1. The power line is an important way of EMI in and out of the circuit. Through the power line, external interference can be passed into the internal circuit, affecting the RF circuit index. To reduce electromagnetic radiation and coupling, the loop area of the primary side, secondary side, and load side of the DC-DC module must be minimum. No matter how complex the form of the power circuit, its large current loop should be as small as possible. Power and ground cables should always be placed close together.
2. If a switching power supply is used in the circuit, the layout of the peripheral devices of the switching power supply should comply with the principle of the shortest power backflow path. The filter capacitance should be close to the switch power supply pin. Use common-mode inductance, close to the switching power module.
3. Long distance power lines on the board should not be near or pass near the output and input terminals of the cascade amplifier (gain greater than 45dB) at the same time. Do not use power cables as RF signal transmission channels, which may cause self-excitation or reduce sector isolation. High frequency filter capacitors are required at both ends of the long distance power line, and even in the middle.
4. The power inlet of RF PCB is combined with three filter capacitors in parallel, and the advantages of these three capacitors are utilized to filter the low, medium and high frequencies on the power line respectively. For example: 10UF, 0.1UF, 100PF. And close to the input pins of the power supply in descending order.
5. Feed the small signal cascade amplifier with the same set of power supply, should start from the last stage and supply power to the front stage in turn, so that the EMI generated by the last stage circuit has less influence on the front stage. And each level of power filter has at least two capacitors: 0.1uF and 100PF. When the signal frequency is higher than 1GHz, the filter capacitor of 10pF should be added.
6. Commonly used in low-power electronic filters, filter capacitance should be close to the triode pin, high-frequency filter capacitance closer to the pin. The triode uses a lower cutoff frequency. If the triode in the electronic filter is a high frequency tube, working in the amplification area, and the layout of peripheral devices is not reasonable, it is easy to produce high frequency oscillation at the output end of the power supply.
The same problem can occur in linear voltage regulator modules because there are feedback loops in the chip and the internal triode operates in the amplification region. The high frequency filter capacitor is required to be close to the pin during the layout to reduce the distributed inductance and destroy the oscillation condition.
7. The copper foil size of the POWER part of the PCB is in accordance with the maximum current it flows, and allowance is taken into account (1A/mm line width is generally referred to).
8. Input and output of power cables cannot be crossed.
9. Pay attention to power decoupling and filtering to prevent interference from different units through power cables. Power cables should be isolated from each other when wiring power cables. The power line is isolated from other strong interference lines (such as CLK).
10. The wiring of the power supply of the small signal amplifier needs to be isolated by ground copper skin and ground hole to avoid the intrusion of other EMI interference and the deterioration of the signal quality.
11. Different power layers should avoid overlapping in space. Mainly in order to reduce the interference between different power supplies, especially between some power supplies with very different voltages, the overlapping problem of power supply planes must be tried to avoid, when it is difficult to avoid can be considered in the interval stratum.
12. For a four-layer PCB (a circuit board commonly used in WLAN), in most applications, components and RF leads are placed on the top layer of the board, the second layer is systematically placed on the third layer, and any signal cables can be distributed on the fourth layer.
The use of a continuous ground plane layout in the second layer is necessary to establish an impedance-controlled RF signal path. It also facilitates the shortest possible ground loop, providing a high degree of electrical isolation for the first and third layers and minimizing the coupling between the two layers. Of course, other board layer definitions could be used (especially if the board has a different number of layers), but this structure is a proven success story.
13. A large power layer can make Vcc wiring easy, but this configuration is often the trigger for system performance deterioration, and connecting all the power leads together on a large plane will not avoid noise transmission between the pins. Conversely, using a star topology reduces the coupling between different power pins. Good power decoupling technology combined with rigorous PCB layout and Vcc leads (star topology) can provide a solid foundation for any RF system design. Having a "noise-free" power supply is essential to optimizing system performance, although there may be other factors in the actual design that reduce system performance metrics.
