PCB wiring technology to reduce signal coupling in

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PCB wiring skills to reduce signal coupling in RF Design

a new round of demand for Bluetooth devices, cordless and cellular is prompting Chinese electronic engineers to pay more and more attention to RF circuit design skills. The design of RF circuit board is the most troublesome part for design engineers. If you want to succeed at one time, careful planning and attention to details are two key design rules that must be paid great attention to

radio frequency (RF) circuit board design is often described as a black art because of many uncertainties in theory, but this view is only partially correct. RF circuit board design also has many guidelines that can be followed and rules that should not be ignored. However, in actual design, the real practical skill is how to compromise these guidelines and rules when they cannot be accurately implemented due to various design constraints

of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulation materials and laminates, as well as wavelength and standing wave. However, this paper will focus on various issues related to the zoning design of RF circuit boards

today's cellular design integrates everything in various ways, which is very detrimental to RF circuit board design. Now the competition in the industry is very fierce. Everyone is looking for ways to integrate the most functions with the smallest size and the smallest cost. Analog, digital and RF circuits are closely packed together, so the space used to separate their problem areas is very small, and considering the cost factor, the number of circuit board layers is often minimized. It is inconceivable that the multi-purpose chip can integrate various functions on a very small bare chip, and the pins connecting the outside world are closely arranged. Therefore, RF, if, analog and digital signals are very close, but they are usually electrically independent. Power distribution may be a nightmare for designers. In order to prolong battery life, different parts of the circuit work time-sharing according to needs, and software controls the conversion. This means that you may need to provide 5 to 6 kinds of working power for your cell

rf layout concept

when designing RF layout, there are several general principles that must be met first:

isolate high-power RF amplifier (HPA) and low-noise amplifier (LNA) as far as possible, in short, keep high-power RF transmitting circuit away from low-power RF receiving circuit. If you have a lot of physical space on your PCB, you can easily do this, but usually there are many components and the PCB space is small, so this is usually impossible. You can regularly apply a thin layer of mos22 molybdenum sulfide and grease on both sides of the PCB, or let them work alternately instead of at the same time. High power circuits may sometimes also include RF buffers and voltage controlled oscillators (VCOs)

ensure that there is at least a whole piece of land in the high-power area on the PCB, preferably without holes on it. Of course, the more copper sheets, the better. Later, we will discuss how to break this design principle as needed and how to avoid the problems that may arise from it

chip and power supply are also essential features to meet the manufacturer's high requirements for the number of plugging cycles. Decoupling is also extremely important. Several methods to realize this principle will be discussed later

rf output usually needs to be far away from RF input, which will be discussed in detail later

sensitive analog signals should be as far away from high-speed digital signals and RF signals as possible

how to partition

the design partition can be divided into physical partition and electrical partition. Physical zoning mainly involves the layout, orientation and shielding of components and parts; Electrical partitions can continue to be decomposed into partitions for power supply for low-precision distribution, RF wiring, sensitive circuits and signals, grounding, etc

first, let's discuss the physical partition. The layout of components is the key to an excellent RF design. The most effective technology is to first fix the components on the RF path and adjust its orientation to minimize the length of the RF path, keep the input away from the output, and separate the high-power circuit and low-power circuit as far as possible

the most effective stacking method of circuit boards is to arrange the main ground (main ground) on the second layer below the surface layer, and walk the RF line on the surface layer as much as possible. Minimizing the via size on the RF path can not only reduce the path inductance, but also reduce the false solder joints on the main ground, and reduce the chance of RF energy leakage to other areas in the laminate

in physical space, linear circuits such as multistage amplifiers are usually enough to isolate multiple RF regions from each other, but duplexers, mixers, and if Amplifiers/mixers always have multiple rf/if signals interfering with each other, so this effect must be carefully minimized. The routing of RF and if should cross as much as possible and be separated from each other as much as possible. Correct RF path is very important for the performance of the whole PCB, which is why component layout usually takes up most of the time in the design of honeycomb PCB

on the honeycomb PCB board, it is usually possible to place the low-noise amplifier circuit on one side of the PCB board and the high-power amplifier on the other side, and finally connect them to the antenna at the RF end and the baseband processor end on the same side through the duplexer. Some skills are needed to ensure that the through hole will not transfer RF energy from one side of the board to the other. The common technology is to use blind holes on both sides. The adverse effect of the through-hole can be minimized by arranging the through-hole in the area where both sides of the PCB are free from RF interference

sometimes it is impossible to ensure sufficient isolation between multiple circuit blocks. In this case, it is necessary to consider using a metal shield to shield RF energy in the RF area, but the metal shield also has problems, such as: its own cost and assembly cost are very expensive

the metal shield with irregular shape is difficult to ensure high precision during manufacturing, and the rectangular or square metal shield also restricts the layout of components; Metal shield is not conducive to component replacement and fault location; Since the metal shield must be welded to the ground and must be kept at a proper distance from the components, it needs to occupy 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 go through the inner layer as much as possible, and the PCB layer below the wiring layer is the layer. RF signal wires can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but more ground should be laid around the gap as much as possible, and the ground on different layers can be connected together through multiple vias

despite the above problems, metal shielding is very effective and often the only solution to isolate critical circuits

in addition, proper and effective chip power decoupling is also very important. Many RF chips integrated with linear circuits are very sensitive to power supply noise. Usually, each chip needs up to four capacitors and an isolation inductor to ensure that all power supply noise is filtered out)

the minimum capacitance value usually depends on its self resonant frequency and low pin inductance, and the value of C4 is selected accordingly. The values of C3 and C2 are relatively larger due to their own pin inductance, so the effect of RF decoupling is worse, but they are more suitable for filtering low-frequency noise signals. Inductance L1 prevents RF signals from being coupled to the chip from the power line. Remember: all cabling is a potential antenna that can receive and transmit RF signals. In addition, it is also necessary to isolate the induced RF signals from the critical lines

the physical location of these decoupling elements is usually also critical. The layout principle of these important elements is: C4 should be as close to the IC pin and grounded as possible, C3 must be closest to C4, C2 must be closest to C3, and the connection route between the IC pin and C4 should be as short as possible. The grounding terminals of these elements (especially C4) should usually be connected to the chip's grounding pin through the next layer. The via connecting the components to the ground should be as close to the component pad on the PCB as possible. It is best to use the blind hole punched on the pad to minimize the inductance of the connecting line, and 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 power supply. The same principle also applies to decoupling the power supply at this inductor end. Some chips require multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them respectively. If there is not enough space around the chip, you may encounter some problems

remember that inductors are rarely close together in parallel, because this will form a hollow core transformer and induce interference signals with each other, so the distance between them should be at least equal to the height of one of the devices, or arranged at right angles to minimize mutual inductance

the principle of electrical zoning is basically the same as that of physical zoning, but it also includes some other factors. Some parts of modern honeycomb adopt different working voltages and are controlled by software to prolong the working life of the battery. This means that the cell needs to run a variety of power supplies, which brings more problems to isolation. The power supply is usually introduced from the connector and immediately decoupled to filter out any noise from the outside of the circuit board, and then distributed after passing through a group of switches or regulators

the DC current of most circuits in the cell is quite small, so the routing width is usually not a problem. However, it is necessary to route a large current line as wide as possible for the power supply of the high-power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, multiple vias are needed to transfer current from one layer to another. In addition, if it cannot be fully decoupled at the power pin end of the high-power amplifier, the high-power noise will radiate to the whole board and cause various problems. The grounding of high power amplifier is very critical, and it is often necessary to design a metal shield for it

in most cases, it is also critical to ensure that the RF output is away from the RF input. This also applies to amplifiers, buffers, and filters. In the worst case, if the outputs of amplifiers and buffers are fed back to their inputs with appropriate phase and amplitude, they may produce self-excited oscillation. At best, they will be able to work stably at any temperature and voltage. In fact, they may become unstable and add noise and intermodulation signals to the RF signal

if the RF signal line has to be bypassed from the input to the output of the filter, it may seriously damage the bandpass characteristics of the filter. In order to isolate the input and output well, a circle of ground must be laid around the filter first, and then a piece of ground must be laid in the lower area of the filter and connected with the main ground surrounding the filter. It is also a good method to keep the signal line that needs to pass through the filter as far away from the filter pin as possible. In addition, the grounding of all parts of the whole board should be very careful, otherwise you may unknowingly introduce a coupling channel that you don't want to happen. Figure 3 illustrates this grounding method in detail

sometimes single ended or balanced RF can be selected

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