Study on EMC of Pulse Radar Transmitter

1 Introduction
The integration and density of modern electronic technology and information technology are getting higher and higher. The interference problems between circuit modules and devices are becoming increasingly prominent, and have reached the point where they seriously affect the functions of the equipment. On the other hand, the rapid increase in electronic equipment has also led to a further deterioration of the electromagnetic environment. Electromagnetic radiation not only has adverse effects on electronic equipment, but also harms human health and affects the natural environment on which humans depend for survival.

In order to solve the problem of electromagnetic compatibility, China has enforced the electromagnetic compatibility standard. This standard restricts the electromagnetic interference generated by electronic equipment and also puts forward requirements for the anti-interference performance of electronic equipment. The national standard and the national military standard have made detailed specifications and standards for the electromagnetic compatibility of military electronic equipment. The pulse radar transmitter has large input and output power and works in a pulse working state. It is the device with the most serious electromagnetic radiation in the radar system. Good electromagnetic compatibility design is the prerequisite for the stable operation of the transmitter itself, the radar system and even other related electronic equipment. Starting from the three aspects of electromagnetic compatibility (interference source, sensitive source, and coupling path), this article briefly analyzes and introduces the electromagnetic compatibility design ideas and methods of pulse radar transmitters from the perspectives of telecommunications design and structural process design.

2 Analysis of interference sources of pulse radar transmitters

Regardless of whether the pulse radar transmitter adopts the gate modulation method or the cathode modulation method, it works in a high-voltage and high-current pulse state. It is generally composed of a transmitting tube, a pulse modulator, a DC high-voltage power supply, and a control protection module
(see Figure 1 for the block diagram).

When the transmitter is working normally, it is a strong interference source, and the interference sources mainly come from the following aspects.
2.1 Pulse modulation interference
When the transmitter is working, under the control of the timing signal, the pulse modulator provides the transmitting tube with a video modulation pulse with the required performance, converting the energy of the DC high-voltage power supply into pulse energy. This working state is equivalent to an electrically controlled capacitor discharge pulse source, and the electrical principle is shown in Figure 2.
The equivalent circuit of the loop is shown in Figure 3, which is a standard RLC series discharge loop. When the switch S is turned on at !=0, the circuit equation is

From the formula, we can know that the peak value of the pulse current is related to the loop inductance L, capacitance W and load resistance R. Since the transmitter is in a periodic pulse state, the pulse voltage is high and the current is large. Electromagnetic pulses are generated at the moment when the modulation switch and the transmitting tube are turned on and off. The electromagnetic pulse energy that enters the adjacent circuit through various coupling pathways will establish current and voltage on the equipment components or the input end of the component. Once it exceeds a certain threshold, the circuit will be disturbed at the least, and the components or components will be damaged at the worst.

2.2 High-voltage switching power supply interference

In order to improve the efficiency of the whole machine and reduce the size, the high-voltage power supply of the transmitter of modern radar generally adopts switching power supply. There are many forms of switching power supply, but no matter what form of switching power supply, its core part is a high-voltage, high-current controlled pulse signal source. The harmonic level generated by the pulse signal in the switching power supply is an EMI signal for other electronic equipment; in addition, because its switching device works in a high-frequency on-off state, the high-frequency fast transient process itself is an electromagnetic disturbance (EMD) source, and the EMI signal it generates has a wide frequency range and a certain amplitude. Strong electromagnetic disturbance signals interfere with nearby sensitive equipment through space radiation and power line conduction. In addition to power switch tubes and high-frequency rectifier diodes, the main components that generate radiation interference include pulse transformers and filter inductors.

2. 3 Microwave leakage
According to the mission and function of different radar systems, the operating frequency band of the transmitter will be different. There are two main ways to transmit electromagnetic waves, one is waveguide transmission and the other is coaxial transmission. No matter which transmission method is used, there will be different degrees of microwave leakage at the interface; in addition, microwave matching, detection and other devices will also cause leakage.

3 Electromagnetic compatibility design
There are two main ways of interference inside the transmitter and to external related equipment: conducted interference and radiated interference. Conducted interference signals can be divided into two categories: one is common mode interference signal, which is a signal with
equal potential and the same phase between the phase line and the ground line and between the neutral line and the ground line; the second is differential mode interference signal, which has a phase difference of 1800 between the phase line and the neutral line. The main way to reduce electromagnetic interference is to reduce the radiation of the interference source, cut off
the interference path, and improve the anti-interference ability of the equipment. The following briefly introduces the ideas and methods of electromagnetic compatibility design of the transmitter from the aspects of circuit design, cable design, grounding design, printed circuit board design, and structural design.
3.1 Circuit design
The main purpose of circuit design is to suppress conducted interference and radiated interference and enhance the anti-interference ability of the circuit itself.
(1) Conducted interference suppression
It is relatively easy to suppress conducted interference. As long as an appropriate EMI filter is used, the EMI signal level on the power line can be suppressed within the limit specified by the relevant standards. The principle of selecting power supply filter is
"impedance mismatch". In order to make the EMI filter have the best attenuation effect on EMI interference signal, the impedance of the filter terminal should make the filter in a serious mismatch state. The more severe the mismatch, the more ideal the attenuation achieved and the better the insertion loss performance. That is, if the noise source and the load internal resistance are low impedance, the input and output impedance of the filter connected to them should be high impedance; if the noise source and the load internal resistance are high impedance, the input and output impedance of the filter connected to them should be low impedance. According to the above analysis of the electromagnetic interference source in the transmitter, it can be seen that the source of conducted interference is mainly the high-voltage switching power supply. Therefore, it is mainly necessary
to add appropriate EMI power supply filter to the input port of the high-voltage switching power supply. Common mode interference in the switching power supply is generated by the potential difference between the current-carrying conductor and the earth. Its characteristic is that the noise voltage on the two lines is at the same potential and in the same direction; while differential mode
interference is generated by the potential difference between the current-carrying conductors. Its characteristic is that the noise voltage on the two lines is at the same potential and in the opposite direction. These two components of interference voltage on the line usually exist at the same time. Due to the imbalance of line impedance
, the two components will transform into each other during transmission, which is very complicated. A typical EMI filter contains two suppression circuits for common-mode noise and differential-mode noise.
In actual use, since the common-mode and differential-mode components generated by the equipment are different, the filter elements can be appropriately increased or decreased. The interference generated by the switching power supply is mainly common-mode interference. When designing the filter circuit, you can
try to remove the differential-mode inductor and add a common-mode filter inductor. The high-voltage power supply of the transmitter is generally large in power and often uses a three-phase power supply. Therefore, the power filter often uses the filter circuit shown in Figure 4.
In addition, according to the operating frequency of the high-voltage switching power supply, the power filter is selected from both low-frequency harmonics and high-frequency harmonics. The adjustment of the specific circuit generally requires EMI testing to obtain satisfactory results.
After selecting a suitable power filter, whether the power filter can be correctly installed also has a great influence on the filtering effect. The installation location of the power filter should be selected at the entrance of the equipment, and the input line should be short to reduce radiation.

(2) Radiated interference suppression
As mentioned above, the pulse modulator and the switching power supply are two very strong interference sources. To reduce radiated interference, a voltage buffer circuit (such as connecting an RCD buffer circuit in parallel at both ends of the switch tube) or a current buffer circuit (such as
connecting an appropriate amount of inductance in series with the collector of the switch tube) can be applied. The inductor can prevent the collector current from suddenly increasing when the power switch tube is turned on, and can also reduce the impact of the impact current in the rectifier circuit.
Rectifier diodes with small recovery charge and short reverse recovery time should be used. In addition, magnetic beads and parallel RC absorption networks at both ends of the rectifier diode can reduce some interference. The values ​​of resistance and capacitance can be several ohms and thousands of picofarads. The capacitor leads should be as short as possible to reduce the lead inductance. The larger the load current, the larger the current flowing through the rectifier diode at the end of the freewheeling, the longer the diode reverse recovery time, and the greater the impact of the peak current. Using multiple rectifier diodes in parallel to share the load current can reduce the impact of the short-circuit peak current. Placing high-frequency pulse transformers, high-frequency output rectifier filters and other components in the oil tank can not only reduce the withstand voltage space, but also limit the magnetic lines of force to the shielding body with low magnetic resistance.
(3) Circuit anti-interference
Filters are very effective in filtering out continuous radio frequency interference, but in actual operation, another type of interference is often encountered: transient interference. Transient interference refers to electromagnetic interference with a very short duration but large amplitude. This requires
the use of overvoltage protectors to protect sensitive circuits. Overvoltage protectors can generally be selected from varistors, transient absorption diodes, gas discharge tubes, etc. Varistors have large parasitic capacitance and are not suitable for high-frequency occasions;
transient absorption diodes have short response time, low clamping voltage, small peak current, and large parasitic capacitance of the device. If used in high-speed data lines, special low-capacitance devices must be used; gas discharge tubes withstand large currents, small parasitic capacitance, and long response time. Since the on-state maintenance voltage is very low, there will be a follow current and it cannot be used in a DC environment (the discharge tube cannot be disconnected). Care should also be taken when using it in AC (the follow current will exceed the rated power value of the device). A resistor can be connected in series in the discharge circuit to limit the current amplitude.

The input and output signals should be transmitted in isolation as much as possible. The interface that needs to be connected to the digital circuit should use a buffer. The ideal interface is a photoelectric coupler. Appropriately connect current limiting resistors, decoupling inductors and
decoupling capacitors in series on the power supply to prevent high-frequency interference from entering the device through the power line and damaging the device. The power supply end and signal output end of the integrated circuit chip are connected in parallel with high-frequency capacitors to the ground, which can effectively filter out high-frequency interference.
3. 2 Cable design
. Shielded cables should be used as much as possible for power input cables. When welding, both ends of the cable shielding layer should be grounded nearby.
. Twisted pairs should be used as much as possible for signal cables.
. When using coaxial cables, pay attention to the termination of the outer layer and the grounding of the circuits at both ends, and do not form a second return path other than the outer layer.
. For cables outside the equipment, ensure low-impedance overlap between the shielding layer and the shielding chassis.
. Try not to arrange signal lines of different properties in one connector or cable.
3. 3 Grounding design
The ideal reference ground is a physical entity with zero potential and zero impedance. No voltage drop will be generated when any current passes through it, but the ideal reference ground does not exist, and the so-called ideal ground plane is only relative and approximate.
In the transmitter cabinet, the reference ground of the equipment is actually the cabinet shell. A three-ground separation design is adopted in the transmitter cabinet, that is, the digital ground, analog ground and chassis ground are separated, and the three grounds are finally connected to the earth line.
The digital ground and analog ground of each module are connected in parallel with the single-point grounding method inside the cabinet, which can minimize the mutual interference caused by the ground current between different circuits. In addition, the high-voltage main circuit wiring is
separated from the low-voltage and signal lines, combined with the three-ground separation design, which can effectively reduce radiation and conduction interference.
3. 4 PCB design
. According to the circuit operating frequency, level size, and digital circuit/analog circuit division, arrange circuits of different nature in different areas of the circuit board to keep interference circuits away from sensitive circuits;
. Circuits in different areas use different ground wires, and different ground wires are connected at one point;
. Set up return lines near key signal lines; .
Set up ground wire grids on double-layer circuit boards;
, high-speed clock circuits are as far away from I/0 ports as possible, high-speed clock lines are as short as possible, do not change layers, and do not make 900 corners;
. The heat sink installed on the chip should be connected to the signal ground at multiple points;
. The leads between the power decoupling capacitor and the chip power pin and ground pin are as short as possible.
3. 5 Structural electromagnetic compatibility design
Shielding is one of the main means to suppress electromagnetic interference and realize electromagnetic radiation protection. Its purpose is to use a shielding body to surround the electromagnetic interference source to suppress the interference of the electromagnetic interference source to the receiver in the surrounding space, or to use a shielding body
to surround the receiver to avoid interference from the interference source〔"". In order to effectively suppress the electromagnetic interference of the transmitter, the electromagnetic shielding design of the cabinet and printed circuit board module should be considered when designing the transmitter structure.
.Cabinet shielding design
In the radar system, since the functions of the transmitter are relatively independent, they are generally composed of a cabinet alone. In this way, the shielding design of the transmitter cabinet is also relatively independent. When designing the shielding, as long as
the interfaces of ventilation, heat dissipation, power supply and communication wires and the parts that cannot be completely shielded due to indication, detection and other reasons are considered, targeted shielding measures can achieve better shielding effects. Specific measures are:
The metal surface of the cabinet_[activity joint must be conductive, and conductive paint and electromagnetic sealing gaskets should be used as needed; consider the electrochemical compatibility of the electromagnetic sealing gasket and the shielding body substrate. When the electrochemical is incompatible, use appropriate environmental sealing measures
to isolate moisture; the cabinet is used as the reference ground of the system, and the impedance of all joints must be controlled very low; the gaps or holes on the cabinet should be kept as far away from strong radiation sources or sensitive circuits as possible; shielding nets or honeycomb panels are used on the vents,
and electromagnetic sealing gaskets must be used between the honeycomb panels and the cabinet.
.Electromagnetic shielding design of modules
The small signal and control circuit printed circuit modules in the cabinet are shielded with a shielding box; the rectifier and inverter parts of the high-voltage switching power supply are shielded with a shielding box.

4 Conclusion
Good electromagnetic compatibility design is an important way to improve the reliability of radar transmitters. At present, most products do not take electromagnetic compatibility into consideration at the beginning of design. Various electromagnetic
interference problems are found after the product design is finalized. It is very difficult to transform the original product while maintaining the circuit and structure of the original product. Based on a lot of practice, this article proposes effective EMI
suppression measures from the design perspective.