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About the layout and wiring principles of the RF board PCB.

September 02, 2020


Today we will talk about the layout and wiring principles of the RF board PCB...

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 location 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 directly dissipated by the chassis), etc.

In addition, the power, heat dissipation, gain, isolation, sensitivity and other indicators of the main RF components and the connections 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 considering the general layout rules, it is also necessary to consider how to reduce the mutual interference and anti-interference ability between various parts, to ensure sufficient isolation of multiple circuits, and to consider the use of circuit modules with insufficient or sensitive isolation and strong radiation sources. The metal shield shields 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 radio frequency trace refers to a large area ground plane, and route the radio frequency line on the surface as much as possible.

2. The digital and analog signal lines are not wired across regions. If the radio frequency wiring must pass through the signal line, it is preferable to arrange 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 more ground around each RF trace and connect 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 punched 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), 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. .

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

8. The wiring of the baseband radio frequency interface line (IQ line) should be wider, preferably above 10mil. 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.