PCB Tech - RF layout skills in mobile phone PCB design

The increase of mobile phone functions has higher requirements for PCB boarddesign. With the advent of Bluetooth devices, cellular phones and the 3G era, engineers are paying more and more attention to the design skills of RF circuits.Radio frequency (RF) circuit board 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. However, in actual design, the really practical skill is how to compromise these guidelines and rules when they cannot be implemented accurately 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 laminates, wavelength and standing waves, so these have a great impact on the EMC and EMI of mobile phones. The conditions that must be met when designing the RF layout are summarized:

1. Separate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible

Simply put, it is to keep the high-power RF transmitting circuit away from the low-power RF receiving 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.

2. 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 routing, sensitive circuits and signals, and grounding.

2.2.1 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 circuit board 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.

2.2.2 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 PCBboard, 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. 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 to the inner layer as much as possible, and it is best that the PCB layer below the wiring layer is the ground layer. RF signal lines 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 around the gap, the ground on different layers can be connected together through multiple vias .

2.2.3 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. Some chips require multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them separately. The inductors are rarely close together in parallel, because this will form an air-core transformer and induce interference with each other. Signals, so the distance between them must be at least equal to the height of one of the devices, or arranged at right angles to minimize their mutual inductance.

2.2.4 The principle of electrical zoning is generally the same as that of physical zoning, but it also contains some other factors. Some parts of the mobile phone use different working voltages and are controlled by software to extend the battery life. This means that mobile phones need to run multiple power sources, and this brings more problems to isolation. The power is usually introduced from the connector, and is immediately decoupled to filter out any noise from the outside of 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 routed 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 current from one layer to another. In addition, if it cannot be sufficiently decoupled at the power supply pin of the high-power amplifier, high-power noise will radiate to the entire board and cause various problems. The grounding of high-power amplifiers is 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 away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, if the output of the amplifier and buffer is fed back to their input with appropriate phase and amplitude, then they may have 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 from the input end of the filter back to the output end, this may seriously damage the bandpass characteristics of the filter. In order to get a good isolation between the input and the output, a ground must be laid around the filter first, and then a ground must be laid in the lower layer area of the filter and connected to the main ground surrounding the filter. It is also a good way to keep the signal lines that need to pass through the filter as far away as possible from the filter pins.

In addition, the grounding of various places on the whole board must be very careful, otherwise a coupling channel will be introduced. Sometimes you can choose to take single-ended or balanced RF signal lines. The principles of cross-interference and EMC/EMI are also applicable here. Balanced RF signal lines can reduce noise and cross-interference if they are routed correctly, but their impedance is usually high, and a reasonable line width must be maintained to obtain a matching signal source, trace and load impedance. The actual wiring may be There will be some difficulties. The buffer can be used to improve the isolation effect, because it can divide the same signal into two parts and used to drive different circuits, especially 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. The buffer can well isolate the impedance changes at different frequencies, so that the circuits will not interfere with each other. Buffers are very helpful to the design. They can follow 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 easy to interfere with other on the board. The circuit is causing interference. Voltage-controlled oscillators (VCOs) can convert varying voltages into varying frequencies. This feature is used for high-speed channel switching, but they also convert trace noise on the control voltage into tiny frequency changes, which gives The RF signal adds noise.

2.2.5 To ensure that noise is not increased, 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 wide-band noise through filtering; secondly, VCO The control line is usually part of a feedback loop that controls the frequency. It may introduce noise in many places, so the VCO control line must be handled very carefully. Make sure that the ground below the RF trace is solid, and all components are firmly connected to the main ground and isolated from other traces that may cause noise. In addition, it is necessary to ensure that the power supply of the VCO has been sufficiently 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, VCO is often placed at the end of the RF area, and sometimes it 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, which helps to set the VCO operating frequency and modulate the voice or data to the RF signal. All VCO design principles also apply to resonant circuits. Because the resonant circuit contains a considerable number of components, has a wide distribution area on the board, and usually runs at a very high RF frequency, the resonant circuit is usually very sensitive to noise. 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 these inductors and capacitors to be located very close and connected back On a control loop that is sensitive to noise. It is not easy to do this.

Automatic gain control (AGC) amplifier is also a problem-prone place, whether it is a transmitting or receiving circuit will have an AGC amplifier. AGC amplifiers can usually effectively filter out noise, but because mobile phones have the ability to deal with the 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 to introduce AGC amplifiers on some key circuits Noise. Designing AGC circuits must comply with good analog circuit design techniques, which are related to the short op amp input pins and short feedback paths, both of which must be far away from RF, IF, or high-speed digital signal traces. Similarly, good grounding is also essential, and the chip's power supply must be well decoupled. If it is necessary to run a long wire at the input or output end, it is best to go at the output end. Usually, the impedance of the output end is 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. Common analog ground and ground used to shield and separate signal lines are usually equally important. Therefore, in the early stages of design, careful planning, well-thought-out component layout and thorough layout * evaluation are all very important. The same should be used for RF Keep the line away from analog lines and some very critical digital signals. All RF traces, pads and components should be filled with grounded 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 route a layer of ground connected to the main ground along the RF trace between them. If it is not possible, make sure that they are crossed. This minimizes capacitive coupling. At the same time, place as much ground as possible around each RF trace and connect them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. When a solid ground plane is placed directly on the first layer below the surface, the isolation effect is best, although other methods of designing with care will also work. On each layer of the PCB board, place as many grounds as possible and connect them to the main ground. Place the traces as close together as possible to increase the number of plots of the internal signal layer and power distribution layer, and adjust the traces appropriately so that you can arrange the ground connection vias to the isolated plots on the surface. Free ground should be avoided on the various layers of the PCB because they can 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.

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

3.3.1 Treatment of power supply and ground wire

Even if the wiring in the entire PCB board is completed very well, 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 slender width can reach 0.05～0.07mm, and the power cord is 1.2～2.5 mm. 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)

(3) Use a large area of copper layer as ground wire, and connect the unused places on the printed circuit board as ground wire. Or it can be made into a multi-layer board, and the power supply and ground wire occupy one layer each.

3.3.2 Common ground processing of digital circuit and analog circuit

Many PCBs are no longer single-function circuits (digital or analog circuits), but 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 interference on the ground wire. 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 should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated and they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.

3.3.3 The signal line is laid on the electric (ground) layer

In the multi-layer printed board wiring, because there are not many wires left in the signal line layer that have not been laid out, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.

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 the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding and assembly of components, such as: 1. Welding requires high-power heaters. 2. It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, called heat shields, commonly known as thermal pads (Thermal), so that virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multilayer board is the same.

3.3.5 The role of the network system in cabling

In many CAD systems, wiring is determined by the network system. The grid is too dense and the path has increased, but the step is too small, and the amount of data in the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-based electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a well-spaced and reasonable grid system to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54 mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or an integral multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.

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

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

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

4.4.3 PCB design specifications related to high-precision etching should be improved. 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.

4.4.4 The protruding leads have tap inductance, so avoid using components with leads. In high frequency environments, it is best to use surface mount components.

4.4.5 For signal vias, avoid using the via processing (pth) process on sensitive boards, because this process will cause lead inductance at the vias.

4.4.6 Provide abundant ground planes. Use molded holes to connect these ground planes to prevent the 3D electromagnetic field from affecting the circuit board.

4.4.7 To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating. This kind of electroplated surface can provide better skin effect for high frequency current (Figure 2). In addition, this highly solderable coating requires fewer leads, which helps reduce environmental pollution.

4.4.8 The solder mask can prevent the flow of solder paste. However, due to the uncertainty of the thickness and the unknown of the insulation performance, the entire surface of the board is covered with solder mask material, which will cause a large change in the electromagnetic energy in the microstrip design. Generally, a solder dam is used as the solder mask. The electromagnetic field. In this case, we manage the conversion from microstrip to coaxial cable. In the coaxial cable, the ground layer is interwoven ring-shaped and evenly spaced. In microstrip, the ground plane is below the active line. This introduces certain edge effects, which need to be understood, predicted and considered during design. Of course, this mismatch will also cause return loss, and this mismatch 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 all kinds of external interference, so that the electronic equipment can work normally in a specific electromagnetic environment, and at the same time to reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

5.5.1 Choose a reasonable wire width

Since the impact interference generated by the transient current on the printed lines is mainly caused by the inductance of the printed wires, the inductance of the printed wires should be minimized. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress 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 is about 1.5mm, which can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2mm and 1.0mm.

5.5.2 Adopt the correct wiring strategy

The use of equal routing can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout permits, it is best to use a grid-shaped wiring structure. The specific method is to wire one side of the printed board horizontally and the other side of the printed board. Then connect with metallized holes at the cross holes.

5.5.3 In order to suppress the crosstalk between the conductors of the printed circuit board, when designing the wiring, try to avoid long-distance equal wiring, and extend the distance between the wires as much as possible, and the signal wire and the ground wire and the power wire should be as far as possible. Do not cross. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.

5.5.4 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, for example, do not change the width of the wires, and the corners of the wires should be greater than 90 degrees to prohibit circular routing.

(2) The clock signal lead is most likely to produce electromagnetic radiation interference. When routing the wire, it 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 to be driven. For those leads that leave the printed circuit board, the driver should be next to the connector.

(4) The wiring of the data bus should clamp a signal ground wire between every two signal wires. 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 arranging high-speed, medium-speed and low-speed logic circuits on the printed board, the devices should be arranged in the manner shown in Figure 1.

5.5.5 Suppress reflection interference

In order to suppress the reflection interference that appears at the terminal of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used. Terminal matching can be added when necessary, that is, a matching resistor of the same resistance is added to the end of the transmission line to the ground and the power terminal. According to experience, for general faster TTL circuits, terminal matching measures should be adopted when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output drive current and the absorption current of the integrated circuit.

5.5.6 Adopt differential signal line routing strategy in the circuit board design process

Differential signal pairs with very close wiring 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 usually apply. This is especially true for the routing of differential signals, especially when designing signal lines for transmission lines. This means that we must carefully design the wiring of the signal line to ensure that the characteristic impedance of the signal line is continuous and constant along the signal line. In the layout and routing process of the differential pair, we hope that the two PCB lines in the differential pair are exactly the same. This means that in practical applications, the greatest effort should be made 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 routed in pairs, and the distance between them is kept constant at any position along the line pair direction. Under normal circumstances, the placement and routing of differential pairs is always as close as possible.