PCB Tech - Do you really understand the design rules of RF board PCB?

As a competent PCB design engineer, the general PCB layout rules must be understood by everyone. However, do you all know the design rules of the RF board PCB? Next, I will share with you some of the layout and wiring principles of the RF board PCB, I hope it will be helpful to you.

RF board PCB layout principles

1. Layout determination: Before layout, you should have a detailed understanding of board functions, working frequency bands, current and voltage, main RF device types, EMC, related RF indicators, etc., and clarify the laminated structure, impedance control, external structure size, shielding cavity and cover The size and position of the device, the processing instructions of special devices (such as the size and location of the device that needs to be hollowed out, and the case is directly dissipated), etc.

In addition, the power, heat dissipation, gain, isolation, sensitivity and other indicators of the main RF components and the connection of filtering, bias, and matching circuits should be clarified. For the power amplifier circuit, the matching wiring requirements recommended by the device manual or the RF field analysis software simulation should also be obtained. Get the impedance matching circuit guide.

2. Physical partition: The key is to arrange the main components according to the main signal flow of the board. First, fix the components on the RF path according to the position of the RF port, and adjust their orientation to minimize the length of the RF path. In addition to the general layout rules, it is necessary to consider how to reduce the mutual interference and anti-interference ability between various parts to ensure sufficient isolation of multiple circuits. For circuit modules with insufficient isolation or sensitive and strong radiation sources, a metal shield should be considered. Shield the radio frequency energy in the RF area.

3. Electrical partition: The layout is generally divided into three parts: power supply, digital and analog. They must be separated in space, and the layout cannot cross the area. Separate strong and weak signals as much as possible, and separate digital and analog. Circuits that perform the same function should be arranged within a certain range as much as possible to reduce the signal loop area.

RF board PCB wiring principles

1. Keep the digital circuit away from the analog circuit as much as possible, make sure that the RF trace refers to a large-area ground plane, and route the RF trace on the surface as much as possible.

2. Digital and analog signal lines do not cross-regional wiring. If the radio frequency wiring must pass through the signal line, it is preferable to route a layer of ground connected to the main ground along the radio frequency wiring between them; second choice, to ensure the radio frequency line Cross with the signal line to minimize capacitive coupling. At the same time, place as much ground as possible around each RF trace and connect it to the main ground.

Generally, the radio frequency printed lines should not be wired in parallel and should not be too long. If parallel wiring is really needed, a ground wire should be added between the two wires (the ground wire is perforated to ensure a good grounding). Radio frequency differential lines, parallel lines, two parallel lines with a ground wire (the ground wire is punched to ensure good grounding), and the characteristic impedance of the printed line is designed according to the requirements of the device.

3. The basic sequence of radio frequency printed circuit board wiring: radio frequency line - baseband radio frequency interface line (IQ line) - clock line - power supply part - digital baseband part - ground.

4. Considering that green oil will affect the performance and signal of the microstrip line, it is recommended that the higher frequency single-board microstrip line does not need to be coated with green oil, and the medium and low frequency single-board microstrip line is recommended to be coated with green oil. .

PCB board

5. Radio frequency traces are usually not perforated. If RF traces must be replaced with layers, the size of the vias should be minimized. This not only reduces the path inductance, but also reduces the chance of RF energy leaking to other areas in the laminate.

6. There are always multiple RF/IF signals interfering with each other in duplexers, intermediate frequency amplifiers, and mixers. The RF and IF traces should be crossed as much as possible, and a ground should be placed between them.

7. Except for special purposes, it is forbidden to extend excess wires on RF signal traces.

8. The wiring of the baseband radio frequency interface line (IQ line) should be wider, preferably above 10mil. In order to avoid phase error, the line length should be as equal as possible and the spacing should be equal as much as possible.

9. The radio frequency control line requires the wiring to be as short as possible, and the wiring length is adjusted according to the input and output impedance of the transmission control signal device to reduce the introduction of noise. Keep traces away from RF signals, non-metallized holes and "ground" edges. Do not drill ground vias around the traces to prevent signals from coupling to the RF ground through the vias.

10. Keep digital wiring and power wiring away from radio frequency circuits as much as possible; clock circuits and high-frequency circuits are the main sources of interference and radiation, and must be arranged separately and away from sensitive circuits.

11. The main clock wiring is required to be as short as possible, the line width is recommended to be more than 10mil, and the two sides of the wiring are covered with ground to prevent interference from other signal lines. It is recommended to use a strip line for routing.

12. The control line of the voltage controlled oscillator (VCO) must be far away from the RF signal. If necessary, the VCO control line can be grounded.