RF circuit board design Although there are many uncertainties in theory, there are many rules that can be followed in RF board design. However, in practical design, the real practical skills is when these rules can not be implemented due to various restrictions, how to deal with them, this paper will focus on RF circuit board design and related issues.
1, the type of micro-vias
Circuites of different properties on the circuit board must be separated, but also connected without the best possible electromagnetic interference, which requires the use of microvia. Typically, the micropores have a diameter of 0.05 mm to 0.20 mm. These vias are generally classified into three categories: blind via, bury via, and via via. The blind holes are located on the top and bottom surfaces of the printed circuit board and have a depth for the connection of the surface lines and the underlying inner lines, which typically do not exceed a certain ratio (aperture). The buried hole refers to the connection hole located in the inner layer of the printed circuit board and does not extend to the surface of the circuit board. The two types of holes are located in the inner layer of the circuit board, laminated through the use of through-hole molding process to complete the process of forming the hole may also overlap several inner layers. The third is called through-hole, which passes through the entire circuit board and can be used to achieve internal interconnection or as a component of the adhesive positioning hole.
2, the use of partitioning skills
When designing an RF circuit board, isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible, simply by leaving the high-power RF transmitter circuit away from the low-noise receiver circuit. If there is a lot of space on the PCB board, then it can be done easily. But usually a lot of components, PCB manufacturing space will become very small, so it is difficult to achieve. They can be placed on both sides of the PCB board, or let them work alternately, rather than work at the same time. High power circuits can sometimes include RF buffers and voltage controlled oscillators (VCOs).
Design partition can be divided into physical partitioning (physical partitioning) and electrical partition (Electrical partitioning). The physical partition mainly involves the component layout, orientation and shielding and other issues; electrical partition can continue to be divided into power distribution, RF traces, sensitive circuits and signals, grounding and other partitions.
3, physical partition
Component layout is the key to achieving an excellent RF design. The most efficient technique is to first fix the components on the RF path and adjust their orientation to minimize the length of the RF path. And the RF input away from the RF output, and as far as possible away from high-power circuits and low noise circuit.
The most efficient circuit board stacking method is to arrange the main ground in the second layer under the surface, and as far as possible the RF line on the surface. Minimizing the vias on the RF path not only reduces the path inductance, but also reduces the potential solder bumps on the main ground and reduces RF energy leakage to other areas of the board.
In a physical space, a linear circuit such as a multi-stage amplifier is usually sufficient to isolate multiple RF regions from each other, but the duplexer, mixer, and intermediate frequency amplifiers always have multiple RF / IF signals interfering with each other, So care must be taken to minimize this effect. The RF and IF traces should be crossed as far as possible and, as far as possible, be separated by a grounded area. The correct RF path is very important for the performance of the entire PCB board, which is why the component layout is usually in the mobile phone PCB board design accounted for most of the time reasons.
On a mobile phone PCB board, a low noise amplifier circuit can usually be placed on one side of a PCB pattern, while the high power amplifier is placed on the other side and eventually connected to the RF antenna by the duplexer on the same side One end and the other end of the baseband processor. This requires some skill to ensure that RF energy does not pass through the hole, from the side of the board to the other side, the commonly used technology is used in both sides of the blind hole. The adverse effects of vias can be minimized by arranging blind holes in areas where both sides of the PCB are not subject to RF interference.
4, metal shield
In some cases, it is unlikely that sufficient segregation will be allowed between multiple circuit blocks, in which case it is necessary to consider the use of metal shields to shield RF energy into the RF area, but the metal shield also has side effects such as manufacturing Cost and assembly costs are high.
Irregular metal shields in the manufacture of difficult to ensure that high-precision, rectangular or square metal shield and make the layout of components subject to some restrictions; metal shield is not conducive to component replacement and fault displacement; because the metal shield must Welding on the ground surface, and must be with the components to maintain an appropriate distance, so need to take up valuable PCB board space.
As far as possible to ensure that the integrity of the metal shield is very important, so enter the metal shield digital signal line should be as far as possible inside the inner layer, and it is best to signal layer layer layer as the ground layer. The RF signal line can be routed from the small notch at the bottom of the metal shield and the wiring layer at the ground notch, but the gap around it should be surrounded by a large area of ​​ground, and the ground on the different signal layers can be routed through multiple vias Even in the beginning. Despite the above shortcomings, metal shields are still very effective and are often the only solution to isolate critical circuits.
5, power decoupling circuit
Proper and efficient chip power decoupling circuits are also important. Many RF chips that integrate linear lines are very sensitive to noise from the power supply, and typically each chip requires up to four capacitors and an isolated inductor to filter out all of the power supply noise.
The minimum capacitance value usually depends on the resonant frequency of the capacitor itself and the pin inductance, and the value of C4 is chosen accordingly. The values ​​of C3 and C2 are relatively large due to their own inductor inductance, and the RF decoupling effect is worse, but they are more suitable for filtering out lower frequency noise signals. RF decoupling is done by the inductor L1, which makes RF signals unable to be coupled from the power supply line to the chip. Because all traces are a potential antenna that can either receive or transmit RF signals, it is necessary to isolate the RF signal from critical lines and components.
The physical location of these decoupling components is also often critical. The principle of the layout of several important components is: C4 as close as possible to the IC pin and ground, C3 must be closest to C4, C2 must be closest to C3, and IC pin and C4 connection to be as short as possible, The ground terminal (especially C4) of a component should normally be connected to the ground pin of the chip by the first ground plane under the board. The vias connected to the ground plane should be as close as possible to the component pads on the PCB. It is best to use the blind holes on the pads to minimize the inductance of the cables. The inductor L1 should be close to C1.
An integrated circuit or amplifier often has an open collector collector, so a pullup inductor is required to provide a high impedance RF load and a low impedance DC power supply. The same principle applies to this The power supply of the inductor is decoupled. Some chips require multiple power supplies to work, so two or three sets of capacitors and inductors are required to decouple them separately. If there is not enough space around the chip, the decoupling effect may be poor. In particular, it is important to note that the inductors are very close together in parallel, since this will form an air-core transformer and inductively generate an interfering signal, so the distance between them is at least equal to one of the heights or at right angles Arranged to minimize their mutual inductance.