PCB Technical - The layout principle of RF circuit board

The layout principle of RF circuit board

When designing the RF layout, the following general principles must be met first:

1. The RF output usually needs to be far away from the RF input.

2. Sensitive analog signals should be as far away as possible from high-speed digital signals and RF signals.

3. Separate the high-power RF amplifier HPA from the low-noise amplifier LNA as much as possible. Simply put, keep the high-power RF transmitter circuit away from the low-power RF receiver circuit.

4. Make sure that there is at least a whole piece of ground in the high-power area on the PCB, preferably without vias. Of course, the larger the area of the copper foil, the better.

5. Circuit and power decoupling are also extremely important.

6. The design partition can be decomposed into physical partition and electrical partition. The physical partition mainly involves component layout, direction, and shielding. Electrical partitions can continue to be decomposed into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding.

Principle of Physical Partition

1. The principle of component location layout. Component layout is the key to achieving a good RF design. The most effective technique is to first fix the components on the RF path and adjust their direction to minimize the length of the RF path, keep the input away from the output, and as far as possible Ground separation of high-power circuits and low-power circuits.

2. PCB stacking design principles. The most effective circuit board stacking method is to arrange the main ground plane on the second layer below the surface layer, and arrange the RF lines on the surface layer as much as possible. Minimizing the size of the vias on the RF path can not only reduce the path inductance, but also reduce the virtual solder joints on the main ground and reduce the chance of RF energy leaking to other areas in the laminate.

3. Principles of radio frequency devices and their RF wiring layout. In physical space, linear circuits like multi-stage amplifiers are usually sufficient to isolate multiple RF zones from each other, but duplexers, mixers, and intermediate frequency amplifiers/mixers always have multiple RF/IFs. The signals interfere with each other, so care must be taken to minimize this effect. The RF and IF traces should cross as much as possible, and place a ground between them as much as possible. The correct RF path is very important to the performance of the entire PCB, which is why component layout usually takes up most of the time in cellular phone PCB design.

4. Design principles to reduce the interference coupling of high/low power devices. On the cellular phone PCB, you can usually put the low-noise amplifier circuit on one side of the PCB, and put the high-power amplifier on the other side, and finally connect them to the RF end and the baseband processor on the same side through a duplexer. On the antenna at the end. Skills must be used to ensure that vias do not transfer RF energy from one side of the board to the other. A common technique is to use blind holes on both sides. The adverse effects of the through holes can be minimized by arranging the through holes in areas that are free from RF interference on both sides of the PCB.

Principles of Electrical Zoning

1. The principle of power transmission. Most circuits in cellular phones have relatively small DC currents, so wiring width is usually not a problem. However, it is necessary to separately set a large current line as wide as possible for the power supply of the high-power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, multiple vias are needed to transfer current from one layer to another.

2. Power supply decoupling for high-power devices. If it cannot be fully decoupled at the power supply pin of the high-power amplifier, high-power noise will radiate to the entire board and cause various problems. The grounding of high-power amplifiers is very critical, and it is often necessary to design a metal shield for it.

3. The principle of RF input/output isolation. In most cases, it is also critical to ensure that the RF output is far away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, if the output of the amplifier and buffer is fed back to their input with appropriate phase and amplitude, then they may have self-oscillation. In the best case, they will be able to work stably under any temperature and voltage conditions. In fact, they may become unstable and add noise and intermodulation signals to the RF signal.

4. The principle of filter input/output isolation. If the RF signal line has to be looped from the input end of the filter back to the output end, then this may seriously damage the bandpass characteristics of the filter. In order to isolate the input and output well, a ground must be placed around the filter first, and then a ground must be placed in the lower area of the filter and connected to the main ground surrounding the filter. It is also a good way to keep the signal lines that need to pass through the filter as far away as possible from the filter pins. In addition, the grounding of various places on the entire board must be very careful, otherwise an undesired coupling channel may be introduced unknowingly.

5. Digital circuit and analog circuit are isolated. In all PCB designs, it is a general principle to keep digital circuits away from analog circuits as much as possible, and it also applies to RFPCB design. The common analog ground and the ground used for shielding and separating signal lines are usually equally important. Design changes caused by negligence may cause the design to be completed to be overturned and rebuilt. The RF circuit should also be kept away from analog circuits and some very critical digital signals. All RF traces, pads and components should be filled with ground copper as much as possible, and connected to the main ground as much as possible. If the RF trace must pass through the signal line, try to arrange a layer of ground connected to the main ground along the RF trace between them. If it is not possible, make sure that they are crossed. This can minimize capacitive coupling. At the same time, place as much ground as possible around each RF trace and connect them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling.