RF (Radio Frequency) PCB design is often described as a “black art” because there are still many uncertainties in theory, but this view is only partially correct. There are also many rules and regulations that can be followed in RF PCB design and should not be ignored.

However, in actual design, the real practical skill is how to compromise these rules and regulations when they cannot be accurately implemented due to various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulation layer materials and laminates, as well as wavelength and standing waves, so these have a great impact on the EMC and EMI of mobile phones. The following is a summary of the conditions that must be met when designing the RF layout of mobile phone PCB boards:

1.1 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 more functions and many components, but the PCB space is small. At the same time, considering that the design process of wiring is the highest, 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 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, the better. Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible.

1.2 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.

1.2.1 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 PCB circuit board is to arrange the main ground plane (main ground) on the second layer under the surface layer and run the RF line on the surface layer as much as possible. Minimizing the size of vias on the RF path not only reduces the path inductance, but also reduces the number of cold solder joints on the main ground and reduces the chance of RF energy leaking into other areas of the stacked board. In physical space, linear circuits such as 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/IF signals interfering with each other, so this effect must be carefully minimized.

1.2.2 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 to 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 straight through holes can be minimized by arranging them in areas where both sides of the PCB 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 lines entering the metal shield should be routed on the inner layer as much as possible, and it is best that 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.

1.2.3 Proper and effective chip power decoupling is also very important. Many RF chips that integrate linear circuits are very sensitive to power supply noise. Usually, each chip requires 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 needed to provide a high-impedance RF load and a low-impedance DC power supply. The same principle applies to decoupling the power supply at the 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.

1.2.4 The principles of electrical partitioning are generally the same as physical partitioning, but there are 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 board is quite small, so the trace width is usually not a problem. However, a large current line as wide as possible must be run separately 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 the current from one layer to another. In addition, if it is not adequately decoupled at the power pin end of the high-power amplifier, the high-power noise will be radiated to the entire board and cause various problems. The grounding of the high-power amplifier is quite critical, and a metal shielding cover is often required for it. In most cases, it is also critical to ensure that the RF output is 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 work 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 passband characteristics of the filter. In order to achieve good isolation between input and output, first of all, a circle of ground must be laid around the filter, and then a piece of 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 can choose to run single-ended or balanced RF signal lines. The principles of cross-interference and EMC/EMI also apply here. Balanced RF signal lines can reduce noise and cross-interference if they are routed correctly, but their impedance is usually higher, and a reasonable line width must be maintained to obtain an impedance that matches the signal source, routing and load. The actual routing may be somewhat difficult. Buffers can be used to improve the isolation effect because they can divide 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 well isolate the impedance changes at different frequencies so that the circuits will not interfere with each other. Buffers are very helpful for design. They can be placed right behind the circuit to be driven, making the high-power output traces very short. Since the input signal level of the buffer is relatively low, they are not easy to interfere with 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 tiny noise on the control voltage into tiny frequency changes, which adds noise to the RF signal.

1.2.5 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 through 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 is adequately 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 shield. 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. In simple terms, 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 also apply to the resonant circuit. Because the resonant circuit contains a considerable number of components, a wide distribution area on the board, and usually operates at a very high RF frequency, the resonant circuit is usually very sensitive to noise. The 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 and connected back to a control loop that is very sensitive to noise. This is not easy to do.

The automatic gain control (AGC) amplifier is also a place where problems easily occur. There will be an AGC amplifier in both the transmitting and receiving circuits. The AGC amplifier can usually filter out noise effectively, but because the mobile phone has the ability to handle rapid changes in the strength of the transmitted and received signals, the AGC circuit is required to have a fairly wide bandwidth, which makes it easy for the AGC amplifier on some key circuits to introduce noise. The design of the AGC line must comply with good analog circuit design techniques, which is related to very short op amp input pins and very short feedback paths. Both of these must be kept away from RF, IF or high-speed digital signal traces. Similarly, good grounding is also essential, and the power supply of the chip must be well decoupled. If a long line must be run at the input or output end, it is best to go at the output end, 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, which also applies to RFPCB design. Common analog ground and ground used to shield and separate signal lines are usually equally important, so careful planning, thoughtful component placement and thorough layout * evaluation are very important in the early stages of design. Also, RF lines should be kept away from analog lines and some critical digital signals. All RF traces, pads and components should be filled with as much ground copper as possible and connected to the main ground as much as possible. If RF traces must cross signal lines, try to lay a layer of ground connected to the main ground along the RF traces between them. If this is not possible, make sure they are crisscrossed, which can minimize capacitive coupling. At the same time, as much ground as possible should be laid around each RF trace and connected to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. A solid, monolithic ground plane is placed directly under the surface layer on the first layer, and the isolation effect is best, although other practices can also work when carefully designed. On each layer of the PCB board, as much ground as possible should be laid and connected to the main ground. Place the traces as close together as possible to increase the number of plots on the internal signal layer and power distribution layer, and adjust the traces appropriately so that you can place the ground connection vias to the isolated plots on the surface layer. Avoid generating free ground on each layer of the PCB because they will pick up or inject noise like a small antenna. In most cases, if you cannot connect them to the main ground, then you’d better remove them.

1.3 When designing a mobile phone PCB board, the following aspects should be given great attention

1.3.1 Treatment of power and ground lines

Even if the wiring in the entire PCB board is completed well, the interference caused by the lack of consideration of the power and ground lines 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 lines should be taken seriously, and the noise interference generated by the power and ground lines 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 line and the power line. Now we will only describe the reduction method to suppress the noise:

(1) It is well known that decoupling capacitors are added between the power and ground lines.

(2) Try to widen the width of the power supply and ground wires. It is best that the ground wire is wider than the power supply wire. The relationship between them is: ground wire > power supply 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 supply wire is 1.2~2.5mm. For the PCB of digital circuits, a wide ground wire can be used to form a loop, that is, to form a ground network for use (the ground of analog circuits cannot be used in this way)

(3) Use a large area of ​​copper layer as a ground wire, and connect all unused areas on the printed circuit board to the ground as a ground wire. Or make a multi-layer board, with the power supply and ground wire occupying one layer each.

1.3.2 Common ground processing of digital circuits and analog circuits

Now many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of 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 kept as far away from sensitive analog circuit devices as possible. For ground lines, the entire PCB has only one node to the outside world, so the problem of digital and analog common ground must be handled inside the PCB. In fact, the digital ground and analog ground are separated inside the board. They are not connected to each other, but 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 is not grounded. This is determined by the system design fixed.

1.3.3 Signal lines are laid on the power (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 power (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.

1.3.4 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 bad hidden dangers for 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.

1.3.5 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 should be used to support the wiring. The distance between the two legs of a standard component is 0.1 inch (2.54 mm), so the basis of the grid system is generally set to 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.

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

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

1.4.2 High-performance insulating circuit boards with strictly controlled insulation constant values ​​according to the level should be used. This method is conducive to the effective management of the electromagnetic field between the insulating material and the adjacent wiring.

1.4.3 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 (conductor) geometry and coating surface is very important for solving the skin effect problems related to microwave frequencies and achieving these specifications.

1.4.4 Protruding leads have tap inductance, so avoid using components with leads. In high-frequency environments, it is best to use surface-mounted components.

1.4.5 For signal vias, avoid using the through-hole processing (PTH) process on sensitive boards because it will cause lead inductance at the vias.

1.4.6 Provide abundant ground planes. Use molded holes to connect these ground planes to prevent the influence of 3D electromagnetic fields on the circuit board.

1.4.7 Choose electroless nickel plating or immersion gold plating processes, and do not use HASL for electroplating. This electroplated surface can provide better skin effect for high-frequency currents (Figure 2). In addition, this highly solderable coating requires fewer leads, which helps reduce environmental pollution.

1.4.8 The solder mask prevents the flow of solder paste. However, due to the uncertainty of thickness and the unknown insulation performance, covering the entire board surface with solder mask will lead to large changes in electromagnetic energy in microstrip designs. Solder dam is generally used as solder mask. electromagnetic field. In this case, we manage the transition from microstrip to 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 some edge effects that need to be understood, predicted, and considered during design. Of course, this mismatch also causes return loss, which must be minimized to avoid noise and signal interference.

1.5 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, while reducing the electromagnetic interference of electronic equipment itself to other electronic equipment.

1.5.1 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 to suppress interference. The signal lines of clock leads, row drivers or bus drivers often carry large transient currents, so the printed conductors should be as short as possible. For discrete component circuits, the printed conductor width of about 1.5mm can fully meet the requirements; for integrated circuits, the printed conductor width can be selected between 0.2 and 1.0mm.

1.5.2 Use the correct wiring strategy

Using equal routing can reduce the wire inductance, 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.

1.5.3 In order to suppress 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.

1.5.4 In order to avoid electromagnetic radiation generated when high-frequency signals pass through printed conductors, the following points should be noted when wiring printed circuit boards:

(1) Minimize the discontinuity of printed conductors, such as the width of the conductor should not change suddenly, the corner of the conductor 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) The bus driver should be close to the bus it is intended 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 as shown in Figure 1.

1.5.5 Suppressing reflection interference

In order to suppress the reflection interference appearing 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. Terminal matching can be added when necessary, that is, a matching resistor of the same resistance value 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.

1.5.6 Using differential signal line routing strategy during PCB circuit board design

Differential signal pairs that are routed very close to each other will also be tightly 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 design the routing of signal lines very carefully to ensure that the characteristic impedance of the signal line is continuous and constant along the signal line. During the layout and routing of differential line pairs, we hope that the two PCB lines in the differential line pair are completely consistent. This means that in practical applications, we should make every effort to ensure that the PCB lines in the differential line pair have exactly the same impedance and the wiring length is also completely consistent. 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. Normally, the layout and routing of differential line pairs are always as close as possible.