Design principles in the design of RF radio frequency circuits

In the design of RF layout, must give priority to meet the following general principles:

As far as possible to the high power RF amplifier (HPA) and low noise amplifier (LNA) isolated, simply say, is to allow high power RF emission circuits away from the low power RF receiver circuit:

Make sure the PCB board on high power area at least one piece, the best there is no vias, of course, the bigger the better copper area;Circuit and power decoupling are also extremely important;RF outputs are typically required to stay away from the RF input;

The sensitive analog signal should be far away from the high speed digital signal and RF signal.

Design partitions can be divided into physical partitions and electrical partitions. Physical partitions mainly related to the layout, direction and shielding, etc.; Electrical Division can continue to be divided into power distribution, RF cable, sensitive circuit and signal, and then to the ground, and so on.

Component location layout principle. Components layout is the key to a good RF design. The most effective technology is firstly fixed on the components in the RF path and adjust the direction, so that the RF path length is reduced to a minimum, so that the input from the output. Separation of high power and low power circuits as far as possible.

PCB stack design principles. The most effective way to stack the circuit board is to place the main ground (the ground) on the surface layer of the second layer, and as far as possible, the RF line is arranged on the surface layer. The RF path over pore size is reduced to a minimum, which can not only reduce the inductance path, but also can reduce the main weld point, and can reduce the RF energy leakage to opportunities in other areas of the laminated plate.

Radio frequency device and RF wiring layout principle. In the physical space, like a multistage amplifier such a linear circuit is usually sufficient to the multiple RF area between isolated from each other, but duplexer, a mixer and an IF amplifier / mixer always have multiple RF / if signals interfere with each other. So it is necessary to carefully to minimize this effect. RF and IF trace should be crossed, and as far as possible in between there is a piece of land. The correct RF path to the entire PCB performance is very important, this is the component layout is usually in the cellular phone PCB design for most of the time.

Design principles for reducing the interference coupling of high / low power devices. In the cellular phone PCB can usually be low noise amplifier circuit on the PCB of the one side, and the high power amplifier on the other side, and finally through the duplexer them in the same side is connected to the RF and baseband processor end of the antenna. To use skills to make sure that the hole is not the RF energy is transferred from one surface of a plate to the other side, a common technique is to use the blind hole on the second surface. The adverse effect of the through hole can be reduced to the minimum by arranging the through hole arranged in the PCB board to be free from the area of the RF interference.


Principle of power transmission. The DC currents of most circuits in a cellular phone are fairly small, so the wiring width is usually not a problem. However, it is necessary to set a high current line as wide as possible for the power amplifier power supply to minimize the transmission voltage drop. In order to avoid too much current loss, a plurality of through holes are needed to transfer the current from one layer to another.

Power decoupling of high power devices. If it is not sufficient to decouple the power supply pin at the high power amplifier, the high power noise will radiate to the whole plate and bring many problems. The grounding of high power amplifier is very important, and it is often needed to design a metal shield.

RF input, output isolation principle. In most cases, the same key is to ensure that the RF output is far away from the RF input. This applies to amplifiers, buffers, and filters. In the worst case, if the output of the amplifier and the buffer is fed back to their input with appropriate phase and amplitude, they are likely to generate a self excited oscillation. In the best case, they will be able to work stably under any temperature and voltage conditions. Actually。 They may become unstable, and the noise and the intermodulation signal is added to the RF signal.

Filter input, output isolation principle. If the RF signal line had to from the input end of the filter wraparound output end, then this may seriously damage the filter bandpass characteristics. In order to isolate the input and output. Must first be placed around the filter enclosure. Secondly, the lower area of the filter is also arranged in a piece of ground, and connected with the main ground of the filter. It is a good way to get the signal lines that need to pass through the filter as far away from the filter pins as possible. In addition, the ground floor of each place on the whole board should be very careful, otherwise it may be in the sense of the introduction of a do not want to occur in the coupling channel.

Digital circuit and analog circuit isolation. In all PCB designs, as far as possible to the digital circuit away from analog circuits is a general principle, it is also applicable to the design of PCB RF. The design changes that may lead to the completion of the design and the design changes that may lead to the completion of the design must be pulled back again, as is often the same as the common simulation ground and the ground used for shielding and separating the signal lines. Also make the RF line far away from the analog circuit and some very critical digital signal. All the RF cable, pads and components should be as much as possible to fill the ground. If the RF cable must pass through the signal line, try to arrange a layer of ground with the main ground as far along the RF as possible. If this is not possible, be sure to ensure that they are crossed. The capacitive coupling is reduced to a minimum. At the same time, as far as possible in each root RF lines around daub some and connecting them to the Lord. In addition. Minimizing the distance between parallel RF lines to minimize the inductive coupling.



(Source: Inside information)

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