How do You Make High-Frequency PCB of RF?

This article is the following article "Summary of RF PCB Layout and Louting", mainly describes the skills and methods of high-frequency PCB design, electromagnetic compatibility design, and the use of differential signal line routing strategy in the circuit board design process.

The skills and methods for high-frequency PCB design

1. The corners of the transmission line should be 45° to reduce the return loss
2. High-performance insulated circuit boards whose insulation constant values ​​are strictly controlled according to the level shall be adopted. This method is conducive to the effective management of the electromagnetic field between the insulating material and the adjacent wiring.
3. To improve the PCB design specifications related to high-precision etching. It is necessary to consider that the total error of the specified line width is +/-0.0007 inches, the undercut and cross-section of the wiring shape should be managed, and the plating conditions of the wiring sidewall should be specified. The overall management of wiring (wire) geometry and coating surface is very important to solve the skin effect problem related to microwave frequency and realize these specifications.
4. The protruding leads have tap inductance, so avoid using components with leads. In high-frequency environments, it is best to use surface mount components.
5. For signal vias, avoid using a via processing (pth) process on sensitive boards because this process will cause lead inductance at the vias.
6. To provide a rich ground plane. Use molded holes to connect these ground planes to prevent the 3D electromagnetic field from affecting the circuit board.
7. To choose electroless nickel plating or immersion gold plating process, do not use the HASL method for electroplating. This kind of electroplated surface can provide a better skin effect for high-frequency current (Figure 2). In addition, this highly solderable coating requires fewer leads, which helps reduce environmental pollution.
8. The solder mask can prevent the flow of solder paste. However, due to the uncertainty of the thickness and the unknown of the insulation performance, the entire surface of the board is covered with solder mask material, which will cause a large change in the electromagnetic energy in the microstrip design. Generally, a solder dam is used as the solder mask. The electromagnetic field. In this case, we manage the conversion from microstrip to coaxial cable. In the coaxial cable, the ground layer is interwoven ring-shaped and evenly spaced. In microstrip, the ground plane is below the active line. This introduces certain edge effects, which need to be understood, predicted, and considered during design. Of course, this mismatch will also cause return loss, and this mismatch must be minimized to avoid noise and signal interference.

Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress all kinds of external interference so that the electronic equipment can work normally in a specific electromagnetic environment, and at the same time to reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

Choose a reasonable wire width

Since the impact interference generated by the transient current on the printed lines is mainly caused by the inductance of the printed wires, the inductance of the printed wires should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. The signal lines of clock leads, row drivers, or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, the printed wire width is about 1.5mm, which can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2mm and 1.0mm.

Adopt the correct wiring strategy

The use of equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a grid-shaped wiring structure. The specific method is to wire one side of the printed board horizontally and the other side to wire vertically. Then connect with metalized holes at the cross holes.

Suppress the crosstalk between the conductors of the printed circuit board

To suppress the crosstalk between the conductors of the printed circuit board, when designing the wiring, try to avoid long-distance equal routing, and extend the distance between the lines as much as possible, and the signal line, the ground line, and the power line Do not cross. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.

Avoid electromagnetic radiation generated when high-frequency signals pass through printed wires

To avoid electromagnetic radiation generated when high-frequency signals pass through printed wires, the following points should also be noted when wiring the printed circuit board:

• (1) Minimize the discontinuity of printed wires. For example, the width of the wires should not change suddenly, and the corners of the wires should be greater than 90 degrees to prohibit circular routing.
• (2) The clock signal lead is most likely to produce electromagnetic radiation interference. When routing the wire, it should be close to the ground loop, and the driver should be close to the connector.
• (3) The bus driver should be close to the bus to be driven. For those leads that leave the printed circuit board, the driver should be next to the connector.
• (4) The wiring of the data bus should clamp a signal ground wire between every two signal wires. It is best to place the ground loop next to the least important address lead because the latter often carries high-frequency currents.
• (5) When arranging high-speed, medium-speed, and low-speed logic circuits on the printed board, the devices should be arranged in the manner shown in Figure 1.

Suppress reflection interference

To suppress the reflection interference that appears at the terminal of the printed line, in addition to special needs, the length of the printed line should be as short as possible and a slow circuit should be used. Terminal matching can be added when necessary, that is, a matching resistor of the same resistance is added to the end of the transmission line to the ground and the power terminal. According to experience, for general faster TTL circuits, terminal matching measures should be adopted when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output drive current and the absorption current of the integrated circuit.

Adopt differential signal line routing strategy in the circuit board design process

Differential signal pairs with very close wiring will also be tightly coupled to each other. This mutual coupling will reduce EMI emissions. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the routing of differential signals, especially when designing signal lines for transmission lines. This means that we must carefully design the wiring of the signal line to ensure that the characteristic impedance of the signal line is continuous and constant along the signal line. In the layout and routing process of the differential pair, we hope that the two PCB lines in the differential pair are the same. This means that in practical applications, the greatest effort should be made to ensure that the PCB lines in the differential pair have the same impedance and the length of the wiring is the same. Differential PCB lines are usually routed in pairs, and the distance between them is kept constant at any position along the line pair direction. Under normal circumstances, the placement and routing of differential pairs are always as close as possible.