RF layout skills in mobile phone PCB design

1. Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit. Mobile phones have many functions and many components, but the PCB space is small. At the same time, considering the highest design process limit of wiring, all of these require higher design skills. At this time, it may be necessary to design a four-layer to six-layer PCB so that they can work alternately instead of working at the same time. High-power circuits sometimes also include RF buffers and voltage-controlled oscillators (VCOs). Make sure that there is at least one whole ground in the high-power area of ​​the PCB board, preferably without vias on it. Of course, the more copper foil, the better. Sensitive analog signals should be kept as far away from high-speed digital signals and RF signals as possible.

2 Design partitions can be decomposed into physical partitions and electrical partitions. Physical partitions mainly involve issues such as component layout, orientation, and shielding; electrical partitions can be further decomposed into partitions for power distribution, RF routing, sensitive circuits and signals, and grounding.
3.2.1 Let's discuss the issue of physical partitions. Component layout is the key to achieving an excellent 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 separate high-power circuits and low-power circuits as far as possible. The
most effective way to stack a circuit board is to arrange the main ground plane (main ground) on the second layer under the surface and run the RF line on the surface 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 number of cold solder joints on the main ground, and reduce the chance of RF energy leaking to other areas within the stacked board. In physical space, linear circuits such as multi-stage amplifiers are usually sufficient to isolate multiple RF areas from each other, but duplexers, mixers, and intermediate frequency amplifiers/mixers always have multiple RF/IF signals interfering with each other, so this effect must be carefully minimized.

3.2.2 RF and IF traces should be crossed as much as possible, and a ground plane should be placed between them as much as possible. The correct RF path is very important for the performance of the entire PCB board, which is why component layout usually takes up most of the time in mobile phone PCB board design. In mobile phone PCB board design, the low noise amplifier circuit can usually be placed on one side of the PCB board, and the high power amplifier on the other side, and finally connected to the antenna on the RF end and the baseband processor end on the same side through the duplexer. Some skills are needed to ensure that the straight through hole does not transfer RF energy from one side of the board to the other side. The commonly used technique is to use blind holes on both sides. The adverse effects of the straight through hole can be minimized by arranging the straight through hole in an area where both sides of the PCB board are not subject to RF interference. Sometimes it is not possible to ensure sufficient isolation between multiple circuit blocks. In this case, it is necessary to consider using 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 at an appropriate distance from the components, so it takes up valuable PCB board space. It is very important to ensure the integrity of the shield as much as possible. The digital signal line entering the metal shield should be routed on the inner layer as much as possible, and it is best if the PCB layer below the routing layer is the ground layer. The RF signal line can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but as much ground as possible should be laid around the gap. The ground on different layers can be connected together through multiple vias.

3.2.3 Proper and effective chip power decoupling is also very important. Many RF chips with integrated linear circuits are very sensitive to power supply noise. Usually, each chip needs to use up to four capacitors and an isolation inductor to ensure that all power supply 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 also applies to decoupling the power supply at this inductor end. Some chips require multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them separately. Inductors are rarely placed in parallel because this will form an air-core transformer and induce interference signals. Therefore, the distance between them should be at least equivalent to the height of one of the devices, or arranged at right angles to minimize their mutual inductance.

3.2.4 The principles of electrical partitioning are generally the same as physical partitioning, but there are also some other factors. Some parts of the mobile phone use different operating voltages and are controlled by software to extend battery life. This means that the mobile phone needs to run multiple power supplies, which brings more problems to isolation. The power supply is usually introduced from the connector and immediately decoupled to filter out any noise from outside the circuit board, and then distributed after passing through a set of switches or regulators. The DC current of most circuits on the mobile phone PCB is quite small, so the trace width is usually not a problem. However, a separate high-current line as wide as possible must be run 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. In addition, if the power supply pin of the high-power amplifier is not adequately decoupled, high-power noise will radiate to the entire board and cause various problems. The grounding of the high-power amplifier is quite critical and often requires a metal shielding case to be designed for it. 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 outputs of amplifiers and buffers are fed back to their inputs with appropriate phase and amplitude, they may produce self-oscillation. In the best case, they will be able to operate stably under any temperature and voltage conditions. In fact, they may become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to be looped back from the input of the filter to the output, this may seriously damage the bandpass characteristics of the filter. In order to achieve good isolation between the input and output, first a circle of ground must be laid around the filter, and secondly a ground must be laid in the lower area of ​​the filter and connected to the main ground around 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, the grounding of various places on the entire board must be very careful, otherwise a coupling channel will be introduced. Sometimes you have the choice of running single-ended or balanced RF signal lines, and the same principles about crosstalk and EMC/EMI apply here. Balanced RF signal lines can reduce noise and crosstalk if they are routed correctly, but their impedance is usually higher, and it may be difficult to maintain a reasonable line width to obtain an impedance match between the signal source, the line, and the load. Buffers can be used to improve isolation because they can split the same signal into two parts and use them to drive different circuits. In particular, the local oscillator may need a buffer to drive multiple mixers. When the mixer reaches the common mode isolation state at the RF frequency, it will not work properly. Buffers can isolate the impedance changes at different frequencies very well, so that the circuits will not interfere with each other. Buffers are very helpful in design. They can be placed right after the circuit that needs to be driven, so that the high-power output traces are very short. Because the input signal level of the buffer is relatively low, they are not likely to cause interference to other circuits on the board. Voltage-controlled oscillators (VCOs) can convert changing voltages into changing frequencies. This feature is used for high-speed channel switching, but they also convert traces of noise on the control voltage into small frequency changes, which adds noise to the RF signal.

3.2.5 To ensure that no noise is added, the following aspects must be considered: First, the expected bandwidth of the control line may range from DC to 2MHz, and it is almost impossible to remove such a wide bandwidth of noise by filtering; second, the VCO control line is usually part of a feedback loop that controls the frequency, which may introduce noise in many places, so the VCO control line must be handled very carefully. Make sure that the ground of the lower layer of the RF trace is solid, and all components are firmly connected to the main ground and isolated from other traces that may bring noise. In addition, make sure that the power supply of the VCO has been fully decoupled. Since the RF output of the VCO is often a relatively high level, the VCO output signal can easily interfere with other circuits, so special attention must be paid to the VCO. In fact, the VCO is often placed at the end of the RF area, and sometimes it also requires a metal shielding cover. The resonant circuit (one for the transmitter and the other for the receiver) is related to the VCO, but it also has its own characteristics. Simply put, the resonant circuit is a parallel resonant circuit with a capacitive diode, which helps set the VCO operating frequency and modulate voice or data onto the RF signal. All VCO design principles apply to resonant circuits. Resonant circuits are usually very sensitive to noise because they contain a relatively large number of components, have a wide distribution area on the board, and usually operate at a very high RF frequency. Signals are usually arranged on adjacent pins of the chip, but these signal pins need to work with relatively large inductors and capacitors, which in turn requires that these inductors and capacitors must be located very close to each other and connected back to a control loop that is very sensitive to noise. This is not easy to achieve.
The automatic gain control (AGC) amplifier is also a problem area. AGC amplifiers are present in both transmit and receive circuits. AGC amplifiers are usually effective in filtering noise, but because mobile phones have the ability to handle fast changes in transmit and receive signal strength, the AGC circuits require a fairly wide bandwidth, which makes it easy for AGC amplifiers on certain critical circuits to introduce noise. Designing AGC circuits must follow good analog circuit design techniques, which are related to very short op amp input pins and very short feedback paths, both of which must be far away from RF, IF or high-speed digital signal traces. Similarly, good grounding is essential, and the chip power supply must be well decoupled. If you have to run a long line at the input or output, it is best to run it at the output, where the impedance is usually much lower and it is not easy to induce noise. Generally, the higher the signal level, the easier it is to introduce noise into other circuits. 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 RF PCB design. The common analog ground and the ground used to shield and separate signal lines are usually equally important, so careful planning, thoughtful component layout and thorough layout evaluation are very important in the early stages of design. RF lines should also be kept away from analog lines and some 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 lay a layer of ground connected to the main ground along the RF trace between them. If this is not possible, make sure they are cross-crossed, which can minimize capacitive coupling. At the same time, try to lay more ground around each RF trace and connect them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. Isolation works best when a solid, monolithic ground plane is placed directly under the surface layer on the first layer, although other approaches can work with careful design. On each layer of the PCB, place as many ground planes as possible and connect them to the main ground plane. Place traces as close together as possible to increase the number of land planes on internal signal layers and power distribution layers, and adjust traces appropriately so that you can place ground connection vias to isolated land planes on the surface layer. Avoid creating free ground planes on each PCB layer because they can pick up or inject noise like a small antenna. In most cases, if you can't connect them to the main ground, then you're better off removing them.

3 When designing a mobile phone PCB, the following aspects should be given great attention.
3.3.1 Treatment of power and ground wires.
Even if the wiring in the entire PCB board is completed well, the interference caused by the inconsiderate consideration of the power and ground wires will reduce the performance of the product and sometimes even affect the success rate of the product. Therefore, the wiring of the power and ground wires should be taken seriously, and the noise interference generated by the power and ground wires should be reduced to a minimum 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. Now we will only describe the noise reduction method:
(1) It is well known that decoupling capacitors are added between the power and ground wires.
(2) Try to widen the width of the power and ground wires. It is best that the ground wire is wider than the power wire. The relationship between them is: ground wire>power wire>signal wire. Usually the signal wire width is: 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power wire is 1.2~2.5 mm. For digital circuit PCB, wide ground conductors can be used to form a loop, that is, to form a ground network (analog circuit ground cannot be used in this way)
(3) Use a large copper layer as the ground wire, and connect all unused areas on the printed circuit board to the ground as the ground wire. Or make a multi-layer board, with the power supply and ground wire each occupying one layer.

3.3.2 Common ground processing of digital circuits and analog circuits
Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are a mixture of digital circuits and analog circuits. Therefore, when wiring, it is necessary to consider the mutual interference between them, especially the noise interference on the ground wire. The frequency of digital circuits is high and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. For ground wires, the entire PCB has only one node to the outside world, so the problem of digital and analog common ground must be processed inside the PCB. In fact, the digital ground and analog ground are separated inside the board. They are not connected to each other, but only at the interface where the PCB is connected to the outside world (such as plugs, etc.). There is a short circuit between the digital ground and the analog ground. Please note that there is only one connection point. There are also cases where the PCB does not share a common ground, which is determined by the system design.

3.3.3 Signal lines are laid on the electrical (ground) layer
When wiring a multi-layer printed circuit board, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the workload of production, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. First, you should consider using the power layer, and then the ground layer. Because it is best to preserve the integrity of the ground layer.

3.3.4 Treatment of connecting legs in large-area conductors
In large-area grounding (electricity), the legs of common components are connected to it. The treatment of connecting legs needs to be comprehensively considered. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface, but there are some adverse risks in the welding and assembly of components, such as: ① Welding requires a high-power heater. ② It is easy to cause cold solder joints. Therefore, taking into account both electrical performance and process requirements, a cross-shaped solder pad is made, which is called heat shield, commonly known as thermal pad. In this way, the possibility of producing cold solder joints due to excessive heat dissipation in the cross section during welding can be greatly reduced. The treatment of the power (ground) layer legs of multilayer boards is the same.

3.3.5 The role of the network system in wiring
In many CAD systems, wiring is determined by the network system. If the grid is too dense, the number of pathways will increase, but the step is too small, and the amount of data in the drawing field is too large, which will inevitably have higher requirements for the storage space of the equipment, and also have a great impact on the computing speed of computer-related electronic products. Some pathways are invalid, such as those occupied by the pads of the component legs or by the mounting holes and fixed holes. If the grid is too sparse, too few pathways will have a great impact on the wiring rate. Therefore, a grid system with reasonable density is required to support the wiring. The distance between the legs of a standard component is 0.1 inch (2.54 mm), so the basis of the grid system is generally set at 0.1 inch (2.54 mm) or an integer multiple of less than 0.1 inch, such as 0.05 inch, 0.025 inch, 0.02 inch, etc.

4 The following are the tips and methods for high-frequency PCB design :
3.4.1 The corners of the transmission lines should be 45° to reduce return loss.
3.4.2 High-performance insulating circuit boards with strictly controlled insulation constant values ​​should be used. This method is conducive to the effective management of the electromagnetic field between the insulating material and the adjacent wiring.
3.4.3 The PCB design specifications for high-precision etching should be improved. Consider the total error of the line width of +/-0.0007 inches, manage the undercut and cross-section of the wiring shape, and specify the plating conditions of the wiring sidewall. Overall management of the wiring (wire) geometry and coating surface is very important to solve the skin effect problems related to microwave frequencies and achieve these specifications.
3.4.4 There is a tap inductance on the protruding lead, and components with leads should be avoided. In high-frequency environments, it is best to use surface-mount components.
3.4.5 For signal vias, avoid using the through-hole processing (PTH) process on sensitive boards because this process will cause lead inductance at the via.
3.4.6 Provide abundant ground planes. Use molded holes to connect these ground planes to prevent the impact of 3D electromagnetic fields on the board.
3.4.7 Choose electroless nickel or immersion gold plating instead of HASL plating. This electroplated surface provides better skin effect for high-frequency currents (Figure 2). In addition, this highly solderable coating requires fewer leads, which helps reduce environmental pollution.
3.4.8 Solder mask prevents the flow of solder paste. However, due to the uncertainty of thickness and unknown insulation properties, covering the entire board surface with solder mask will cause a large variation in electromagnetic energy in microstrip designs. Solder dam is generally used as solder mask. Electromagnetic field. In this case, we manage the transition between microstrip and coaxial cable. In coaxial cable, the ground plane is annular and evenly spaced. In microstrip, the ground plane is below the active line. This introduces certain edge effects that need to be understood, predicted and considered during design. Of course, this mismatch also results in return loss, which must be minimized to avoid noise and signal interference.

5 Electromagnetic compatibility design Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interferences, so that electronic equipment can work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself to other electronic equipment. 3.5.1 Choose a reasonable wire width Since the impact interference generated by transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and fine wires are beneficial for suppressing interference. The signal lines of clock leads, row drivers or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, the printed wire width can fully meet the requirements when it is about 1.5mm; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm. 3.5.2 Use the correct wiring strategy Using equal routing can reduce the inductance of the wires, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a tic-tac-toe mesh wiring structure. The specific method is to wire horizontally on one side of the printed board and vertically on the other side, and then connect them with metalized holes at the cross holes. 3.5.3 In order to suppress the crosstalk between the wires of the printed board, long-distance equal routing should be avoided as much as possible when designing the wiring, and the distance between the wires should be kept as far as possible. The signal line should not cross the ground line and the power line as much as possible. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk. 3.5.4 In order to avoid electromagnetic radiation generated when high-frequency signals pass through printed wires, the following points should also be noted when wiring the printed circuit board: (1) Minimize the discontinuity of the printed wires, such as the wire width should not change suddenly, the wire corner should be greater than 90 degrees, and loop routing is prohibited. (2) Clock signal leads are most likely to generate electromagnetic radiation interference. When routing, they should be close to the ground loop, and the driver should be close to the connector. (3) Bus drivers should be close to the bus they want to drive. For those leads that leave the printed circuit board, the driver should be close to the connector. (4) The data bus should be routed with a signal ground wire between every two signal lines. It is best to place the ground loop close to the least important address lead because the latter often carries high-frequency current. (5) When arranging high-speed, medium-speed and low-speed logic circuits on the printed circuit board, the devices should be arranged in the manner shown in Figure 1. 3.5.5 Suppressing reflection interference In order to suppress the reflection interference that appears at the terminal of the printed line, the length of the printed line should be shortened as much as possible and a slow circuit should be used except for special needs. If necessary, terminal matching can be added, that is, a matching resistor of the same resistance is added to the ground and power supply ends at the end of the transmission line. According to experience, for TTL circuits with generally faster speeds, terminal matching measures should be adopted when the printed line is longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output drive current and absorption current of the integrated circuit. 3.5.6 Use differential signal line routing strategy during circuit board design Differential signal pairs that are routed very close to each other will also be closely coupled to each other. This mutual coupling will reduce EMI emission. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules are usually applicable to differential signal routing, especially when designing signal lines of transmission lines. This means that we must be very careful in designing the routing of signal lines to ensure that the characteristic impedance of the signal line is continuous and constant everywhere along the signal line. In the process of layout and routing of differential line pairs, we hope that the two PCB lines in the differential line pair are exactly the same. This means that in practical applications, every effort should be made to ensure that the PCB lines in the differential line pair have exactly the same impedance and the routing length is exactly the same. Differential PCB lines are usually always routed in pairs, and the distance between them is kept constant at any position along the direction of the line pair. In general, the layout and routing of differential line pairs are always as close as possible.