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Some Skills for RFID Circuit Board Design

Although there are many uncertainties in the theory of RF circuit board design, there are still many rules that can be followed in RF circuit board design. However, in the actual design, the real practical skill is how to compromise these rules when they cannot be implemented due to various limitations. This paper will focus on various issues related to RF circuit board partition design.

Types of micropores

The circuits with different properties on the circuit board must be separated, but they must be connected under the best condition without electromagnetic interference, which requires the use of micro via. Generally, the diameter of micro vias is 0.05mm ~ 0.20mm. These vias are generally divided into three categories, namely blind via, bury via and through via.

  1. The blind hole is located on the top and bottom surface of the printed circuit board and has a certain depth. It is used for the connection of the surface circuit and the bottom inner circuit. The depth of the hole usually does not exceed a certain ratio (aperture).
  2. Buried hole refers to the connection hole in the inner layer of printed circuit board, which will not extend to the surface of printed circuit board. The above two types of holes are located in the inner layer of the circuit board. Before laminating, the through-hole forming process is used to complete. In the process of through-hole formation, several inner layers may be overlapped.
  3. The third is called through-hole, which passes through the whole circuit board and can be used to realize internal interconnection or as the adhesive positioning hole of components.

Using Zoning Techniques

In the design of RF circuit board, high power RF amplifier (HPA) and low noise amplifier (LNA) should be isolated as far as possible. In short, high power RF transmitting circuit should be kept away from low noise receiving circuit. If there is a lot of space on the PCB, it can be easily done. But usually when there are many components, PCB manufacturing space will become very small, so it is difficult to achieve. You can put them on both sides of the PCB, or let them work alternately instead of working at the same time. High power circuits may also include RF buffers and VCOs.\

Design partition can be divided into physical partition and electrical partition. The physical partition mainly involves the layout, orientation and shielding of components; the electrical partition can continue to be divided into power distribution, RF wiring, sensitive circuit and signal, grounding and other partitions.

Entity partition

The layout of components 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. And keep RF input away from RF output, and away from high-power circuit and low-noise circuit as far as possible.

The most effective method of circuit board stack is to arrange the main grounding at the second layer under the surface layer and to walk the RF line on the surface layer as much as possible. Reducing the via size of RF path to the minimum can not only reduce the path inductance, but also reduce the solder joints on the main ground, and reduce the chance of RF energy leakage to other areas in the laminate.

The layout of components 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. And keep RF input away from RF output, and away from high-power circuit and low-noise circuit as far as possible.

The most effective method of circuit board stack is to arrange the main grounding at the second layer under the surface layer and to walk the RF line on the surface layer as much as possible. Reducing the via size of RF path to the minimum can not only reduce the path inductance, but also reduce the solder joints on the main ground, and reduce the chance of RF energy leakage to other areas in the laminate.

Metal shield

Sometimes, it is unlikely to keep enough separation between multiple circuit blocks. In this case, it is necessary to consider using metal shield to shield RF energy in RF area. However, metal shield also has side effects, such as high manufacturing cost and assembly cost.

It is difficult to ensure the high precision of the metal shield with irregular shape in manufacturing. The rectangular or square metal shield also limits the layout of the components; the metal shield is not conducive to the replacement and fault displacement of the components; because the metal shield must be welded on the ground and keep a proper distance from the components, it needs to occupy precious PCB space.

It is very important to ensure the integrity of the metal shield as much as possible, so the digital signal line entering the metal shield should go through the inner layer as much as possible, and it is better to set the next layer of the signal line layer as the ground plane. The RF signal line can be routed out from the small gap at the bottom of the metal shield and the wiring layer at the grounding gap, but the gap should be surrounded by a large grounding area as much as possible, and the grounding on different signal layers can be connected by multiple vias. Despite the above disadvantages, the metal shield is still very effective and is often the only solution to isolate critical circuits.

Power Decoupling Circuit

A proper and effective decouple circuit is also very important. Many RF chips integrated with linear circuits are very sensitive to the noise of power supply. Generally, each chip needs to use up to four capacitors and an isolated inductor to filter all the power supply noise.

The minimum capacitance value usually depends on the resonance frequency of the capacitor itself and the inductance of the grounding pin, and the value of C4 is selected accordingly. The values of C3 and C2 are relatively large due to the inductance of their own pins, so the RF decoupling effect is worse, but they are more suitable for filtering the noise signal of lower frequency. RF decoupling is accomplished by inductance L1, which makes RF signal unable to be coupled from power line to chip. Because all the wires are potential antennas that can receive and transmit RF signals, it is necessary to isolate RF signals from key circuits and components.

The physical location of these decoupling components is also often critical. The layout principles of these important components are as follows: C4 must be as close to IC pin as possible and grounded, C3 must be as close to C4, C2 must be as close to C3, and the connecting line between IC pin and C4 must be as short as possible. The grounding terminals (especially C4) of these components should generally be connected to the chip’s grounding base through the first ground plane below the board. The through-hole connecting the assembly to the ground plane should be as close to the assembly pad on the PCB as possible, and it is better to use the blind hole punched on the pad to minimize the inductance of the connecting wire, and the inductance L1 should be close to C1.

An integrated circuit or amplifier often has an open collector, so a pull-up inductor is needed to provide a high impedance RF load and a low impedance DC power supply. The same principle also applies to decouple the power supply end of this inductor. Some chips need more than one power supply to work, so they may need two or three sets of capacitors and inductors to decouple them respectively. If there is not enough space around the chip, the decouple effect may be poor. In particular, it should be noted that the inductors are rarely parallel together, because this will form an empty core transformer and generate interference signals through mutual induction, so the distance between them should be at least equal to the height of one of them, or arranged at right angles to minimize their mutual inductance.

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