Design Skills of Radio Frequency Circuit Board
From the past to the present, RF circuit board design, like electromagnetic interference (EMI) problems, has been the most difficult part for engineers to control, even a nightmare. If you want to succeed in design at one time, you must plan carefully and pay attention to details before it works.
Radio Frequency (RF) circuit board design is often described as a "black art" because of many theoretical uncertainties. However, this is only a partial view, RF circuit board design still has many rules to follow. However, in practical design, the real practical skill is how to compromise these rules when they cannot be implemented due to various constraints. Important RF design topics include impedance and impedance matching, insulation materials and laminates, wavelength and harmonics, etc. This paper will focus on various issues related to RF circuit board partition design.
Types of micropore
Circuits of different qualities on circuit boards must be separated, but they must be connected in the best condition without electromagnetic interference, which requires the use of microvia. Usually, the diameter of micropore is 0.05 mm to 0.20 mm. These micropore can be divided into three categories: blind via, bury via and through via. Blind holes are located on the top and bottom surfaces of printed circuit boards, and have a certain depth. They are used to connect the surface circuit with the underlying inner circuit. The depth of holes usually does not exceed a certain ratio (aperture). Buried hole refers to the connecting hole located in the inner layer of PCB, which does not extend to the surface of PCB. These two kinds of holes are located in the inner layer of the circuit board. Before lamination, through-hole forming process is used to complete, and several inner layers may be overlapped in the process of through-hole forming. The third type is called through hole, which passes through the entire circuit board and can be used to realize internal interconnection or as an adhesive positioning hole for components.
Using partitioning techniques
When designing RF circuit boards, high power RF amplifiers (HPA) and low noise amplifiers (LNAs) should be isolated as far as possible. Simply speaking, RF connection is to keep the high power RF transmitting circuit away from the low power receiving circuit. If there's a lot of space on the PCB board, it's easy to do that. But usually when there are many components, PCB space becomes very small, so it is difficult to achieve. They can be placed on both sides of the PCB board, or they can work alternately rather than simultaneously. High power circuits sometimes include RF buffers and voltage controlled oscillators (VCO).
Design partitions can be divided into physical partitioning and electrical partitioning. Entity partition mainly involves the layout of components, orientation and shielding, etc. Electrical partition can continue to be divided into power distribution, RF routing, sensitive circuit and signal, grounding and other partitions.
Entity partition
Component layout 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. It also keeps RF input away from RF output and away from high power and low power circuits as far as possible.
The most effective way to stack circuit boards is to arrange the main grounding in the second layer below the surface, and to walk the RF line on the surface as far as possible. Minimizing the size of the through hole on the RF path not only reduces the path inductance, but also reduces the virtual solder joints on the main grounding, and reduces the chances of RF energy leakage to other areas in the laminate.
In physical space, linear circuits such as 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, so this effect must be carefully minimized. RF and IF routes should be crossed as far as possible, and a ground area should be separated between them as far as possible. The correct RF path is very important for the performance of the whole PCB board, which is why the component layout usually takes up most of the time in the design of mobile phone PCB board.
In mobile phone PCB board, low noise amplifier circuit can be placed on one side of PCB board, while high power amplifier circuit can be placed on the other side, and eventually connected to one end of RF antenna and the other end of baseband processor by duplexer on the same side. This requires some techniques to ensure that RF energy is not transmitted from one side of the board to the other through holes. The common technique is to use blind holes on both sides. Blind holes can be arranged on both sides of PCB board without RF interference to minimize the adverse effects of through holes.
Metal shield
Sometimes, it is impossible to retain enough segregation 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 and assembly costs.
It is difficult to ensure high precision of irregular metal shielding cover in manufacturing. The layout of components is limited by rectangular or square metal shielding cover. Metal shielding cover is not conducive to component replacement and fault displacement. Because metal shielding cover must be welded on the ground surface, and it must keep an appropriate one with the components. It takes up precious PCB board space because of its distance.
It is very important to ensure the integrity of the metal shield as far as possible, so the digital signal line entering the metal shield should go as far as possible to the inner layer, and it is better to set the next layer of the signal line layer as the grounding layer. RF signal line can go out from the wiring layer of the small gap at the bottom of the metal shield cover and the grounding gap, but the gap should be surrounded by as much ground area as possible. Grounding on different signal layers can be connected by multiple holes.
Despite these shortcomings, metal shielding is still very effective and often the only solution to isolate critical circuits.
Power decoupling circuit
In addition, proper and effective decouple circuit of chip power supply 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 one isolation inductor to filter out all power supply noise.
The minimum capacitance value usually depends on the resonant frequency of the capacitor itself and the inductance of the contact. The value of C4 is chosen accordingly. The values of C3 and C2 are relatively large because of the connection inductance, so the RF decoupling effect is worse, but they are more suitable for filtering low frequency noise signals. RF decoupling is accomplished by inductor L1, which makes it impossible for RF signals to be coupled from power lines to chips. Because all routes are potential antennas that can receive and transmit RF signals, it is necessary to isolate the RF signals from key circuits and components.
The physical location of these decoupled components is also often critical. The layout principle of these important components is that if C4 is to be as close as possible to IC connector and grounded, C3 must be nearest to C4, C2 must be nearest to C3, and the connection between IC connector and C4 should be as short as possible. The grounding terminals of these components (especially C4) should usually be connected with the chip's grounding foot by the first grounding layer under the board. The holes connecting the components to the grounding layer should be as close as possible to the component pad on the PCB board. It is better to use blind holes on the pad to minimize the inductance of the connection line, and the inductance 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 power supply. The same principle applies to decoupling the power end of the inductor. Some chips require multiple power sources to work, so two or three capacitors and inductors may be needed to decouple them separately. If there is not enough space around the chip, the decoupling effect may not be good.
Particular attention should be paid to the fact that inductances are seldom parallel to each other, because they will form a hollow transformer and induce interference signals to each other, so the distance between them should be at least equal to the height of one of them, or arranged at right angles to minimize mutual inductance.
Electrical Zoning
Electrical zoning is in principle the same as physical zoning, but it also contains some other factors. Some parts of modern mobile phones use different operating voltages and are controlled by software to prolong battery life. This means that mobile phones need to run multiple power sources, which creates more isolation problems. The power supply is usually introduced by a connector and decoupled immediately to remove any noise from the outside of the circuit board. It is then distributed through a set of switches or regulators.
In mobile phones, the DC current of most circuits is quite small, so the wiring width is usually not a problem. However, it is necessary to design a single high current circuit as wide as possible for the power supply of high power amplifiers to minimize the voltage drop at launch. In order to avoid too much current loss, it is necessary to transfer current from one layer to another through multiple holes. In addition, if it can not be decoupled adequately at the power terminal of the high power amplifier, the high power noise will radiate to the whole circuit board and bring various problems. Grounding of high power amplifier is very important, and it is often necessary to design a metal shield for it.
RF output must be away from RF input
In most cases, RF output must be kept away from RF input. This principle also applies to amplifiers, buffers and filters. In the worst case, if the outputs of amplifiers and buffers are fed back to their input terminals with appropriate phase and amplitude, they may generate self-excited oscillations. They may become unstable and add noise and intermodulation products to RF signals.
If the RF signal line goes back from the input end of the filter to the output end, it may seriously damage the band-pass characteristics of the filter. In order to isolate the input and output well, first of all, a main grounding area must be around the filter. Secondly, the lower layer of the filter must be a grounding area, and the grounding area must be connected with the main grounding area around the filter. It is also a good way to keep the signal lines that need to pass through the filter as far away as possible from the filter pins. In addition, the grounding of all parts of the circuit board should be very careful, otherwise an unwanted coupling channel may be introduced unconsciously.
Sometimes you can choose to go single-ended or balanced RF traces. The principles of crosstalk and EMC/EMI apply here as well. Balanced RF signal lines can reduce noise and crosstalk if they are properly aligned, but their impedance is usually high. Moreover, in order to obtain an impedance matching signal source, routing and load, it is necessary to maintain a reasonable line width, which may be difficult in actual wiring.
Buffer
Buffers can be used to improve isolation because they can divide the same signal into two parts and drive different circuits. In particular, local oscillators may require buffers to drive multiple mixers. When the mixer reaches the common mode isolation state at the RF frequency, it will not work properly. Buffers can isolate the impedance changes at different frequencies so that the circuits do not interfere with each other.
Buffers are very helpful to the design. They can follow the circuit that needs to be driven, so that the high power output line is very short. Because the input signal level of the buffer is relatively low, they are not easy to interfere with other circuits on board.
Voltage controlled oscillator
Voltage controlled oscillators (VCO) can convert varying voltages into varying frequencies. This feature is used for high-speed channel switching, but they also convert micro-noise on the control voltage into small frequency changes, which adds noise to RF signals. In short, after the VCO has been processed, there is no way to remove noise from the RF output signal. The difficulty is that the expected bandwidth of the VCO control line may range from DC to 2MHz, and it is almost impossible to remove such wide bandwidth noise by means of filters. Secondly, the VCO control line is usually part of a feedback loop of the control frequency, which may introduce noise in many places, so it must be very strong. Careful handling of VCO control lines.
Resonant circuit
Tank circuit is used in transmitter and receiver. It is related to VCO, but it has its own characteristics. Simply put, the resonant circuit is a series of diodes with inductance and capacitance connected in parallel. It helps to set VCO operating frequency and adjust voice or data to RF carrier.
All VCO design principles are also applicable to resonant circuits. Because the resonant circuit contains a considerable number of components, occupies a large area, and usually operates at a high RF frequency, the resonant circuit is usually very sensitive to noise. Signals are usually arranged on the adjacent pins of the chip, but these signal pins need to cooperate with larger inductances and capacitors to work. Instead, they need to be located as close as possible to the signal pins and connected back to a noise-sensitive control loop, while avoiding noise interference as much as possible. It's not easy to do that.
Automatic Gain Control Amplifier
Automatic Gain Control (AGC) amplifier is also a problem-prone place, whether the transmitting or receiving circuit will have an AGC amplifier. AGC amplifiers are usually effective in filtering noise, but because mobile phones have the ability to handle the rapid changes in the intensity of transmitted and received signals, AGC circuits are required to have a fairly large bandwidth, which makes AGC amplifiers easy to introduce noise.
The design of AGC circuits must follow the design principles of analog circuits, even with very short input pins and very short feedback paths, and both must be far away from RF, IF or high-speed digital signal lines. Similarly, good grounding is also essential, and the power supply of the chip must be well decoupled. If a long line must be designed at the input or output, it is better to implement it at the output, because the impedance of the output is usually much lower than that of the input, and noise is not easy to introduce. Usually the higher the signal level, the easier it is to introduce noise into other circuits.
Ground Connection
Make sure that the grounding of the lower layer of the RF line is solid, and that all components are firmly connected to the main grounding and isolated from other noisy lines. In addition, to ensure that VCO power supply has been fully decoupled, because the RF output of VCO is often a fairly high level, VCO output signal is easy to interfere with other circuits, so special attention must be paid to VCO. In fact, VCO is often placed at the end of the RF area, sometimes it also needs a metal shield.
In all PCB designs, it is a major principle to keep the digital circuit away from the analog circuit as far as possible. It is also applicable to RF PCB designs. Common analog grounding and grounding for shielding and separating signal lines are usually equally important. Also, RF lines should be kept away from analog lines and some key digital signals. All RF lines, pads and components should be surrounded by grounded copper sheets as far as possible and connected to the main grounding as possible. Microvia construction board is very useful in RF line development stage. It can use many crossings at will without any cost. Otherwise, drilling on ordinary PCB board will increase development cost, which is not economical in mass production.
When a solid grounding surface is directly placed on the first layer below the surface, the isolation effect is the best. When grounding surface is divided into several blocks to isolate analog, digital and RF lines, the effect is not good, because ultimately there are always some high-speed signal lines to pass through these separate grounding surfaces, which is not a good design.
Relevant Reading
Signal Integrity in High Speed Circuit Board Design
Analysis and Processing of Transmission Line Effect in High Speed Circuits
Ten PCB Routing Rules for High Frequency Circuits
Re-interpretation of High Speed PCB
Resistance Termination in High Speed Circuits
Interference Analysis and Countermeasure in High-Frequency PCB Design
Related products
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