
Another major source of radiated emissions is the crystal oscillator in the EUT circuit. Generally speaking, it is very simple to judge whether the radiated emission is from a crystal oscillator, because the frequency of the crystal oscillator is known and very accurate, such as the radiated frequency measured in the RE102 project test is exactly the same as the crystal oscillator frequency or its integer times, that is interference from the fundamental or harmonic of the oscillator. However, there are exceptions. If multiple crystal oscillators with different frequencies are used in the EUT, the frequency of each crystal oscillator may be cross-modulated, which complicates the radiation spectrum and causes radiation interference at a large number of frequency points. To reduce the radiation emission of the crystal oscillator, the first is to select a high-quality crystal oscillator and make it work in a low-voltage and low-power state; secondly, to correctly design the oscillator circuit to reduce the harmonics of the crystal oscillator, and if necessary, perform board-level operation on the crystal oscillator circuit. shield. Try to avoid using multiple oscillator sources in the circuit, and use the technique of deriving the remaining desired frequencies from one oscillator. These measures can greatly reduce the external radiation of the crystal oscillator. At present, there is a crystal oscillator with spread spectrum energy, which can disperse the radiated energy of the crystal oscillator into the frequency band around the main oscillation frequency, so as to reduce the peak radiated energy at a certain frequency point. Sometimes this crystal oscillator can be considered.
For most military electronic equipment, there is no condition to generate strong magnetic field radiation, and it is generally not difficult for the RE101 project to reach the standard.
3.4 Radiation sensitivity test
Radiation sensitivity tests include magnetic field radiation sensitivity item RS101 and electric field radiation sensitivity item RS103. For communication equipment and automatic control equipment that need to receive or detect weak electrical signals, electric field radiation sensitivity is an extremely critical test item, and it can be said to be the most difficult test in all EMC tests.
To make the RS103 test up to standard, still work on the power supply and shielding. The aforementioned anti-conducted interference measures for power supplies can also be applied to radiated interference. In order to prevent external radiation interference from entering the chassis through the power cable, the power cable must have a shielding layer, and the shielding layer must be grounded outside the chassis, and cannot be grounded when the power cable enters the chassis.
For the shielding of the chassis, it has been mentioned earlier that we should try to maintain the electrical continuity of the entire chassis, and carefully handle every seam and opening on the chassis. The seams of the chassis are preferably welded. If they cannot be welded for maintenance and disassembly, the seams must be pressed tightly. The author once did such an experiment: tune one FM radio to the radio state and put it in the iron chassis, and let the sound come out through the small holes on the surface of the chassis. When the case is closed, the radio can still receive radio waves. Then start to press the cover of the case. Each time it is pressed a little, the broadcast sound of the radio will be a little lighter. When it is pressed to a certain extent, the broadcast will not be received at all, and only the static noise of the radio will be heard. The importance of pressing the seams can be seen. In order to fill the small gaps between the seams, conductive rubber gaskets with silver aluminum fillers can be used at the seams.
The opening of the chassis has display holes and cable entry and exit holes. The LED display hole below 3 mm has little effect on the shielding effect, and the LCD display area is large. Without shielding, the external electric field radiation will enter the chassis. The methods of shielding include sticking a transparent conductive film on the display screen or adding glass with a wire mesh. The former is easy to use but has limited shielding effect, while the latter has better shielding effect but has an impact on light transmittance. No matter which method is used, pay attention to the good electrical continuity between the shielding layer and the chassis. It is best to add a shielding cover behind the display, and use a high-frequency feedthrough capacitor to filter the signal lines passing through the rear shielding cover.
The cable hole is also a weak point for the interference of external electric fields to penetrate into the chassis. The shielding effectiveness will be reduced by more than 30 dB when an unmeasured cable is passed through the shield [4]. Now some standard military connectors can be equipped with special shielded cable accessories. The use of such accessories can ensure good electrical continuity between the outer shielding layer of the cable and the connector shell.
Compared with the electric field radiation, the magnetic field radiation sensitivity RS101 test is less required. However, it should be noted that if there are devices that are sensitive to magnetic field radiation, such as inductive coils or electromagnetic sensors, it may not meet the standard in the RS101 test. The author once installed a communication device on the bulkhead of a certain platform, and as a result, there was a 400 Hz interference sound, and there was no interference when it was removed. At first it was suspected that there was electric field interference at the installation site, but the device has passed the RS103 test and no amount of improvement in shielding and grounding will help. Later, it was learned that a 400 Hz power cable was laid in the bulkhead of the installation location, and the large current generated a strong magnetic field, which belonged to magnetic field interference rather than electric field interference. Because of the different protection requirements for electrical and magnetic shielding, the usual hermetically sealed metal enclosures are not immune to magnetic field radiation. Finally, the device's moving-coil voice sensor was replaced with an electret-type sensor that was not sensitive to magnetic fields, and the interference disappeared immediately.
4 EMC design example of military electronic equipment
In recent years, with reference to the above-mentioned technologies, combined with the selection of appropriate EMC equipment, the author has carried out the EMC design conforming to the GJB151A-97 standard for a variety of military electronic equipment used in land, sea and air, and achieved good results. An example is given below. A certain communication equipment consists of a host and several slaves and extensions, and is required to comply with GJB151A. 97 standard to do CE102, CE107, CS101, CS106, CS114, CS115, CS116, RE102, RS103 a total of 9 EMC tests.
4.1 Main design considerations and measures
1) Chassis: Considering that this equipment is quite sensitive to weight, it is decided to use ZL110 aviation cast aluminum to manufacture the main chassis. In order to reduce the gap, except for the front panel and the upper cover, the chassis is integrally cast and then finished. The front panel and the upper cover are milled from LY12 aluminum plate, and a groove is milled at the junction of the chassis, and the EMC special elastic alloy stainless steel spiral tube is embedded in the groove. When the panel and the upper cover are installed on the chassis, the spiral tube is moderately compressed to maintain the electrical continuity of the contact surface. In order to ensure the high conductivity of the chassis surface, the chassis and cover are chemically cleaned after the gold processing is completed, and then conductive oxidation treatment is performed. The chassis of the slave and extension are also manufactured by a similar process.
2) Power supply: The power supply adopts a shielded power connector to connect to the chassis, and the fuse holder is equipped with a shielding cover. Immediately after the power cord enters the chassis, connect it to a double-section power filter, and the filter output is connected to an isolation transformer with three layers of shielding (primary, primary and secondary, and secondary three-layer shielding). The power filter and the isolation transformer are integrally shielded in the left rear corner of the chassis with an integrated sealed aluminum box. The output of the transformer is connected to the rectifier bridge with a twisted pair through a high-frequency magnetic core common mode choke coil. Each diode of the rectifier bridge is connected in parallel with a high-frequency bypass capacitor, and each output of the secondary power supply is connected in series with a DC filter.
3) Chassis connector: The chassis socket adopts the military standard XC series socket, and the socket is fixed on the chassis with 4 screws through the base square plate. The cable plug is fitted with a shielding sleeve. Receptacles and plugs are cadmium plated, pins are gold plated. Install CONCIL-A type aluminum silver-plated particles on the contact surface of socket square plate and chassis, fill with fluorosilicon conductive rubber gasket and press it tightly.
4) Cables between systems: All cables in the system are made of twisted-pair double-shielded radiation cross-linked fluoroplastic sheathed aviation cables. The outer shield layer is connected to the structural ground of the installation platform. The inner shield layer selects the grounding method according to the nature of the signal carried by the cable. . After entering the chassis, all cables pass through feed-through filters or chip three-terminal filters to filter out high-frequency interference signals; differential signal lines pass through common-mode choke coils to filter out common-mode interference signals; weak audio signals input from microphones pass through A closed magnetic circuit audio transformer is used for isolation and amplification.
5) Circuit board: 4-layer printed circuit board, SMD components.
6) Electronic circuit: select low-voltage, low-current devices, carefully design the circuit, the whole machine works in a low-power state, and the cooling system can be omitted to facilitate the shielding of the chassis. Because the power consumption is small, a linear secondary power supply can be used, which eliminates the electromagnetic radiation of the switching power supply.
After taking the above measures, the equipment has successfully passed the EMC tests of 9 items in accordance with the GJB151A-97 standard while ensuring various technical indicators. Has worked well in complex electromagnetic environments for many years.
5 Conclusion
Refer to the national military standard GJB151A. 97, combined with actual work, summarizes some experience in EMC design and testing of military electronic equipment. Electromagnetic compatibility is a technology with strong theory and practice. When designing military electronic equipment, if you can fully predict the various possible forms of EMI based on theory and experience and take corresponding EMC countermeasures, it will make the entire design. The process is more reasonable and effective, and there will be no major repetitions when completing the equipment manufacturing and electromagnetic compatibility test, ensuring the quality and progress of the work
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