PCB design partitions of Mobile phone

1. We discuss physical partitioning issues. 

The component layout is the key to achieving an excellent 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, keeping the input away from the output and as far as possible. Ground high power circuits and low power circuits.

The most efficient method of board stacking is to arrange the main ground plane (main ground) in the second layer below the surface and walk the RF lines as far as possible on the surface. Minimizing the via size on the RF path not only reduces path inductance but also reduces the number of solder joints on the primary ground and reduces the chance of RF energy leaking into other areas of the laminate.

In physical space, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but duplexers, mixers, and IF amplifiers/mixers always have multiple RF/IFs. The signals interfere with each other, so care must be taken to minimize this effect.

2. RF and IF traces should be crossed as much as possible, and as far as possible between them. 

The correct RF path is very important for the performance of the entire PCB board, which is why the component layout is usually the majority of the time in the design of the mobile phone PCB. In the design of a mobile phone PCB board, it is usually possible to place the low noise amplifier circuit on one side of the PCB board, while the high power amplifier is placed on the other side, and finally connect them to the RF side and baseband processing on the same side through the duplexer on the antenna of the device. Some tricks are needed to ensure that the straight-through hole does 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 straight-through holes can be minimized by arranging straight-through holes in areas where both sides of the PCB are free of RF interference. Sometimes it is not possible to ensure adequate isolation between multiple circuit blocks. In this case, it is necessary to consider shielding the RF energy in the RF area with a metal shield. The metal shield must be soldered to the ground and must be kept with the components. A proper distance, so it takes up valuable PCB space.

It is very important to ensure the integrity of the shield as much as possible. The digital signal line entering the metal shield should be as far as possible inside the layer, and it is better than the PCB below the trace layer is the ground layer. The RF signal line can go out from the small notch at the bottom of the metal shield and the wiring layer at the ground notch, but the ground around the notch should be as much as possible, and the ground on different layers can be connected through multiple vias. 

3. Proper and effective chip power supply decoupling is also very important.

Many RF chips with integrated linear circuitry are very sensitive to power supply noise. Typically, each chip requires up to four capacitors and an isolated inductor to ensure that all power supply 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 supply. The same principle applies to decoupling the inductor's power supply.

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. The inductors are rarely tied together in parallel because this creates an air core transformer and induces 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.

4. The electrical partitioning principle is generally the same as the physical partition, but it also contains some other factors. 

Some parts of the phone use different operating voltages and are controlled by software to extend battery life. This means that the phone needs to run multiple power supplies, which brings more problems to the isolation. The power supply is typically introduced from the connector and immediately decoupled to filter out any noise from outside the board and then distribute it after passing through a set of switches or regulators.

The DC current of most circuits on the PCB board is quite small, so the trace width is usually not a problem. However, it is necessary to separate a large current line as wide as possible for the power supply of the high power amplifier to minimize the transmission voltage drop. . To avoid too much current loss, multiple vias are required to transfer current from one layer to another. In addition, if the high power amplifier's power pin is not fully decoupled, high power noise will radiate across the board and cause a variety of problems. The grounding of high power amplifiers is critical and often requires 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 worst case, if the outputs of the amplifier and buffer are fed back to their inputs with the appropriate phase and amplitude, they are likely to generate self-oscillation. In the best case, they will work stably under any temperature and voltage conditions. In fact, they can become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to be wound back from the input of the filter, this can seriously damage the bandpass characteristics of the filter. In order to get good isolation between the input and the output, it is first necessary to make a circle around the filter, and then the lower layer of the filter is also grounded and connected to the main ground surrounding the filter. It is also a good idea to keep the signal lines that need to pass through the filter as far as possible from the filter pins.

The grounding of each part of the board should be very careful, otherwise, a coupling channel will be introduced.

Sometimes you can choose to go with single-ended or balanced RF signal lines. The same principles for cross-interference and EMC/EMI apply here. Balanced RF signal lines can reduce noise and crosstalk if the traces are correct, but their impedance is usually high, and a reasonable linewidth is required to get a matching source, trace, and load impedance. Actual wiring may be There will be some difficulties.

The buffer can be used to improve isolation because it splits the same signal into two parts and drives different circuits. In particular, the local oscillator may require 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 buffers are well isolated to isolate impedance changes at different frequencies so that the circuits do not interfere with each other. Buffers are very helpful in the design, they can be followed by the circuit that needs to be driven, so that the high power output traces are very short, because the buffer input signal level is relatively low, so they are not easy to the other on the board.

The circuit causes interference. Voltage-controlled oscillators (VCOs) convert varying voltages into varying frequencies. This feature is used for high-speed channel switching, but they also convert small amounts of noise on the control voltage into small frequency changes, which gives The RF signal adds noise.