From the past to the present, RF board design, like electromagnetic interference (EMI) issues, has been the most difficult part of engineers, even nightmares. If you want to design successfully once, you must plan carefully and pay attention to details before you can work.
Radio frequency (RF) board design is often described as a "black art" because of the many theoretical uncertainties. But this is just a partial view, RF board design still has many rules to follow. However, in practical design, the really practical technique is how to compromise these rules when they cannot be implemented due to various restrictions. Important RF design topics include impedance and impedance matching, insulation materials and laminates, wavelengths and harmonics, etc. This article will focus on various issues related to RF board partition design.
Types of microvias Circuits of different nature on the board must be separated, but connected in the best case without electromagnetic interference, which requires the use of microvias. Typically, the micro vias have a diameter of 0.05 mm to 0.20 mm. These vias are generally classified into three types, namely, blind vias, bury vias, and through vias. The blind holes are located on the top and bottom surfaces of the printed wiring board and have a depth for the connection of the surface lines and the underlying inner lines, and the depth of the holes usually does not exceed a certain ratio (aperture). Buried hole refers to a connection hole located in the inner layer of the printed wiring board, which does not extend to the surface of the circuit board. The above two types of holes are located in the inner layer of the circuit board, and are completed by a through hole forming process before lamination, and several inner layers may be overlapped during the formation of the via holes. The third type is called a through hole, which passes through the entire circuit board and can be used to achieve internal interconnection or as an adhesive positioning hole for the assembly.
Partitioning techniques When designing RF boards, high-power RF amplifiers (HPAs) and low-noise amplifiers (LNAs) should be isolated as much as possible. Simply speaking, the RF connection is to keep the high-power RF transmitting circuit away from the low-power receiving circuit. This can be done easily if there is a lot of space on the PCB. However, when there are many components, the PCB space will become very small, so this 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 can sometimes also include RF buffers and voltage controlled oscillators (VCOs).
Design partitions can be divided into physical partitioning and electrical partitioning. Entity partitioning mainly involves issues such as component placement, orientation, and shielding; electrical partitioning can continue to be divided into power distribution, RF traces, sensitive circuits, and signal, ground, and other partitions.
The physical partitioning 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. Keep the RF input away from the RF output and keep it as far away as possible from high power circuits and low power circuits.
The most efficient method of board stacking is to place the main ground on 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 dummy pads on the main 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 always have multiple RF/IF signals interfering with each other. Therefore, this effect must be carefully minimized. The RF and IF traces should be crossed as much as possible, and as much as possible a grounded area between them. The correct RF path is very important for the performance of the entire PCB, which is why component placement is often the most important part of a mobile phone PCB design.
On a mobile phone PCB, it is usually possible to place the low noise amplifier circuit on one side of the PCB, while the high power amplifier is placed on the other side, and finally connect them to the end of the RF antenna on the same side by the duplexer. And the other end of the baseband processor. This requires some skill to ensure that RF energy is not transmitted through the vias from one side of the board to the other. A common technique is to use blind holes on both sides. The adverse effects of vias can be minimized by arranging blind vias in areas where both sides of the PCB are immune to RF interference.
Metal shields are sometimes less likely to leave sufficient separation between multiple circuit blocks. In this case, metal shields must be considered to shield RF energy in the RF region, but metal shields also have side effects. For example: manufacturing costs and assembly costs are high.
Irregular metal shields are difficult to guarantee high precision during manufacture. Rectangular or square metal shields also limit the layout of components; metal shields are not conducive to component replacement and fault displacement; Solder on the ground plane and must be kept at an appropriate distance from the components, thus taking up valuable 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 be as far as possible inside the layer, and it is better to set the next layer of the signal layer as the ground plane. The RF signal line can be routed from the small notch at the bottom of the metal shield and the wiring layer at the grounding notch, but the periphery of the notch should be surrounded by a large grounding area as much as possible. The grounding on different signal layers can be through multiple vias. connected together.
Despite these shortcomings, metal shields are still very effective and often the only solution to isolate critical circuits.
Power Supply Decoupling Circuits In addition, proper and efficient chip power supply decoupling circuits are also important. Many RF chips that incorporate linear lines are very sensitive to power supply noise. Typically, each chip requires up to four capacitors and an isolated inductor to filter out all power supply noise. (Figure 1)
Figure 1 Chip Power Decoupling Circuit
The minimum capacitance value usually depends on the resonant frequency of the capacitor itself and the pin inductance. The value of C4 is chosen accordingly. The values ​​of C3 and C2 are relatively large due to their own pin inductance, and the RF decoupling effect is worse, but they are more suitable for filtering out lower frequency noise signals. RF decoupling is accomplished by inductor L1, which prevents RF signals from being coupled into the chip from the power line. Since all traces are a potential antenna that can both receive and transmit RF signals, it is necessary to isolate the RF signal from critical lines and components.
The physical location of these decoupling components is often also critical. The layout principle of these important components is: C4 should be as close as possible to the IC pin and grounded, C3 must be closest to C4, C2 must be closest to C3, and the connection between IC pin and C4 should be as short as possible. The ground terminal of the component (especially C4) should normally be connected to the ground pin of the chip by the first ground plane under the board. The vias that connect the component to the ground plane should be as close as possible to the component pads on the PCB. It is best to use a blind via on the pad to minimize the inductance of the trace. The inductor L1 should be close to C1.
An integrated circuit or amplifier often has an open collector output, so a pullup inductor is needed to provide a high impedance RF load and a low impedance DC supply. The same principle applies to this The power supply terminal of the inductor is decoupled. Some chips require multiple power supplies to operate, so two or three sets of capacitors and inductors may be 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 rarely parallel together because they will form an air core transformer and induce mutual interference signals, so the distance between them must be at least equal to one of the heights, or at right angles. Arrange to minimize mutual inductance.
The electrical partition electrical partition is in principle the same as the physical partition, but it also contains some other factors. Some parts of modern mobile phones use different operating voltages and are controlled by software to extend battery life. This means that mobile phones need to run multiple power supplies, which creates more isolation problems. The power supply is usually introduced by a connector and immediately decoupled to filter out any noise from outside the board and then pass through a set of switches or regulators before power distribution.
In mobile phones, the DC current of most circuits is quite small, so the trace width is usually not a problem. However, it is necessary to design a wide current line as wide as possible for the power supply of the high power amplifier to make the voltage at the time of emission. The voltage drop can be minimized. 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 important and often requires a metal shield.
The RF output must be kept away from the RF input. In most cases, the RF output must be kept away from the RF input. This principle 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. They can become unstable and add noise and intermodulation products to the RF signal.
If the RF signal line wraps around the output of the filter, this can severely damage the bandpass characteristics of the filter. In order to isolate the input and output well, first there must be a main grounding area around the filter, and the lower layer of the filter must also be a grounding area, and this grounding area must be 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. In addition, the grounding of each place on the entire board must be very careful, otherwise it may unwittingly introduce an undesired coupling channel. (Figure 2) details this grounding method.
Sometimes you can choose to use single-ended or balanced RF traces. The same principles for crosstalk and EMC/EMI apply here. Balancing RF signal lines can reduce noise and crosstalk if the traces are correct, but their impedance is usually high. Moreover, in order to obtain an impedance-matched signal source, trace, and load, it is necessary to maintain a reasonable line width, which may be difficult in actual wiring.
Figure 2 The filter is surrounded by a ground plane (green area)
buffer
The buffer can be used to improve isolation because it divides 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 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 Oscillator Voltage-Controlled Oscillator (VCO) converts 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. This adds noise to the RF signal. In summary, after the voltage controlled oscillator has been processed, there is no way to remove the noise from the RF output signal. The difficulty is that the desired bandwidth of the VCO control line may range from DC to 2 MHz, and it is almost impossible to remove such wide band noise by filters; secondly, the VCO control line is usually a feedback frequency control. Part of the loop, which can introduce noise in many places, so the VCO control line must be handled very carefully.
Resonant circuit A tank circuit is used for the transmitter and receiver. It is related to the VCO, but it also has its own characteristics. Simply put, a resonant circuit is a resonant circuit that is connected by a series of diodes with inductors and capacitors. It helps to set the VCO operating frequency and modulate voice or data onto the RF carrier.
The design principles of all VCOs also apply to resonant circuits. Resonant circuits are often very sensitive to noise because they contain a significant number of components, a large footprint, and typically operate at a very high RF frequency. The signals are usually arranged on the adjacent pins of the chip, but these signal pins need to work with larger inductors and capacitors to work. Instead, the inductors and capacitors need to be placed as close as possible to the signal pins. A noise-sensitive control loop, but try to avoid noise interference. It is not easy to do this.
The automatic gain control amplifier automatic gain control (AGC) amplifier is also a problem that is prone to problems. Both the transmitting and receiving circuits have an AGC amplifier. AGC amplifiers typically filter out noise effectively, but because mobile phones have the ability to handle the rapid changes in transmit and receive signal strength, the AGC circuit is required to have a fairly large bandwidth, which makes the AGC amplifier easy to introduce noise.
Designing AGC lines must follow the design principles of analog circuits, that is, using very short input pins and very short feedback paths, and both must be away from RF, IF, or high-speed digital signal lines. Also, good grounding is essential and the chip's power supply must be well decoupled. If you have to design a long trace on the input or output, it is best to choose to implement it at the output, because usually the output has a much lower impedance than the input and it is not easy to introduce noise. Usually the higher the signal level, the easier it is to introduce noise into other circuits.
Grounding is to ensure that the grounding of the lower layer of the RF trace is solid and that all components are securely connected to the main ground and isolated from other traces that may cause noise. In addition, to ensure that the VCO's power supply is fully decoupled, since the VCO's RF output tends to be a fairly high level, the VCO output signal can easily interfere with other circuits, so special attention must be paid to the VCO. In fact, the VCO is often placed at the end of the RF area, and sometimes it requires a metal shield.
In all PCB designs, it is a big principle to keep digital circuits away from analog circuits as much as possible. It is also suitable for RF PCB design. Common analog grounding and grounding for shielding and spacing signal lines are often equally important. The RF line should also be kept away from analog lines and some critical digital signals. All RF traces, pads, and components should be grounded as much as possible and connected to the main ground as much as possible. Microvia construction boards are useful during the RF line development phase. They can use as many vias as you need without any overhead. Otherwise, drilling on a common PCB will increase development costs. I.
The isolation is best when a solid monolithic ground plane is placed directly on the first layer below the surface. When the ground plane is divided into several pieces to isolate the analog, digital, and RF lines, the effect is not good, because there are always some high-speed signal lines to pass through these separate ground planes, which is not a good design.
There are many topics related to signals and control lines that require special attention, but they are beyond the scope of this article.
Conclusion Regardless of whether RF PCB design is a "black art", adhering to some basic RF design rules and reference to some excellent design examples will help complete the RF design work. Successful RF design must pay careful attention to every step and every detail of the entire design process, which means thorough and careful planning must be carried out at the beginning of the design, and a comprehensive and continuous assessment of the progress of each design step must be carried out. . This meticulous design technique is lacking in most domestic electronics cultures.
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