Today's cellular phone design integrates everything in a variety of ways, which is detrimental to RF circuit board design.
Nowadays, the competition in the industry is very fierce. Everyone is looking for ways to integrate the most functions with the smallest size and the smallest cost.
Analog, digital and RF circuits are closely packed together, and the space used to separate the problem areas is very small, and the number of circuit board layers is often minimized considering cost factors.
Incredibly, multipurpose chips can integrate multiple functions on a very small bare chip, and the pins connecting the outside world are arranged very closely, so RF, IF, analog and digital signals are very close, but they are usually electrically unrelated.
Power distribution can be a nightmare for designers. In order to prolong battery life, different parts of the circuit work on a time-sharing basis and are controlled by software. This means that you may need to provide five or six power sources for your cellular phone.
In designing RF layouts, there are several general principles that must be addressed first:
Separate the high power RF amplifier (HPA) from the low noise amplifier (LNA) as far as possible. In short, keep the high power RF transmitting circuit away from the low power RF receiving circuit.
If you have a lot of physical space on your PCB board, you can easily do this, but usually there are many components, and PCB space is small, so this is usually impossible. You can put them on both sides of the PCB board or have them work alternately instead of simultaneously. High power circuits sometimes include RF buffers and voltage controlled oscillators (VCO).
Make sure that there is at least one whole area in the high power area of the PCB board, preferably no holes in it. Of course, the more copper, the better. Later, we will discuss how to break this design principle as needed and how to avoid the problems that may arise from it.
Chip and power decoupling are equally important, and several ways to implement this principle will be discussed later.
RF output usually needs to be away from RF input, which we will discuss in detail later.
Sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible.
How to partition?
Design partitions can be decomposed into physical and electrical partitions. Physical zoning mainly involves the layout, orientation and shielding of components; electrical zoning can continue to be decomposed into power distribution, RF routing, sensitive circuits and signals, as well as grounding zoning.
First, we discuss physical partitioning. Component layout is the key to achieve an excellent RF design. The most effective technology is to fix the components on the RF path and adjust their orientation to minimize the length of the RF path, keep the input away from the output, and separate the high-power and low-power circuits as far as possible.
The most effective way to stack circuit boards is to arrange the main ground surface (main ground) on the second layer below the surface, and to move the RF line on the surface as far as possible. Minimizing the size of the through hole in the RF path not only reduces the path inductance, but also reduces the virtual solder joints on the main floor, and reduces the chances of RF energy leakage to other areas in the laminate.
In physical space, linear circuits such as multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but duplexers, mixers and IF amplifiers/mixers always have multiple RF/IF signals interfering with each other, so this effect must be carefully minimized. RF and IF routes should be crossed as far as possible, and as far as possible separated from each other. The correct RF path is very important for the performance of the whole PCB board, which is why the component layout usually takes up most of the time in the PCB board design of cellular phone.
In cellular telephone PCB board, low noise amplifier circuit can be placed on one side of PCB board, while high power amplifier circuit can be placed on the other side, and eventually connected to the antenna of RF and baseband processor on the same side through duplexer. Some techniques are needed to ensure that the RF energy is not transferred from one side of the board to the other through the hole. The common technique is to use blind holes on both sides. By arranging the straight through hole in the area of the PCB board which is not disturbed by RF, the adverse effect of the straight through hole can be minimized.
Sometimes it is impossible to ensure adequate isolation between multiple circuit blocks. In this case, it is necessary to consider the use of metal shield to shield RF energy in the RF area. However, there are also problems with metal shield, such as: both its own cost and assembly cost are very expensive;
Metal shield with irregular shape is difficult to ensure high precision in manufacturing, and the layout of components is limited by rectangular or square metal shield. Metal shield is not conducive to component replacement and fault location. Because metal shield must be welded to the ground, it must keep a proper distance from components, so it is necessary. Take up valuable PCB board space.
It is very important to ensure the integrity of the shield as far as possible. The digital signal line entering the metal shield should go as far as possible to the inner layer, and the PCB layer below the wire layer is the best layer. RF signal lines can go out of the wiring layers at the small notches at the bottom of the metal shield cover and the ground notches, but the areas around the notches should be as many as possible, and the ground on different layers can be connected through multiple holes.
Despite these problems, metal shielding is very effective and often the only solution to isolate critical circuits.
In addition, proper and effective decoupling of chip power supply is also very important. Many RF chips integrated with linear circuits are very sensitive to power supply noise. Usually each chip needs up to four capacitors and one isolation inductor to ensure that all power supply noise is filtered out.
The minimum capacitance usually depends on its self-resonant frequency and low-pin inductance, and the value of C4 is chosen accordingly. The values of C3 and C2 are relatively larger due to the relationship between their pin inductances, so the RF decoupling effect is worse, but they are more suitable for filtering low frequency noise signals. Inductance L1 prevents RF signals from being coupled from power lines to chips. Keep in mind that all routes are potential antennas that can receive and transmit RF signals, and it is also necessary to isolate the induced RF signals from the key lines.
The physical location of these decoupling elements is usually also critical. The layout principles of these important elements are as follows: C4 must be as close to IC pin as possible and grounded, C3 must be nearest to C4, C2 must be nearest to C3, and the connection between IC pin and C4 should be as short as possible. The grounding end of these elements (especially C4) should usually pass through the next place. The layer is connected to the ground pin of the chip. The holes connecting the elements to the formation should be as close as possible to the component pad on the PCB board. It is better to use blind holes on the pad to minimize the inductance of the connection line and 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 end. Some chips require multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them separately. If there is not enough space around the chip, you may encounter some problems.
Keep in mind that inductances are rarely parallel because they form a hollow transformer and interact with each other to produce interference signals, so the distance between them should be at least equal to the height of one of the devices, or arranged at right angles to minimize their mutual inductance.
The principle of electrical zoning is basically the same as that of physical zoning, but it also contains some other factors. Some parts of modern cellular phones use different operating voltages and are controlled by software to prolong battery life. This means that cellular phones need to run multiple power sources, which brings more problems to isolation. The power supply is usually introduced from the connector and decoupled immediately to remove any noise from the outside of the circuit board, which is then allocated after a set of switches or regulators.
The DC current of most circuits in cellular telephones is quite small, so the wiring width is usually not a problem. However, a large current line as wide as possible must be separately used for the power supply of high power amplifiers to minimize the transmission voltage. In order to avoid too much current loss, it is necessary to use multiple through holes to transfer current from one layer to another. In addition, if it can not be decoupled adequately at the pin end of the high power amplifier, the high power noise will radiate to the whole board and bring various problems. Grounding of high power amplifier is very important, and it is often necessary to design a metal shield for it.
How should RF(Radio Frequency) circuit boards be partitioned?
2019 09/16
