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With the advancement of science and technology, the types of printed circuit boards have become more and more diverse, and their functions have become more and more complete. Among them, RF-printed circuit boards are growing rapidly, and their application fields are widespread. This article will take you to explore the mysteries of RF circuit boards.
RF stands for Radio Frequency, which indicates the frequency of electromagnetic waves that can be radiated into space, ranging from 300kHz to 300GHz. Radiofrequency is radio frequency current, abbreviation of a high-frequency alternating electromagnetic wave. An alternating current that changes less than 1,000 times per second is called a low-frequency current, and an alternating current that changes more than 10,000 times per second is called a high-frequency current. And radio frequency is such a high-frequency current.
In electronics theory, electromagnetic waves refer to the electromagnetic fields formed by alternating current passing through a conductor.
When the frequency of electromagnetic waves is lower than 100kHz, the Earth's surface absorbs them and cannot form effective transmission. However, when electromagnetic wave's frequency is higher than 100kHz, they can spread in the air and can transmit signals over long distances. High-frequency electromagnetic waves with long-distance transmission capabilities are called radio frequencies.
RF's function is to transmit information wirelessly. It uses high-frequency electromagnetic waves (radio frequency) to achieve long-distance communication. It can propagate in the air and reflect through the ionosphere at the outer edge of the atmosphere, thus forming an effective long-distance transmission capability. Radio frequency technology is widely used in wireless communications, radar systems, medical equipment, and other fields.
Radio-frequency (RF)has the following characteristics:
High frequency: The frequency of radio frequency waves is high and can transmit more data information.
Strong long-distance transmission capability: The transmission distance of radio frequency waves is longer than that of ordinary electromagnetic waves and is suitable for communication equipment with a wide range.
Strong penetration: Radio frequency waves can penetrate obstacles such as walls, mountains, forests, buildings, etc. They can cover a wide range of communication signals and complete signal transmission.
RF-printed circuit boards are one of the types of high-frequency circuit boards. RF board refers to a high-frequency board made using radio frequency technology. RF circuit board is a very high-speed circuit with a frequency of 100mhz and above, generally between 500mhz-2ghz. Above 2ghz is a microwave PCB.
Therefore, the difference between rfPCB and microwave circuit board lies in frequency. But there are also some differences in circuit board design.
Radio frequency circuits can be designed using lumped parameter components or mixed parameter components.
Since the size of microwave circuits is almost the same as their wavelength, they must be designed using distributed parameter components.
The manufacturing of RF-printed circuit boards is so strict and requires more sophisticated mechanical equipment, such as plasma etching machinery, laser direct imaging (LDI), etc. With the support of this precision equipment and RF technology, RF circuit boards have the following characteristics:
The thermal conductivity of RF-printed circuit boards is lower than that of ordinary printed circuit boards made of FR-4 material. It means they can operate under high-temperature conditions for a long time without being damaged.
Compared with other circuit boards, their electromagnetic wave frequency is higher, so the signal transmission distance is longer and can cover a large area, allowing wireless communication equipment to transmit information in time
The RF PCB substrate material is polytetrafluoroethylene (PTFE), which has a lower coefficient of thermal expansion (CTE) than traditional FR4. Therefore, the RF board is more stable in high-temperature environments.
When designing a PCB, you can mix PTFE materials and FR4 to build a PCB stack up and reduce the board area. Since FR4 is cheaper, it can also reduce production costs.
The material needs to be selected based on factors such as dielectric constant (DK), dissipation factor (DF), coefficient of thermal expansion (CTE), manufacturability, passive intermodulation (PIM), cost considerations, etc. Different materials have different application scenarios, so when designing, it is necessary to consider and evaluate the project in many aspects to meet the design performance requirements.
PTFE is very popular in RF design due to its low thermal expansion coefficient, high dielectric constant, and stable performance, such as Teflon and Rogers materials.
Fr- stands for flame retardant. FR -4 is one of the most common PCB materials. Its price is relatively low compared to PTFE. It is generally suitable for electronic devices. However, in higher-frequency RF applications, the high thermal expansion coefficient of FR-4 will be limited.
It is a specific series of PTFE or glass fiber composite materials. Low Dk and low loss, suitable for high-frequency circuit design, reduce passive intermodulation (PIM) performance and provide better stability.
Similar to RO4000, the RO3000 series is also a PTFE-based material but has different dielectric constants and other properties, suitable for different RF designs. Models include: RO3003, RO3006, RO3010, RO3035 high frequency laminates.
Suitable for high-power radio frequency fields, this circuit board generally uses an aluminum substrate to improve heat dissipation and electrical properties.
It is used in some very high-frequency and microwave-frequency devices. The metalized ceramic PCBs can provide lower dielectric loss and excellent stability.
Radiofrequency technology is significant in wireless communications, such as mobile communications, satellite communications, wireless broadcasting, radar, and other fields.
Radiofrequency technology plays a vital role in electronic devices, such as televisions, radio stations, microwave ovens, and other fields.
Radiofrequency technology is widely used in medical and life science fields, such as medical imaging, drug development, and biological testing.
Radiofrequency technology is widely used in electronic games, such as wireless microphones, wireless controllers, near-field communications, etc.
Radiofrequency technology is also indispensable in artificial satellites and space exploration, such as orbit control, satellite communications, etc.
To sum up, radio frequency technology is widely used in various industrial and scientific fields and plays a crucial role.
There are many factors to consider when designing a circuit board, such as parameter settings, manufacturing costs, manufacturability, etc. Among them, parameter settings are crucial because they affect the practical performance of the circuit board.
In RF board design, you can achieve the best performance of the circuit board.
It is significant to reduce circuit signal loss, and proper impedance matching can ensure power transmission efficiency between components. This process mainly considers the line width, spacing, substrate material, and routing configuration.
Signal transmission is propagated through the traces, so the layout of the traces is significant. It is easily affected. It is generally best to have coplanar and short traces to reduce losses and improve performance.
It may generate parasitic capacitance (unintended capacitance), which can cause signal distortion. Therefore, in the design, vias should be reduced as much as possible, the hole size should be kept uniform, and specific vias should be ensured for the pins or pads.
RF circuit boards require noise reduction to be effective, and power supply decoupling is one way to remove noise. Selecting decoupling capacitors can eliminate all power supply-introduced noise, so it is critical to place the capacitors carefully.
RF components and capacitors should be placed on the same side
Each capacitor must have a ground via
The capacitor with the lowest power is placed near the power supply and is arranged from large to small.
The manufacturing process of RF circuit boards is somewhat different from that of traditional circuit boards. In order to maintain the performance and stability of high-frequency circuits, there are some key manufacturing processes.
Due to its high-frequency performance, it is vital to select the material. Common materials include polytetrafluoroethylene (such as Teflon and RO4000 series materials ).
RF circuit boards have high requirements for precision, so laser imaging machines are used in the imaging stage, and plasma etching machines are used during etching. It ensures the circuit accuracy meets the design requirements.
Board thickness affects the electrical stability between circuits, so it must be consistent during PCB manufacturing flow.
An impedance matching test is a must in the manufacturing process, which affects the signal loss. The test ensures that the circuit impedance is consistent with the designed impedance. Generally, the time domain reflection test (TDR) is used.
Test the loss characteristics of RF PCB within the designed power range to ensure that the signal transmission quality is not affected by excessive loss. Common test methods include:
•Frequency conversion method
•Transmission Line Method
•Heat balance method
•DC Power Measurement Method
Test the circuit board under different temperature and humidity conditions to evaluate its stability and reliability in different environments.
With the rapid development of the Internet of Things, 5G communications, smart homes, and other fields, the demand for the RF PCB market is showing a rapid growth trend, promoting further innovation and market competition in RF technology.
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