(1) RF devices and RF wiring layout principles. In physical space, linear circuitssuch as multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but diplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF signals interfering with each other, so care must be taken to minimize this effect. RF and IF tracks should be crossed as much as possible, with a space 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.
(2) PCB design principles to reduce the interference coupling of high/low power devices. On cellular telephone PCBS, it is usually possible to place the low noise amplifier circuit on one side of the PCB and the high power amplifier on the other side, and eventually connect them to the RF and baseband processor antennas on the same surface via a diplexer. To ensure that the through-hole does not transfer RF energy from one side of the plate to the other, a common technique is to use blind holes on both sides. The adverse effects of the through-hole can be minimized by placing the through-hole in an area where both sides of the PCB board are free from RF interference.
4. Electrical zoning principle
(1) Power transmission principle. Most circuits in cellular phones have fairly small DC currents, so wiring width is usually not an issue. However, the high power amplifier's power supply must be provided with a separate high current line as wide as possible to minimize the transmission voltage drop. To avoid too much current loss, multiple through-holes are needed to transfer the current from one layer to another.
(2) Power decoupling of high power devices. If the high power amplifier is not sufficiently decoupled at the power pin end, then the high power noise will radiate throughout the board and bring a variety of problems. The grounding of high power amplifiers is critical and often requires the design of a metal shield.
(3) RF input/output isolation principle. 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, amplifiers and buffers are likely to generate self-excited oscillations if their outputs are fed back to their inputs at the appropriate phase and amplitude. At best, they will work stably under all temperature and voltage conditions. In fact, they may become unstable and add noise and intermodulation signals to the RF signal.
(4) Filter input/output isolation principle. If the RF signal line has to be wound back from the input to the output of the filter, then this can seriously impair the bandpass characteristics of the filter. In order for the input and output to be well isolated, a field must first be arranged around the filter, and then a field must be arranged in the lower part of the filter and connected to the main ground surrounding the filter. It is also a good idea to keep the signal lines that need to pass through the filter as far away from the filter pins as possible. In addition, careful grounding is required at various points throughout the board, otherwise an unwanted coupling path may be unknowingly introduced.
(5) Digital circuit and analog circuit isolation. It is a general principle in all PCB designs to keep digital circuits as far away from analog circuits as possible, and it applies to RF PCB designs as well. Common analog sites are usually equally important as those used to shield and separate signal lines, and inadvertence of design changes may result in the completion of the design having to be reworked. RF lines should also be kept away from analog lines and some very critical digital signals. All RF lines, pads and components should be filled with ground copper as much as possible around, and connected to the main ground as much as possible. If RF wiring must pass through signal wiring, try to lay a floor between them along the RF wiring that connects to the main ground. If this is not possible, make sure they are crisscrossed to minimize capacitive coupling, while laying as much ground around each RF wire as possible and connecting them to the main ground. In addition, minimizing the distance between parallel RF lines minimizes perceptual coupling.