PCB Blog - Mobile phone PCB board design RF layout skills

The increase of mobile phone functions requires more PCB board design. With the advent of bluetooth devices, cellular phones and 3G, engineers are paying more and more attention to RF circuit design skills. RF board design is often described as a "black art" due to theoretical uncertainties, but this view is only partially true. RF board design has many rules that can be followed and rules that should not be ignored. However, in practical design, the really useful technique is how to compromise these principles and laws when they cannot be implemented precisely because of various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer materials and laminates, as well as wavelength and standing wave, 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 board:

1. As far as possible, the high power RF amplifier (HPA) and low noise amplifier (LNA) isolated, in short, is to let the high power RF transmitting circuit away from the low power RF receiving circuit. Mobile phone features more, a lot of components, but PCB space is small, taking into account the design process of wiring limit, all these requirements for design skills are relatively high. At this point, you might want to design four to six PCB layers to work alternately, rather than simultaneously. High power circuits may also sometimes include RF buffers and voltage controlled oscillators (VCO). Make sure there is at least one whole floor of the high power area on the PCB with no holes in it. Of course, the more copper skin the better. Sensitive analog signals should be kept as far away from high-speed digital signals and RF signals as possible.

2. Design partition can be divided into physical partition and electrical partition. Physical partitioning mainly involves components layout, orientation and shielding, etc. Electrical partitions can continue to be decomposed into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding.

2.1 We discuss physical partitioning. The component layout is the key to implementing an RF design. The effective technique is to first fix the components on the RF path and Orient them so that the length of the RF path is reduced to such that the input is far away from the output and the high-power and low-power circuits are separated as far as possible. An efficient way to stack circuit boards is to place the main ground floor (the main ground) on the second layer below the surface, with RF lines on the surface as much as possible. Reducing the size of the through-holes in the RF path not only reduces path inductance, but also reduces virtual solder joints on the main ground and the chance of RF energy leakage to other areas within the laminate. In physical space, linear circuits such as multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but diplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF signals interfering with each other, so this effect must be carefully reduced to.

2.2 RF and IF should be crossed as far as possible and separated as far as possible. The correct RF path is very important to the performance of the entire PCB, which is why component layout usually takes up most of the time in mobile PCB design. In cell phone PCB designs, it is usually possible to place the low noise amplifier circuit on one side of the PCB and the high power amplifier on the other side, and eventually connect them to the RF and baseband processor antenna on the same side through a dipper. Some tricks are required to ensure that straight through holes does not transfer RF energy from one side of the board to the other, and a common technique is to use blind holes on both sides. The adverse effects of straight through holes can be reduced to a minimum by arranging straight through holes in areas on both sides of the PCB that are free from RF interference. Sometimes it is not possible to ensure adequate isolation between multiple circuit blocks, in which case a metal shield must be considered to shield RF energy within the RF region. The metal shield must be sold-to the ground and kept at a reasonable distance from the components, thus taking up valuable PCB space. It is very important to ensure the integrity of the shield cover as much as possible. The digital signal lines entering the metal shield cover should go through the inner layer as much as possible, and the PCB layer below the wiring layer is the stratum. RF signal line can go out from the small gap at the bottom of the metal shield cover and the wiring layer of the gap, but around the gap as much as possible to cloth some ground, the ground on different layers can be connected through a plurality of holes.

2.3 Proper and effective chip power decoupling is also very important. Many RF chips with integrated linear circuits are very sensitive to power source noise, and typically each chip requires up to four capacitors and an isolating inductor to ensure that all power source noise is filtered. 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 the inductor end. Some chip need more power to work, so you may need two or three sets of capacitance and inductance to decoupling on them respectively, few inductance parallel together, because this will form a tubular transformer and mutual induction interference signal, so the distance between them must be at least equal the height of one of the devices, or a right Angle to the mutual inductance to.

2.4 The principles for electrical zoning are generally the same as for physical zoning, but some other factors are included. Some parts of the phone operate at different voltages and are controlled by software to extend battery life. That means the phone needs to run on multiple power sources, which creates more problems for isolation. Power is usually brought in from the connector, immediately decoupled to filter out any noise coming from outside the circuit board, and then distributed through a set of switches or regulators. The DC current of most circuits on cell phone PCBS is fairly small, so wiring width is usually not an issue, however, a separate high-current line as wide as possible must be run for the high-power amplifier's power supply to reduce the transmission voltage drop to. To avoid too much current loss, a plurality of holes are used to transfer current from one layer to another. In addition, if it is not sufficiently decoupled at the power pin end of the high power amplifier, the high power noise will radiate throughout the board and bring all kinds of problems. Grounding of high power amplifiers is critical and often requires the design of a metal shield. In most cases, it is also critical to ensure that the RF output is kept away from the RF input. This also applies to amplifiers, buffers, and filters. In the bad case, amplifiers and buffers can generate self-excited oscillations if their outputs are fed back to their inputs with the right phase and amplitude. In this case, they will be able to operate stably under any temperature and voltage conditions. In fact, they can become unstable and add noise and intermodulation signals to RF signals. If the RF signal line has to wind back from the input to the output of the filter, this can seriously impair the bandpass characteristics of the filter. In order to achieve good isolation of input and output, a field must first be laid around the filter, and then a field must be laid in the lower region of the filter, and connected to the main ground surrounding the filter. It is also a good idea to place signal lines that need to pass through the filter as far away from the filter pins as possible. In addition, the grounding of the entire board must be very careful, otherwise a coupling channel will be introduced. Sometimes you can choose to run a single-ended or balanced RF signal line, and the principles of cross interference and EMC/EMI also apply here. Balanced RF signal lines can reduce noise and cross interference if routed correctly, but their impedance is usually high, and actual wiring can be somewhat difficult to achieve by maintaining a reasonable line width to get an impedance that matches the signal source, routed, and load. Buffers can be used to improve isolation because they can split the same signal into two parts and be used to drive different circuits, especially if the local oscillator may need a buffer to drive multiple mixers. When the mixer reaches the common-mode isolation state at RF frequency, it will not work properly. Buffers are good at isolating impedance changes at different frequencies so that circuits do not interfere with each other. Buffers are a great help in design because they can stay close to the circuit that needs to be driven, making the high-power output line very short. Because the input signal level of buffers is low, they are less likely to interfere with other circuits on the board. Voltage-controlled oscillators (VCO) convert varying voltages to varying frequencies, a feature used for high-speed channel switching, but they also convert tiny amounts of noise on the control voltage to small frequency changes, which add noise to the RF signal.

2.5 The following aspects must be considered to ensure that the noise is not increased: First, the expected bandwidth range of the control line may be from DC to 2MHz, and it is almost impossible to remove the noise of such wide band through filtering; Second, the VCO control line is usually part of a feedback loop that controls the frequency, and it can introduce noise in many places, so the VCO control line must be handled with great care. Make sure the RF floor is solid and all components are securely connected to the main floor and isolated from other wires that may cause noise. In addition, to ensure that the power supply of the VCO is sufficiently decoupled, special attention must be paid to the VCO because its RF output tends to be at a relatively high level and the VCO output signal can easily interfere with other circuits. In fact, the VCO is often placed at the end of the RF region, and sometimes it requires a metal shield. Resonant circuits (one for the transmitter, the other for the receiver) are related to the VCO, but have their own characteristics. Simply put, a resonant circuit is a parallel resonant circuit with capacitive diodes that help set the VCO operating frequency and modulate voice or data to RF signals. All VCO design principles also apply to resonant circuits. Resonant circuits are usually very sensitive to noise because they contain a large number of components, have a wide distribution area on the board and usually operate at a high RF frequency. Signals are usually arranged on adjacent pins of the chip, but these pins need to be paired with relatively large inductors and capacitors to work, which in turn requires that these inductors and capacitors be located close together and connected back to a noise-sensitive control loop. It's not easy to do that. Automatic gain control (AGC) amplifiers are also a problem spot, both for transmitting and receiving circuits. AGC amplifiers are usually effective in filtering out noise, but the ability of mobile phones to handle rapid changes in the intensity of transmitted and received signals requires a fairly wide bandwidth for AGC circuits, which makes it easy for AGC amplifiers on certain critical circuits to introduce noise. The design of AGC lines must adhere to good analog circuit design techniques, which are associated with very short op-amp input pins and very short feedback paths, both of which must be away from RF, IF, or high-speed digital signal wiring. Good grounding is also essential, and the power supply to the chip must be well decoupled. If you have to run a long line through either the input or output, it's at the output, where the impedance is usually much lower and less susceptible to noise. Generally, the higher the signal level, the easier it is to introduce noise to other circuits. In all PCB designs, it is a general principle to keep digital circuits away from analog circuits as much as possible, and this applies to RF PCB designs as well. Public to simulate and used for shielding and separated signal lines are usually equally important, in the early design stage, therefore, careful planning and thoughtful components layout and complete the layout of * assessment is very important, also should make the RF circuit away from analog circuit and some essential digital signal, and all of the RF cables, welding plate and components should be around as much as possible to fill the grounding copper sheet, And as connected to the Lord as possible. If RF cables must cross signal cables, try to lay a layer of ground between them along the RF cables connected to the main ground. If this is not possible, make sure they are crossed, which reduces capacitive coupling to a minimum, while placing as much ground as possible around each RF line and connecting them to the main ground. In addition, reducing the distance between parallel RF lines can reduce perceptual coupling to. A solid whole ground floor can isolate the effect when placed directly beneath the surface, although other design methods can also be used with care. Cover as much ground as possible on each layer of the PCB and connect them to the main floor. Put the wiring together as much as possible to increase the number of blocks in the internal signal layer and power distribution layer, and adjust the wiring so that you can place the ground connection holes into the isolated blocks on the surface. Free ground should be avoided on PCB layers because they pick up or inject noise like a small antenna. In most cases, if you can't connect them to the main ground, then you take them out.

3. Great attention should be paid to the following aspects in the design of mobile phone PCB board

3.1 Processing of power supply and ground wire

Even if the wiring in the whole PCB board is completed well, but the interference caused by the power supply and ground wire is not considered well, the performance of the product will decline, and sometimes even affect the success rate of the product. So the wiring of electricity, ground wire should be treated seriously, the noise interference that electricity, ground wire place produces falls to limit, in order to ensure the quality of the product. For every engineer who is engaged in the design of electronic products, it is clear that the reason for the noise between ground wire and power line is generated. Now, the reduced noise suppression is only described as follows:

(1) it is well known that the decoupling capacitor is added between the power supply and ground wire.

(2) As far as possible to widen the width of power supply, ground wire is wider than the power line, their relationship is: ground wire > power line > signal line, usually signal line width is: 0.2 ~ 0.3mm, the fine width can reach 0.05 ~ 0.07mm, power line is 1.2 ~ 2.5mm. The PCB of a digital circuit can be used as a circuit with wide ground conductors, that is, to form a ground network for use (analog ground cannot be used this way)

(3) With a large area of copper layer as ground wire, in the printed board is not used in the place are connected with the ground wire. Or make it multi-layer board, power supply, grounding line each occupy a layer.

3.2 Common ground processing of digital circuit and analog circuit

Many PCBS are no longer single-function circuits (digital or analog), but are a mix of digital and analog circuits. Therefore, when wiring, we need to consider the interference between them, especially the noise interference on the ground line. The sensitivity of the high frequency digital circuits, analog circuits, the signal wire, high-frequency signal lines as far as possible away from the sensitive analog devices, for the ground, moving the PCB to the outside world is only one node, so must be within the PCB processing, mold has problem, and inside the plate to digital and analog is actually are divided between them, Only in the PCB and external connection interface (such as plug, etc.). There is a bit of a short connection between the digital ground and the analog ground. Note that there is only one connection point. There are also incongruent ones on the PCB, depending on the system design.

3.3 Signal Cables are laid on electrical (ground) layers

In the multi-layer PCB wiring, because there is no finished line left in the signal line layer, and then add layers will cause waste will also increase the production of a certain amount of work, the cost also increased accordingly, in order to solve this contradiction, you can consider wiring in the electrical (ground) layer. The power zone should be considered first, and the formation second. Because it preserves the integrity of the formation.

3.4 Processing of connecting legs in large-area conductors

In the large area of grounding (electricity), the legs of common components are connected with it. The processing of the connecting legs needs to be considered comprehensively. In terms of electrical performance, the pads of component legs are fully connected with the copper surface, but there are some hidden dangers for the welding assembly of components, such as: (1) the welding needs a high power heater. (2) Easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process needs, make a cross welding pad, called heat shield, commonly known as Thermal, so that the possibility of virtual welding spot due to excessive heat dissipation of the section during welding can be greatly reduced. The electrical (ground) leg of the multilayer is treated the same.

3.5 The role of network system in cabling

In many CAD systems, wiring is determined by the network system. The grid is too dense, the path is increased, but the step is too small, the data volume of the graph field is too large, which will inevitably have higher requirements for the storage space of the equipment, but also has a great impact on the computing speed of computer electronic products. Some paths are invalid, such as those occupied by the pads of component legs or by mounting holes, setting holes, etc. Too sparse grid and too few paths have a great influence on the distribution rate. Therefore, it is necessary to have a reasonably dense grid system to support the wiring. The legs of standard components are 0.1 inch (2.54mm) apart, so the base of grid systems is usually 0.1 inch (2.54mm) or integral multiples of less than 0.1 inch (e.g. 0.05 inch, 0.025 inch, 0.02 inch, etc.).

4. Techniques and methods for hf PCB design are as follows:

4.1 Transmission line corners shall be at 45° angles to reduce return loss

4.2 High performance insulating circuit board with insulation constant value strictly controlled according to levels shall be adopted. This method is beneficial for effective management of electromagnetic field between insulating material and adjacent wiring.

4.3 PCB design specifications for high-precision etching shall be improved. Consider specifying a total line width error of +/-0.0007 inches, managing undercut and cross sections of wiring shapes and specifying wiring side wall plating conditions. Overall management of wiring (wire) geometry and coating surfaces is important to address skin effects related to microwave frequencies and to implement these specifications. Lead assemblies with tap inductance in protruding leads should be avoided. In high frequency environments, use surface mounted components.

4.5 For signal through-holes, the use of through-hole machining (PTH) on sensitive plates should be avoided as this process can cause lead inductance at the through-holes.

4.6 Abundant grounding should be provided. Moulded holes are used to connect these grounding layers to prevent 3d electromagnetic fields from affecting the circuit board.

4.7 Non-electrolysis nickel plating or immersion gold plating should be selected instead of HASL plating method. This electroplated surface provides a better skin effect for high-frequency currents (Figure 2). In addition, this highly weldable coating requires fewer leads, helping to reduce environmental pollution.

4.8 Solder resistance layer can prevent solder paste from flowing. However, due to the uncertainty of thickness and unknown insulation performance, covering the entire plate surface with solder resistance material will lead to a large change in electromagnetic energy in microstrip design. Generally, solder dam is used as solder resistance layer. The electromagnetic field of. In this case, we manage the conversion from microstrip to coaxial cable. In coaxial cables, the ground layers are interlaced in rings and evenly spaced. In microbelts, the grounding layer is below the active line. This introduces certain edge effects that need to be understood, predicted, and considered at design time. Of course, this mismatch also leads to backloss and must be minimized to avoid noise and signal interference.

5. Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic equipment to work harmoniously and effectively in various electromagnetic environments. The purpose of electromagnetic compatibility design is to make the electronic equipment can not only suppress all kinds of external interference, so that the electronic equipment can work normally in a specific electromagnetic environment, but also reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

5.1 Select a reasonable conductor width

Since the impulse interference caused by transient current on the printed line is mainly caused by the inductance composition of the printed wire, the inductance of the printed wire should be minimized. The inductance of printed wire is directly proportional to its length and inversely proportional to its width, so a short and precise wire is favorable to suppress interference. Signal lines for clock leads, line drives, or bus drives often carry large transient currents, and printed leads should be kept as short as possible. For discrete component circuits, printed wire width of about 1.5mm, can fully meet the requirements; For integrated circuits, the printed wire width can be selected between 0.2 mm and 1.0mm.

5.2 Use the correct cabling strategy

The use of equal wiring can reduce the inductance of the wires, but the mutual inductance and distributed capacitance between the wires increases. If the layout allows, the use of well-shaped mesh wiring structure, the specific practice is that one side of the printed board is horizontally wired, the other side is vertically wired, and then connected with the metalized holes at the cross hole.

5.3 In order to suppress crosstalk between PCB wires, long-distance equal wiring should be avoided as far as possible during wiring design, the distance between wires should be extended as far as possible, and the signal line should not cross with ground wire and power line as far as possible. Cross talk can be effectively suppressed by setting a printed line connecting to the ground between some signal lines which are very sensitive to interference.

5.4 In order to avoid electromagnetic radiation caused by high frequency signals passing through printed wires, the following points should also be paid attention to when wiring printed circuit boards:

(1) to minimize the discontinuity of printed wires, such as wire width does not change, wire corner should be greater than 90 degrees to prohibit circular wiring, etc.

(2) The clock signal lead is easy to produce electromagnetic radiation interference, the line should be close to the ground circuit, the driver should be close to the connector.

(3) The bus driver shall be adjacent to the bus it intends to drive. For those leads away from the printed board, the driver should be close to the connector.

(4) The wiring of the data bus shall include a signal ground wire between each two signal lines. The loop is placed close to the unimportant address lead because the latter often carries high frequency current.

(5) Devices shall be arranged in accordance with figure 1 when the printed board arranges high-speed, medium-speed and low-speed logic circuits.

5.5 Suppress reflective interference

In order to suppress the reflective interference at the end of printed lines, except for special needs, the length of printed lines should be shortened as much as possible and slow circuits should be used. When necessary, terminal matching can be added, that is, a matching resistance of the same resistance value can be added at the end of the transmission line to the ground and the power supply end. According to experience, when the printed line length is more than 10cm, terminal matching measures should be adopted for the TTL circuit with higher general speed. The resistance of the matching resistor shall be determined by the value of the output drive current and the absorption current of the integrated circuit

5.6 Routing strategy of differential signal line is adopted in circuit board design

The differential signal on the wiring is very close to each other between each other will also tight coupling, the coupling between each other will reduce EMI emission, usually (of course there are some exceptions) differential signal is high speed signal, so high-speed design rules are often applied to differential signal wiring, especially the design of transmission line. 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 and constant throughout the signal line. In the layout and routing process of the differential line pair, we want the two PCB lines in the differential line pair to be exactly the same. This means that, in practice, every effort should be made to ensure that the PCB lines in the differential line pair have exactly the same impedance and the length of the wiring is exactly the same. Differential PCB board lines are usually routed in pairs, and the distance between them remains a constant at any location along the pair of directions.