RF PCB Layout Guidelines

2023-01-05 11:50:31 rf pcb 63 Viewed

SummaryIn low-power RF PCB design, use standard FR4 materials (good insulation properties, uniform material, dielectric constant ε=4), mainly use 4-layer to 6-layer RF PCB boards, and thickness can be used when the cost budget is low. For the double-sided RF PCB board below 1mm, it is necessary to ensure...

In low-power RF PCB design Guidelines, use standard FR4 materials (good insulation properties, uniform material, dielectric constant ε=4), mainly use 4-layer to 6-layer RF PCB boards, and thickness can be used when the cost budget is low. For the double-sided RF PCB board below 1mm, it is necessary to ensure that the reverse side is a complete ground layer. At the same time, because the thickness of the double-sided RF PCB board is more than 1mm, the FR4 medium between the ground layer and the signal layer is thicker. In order to make the RF signal line The impedance reaches 50 ohms, and the width of the signal traces is often about 2mm, and the spatial distribution of the RF PCB is difficult to control.

For a four-layer RF PCB board, in general, only RF signal lines are used on the top layer, the second layer is a complete ground, and the third layer is a power supply. The bottom layer of the RF PCB generally uses digital signal lines that control the status of RF devices. It is best not to make the power supply of the three layers into a continuous plane, but let the power supply lines of each RF device be distributed in a star shape, and finally connected to one point. The power traces of the third-layer RF devices should not cross the digital lines of the bottom layer.

2) For a mixed-signal RF PCB, the RF part and the analog part should be kept away from the digital part, and the ground of the digital part should be separated from the RF part. It is strictly forbidden to use switching power supply to directly supply power to the RF part. The main reason is that the ripple of the switching power supply will modulate the signal of the RF PCB part.

3) The components of the RF PCB should be closely arranged to ensure the shortest connection between the components. For the ground (GND) pins of each RF device on the RF PCB board, including the pins connected to the ground (GND) by resistors, capacitors, and inductors, holes should be drilled as close as possible to the pins and ground ( Layer 2) connectivity.

4) When RF PCB selects components that work in a high-frequency environment, surface-mount devices should be used as much as possible. This is because surface mount components are generally small in size, and the leads of the components are very short. In this way, the influence of additional parameters caused by component pins and internal wiring of components can be reduced as much as possible. Especially for discrete resistors, capacitors, and inductance components, the use of smaller packages is very helpful to improve the stability and consistency of RF PCB circuits;

5) RF PCB active devices that work in a high-frequency environment often have more than one power supply pin. If the RF PCB space permits, it is recommended to use two decoupling capacitors for each pin, with a capacitance of 1nF and 100nF. Generally, ceramic capacitors made of X5R or X7R are used. the

6) For the feed-in and feed-out of the RF signal on the RF PCB, a special RF coaxial connector must be used. The most commonly used one is the SMA type connector. If the signal frequency of RF PCB is further increased, we need to carefully select RF connecting wires and RF connectors. At this time, the in-line SMA connector may cause relatively large signal insertion loss due to its structure (mainly turning).

7) When designing the RF PCB, there are strict regulations on the width of the traces of the RF signal. When designing, it is necessary to strictly calculate and simulate the impedance of the wiring at the corresponding frequency point according to the thickness and dielectric constant of the RF PCB to ensure that it is 50 ohms (CATV standard is 75 ohms). When it is actually handed over to the RF PCB factory for production, the process used by the RF PCB manufacturer will cause the actual impedance of the RF PCB to differ from the simulation results by thousands of miles.

8) For those RF microstrip circuits that are simulated in ADS, HFSS and other simulation tools on the RF PCB, especially those directional couplers, filters (narrowband filters for PAs), microstrip resonators, and impedance matching Network, etc., you must communicate with the RF PCB factory well, and use boards with strict indicators such as thickness and dielectric constant that are consistent with those used in simulation. The best solution is to find the RF PCB board agent to buy the corresponding board, and then entrust the RF PCB factory to process it.

9) In the circuit of RF PCB, we often use a crystal oscillator as a frequency standard. This crystal oscillator may be TCXO, OCXO or ordinary crystal oscillator. For such a crystal oscillator circuit, it must be kept away from the digital part, and the RF PCB uses a special low-noise power supply system. More importantly, the frequency drift of the crystal oscillator may occur with the change of the ambient temperature. For TCXO and OCXO, this situation will still occur, but to a lesser extent. Especially small-package crystal oscillator products mounted on the RF PCB are very sensitive to ambient temperature. For such a situation, we can add a metal cover to the crystal oscillator circuit to reduce the frequency drift of the crystal oscillator caused by sudden changes in the ambient temperature. Of course, this will lead to an increase in the size and cost of the RF PCB.

The overall requirements for RF PCB layout are: RF signal traces are short and straight, reducing line mutations, drilling fewer vias, not intersecting with other signal lines, and adding as many vias as possible around the signal lines of RF PCB.

Make the ground wire as thick as possible. If conditions permit, each layer of the RF PCB should be grounded as much as possible, and the ground should be connected to the main ground, and more ground vias should be drilled to reduce the impedance of the ground wire as much as possible.

The power supply of the RF PCB circuit should not be divided into planes as much as possible. The entire power plane not only increases the radiation of the power plane to the RF PCB signal, but is also easily interfered by the RF signal. Therefore, the RF PCB power line or plane is generally in the shape of a long strip, which is processed according to the magnitude of the current. It is as thick as possible under the premise of meeting the current capacity, but it cannot be widened without limit. When the RF PCB handles the power line, it must avoid forming a loop.

The direction of the power line and the ground line should be parallel to the direction of the RF PCB signal but cannot overlap. It is best to use a vertical cross where there is a cross.


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