PCBA engineers must see! 13 key points of power supply design for RF circuit.

PCBA engineers must see! 13 key points of power supply design for RF circuit

01

The power line is an important way for EMI to enter and exit the circuit. Through the power line, external interference can be transmitted to the internal circuit and affect the RF circuit index. In order to reduce electromagnetic radiation and coupling, the loop area of primary side, secondary side and load side of DC-DC module is required to be minimum. No matter how complex the form of power circuit is, its high current loop should be as small as possible. The power cord and ground wire should always be placed very close.

02

If switching power supply is used in the circuit, the layout of peripheral devices of switching power supply shall comply with the principle of shortest power return path. The filter capacitor should be close to the relevant pins of the switching power supply. Use a common mode inductor close to the switching power supply module.

03

The long-distance power line on the single board cannot approach or pass near the output and input of the cascade amplifier (gain greater than 45dB) at the same time. Avoid the power line becoming the RF signal transmission path, which may cause self excitation or reduce the sector isolation. High frequency filter capacitors are required at both ends of long-distance power lines, and even in the middle.

04

Three filter capacitors are combined and connected in parallel at the power inlet of RF PCB, and the low, medium and high frequencies on the power line are filtered by using the respective advantages of these three capacitors. For example: 10uF, 0.1uF, 100pF. And close to the input pin of the power supply in order from large to small.

05

When feeding the small signal cascade amplifier with the same group of power supply, it should first 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 impact on the front stage. And the power filter of each stage has at least two capacitors: 0.1uF, 100pF. When the signal frequency is higher than 1GHz, 10PF filter capacitor shall be added.

06

Commonly used in low-power electronic filters, the filter capacitor should be close to the triode pin, and the high-frequency filter capacitor should be closer to the pin. The transistor shall be of low cut-off frequency. If the triode in the electronic filter is a high-frequency tube, which works in the amplification area, and the layout of peripheral devices is unreasonable, it is easy to produce high-frequency oscillation at the output end of the power supply.

The same problem may exist in the linear voltage stabilizing module, because there is a feedback loop in the chip and the internal triode works in the amplification area. In the layout, the high-frequency filter capacitor is required to be close to the pin to reduce the distributed inductance and destroy the oscillation conditions.

07

The copper foil size of the power part of the PCB conforms to the maximum current flowing through it, and the allowance is considered (the general reference is 1A / mm line width).

08

The input and output of the power cord cannot be crossed.

09

Pay attention to power decoupling and filtering to prevent interference from different units through power lines. During power wiring, power lines shall be isolated from each other. The power line is isolated from other strong interference lines (such as CLK) with ground wire.

10

The power wiring of small signal amplifier needs to be isolated by grounding copper sheet and grounding via to avoid other EMI interference, which will deteriorate the signal quality of this level.

11

Different power layers should avoid overlapping in space. The main purpose is to reduce the interference between different power sources, especially between some power sources with large voltage difference. The overlap of power plane must be avoided. If it is difficult to avoid, the interlayer can be considered.

12

PCB layer distribution is convenient to simplify the subsequent wiring processing. For a four layer PCB (the circuit board commonly used in WLAN), in most applications, the top layer of the circuit board is used to place components and RF leads, the second layer is used as the system ground, the power part is placed in the third layer, and any signal lines can be distributed in the fourth layer.

The continuous ground plane layout of the second layer is very necessary for establishing the RF signal path with controlled impedance. It is also convenient to obtain the shortest ground loop as possible, provide a high degree of electrical isolation for the first and third layers, and minimize the coupling between the two layers. Of course, other board layer definitions can also be adopted (especially when the circuit board has different layers), but the above structure is a successful example after verification.

13

A large area of power supply layer can make VCC wiring easier. However, this structure is often the fuse causing the deterioration of system performance. Connecting all power leads together on a large plane will not avoid the noise transmission between pins. Conversely, if a star topology is used, the coupling between different power pins will be reduced. Good power decoupling technology combined with rigorous PCB layout and VCC lead (star topology) can lay a solid foundation for any RF system design. Although there are other factors that reduce the system performance index in the actual design, having a "noise-free" power supply is the basic element to optimize the system performance.