Summary of conditions that must be met when designing RF layouts for mobile PCBs

Radio frequency (RF) circuit board design is often described as a kind of "black art" due to its theoretical uncertainty. However, this view is only partly correct. There are many guidelines that RF circuit board design can follow and should not be followed. Neglected rule. The following summarizes the conditions that must be met when designing an RF layout for a mobile phone PCB.

However, in practical design, the practical and practical technique is how to compromise the rules and laws 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, insulating layer materials and laminated plates, as well as wavelength and standing wave, so these have great impact on the EMC and EMI of mobile phones. The following is on the mobile phone PCB board. The conditions that must be met when designing an RF layout are summarized:

1.1 Isolate high-power RF amplifiers (HPAs) and low-noise amplifiers (LNAs) as much as possible.

Simply put, it is to keep the high-power RF transmit circuit away from the low-power RF receive circuit. The mobile phone has many functions and many components, but the PCB space is small. At the same time, the designing process of the wiring is the most restrictive. All these requirements for design skills are relatively high. It may be necessary to design four- to six-layer PCBs at this time, so that they work alternately instead of working at the same time. High power circuits can sometimes also include RF buffers and voltage controlled oscillators (VCOs). Make sure there is at least a whole piece of land in the high power area on the PCB. It is better to have no 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.

The physical partition mainly involves issues such as component layout, orientation and shielding; the electrical partition can be further decomposed into partitions such as power distribution, RF traces, sensitive circuits and signals, and grounding.

1.2.1 We discuss physical partitioning issues.

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 keep it as far as possible. Separate high-power circuits and low-power circuits.

The most effective board stacking method is to arrange the main ground plane (main ground) in the second layer under the surface layer, and walk the RF line as far as possible on the surface layer. Minimizing the size of vias in the RF path not only reduces the path inductance, but also reduces the number of solder joints on the main ground and reduces the chance of leakage of RF energy into other areas within the laminate. In the physical space, a linear circuit such as a multi-stage amplifier is usually sufficient to isolate multiple RF regions from each other, but duplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF 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 far as possible, and as far as possible at their intervals.

The correct RF path is very important for the performance of the entire PCB board, which is why the component layout is usually the most important part of the design of the mobile phone PCB board. In mobile phone PCB design, low-noise amplifier circuits can usually be placed on one side of the PCB, while high-power amplifiers are placed on the other side, and they are finally connected to the RF side and the baseband processing in the same plane through a duplexer. On the antenna side. Some tricks are needed to ensure that 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 straight through holes can be minimized by arranging straight through holes in areas where the PCB board is not subject to RF interference on both sides. It is not always possible to ensure sufficient isolation between multiple circuit blocks. In this case, it must be considered that a metal shield is used to shield the RF energy in the RF area. The metal shield must be welded to the ground and must be kept in contact with the components. A proper distance therefore requires valuable PCB space. It is very important to ensure the integrity of the shielding cover as much as possible. The digital signal lines entering the metal shielding cover should go as far as possible to the inner layer, and it is better that the lower layer PCB of the wiring layer is the ground layer. The RF signal line can go out from the small notch at the bottom of the metal shield and the wiring layer at the notch of the ground, but as much as possible to put some ground around the notch, and the ground on different layers can be connected through multiple vias. .

1.2.3 Proper and efficient decoupling of the chip supply is also very important.

Many RF chips with integrated linear circuitry are very sensitive to power supply noise. Typically, each chip requires up to four capacitors and an isolation inductor to ensure that all 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 applies to the decoupling of the power supply at this inductor side. Some chips require multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them individually. The inductors are rarely paralleled together because they will form an air-core transformer and induce mutual interference. Signals, so the distance between them should be at least equal to the height of one of the devices, or be arranged at right angles to minimize their mutual inductance.

1.2.4 The principle of electrical zoning is generally the same as the physical zoning, but it also contains some other factors.

Some parts of the phone use different operating voltages and control it with software to extend battery life. This means that mobile phones need to run multiple power supplies, which brings more problems to isolation. The power supply is usually brought in from the connector and is immediately decoupled to filter out any noise from outside the board and then distribute it through a set of switches or regulators. The DC current of most circuits on the mobile PCB is rather small, so the trace width is usually not a problem, but a high-current line as wide as possible must be taken for the power 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 it cannot be fully decoupled from the high-power amplifier's power pin, high-power noise will radiate to the entire board and cause various problems. The grounding of a high power amplifier is quite critical, and it is often necessary to design a metal shield for it. In most cases, it is also critical to ensure that the RF output is far from the RF input. This also applies to amplifiers, buffers, and filters. In the worst case, if the outputs of the amplifier and the buffer are fed back to their inputs with the proper phase and amplitude, then they may generate self-oscillation. In the best case, they will work stably under any temperature and voltage conditions. In fact, they can 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 can seriously impair the bandpass characteristics of the filter. In order to make the input and output well-isolated, we must first place a circle around the filter. Secondly, the lower layer of the filter should also be placed on a piece of ground and connected to the main ground around the filter. It is also a good idea to keep the signal lines that go through the filter as far away as possible from the filter pins.

In addition, grounding must be done carefully everywhere on the board, otherwise a coupling channel will be introduced. Sometimes single-ended or balanced RF signal lines can be chosen, and the same applies to cross-interference and EMC/EMI principles. Balancing RF signal lines can reduce noise and crosstalk if the traces are correct, but their impedance is usually high, and a reasonable line width is needed to obtain a matching signal source, trace, and load impedance. Actual wiring may be possible. There will be some difficulties. The buffer can be used to increase the isolation effect because it divides the same signal into two parts and is used to drive different circuits. In particular, the local oscillator may need a buffer to drive multiple mixers. When the mixer reaches common-mode isolation at the RF frequency, it will not work properly. The buffer can well isolate the impedance changes at different frequencies so that the circuits do not interfere with each other. Buffers are very helpful in design. They can be followed by circuits that need to be driven, so that the high-power output traces are very short. Because the buffer input signal level is relatively low, they are not easy to be on the board. Circuits cause interference. Voltage-controlled oscillators (VCOs) convert varying voltages to varying frequencies. This feature is used for high-speed channel switching, but they also convert the tiny amount of noise on the control voltage into tiny frequency variations. This gives The RF signal adds noise.

1.2.5 To ensure that noise is not increased must be considered from the following aspects:

First, the expected bandwidth of the control line may range from DC up to 2 MHz, and removing such wide-band noise by filtering is almost impossible; secondly, the VCO control line is usually part of a feedback loop that controls the frequency, and it Noise can be introduced everywhere, so VCO control lines must be handled very carefully. Make sure that the ground underneath the RF trace is solid, and that all components are securely connected to the main ground and are isolated from other traces that may cause noise.

In addition, to ensure that the VCO's power supply is fully decoupled, the VCO's RF output tends to be a relatively high level and the VCO output signal can easily interfere with other circuits. Therefore, special attention must be paid to the VCO. In fact, the VCO is often placed at the end of the RF area. Sometimes it also needs 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. Simply put, the resonant circuit is a parallel resonant circuit with a capacitive diode that helps set the VCO operating frequency and modulates the voice or data onto the RF signal. All VCO design principles apply equally to resonant circuits. Since resonant circuits contain a relatively large number of components, the distribution area on the board is wide, and they usually operate at a very high RF frequency, resonant circuits are usually very sensitive to noise. Signals are usually arranged on the chip's adjacent feet, but these signal pins need to work with relatively large inductance and capacitance. This in turn requires that these inductors and capacitors must be located close together and connected back. A noise-sensitive control loop. It is not easy to do this.

Automatic Gain Control (AGC) amplifiers are also a problem that can easily go wrong. There are AGC amplifiers in either the transmit or receive circuits. AGC amplifiers are generally effective in filtering noise, but because mobile phones have the ability to handle rapid changes in transmit and receive signal strength, AGC circuits are required to have a fairly wide bandwidth, which makes it easy to introduce AGC amplifiers on certain critical circuits. noise. Designing an AGC line 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 routed away from RF, IF, or high-speed digital signals. Similarly, good grounding is also essential, and the power supply to the chip must be well decoupled. If it is necessary to take a long line at the input or output, then it is better to be at the output, usually the impedance of the output is much lower, and it is not easy to induce noise. In general, 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 the digital circuit away from the analog circuit as much as possible. It also applies to the RF PCB design. The common analog ground and the ground used to shield and separate the signal lines are usually equally important.

Therefore, in the early design stage, careful planning, thoughtful component layout, and thorough layout evaluation are very important. It is also important to keep RF lines away from analog lines and some very critical digital signals. All RF traces and pads As much copper ground as possible should be placed around the components and connected to the main ground as much as possible. If the RF traces must pass through the signal lines, try to route a layer of ground to the main ground along the RF traces between them. If it is not possible, it must be ensured that they are criss-crossing, which minimizes capacitive coupling while placing as much ground as possible around each RF trace and connecting them to the main site. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. A solid block ground plane is placed directly on the first layer below the surface, the best isolation effect, although careful design of other methods also works. At each level of the PCB, as many grounds as possible should be laid and connected to the main ground. Route 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 connect the ground vias to isolated blocks on the surface. You should avoid creating free ground on all layers of the PCB because they pick up or inject noise like a small antenna. In most cases, if you can't connect them to the main land, then you'd better remove them.

1.3 In the design of mobile phone PCBs, great attention should be given to the following aspects:

1.3.1 Power and Ground Handling

Even though the wiring in the entire PCB board is completed very well, the interference caused by the inconsiderate consideration of the power supply and the ground wire will cause the performance of the product to decline and sometimes even affect the success rate of the product. Therefore, the wiring of power and ground should be taken seriously, and the noise interference generated by the power and ground lines should be minimized to ensure the quality of the products. For every engineering personnel engaged in the design of electronic products, the reason for the noise generated between the ground wire and the power cord is understood. Now, only the reduced noise suppression is expressed as:

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

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

(3) Use a large-area copper layer as a ground wire, and connect the unused ground to the ground as a ground wire on the printed board. Or make a multi-layer board, power and ground each occupy a floor.

1.3.2 Common Processing of Digital Circuits and Analog Circuits

There are many PCBs that are no longer single-function circuits (digital or analog circuits), but instead are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise on the ground. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line is as far away as possible from the sensitive analog circuit device. For the ground line, the whole human PCB has only one node to the outside world, so The problem of dealing with the number and common ground of the PCB must be done inside the PCB, and the digital and analog grounds inside the board are actually separated from each other, but only at the interfaces (such as plugs) where the PCB is connected to the outside world. There is a short connection between the digital ground and the analog ground. Note that there is only one connection point. There is also no common ground on the PCB, which is determined by the system design.

1.3.3 Signal lines are laid on the electric (ground) layer

In the multi-layer PCB layout, because there are not many lines remaining in the signal line layer, there will be less waste when adding more layers, which will add a certain amount of work to the production, and the cost will increase correspondingly. To solve this problem, you can consider wiring on the electrical (ground) layer. First, consider using the power plane, followed by the ground. Because it is best to preserve the integrity of the formation.

1.3.4 Treatment of connecting legs in large area conductors

In a large area of ​​grounding (electricity), the leg of a commonly used component is connected to it, and the handling of the connecting leg requires comprehensive consideration. In terms of electrical performance, the pad of the leg of the component is fully connected to the copper surface, but There are some hidden troubles in the welding assembly of components such as: 1 Welding requires high-power heaters. 2 easy to cause a virtual solder joints. Therefore, taking into account the electrical performance and process needs, a cross-shaped solder pad is formed, which is called a thermal shield (commonly referred to as a thermal pad), so that the possibility of generating a solder joint due to excessive cross-section heat during welding can be achieved. decrease very much. The electrical (ground) leg of the multilayer board is treated the same.

1.3.5 The role of network system in wiring

In many CAD systems, wiring is determined by the network system. Although the grid is too dense and the number of accesses is increased, the step size is too small and the data volume of the map field is too large. This inevitably imposes higher requirements on the storage space of the equipment. At the same time, the computing speed of the target computer electronic products is also high. Great influence. However, some of the vias are ineffective, such as occupied by the pads of the component legs or occupied by the mounting holes, the holes, and the like. The grid is too sparse, and too few channels have a great impact on the distribution rate. Therefore, there must be a dense and reasonable grid system to support the routing. The distance between the two legs of the standard component is 0.1 inch (2.54mm), so the foundation of the grid system is generally set to 0.1 inch (2.54mm) or an integral multiple of less than 0.1 inch, such as: 0.05 inch, 0.025 inch, 0.02 Inches and so on.

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

1.4.1 Use a 45° angle at the corner of the transmission line to reduce the return loss

1.4.2 High-performance insulated circuit boards with tightly controlled levels of insulation constants should be used. This method facilitates effective management of the electromagnetic field between the insulating material and adjacent wiring.

1.4.3 To improve PCB design specifications for high-precision etching. Consider the specified total line width error of +/- 0.0007 inches, manage the undercuts and cross-sections of the wiring shapes, and specify the wiring sidewall plating conditions. The overall management of the wiring (wire) geometry and the coating surface is very important for solving the skin effect problems associated with microwave frequencies and implementing these specifications.

1.4.4 Tapped leads exist in protruding leads, avoid using leaded assemblies. In high frequency environments, it is best to use surface mount components.

1.4.5 For signal vias, avoid using the pth process on the sensitive board, as this process will cause lead inductance at the via.

1.4.6 To provide a rich ground plane. The use of molded holes to connect these ground planes prevents the influence of 3-dimensional electromagnetic fields on the circuit board.

1.4.7 To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating. This plated surface can provide better skin effect for high-frequency currents (Figure 2). In addition, fewer leads are needed for this high solderable coating, helping to reduce environmental pollution.

1.4.8 Solder mask prevents solder paste flow. However, due to the uncertainty of the thickness and the unknown nature of the insulation properties, covering the entire board surface with the solder mask material will result in a large change in the electromagnetic energy in the microstrip design. Solder dams are generally used as solder masks. Electromagnetic field. In this case, we manage the transition from microstrip to coaxial cable. In coaxial cables, the ground layers are circularly interwoven and spaced evenly. In microstrip, the ground plane is below the active line. This introduces some edge effects that need to be understood, predicted, and considered at design time. Of course, this mismatch also results in return loss. This mismatch must be minimized to avoid noise and signal interference.

1.5 Electromagnetic Compatibility Design

Electromagnetic compatibility refers to the ability of electronic devices to work in coordination and effectively in various electromagnetic environments. The purpose of the electromagnetic compatibility design is to enable electronic devices to both suppress various external interferences and enable the electronic devices to work properly in a specific electromagnetic environment while reducing the electromagnetic interference of the electronic devices themselves to other electronic devices.

1.5.1 Choosing a Reasonable Width

Because the impact interference produced by the transient current on the printed lines is mainly caused by the inductance components of the printed conductors, the inductance of the printed conductors should be minimized. The inductance of a printed conductor is proportional to its length and inversely proportional to its width, so a short, fine conductor is good for suppressing interference. Clock lines, row drivers, or bus driver signal lines often carry large transient currents. Printed conductors must be as short as possible. For a discrete component circuit, when the width of the printed conductor is about 1.5mm, it can fully meet the requirements; for the integrated circuit, the width of the printed conductor can be selected between 0.2~1.0mm.

1.5.2 Using the Correct Cabling Strategy

Equivalent traces can reduce the inductance of the wire, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is better to adopt the grid structure of the grid structure. The specific approach is to laterally route one side of the printed circuit board, and the other is to make vertical wiring. Then connect them with metallized holes at the cross holes.

1.5.3 In order to suppress the crosstalk between conductors of the printed circuit board, it is necessary to avoid the long distance equal wiring when designing the wiring, and to try to pull the distance between the wiring as far as possible. The signal and ground lines and the power line should be as far as possible. Do not cross. Placing a grounded trace between signal lines that are sensitive to interference can effectively suppress crosstalk.

1.5.4 In order to avoid the electromagnetic radiation generated when the high-frequency signal passes through the printed conductors, the following points should also be noted when wiring the printed circuit board:

(1) Minimize the discontinuity of printed conductors. For example, do not change the width of the conductors. The corners of the conductors should be greater than 90 degrees to prevent looping.

(2) The clock signal leads are most likely to cause electromagnetic radiation interference. When the wires are routed, they should be close to the ground loop. The driver should be next to the connector.

(3) The bus driver should be close to the bus it wants to drive. For those leads that leave the printed circuit board, the driver should hold the connector tightly.

(4) The data bus should be routed with a signal ground between every two signal lines. It is best to place the ground loop next to the least important address lead because the latter often carries high-frequency currents.

(5) When high-speed, medium-speed, and low-speed logic circuits are arranged on the printed board, the devices should be arranged in the manner shown in Fig. 1.

1.5.5 Suppressing Reflection Interference

In order to suppress the reflection interference appearing at the end of the printed line, in addition to the special needs, the length of the printed line should be shortened as much as possible and the slow circuit should be used. If necessary, terminal matching may be added, that is, a matching resistor with the same resistance is added to the ground and the power supply terminal at the end of the transmission line. According to experience, terminal matching measures should be adopted when the printed lines are longer than 10cm for TTL circuits with fast speeds. The resistance of the matching resistor should be determined based on the maximum value of the output drive current and the sink current of the integrated circuit.

1.5.6 Using Differential Signal Wire Routing Strategies in Board Design

Differential signals that are in close proximity to each other are also closely coupled to each other. This coupling between them reduces EMI emissions. Usually (with some exceptions) differential signals are also high-speed signals, so high-speed design rules are generally applicable This is especially true when wiring differential signals, especially when designing signal lines for transmission lines. This means that we must design the wiring of the signal line very carefully to ensure that the characteristic impedance of the signal line is continuous throughout the signal line and remains constant. In the layout 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, it should do its utmost to ensure that the PCB lines in the differential pair have exactly the same impedance and the length of the wiring is exactly the same. Differential PCB lines are usually always wired in pairs, and the distance between them remains constant at any position along the direction of the line pair. In general, the layout of differential pairs is always as close as possible.

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