How to eliminate internal EMI effects in communication systems
Radio communication is the transfer of energy from one circuit to another in the form of an electromagnetic field. When making circuit or radio communication, what we need is that the energy between the two circuits can be transmitted according to certain requirements, otherwise electromagnetic interference will occur between the devices and inside the device. Especially in the design of more complex equipment circuits and complex units in them, electromagnetic compatibility issues should be considered. Due to the wireless propagation characteristics of electromagnetic waves, it is determined that they will be affected by many factors from the outside and inside.
Here we only discuss the causes of EMI inside the communication system and how to eliminate the effects of EMI. Within the communication system, a variety of electronic devices are used, which has caused EMI to become a more complicated problem. There are many reasons, such as power lines and signal lines, common impedance of signal lines and signal lines, and various signals. Improper grounding, electromagnetic induction of weak signal circuits for weak signal circuits, effects of space electromagnetic wave radiation of high-power signals on other circuits, and mutual interference caused by parallel transmission of multiple signals. Various conductive couplings, spatial radiation and improper grounding in communication systems are the main causes of various interference disturbances and system EMC differences.
1 Several factors that produce EMI
1.1 Electromagnetic radiation
High-power electronic devices and high-frequency signals tend to generate strong high-frequency electromagnetic waves, which diffuse radiation to the surrounding area, which inevitably interferes with other circuits. Especially in electronic equipment systems, many components are concentrated in a small space. It is normal to generate electromagnetic field coupling in the vicinity of weak signal circuits due to the radiation of high power signals. Especially for units with wide signal operating frequency, common signal circuit, dense cell layout, discrete devices and discrete components, sometimes each of its wires generates an antenna that emits electromagnetic waves and receives electromagnetic waves. At this time, it is easier to form the antenna effect of the wire around the sensitive device and at the weak signal circuit.
1.2 Conductive coupling
When the current of two or more loops passes through a common impedance, the voltage formed by the current of one loop on the common impedance affects other circuit loops, that is, the conductive coupling of the circuit. The causes are: ground impedance, common power line internal resistance, common line impedance, and multi-circuit circuit conduction. In practical circuits, the conductive coupling can be divided into electric field coupling, magnetic field coupling and electric field magnetic field coupling, that is, our often equivalent capacitive coupling in parallel with the circuit, inductive coupling in series with the circuit, and capacitive inductance in parallel with the circuit in parallel coupling.
1.3 Grounding interference
In the same circuit, there is always a certain impedance between two different grounding points. The ground current generates a ground voltage on the common impedance, and the ground voltage is directly applied to the circuit to become an interference voltage. In general, the grounding impedance is often very low and can be ignored during design. However, for EMI, the impedance on the ground plane of the ground loop cannot be ignored.
2 technology to improve electromagnetic compatibility
How to reduce the various EMI of the communication system and improve the EMC capability of the system is the premise to ensure the normal operation of the system. There are several methods:
2.1 Reduce radiation interference
The transmission of electromagnetic radiation is large enough between systems that are far apart, as small as the imaginable distance in the system. Therefore, at the beginning of equipment design, large signals and small signals should be strictly separated, sub-unit design or isolation, to the maximum extent. Reduce the mutual electromagnetic interference in the space. The main methods are:
2.1.1 Physical isolation
That is to increase the distance between the signal conductor and the interference source, the weak signal conductor should avoid parallel design with the strong signal conductor. Avoid two wires of the same signal loop, and the weak signal loop and the strong signal loop share the ground wire. For various components and wires of different performance, they should be classified according to their different levels, power and anti-interference ability.
2.1.2 Balance circuit voltage
Using the balanced relationship on the circuit, the two wires that transmit the same signal have the same interference voltage. The interference voltage can be self-cancelled on the load of the two wires. In this way, electromagnetic interference of the external circuit can be effectively suppressed.
2.1.3 Maintain good shielding between signals
Limit the influence of power lines or magnetic lines to a certain range, prevent power lines or magnetic lines from entering a certain range, and separate external interference from the measuring device. The measurement signal is not affected by the external electromagnetic field. When using shielded wires and shielded cables, care must be taken that the shield and the outer layer cannot flow current, and the ground cannot form a loop.
2.1.4 Correct grounding
The ground potential is very complicated. In order to minimize the interference, “one-point grounding” and “multi-point grounding” should be applied correctly.
2.2 Reduce grounding interference
The general requirements for the ground plane are: the ground plane should be zero potential; the ideal ground plane should be a zero-resistance entity, there is no voltage drop when the current passes; there is a large distributed capacitance between the ground plane and the wiring, and the lead inductance of the plane itself Will be small; grounding requirements to minimize the interference voltage generated by the common ground impedance of multiple circuits. At the same time avoid unnecessary loops.
When designing the grounding of small-signal circuits, especially for circuit design with large signal variation range and large frequency variation range, the following aspects should be noted:
2.2.1 weak signal amplifier and signal source select one point to ground
For the signal source and the circuit to which the amplifier is connected. If the source and amplifier are grounded at different points, there is a potential difference between the two places. For the output of the amplifier, this potential difference is small and negligible; but for the input of the amplifier, this potential difference should be much smaller than the output signal of the signal source, otherwise it will cause a lot of interference on the amplifier. .
2.2.2 Multi-level circuit selection one point grounding
A low level circuit is the most susceptible circuit. Especially for multi-stage connected circuits, the input signal is very small compared with the output signal. Therefore, a grounding should be selected to minimize the interference of the low-level circuit.
2.2.3 Grounding point selection of cable shielding layer
For low frequency and high frequency circuits, different grounding methods should be used, especially high frequency shielding wires, so that the loop current only passes through the low resistance shielding layer and not through the inner conductor of the cable. It can effectively suppress the interference of the ground loop.
2.2.4 Grounding selection of resonant circuit
The grounding of the resonant tank and the filter circuit is the most easily overlooked problem. Due to the misdirection of the grounding method, many designers have grounded the capacitive inductance of the resonant circuit. In the parallel resonant circuit, the inductor and capacitor should be grounded a little, so that the resonant circuit itself forms a closed loop, at which time the high-frequency large current will not pass through the ground plane. And suppress the ground loop interference. The correct resonant loop and filter circuit can effectively suppress interference.
2.3 reduce conducted interference
Conducted interference is divided into differential mode interference and common mode interference: the former refers to the interference signal existing between the phase line and the neutral line: the latter refers to the interference signal between each phase line and the neutral line and the ground.
In different circuits, the specific interference suppression method should be selected according to the influence of differential mode interference and common mode interference on the circuit. In a general circuit system, the differential mode interference amplitude is small, the frequency is low, and the interference caused is small; the common mode interference amplitude is large, the frequency is high, and radiation can also be generated through the wire. The resulting interference is large. Therefore, it is necessary to weaken the conducted interference. The most effective way to control the EMI signal below the limit level specified by the EMC standard is to install a filter in the switching power supply input and output circuits. At the same time, filtering is one of the most widely used and effective methods. Currently. The most effective way to suppress conducted interference is passive filtering. Different methods of conduction caused by different coupling modes of the circuit should be eliminated or reduced by different methods. For the interference caused by inductive coupling, increase the release capacitance at the appropriate position to eliminate or reduce the interference caused by mutual inductance. For capacitive coupling, the interference frequency and impedance are determined by the device. The most effective way to suppress capacitive coupling interference is Reduce the coupling capacitance.
2.4 Other measures to reduce electromagnetic interference
In order to minimize the noise of the communication system and eliminate EMI, the following methods should be considered when designing the system:
2.4.1 Select low noise devices and components
In amplification or other circuits. The internal noise of electronic devices plays an important role. Therefore, improving the noise performance of electronic devices and selecting low-noise electronic devices can greatly reduce the noise figure of the circuit.
2.4.2 Correctly selecting transistors
The noise figure of the amplifier stage of the DC stage of the amplifier stage has a large relationship with the DC operating point of the transistor. At the same time, the noise figure NF is also related to the VCE and VCB of the transistor, respectively.
2.4.3 Select the appropriate signal source internal resistance
The first-stage amplifier, the stages of the multi-stage amplifier, the mixer, etc. are all connected to the signal source, and the appropriate internal resistance is selected to obtain not only the minimum noise figure but also the maximum power gain.
2.4.4 Choosing the right working bandwidth
Noise and voltage are related to the passband width. As the bandwidth of the receiver or amplifier increases, the various internal noises of the receiver or amplifier also increase. Therefore, the bandwidth of the receiver or amplifier must be strictly selected. It is neither too narrow, and satisfies the signal's requirement for distortion when passing. The fork will not be too wide. In order to avoid a decrease in signal to noise ratio.
In nature, EMI is ubiquitous, and no matter what measures are taken, the effects of EMI cannot be completely eliminated. What we can do is to minimize its impact and to meet the requirements of the various constraints to meet the needs of the work.