Today's cellular phone design integrates everything together in various ways, which is very detrimental to RF circuit board design. Now the industry is very competitive, and everyone is looking for ways to integrate the most functions with the smallest size and the smallest cost. Analog, digital, and RF circuits are tightly packed together, and the space used to separate their problem areas is very small, and considering cost factors, the number of circuit board layers is often minimized. What is incredible is that multi-purpose chips can integrate multiple functions on a very small die, and the pins that connect to the outside world are arranged very closely, so RF, IF, analog and digital signals are very close. But they are usually electrically irrelevant. Power distribution may be a nightmare for designers. In order to extend battery life, different parts of the circuit work in time-sharing as needed, and the software controls the conversion. This means you may need to provide 5 to 6 working power sources for your cellular phone.
When designing the RF layout, there are several general principles that must be met first:
Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible. Simply put, keep the high-power RF transmitter circuit away from the low-power RF receiver circuit. If you have a lot of physical space on your PCB, you can easily do this, but usually there are many components and the PCB space is small, so this is usually impossible. You can put them on both sides of the PCB board, or let them work alternately instead of working at the same time. High-power circuits sometimes include RF buffers and voltage controlled oscillators (VCO).
How to partition?
The 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 wiring, sensitive circuits and signals, and grounding.
First, 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 line 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. 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 PCB board, which is why the component layout usually accounts for most of the time in the cellular phone PCB board design.On the cellular phone PCB board, 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.
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 radio frequency energy in the RF area. However, metal shields also have problems, such as their own cost and Assembly costs are very expensive;It is difficult to guarantee high precision when manufacturing irregular metal shields. The rectangular or square metal shields restrict the layout of components; the metal shields are not conducive to component replacement and fault location; because the metal shields must be welded on On the ground, a proper distance must be kept from the components, so it takes 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 below the wiring 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 around the gap, and the ground on different layers can be connected together through multiple vias.Despite the above problems, metal shields are very effective and are often the only solution to isolate critical circuits.
In addition, 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. The minimum capacitance value usually depends on its self-resonant frequency and low pin inductance, and the value of C4 is selected accordingly. The values of C3 and C2 are relatively large due to their own pin inductance, so the effect of RF decoupling is worse, but they are more suitable for filtering out lower frequency noise signals. The inductance L1 prevents the RF signal from coupling into the chip from the power line. Remember: all traces are a potential antenna that can both receive and transmit RF signals, and it is also necessary to isolate the induced RF signals from key lines. The physical location of these decoupling components is usually also critical. The layout principle of these important components is: C4 should be as close as possible to the IC pin and grounded, C3 must be the closest to C4, C2 must be the closest to C3, and the IC pin and The connection traces of C4 should be as short as possible, and the ground terminals of these components (especially C4) should usually be connected to the ground pin of the chip through the next ground layer. The vias that connect the components to the ground plane should be as close as possible to the component pads on the PCB. It is best to use blind holes punched on the pads to minimize the inductance of the connection line. The inductance should be close to C1.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 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. If there is not enough space around the chip, you may encounter some troubles.
Remember that inductances are rarely close together in parallel, because this will form an air-core transformer and induce interference signals with each other, so the distance between them must be at least the height of one of the devices, or arranged at right angles to reduce their mutual inductance. To the smallest.The principle of electrical partitioning is basically the same as that of physical partitioning, but it also contains some other factors. Some parts of modern cellular phones use different operating voltages and are controlled by software to extend battery life. This means that cellular phones need to run multiple power sources, and this brings more problems to isolation. The power supply 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 in cellular phones is quite small, so 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 very critical, and it is often necessary to design a metal shield for it.
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