Today's cell phone designs integrate everything together in various ways, which is bad for RF board design. Now the competition in the industry is very fierce, 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 all tightly packed together, leaving little space to separate problem areas from each other, and board layers are often kept to a minimum for cost considerations. It is incredible that a multi-purpose chip can integrate multiple functions on a very small die, and the pins connected 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 can be a nightmare for designers. To preserve battery life, different parts of the circuit are time-shared as needed, with software-controlled transitions. That means you might need 5 or 6 working power sources for your cell phone.
When designing an RF layout, there are several general principles that must be prioritized:
Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible, simply put, keep the high-power RF transmit circuit away from the low-power RF receive circuit. If you have a lot of physical space on your PCB, then you can do this easily, but usually with many components and less PCB space, this is usually not possible. You can put them on both sides of the PCB board, or make them work alternately, not at the same time. High power circuits sometimes also include RF buffers and voltage controlled oscillators (VCOs).
Make sure that there is at least a whole piece of ground in the high power area on the PCB, preferably without vias on 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 result from it.
Chip and power supply decoupling is also extremely important, and several ways to achieve this principle will be discussed later.
The RF output usually needs to be far away from the RF input, which we will discuss in detail later. Sensitive analog signals should be kept as far away as possible from high-speed digital signals and RF signals.
Design partitions can be decomposed into physical partitions and electrical partitions. Physical partitioning mainly involves issues such as component layout, orientation, and shielding; electrical partitioning can be further decomposed into partitions for power distribution, RF routing, sensitive circuits and signals, and grounding.
First we discuss the physical partition issue. Component layout is the key to achieving a good RF design, the most effective technique is to first fix the components located on the RF path, and orient them to minimize the length of the RF path, so that the input is far away from the output, and as far as possible Ground separates high-power circuits from low-power circuits.
The most effective board stacking method is to arrange the main ground plane (main ground) on the second layer under the surface layer, and route the RF lines on the surface layer as much as possible. Minimizing via size on the RF path not only reduces path inductance, but also reduces virtual solder joints on the main ground and reduces the chance of RF energy leaking to other areas within the stackup.
Physically, linear circuits like 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 interfere with each other, so care must be taken to minimize this effect. RF and IF traces should be crossed as much as possible, and a ground should be separated between them as much as possible. The correct RF path is very important to the performance of the overall PCB board, which is why component layout usually takes up most of the time in cellular phone PCB board design.
On a cellular phone PCB, 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 finally connect them to the RF end and baseband processing on the same side through a duplexer on the antenna at the receiver end. Some tricks are required to ensure that the through vias do not transfer RF energy from one side of the board to the other, a common technique is to use blind vias on both sides. The adverse effect of the through-via can be minimized by arranging the through-via in an area free from RF interference on both sides of the PCB.
Sometimes it is not possible to ensure sufficient isolation between multiple circuit blocks. In this case, metal shields must be considered to shield radio frequency energy in the RF area, but metal shields also have problems, such as: their own cost and Assembly costs are expensive;
Metal shields with irregular shapes are difficult to ensure high precision during manufacture, and rectangular or square metal shields restrict the layout of components; metal shields are not conducive to component replacement and fault location; since metal shields must be welded on On the ground, an appropriate distance must be kept from components, thus occupying 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. 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 should be distributed around the gap, and the grounds on different layers can be connected together through multiple vias .
Despite the above issues, metal shields are very effective and often the only solution for isolating critical circuits.
In addition, proper and effective chip power supply decoupling is also very important. Many RF chips with integrated linear circuits are very sensitive to noise from the power supply, usually requiring up to four capacitors and an isolation inductor per chip to ensure that all power supply noise is filtered out).
The minimum capacitance value is usually determined by its self-resonant frequency and low lead inductance, and the value of C4 is chosen accordingly. The values of C3 and C2 are relatively large due to their own pin inductance, so the RF decoupling effect is poor, but they are more suitable for filtering out lower frequency noise signals. Inductor L1 prevents RF signals from being coupled into the chip from the power line. Remember: every trace is a potential antenna that can both receive and transmit RF signals, and it is also necessary to isolate induced RF signals from critical lines.
The physical location of these decoupling components is also often critical. The layout principles for these important components are: C4 should be as close as possible to the IC pin and grounded, C3 must be closest to C4, C2 must be closest to C3, and the IC pin must be connected to the ground. The connection trace 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 connecting 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 connecting wires. 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 source. The same principle applies to decoupling the power supply on this inductor. Some chips require more than one power supply to work, so you may need two or three sets of capacitors and inductors to decouple them separately, and if there is not enough space around the chip, then you may have some troubles.
Remember that inductors are rarely close together in parallel, as this will form an air core transformer and induce interfering signals in each other, so they should be spaced at least as high as one of the devices, or arranged at right angles to reduce their mutual inductance to the minimum.
The principles of electrical zoning are largely the same as physical zoning, but include some additional factors. Certain parts of modern cellular phones operate at different voltages and are controlled by software to extend battery operating life. This means that cellular phones need to run from multiple power sources, which creates even more problems with isolation. Power is typically brought in at a connector and immediately decoupled to filter out any noise from outside the board before being distributed through a set of switches or regulators.
Most circuits in a cell phone draw relatively little dc current, so trace width is usually not an issue, but the high-power amplifier power supply must be run with a separate high-current trace as wide as possible to minimize transmission voltage drops. To avoid too much current loss, multiple vias are required to pass current from one layer to another. Also, if the high power amplifier is not adequately decoupled at its power supply pins, high power noise will radiate across the board and cause all kinds of problems. Grounding of high power amplifiers is critical and often requires a metal shield.
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