Electronic Design - Summary of RF PCB layout and routing experience

Radio frequency (RF) PCB design is often described as a kind of "black art" because there are still many theoretical uncertainties, but this view is only partially correct. RF circuit board design also has many guidelines that can be followed and should not be Ignored law. The following is a summary of the conditions that must be met when designing the RF layout of the mobile phone PCB board:

1.1 Separate the high-power RF amplifier (HPA) and 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. The mobile phone has many functions and many components, but the PCB space is small. At the same time, considering that the wiring design process has the highest limit, all of these have relatively high requirements for design skills. At this time, it may be necessary to design a four- to six-layer PCB and let them work alternately instead of working at the same time. High-power circuits sometimes include RF buffers and voltage controlled oscillators (VCO). Make sure that there is at least a whole piece of ground in the high-power area of the PCB, preferably without vias. Of course, the more copper, the better. Sensitive analog signals should be as far away as possible from high-speed digital signals and RF signals.

1.2 The design partition can be decomposed into physical partition and electrical partition. Physical partitioning mainly involves issues such as component layout, orientation, and shielding; electrical partitioning can continue to be decomposed into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding.

1.2.1 We discuss the issue of physical partitioning. 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 orientation 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.

The most effective PCB stacking method is to arrange the main ground plane (main ground) on the second layer below the surface layer, and route 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. 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.

1.2.2 The RF and IF traces should be crossed as much as possible, and a ground should be placed between them as much as possible. The correct RF path is very important to the performance of the entire PCB board, which is why the component layout usually accounts for most of the time in the mobile phone PCB board design. In the mobile phone PCB board design, usually the low-noise amplifier circuit can be placed on one side of the PCB board, and the high-power amplifier is placed on the other side, and finally they are connected to the RF end and baseband processing on the same side through a duplexer. On the antenna at the end of the device. Some tricks are needed to ensure that the straight through holes 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 straight-through holes can be minimized by arranging the straight-through holes in areas that are free from RF interference on both sides of the PCB board. Sometimes it is impossible to ensure sufficient isolation between multiple circuit blocks. In this case, it is necessary to consider the use of a metal shield to shield the RF energy in the RF area. The metal shield must be soldered to the ground and must be kept with the components. A proper distance, so it needs to take up valuable PCB board space.

1.2.3 Proper and effective chip power decoupling is also very important. Many RF chips with integrated linear circuits are very sensitive to power noise. Usually, each chip needs to use up to four capacitors and an isolation inductor to ensure that all power noise is filtered out. An integrated circuit or amplifier often has an open-drain output, so a pull-up inductor is required to provide a high-impedance RF load and a low-impedance DC power supply. The same principle applies to decoupling the power supply at this inductor side.

1.3 When designing the mobile phone PCB board, great attention should be paid to the following aspects

1.3.1 Treatment of power supply and ground wire

Even if the wiring of the entire PCB board is well completed, the interference caused by the improper consideration of the power supply and the ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the electric and ground wires must be taken seriously, and the noise interference generated by the electric and ground wires should be minimized to ensure the quality of the product. Every engineer engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is described:

(1) It is well-known to add decoupling capacitors between the power supply and ground.

(2) Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire, usually the signal wire width is: 0.2～0.3mm, the most The thin width can reach 0.05～0.07mm, and the power cord is 1.2～2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way)

1.4 The skills and methods for high-frequency PCB design are as follows:

1.4.1 The corners of the transmission line should be 45° to reduce the return loss

1.4.2 High-performance insulated circuit boards whose insulation constant values are strictly controlled according to the level shall be adopted. This method is conducive to effective management of the electromagnetic field between the insulating material and the adjacent wiring.

1.4.3 To improve the PCB design specifications related to high-precision etching. It is necessary to consider that the total error of the specified line width is +/-0.0007 inches, the undercut and cross-section of the wiring shape should be managed, and the plating conditions of the wiring side wall should be specified. The overall management of wiring (wire) geometry and coating surface is very important to solve the skin effect problem related to microwave frequency and realize these specifications.