Getting Started with RF
In actual design, the real practical trick is how to trade off these guidelines and laws 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, insulating layer materials and laminated boards, wavelengths and standing waves, etc. Careful planning under the premise of a comprehensive grasp of various design principles is the guarantee for a successful design at the first time .
Common Problems in RF Circuit Design
1. Interference between digital circuit modules and analog circuit modules
If the analog circuit (RF) and the digital circuit are working separately, each may work fine. However, once you put the two on the same board and work together from the same power supply, the whole system is likely to be unstable. This is primarily because digital signals frequently swing between ground and the positive supply (>3 V), and with extremely short periods, often on the order of nanoseconds. Due to the larger amplitude and shorter switching times. These digital signals contain a large number of high-frequency components independent of the switching frequency. In the analog part, the signal transmitted from the wireless tuning loop to the receiving part of the wireless device is generally less than 1 μV. Therefore, the difference between the digital signal and the RF signal can reach 120 dB. Obviously. If the digital signal cannot be well separated from the radio frequency signal. Weak radio frequency signals can be corrupted, and as a result, the wireless device's performance will deteriorate, or even not work at all.
2. Noise interference from power supply
RF circuits are quite sensitive to power supply noise, especially to glitch voltage and other high frequency harmonics. Microcontrollers will suddenly draw most of the current for a short period of time during each internal clock cycle, because modern microcontrollers are manufactured in a CMOS process. therefore. Suppose a microcontroller runs at an internal clock frequency of 1MHz, at which frequency it will draw current from the power supply. If proper power supply decoupling is not taken. It will inevitably cause a voltage glitch on the power line. If these voltage spikes reach the power pins of the RF part of the circuit, it may cause work failure in severe cases.
3. Unreasonable ground wire
If the ground wire of the RF circuit is not handled properly, some strange phenomena may occur. For digital circuit designs, most digital circuits function well even without a ground plane. And at RF frequencies, even a very short ground lead acts like an inductor. Roughly calculated, the inductance per millimeter of length is about lnH, and the inductance of a 10 toni PCB line at 433 MHz is about 27Ω. Without a ground plane, most ground traces would be long and the circuit would not behave as designed.
4. Radiation interference from the antenna to other analog circuit parts
In PCB circuit design, there are usually other analog circuits on the board. For example, many circuits have an analog-to-digital conversion (ADC) or a digital-to-analog converter (DAC). The high-frequency signal from the antenna of the RF transmitter may reach the analog signal of the ADC. If the processing at the ADC input is not done properly, the RF signal may self-excite within the ESD diodes at the ADC input. Thus causing ADC deviation.
1. Principles of RF circuit layout
When designing RF layout, the following general principles must be met first:
(1) Isolate the high-power RF amplifier (HPA) and the low-noise amplifier (LNA) as much as possible, simply put, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit;
(2) Make sure that there is at least a whole piece of land in the high-power area on the PCB, preferably without vias on it. Of course, the larger the copper foil area, the better;
(3) Circuit and power supply decoupling is also extremely important;
(4) The RF output usually needs to be far away from the RF input;
(5) Sensitive analog signals should be kept as far away as possible from high-speed digital signals and RF signals;
Two, physical partition, electrical partition design partition
It can be decomposed into physical partition and electrical partition. Physical partitioning mainly involves issues such as component layout, orientation, and shielding; electrical partitioning can be further decomposed into partitions for power distribution, RF routing, sensitive circuits and signals, and grounding.
1. We discuss physical partitioning
Component layout is the key to achieving a good RF design, the most effective technique is to first fix the components located on the RF path, and orient them to minimize the length of the RF path, so that the input is far away from the output, and as far as possible Ground separates high-power circuits from low-power circuits.
The most effective board stacking method is to arrange the main ground plane (main ground) on the second layer under the surface layer, and route the RF lines on the surface layer as much as possible. Minimizing via size on the RF path not only reduces path inductance, but also reduces virtual solder joints on the main ground and reduces the chance of RF energy leaking to other areas within the stackup. Physically, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but diplexers, mixers, and IF amplifiers/mixers always have multiple RF/IF Signals interfere with each other, so care must be taken to minimize this effect.
2. The RF and IF traces should be crossed as much as possible, and a ground should be separated between them as much as possible.
The correct RF path is very important to the performance of the entire PCB board, which is why component layout usually takes up most of the time in mobile phone PCB board design. In the design of mobile phone 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 RF terminal and baseband processing on the same side through a duplexer on the antenna at the receiver end.
3. Proper and effective chip power decoupling is also very important
Many RF chips that integrate linear lines are very sensitive to power supply noise, and usually require up to four capacitors and an isolation inductor per chip to
Make sure to filter out all power noise. 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 source. The same principle applies to decoupling the power supply on this inductor.
Some chips require more than one power supply to work, so you may need two or three sets of capacitors and inductors to decouple them separately. Inductors are rarely placed in parallel, because this will form an air core transformer and induce mutual interference. signals, so they should be spaced at least as high as the height of one of the devices, or arranged at right angles to minimize their mutual inductance.
4. The principle of electrical partitioning is generally the same as that of physical partitioning, but it also includes some other factors
Certain parts of the phone operate at different voltages and are controlled by software to extend battery life. That means the phone needs to run from multiple power sources, which creates even more problems with isolation.
Power is typically brought in at a connector and immediately decoupled to filter out any noise from outside the board before being distributed through a set of switches or regulators. The DC current of most circuits on the mobile phone PCB board is quite small, so the trace width is usually not a problem, however, a separate high-current trace as wide as possible must be run for the power supply of the high-power amplifier to minimize the transmission voltage drop . To avoid too much current loss, multiple vias are required to pass current from one layer to another. Also, if the high power amplifier is not adequately decoupled at its power supply pins, high power noise will radiate across the board and cause all kinds of problems.
Grounding of high power amplifiers is critical and often requires a metal shield. In most cases, it is also critical to keep the RF output away from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, amplifiers and buffers have the potential to self-oscillate if their outputs are fed back to their inputs with proper phase and amplitude. In the best case, they will work stably at any temperature and voltage.
In fact, they can become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to loop from the input to the output of the filter, this can seriously damage the bandpass characteristics of the filter. In order to achieve good isolation between the input and output, firstly a circle of ground must be laid around the filter, and secondly, a piece of ground must be laid in the lower layer of the filter and connected to the main ground around the filter. It is also a good idea to keep the signal lines that need to pass through the filter as far away from the filter pins as possible.
5. To ensure no increase in noise, the following aspects must be considered
First, the desired frequency bandwidth of the control line may range from DC up to 2MHz, and it is almost impossible to remove such wide-band noise through filtering; Noise can be introduced anywhere, so great care must be taken with the VCO control lines. Make sure that the ground underneath the RF traces is solid and that all components are firmly connected to the main ground and isolated from other traces that may introduce noise.
In addition, to ensure that the power supply of the VCO has been fully decoupled, because the RF output of the VCO is often a relatively 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.
The resonant circuits (one for the transmitter and the other for the receiver) are related to the VCO, but have their own characteristics. Simply put, a resonant circuit is a parallel resonant circuit with capacitive diodes that helps in setting the VCO operating frequency and modulating voice or data onto the RF signal. All VCO design principles also apply to resonant circuits. Resonant circuits are usually very sensitive to noise due to their relatively high number of components, wide board distribution, and often operation at a high RF frequency.
Signals are usually arranged on adjacent pins of the chip, but these signal pins need to cooperate with relatively large inductors and capacitors to work, which in turn requires that these inductors and capacitors must be located very close and connected back to the chip. on a control loop that is sensitive to noise. It is not easy to do this.
The automatic gain control (AGC) amplifier is also a place that is prone to problems. Both the transmitting and receiving circuits will have AGC amplifiers. AGC amplifiers can usually effectively filter out noise, but due to the ability of mobile phones to deal with rapid changes in the strength of transmitted and received signals, the AGC circuit is required to have a fairly wide bandwidth, which makes it easy to introduce AGC amplifiers on some key circuits. noise. Designing AGC lines must follow good analog circuit design techniques, and this has to do with very short op amp input pins and very short feedback paths, both of which must be kept away from RF, IF, or high-speed digital signal traces.
Likewise, good grounding is essential, and the chip's power supply must be well decoupled. If you must run a long wire at the input or output, it's best at the output, which is usually much lower impedance and less susceptible to noise induction. Generally, the higher the signal level, the easier it is to introduce noise into other circuits. In all PCB designs, it is a general principle to keep digital circuits as far away from analog circuits as possible, and it also applies to RF PCB designs. Common analog grounds are often equally important as grounds used for shielding and separating signal lines, so careful planning, well-thought-out component placement, and thorough layout evaluation are very important in the early stages of design, as should RF The lines should be far away from analog lines and some critical digital signals. All RF lines, pads and components should be filled with ground copper as much as possible and connected to the main ground as much as possible. If the RF traces must pass through the signal lines, try to lay a layer of ground connected to the main ground along the RF traces between them. If not possible, make sure they are criss-crossed, this minimizes capacitive coupling, while spreading as much ground as possible around each RF trace and connecting them to the main ground.
Three, PCB board design should pay attention to several aspects
1. Handling of power supply and ground wire
Every engineer who is engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only expresses the reduced noise suppression:
(1) It is well known that a decoupling capacitor is added between the power supply and the ground.
(2) Widen the width of the power supply and ground wires as much as possible. It is better that the ground wire is wider than the power wire. The thin width can reach 0.05~0.07mm, and the power line is 1.2~2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground network (the ground of the analog circuit cannot be used in this way)
(3) Use a large area of copper layer as the ground wire, and connect the unused places to the ground on the printed board as the ground wire. Or make a multi-layer board, the power supply and the ground wire each occupy one layer.
2. Co-location of digital circuits and analog circuits
Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital circuits and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be as far away from the sensitive analog circuit device as possible. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated, and they are not connected to each other, but only at the interface between the PCB and the outside world (such as plugs, etc.). The digital ground and the analog ground are shorted a bit, please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.
3. The signal line is laid on the electrical (ground) layer
When wiring multi-layer printed boards, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the workload of production, and the cost will increase accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.
4. Treatment of connecting legs in large-area conductors
In large-area grounding (electricity), the legs of commonly used components are connected to it, and the treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface, but for There are some bad hidden dangers in the welding assembly of components. Therefore, taking into account the electrical performance and process requirements, a cross-shaped pad is made, which is called a heat shield, commonly known as a thermal pad (Thermal), so that the possibility of virtual solder joints may be generated due to excessive cross-section heat dissipation during soldering. Sex is greatly reduced. The treatment of the power-connecting (ground) layer legs of the multi-layer board is the same.
5. The role of the network system in wiring
In many CAD systems, wiring is determined according to the network system. If the grid is too dense, although the number of channels increases, the step size is too small, and the data volume of the map field is too large. This will inevitably have higher requirements for the storage space of the device, and at the same time, it will also affect the computing speed of computer electronic products. huge impact. And some paths are invalid, such as those occupied by the pads of component legs or by mounting holes and fixed holes. Too sparse a grid and too few channels have a great impact on the routing rate. Therefore, there must be a grid system with reasonable density to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set at 0.1 inches (2.54 mm) or an integer multiple of less than 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches etc.
4. High-frequency PCB design skills and methods
1. The corner of the transmission line should be 45° to reduce the return loss.
2. It is necessary to adopt high-performance insulating circuit boards whose insulation constant values are strictly controlled according to the level. This approach facilitates efficient management of electromagnetic fields between the insulating material and adjacent wiring.
3. It is necessary to improve the PCB design specifications for high-precision etching. Consider specifying a total line width tolerance of +/-0.0007 inches, managing the undercut and cross-section of the wiring shape, and specifying the wiring sidewall plating conditions. Overall management of wiring (conductor) geometry and coating surfaces is critical to address skin effect issues associated with microwave frequencies and to achieve these specifications.
4. There is tap inductance in the protruding leads, so avoid using components with leads. In high frequency environments, it is best to use surface mount components.
5. For signal vias, avoid using the via processing (pth) process on sensitive boards, because this process will cause lead inductance at the vias.
6. Provide a rich ground plane. Molded holes should be used to connect these ground planes to prevent the influence of 3D electromagnetic fields on the circuit board.
7. To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating.
8. The solder mask can prevent the flow of solder paste. However, due to thickness uncertainty and unknown insulation properties, covering the entire board surface with solder mask material will
Leads to large variations in electromagnetic energy in microstrip designs. Generally, solder dams are used as the electromagnetic constant of the solder mask layer.
In this case, we manage the conversion between microstrip and coax. In coaxial cable, the ground planes are circularly interwoven and evenly spaced.
In microstrip, the ground plane is below the active lines. This introduces certain edge effects that need to be understood, anticipated and accounted for at design time. Of course, this mismatch also causes return loss, which must be minimized to avoid noise and signal interference.
5. Electromagnetic Compatibility Design
Electromagnetic compatibility refers to the ability of electronic equipment to work harmoniously and effectively in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress various external interferences, enable electronic equipment to work normally in a specific electromagnetic environment, and at the same time reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.
1. Choose a reasonable wire width
Since the impact interference generated by the transient current on the printed line is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be reduced as much as possible. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference. Signal lines for clock leads, row drivers, or bus drivers often carry large transient currents, and the traces should be kept as short as possible. For discrete component circuits, the printed wire width can fully meet the requirements when it is about 1.5 mm; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0 mm.
2. Adopt the correct wiring strategy
The use of equal wiring can reduce the inductance of the wires, but the mutual inductance and distributed capacitance between the wires will increase. If the layout allows, it is best to use a well-shaped mesh wiring structure. The specific method is to wire one side of the printed board horizontally and the other side vertically. Then use metallized holes to connect at the intersection holes.
3. Effectively suppress crosstalk
In order to suppress crosstalk between printed circuit board wires, long-distance equal wiring should be avoided as far as possible when designing wiring, and the distance between lines should be opened as much as possible, and the signal lines and ground lines and power lines should not cross as much as possible. Setting a grounded printed line between some signal lines that are very sensitive to interference can effectively suppress crosstalk.
4. In order to avoid electromagnetic radiation generated when high-frequency signals pass through printed wires, the following points should also be paid attention to when wiring printed circuit boards:
(1) Minimize the discontinuity of the printed wires, for example, the width of the wires should not change abruptly, the corners of the wires should be greater than 90 degrees, and ring routing is prohibited.
(2) The clock signal leads are most likely to generate electromagnetic radiation interference. When routing, they should be close to the ground loop, and the driver should be close to the connector.
(3) The bus driver should be close to the bus it wants to drive. For those leads that leave the printed circuit board, the driver should be next to the connector.
(4) The wiring of the data bus should sandwich a signal ground wire between every two signal wires. It is best to place the ground return next to the least critical address lead, which often carries high frequency currents.
(5) When arranging high-speed, medium-speed and low-speed logic circuits on the printed board, the devices should be arranged in the manner shown in Figure 1.
5. Suppress reflection interference
In order to suppress the reflection interference that appears at the terminal of the printed line, in addition to special needs, the length of the printed line should be shortened as much as possible and a slow circuit should be used.
If necessary, terminal matching can be added, that is, a matching resistor of the same resistance value is added to the end of the transmission line to the ground and the power supply end. According to experience, for TTL circuits with generally faster speeds, terminal matching measures should be used when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the maximum value of the output driving current and the sinking current of the integrated circuit.
6. The differential signal line layout strategy is adopted in the circuit board design process
Differential signal pairs that are routed very close to each other will also be tightly coupled to each other. This mutual coupling will reduce EMI emissions. Usually (of course there are some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the routing of differential signals, especially when designing signal lines for transmission lines. This means that we must design the wiring of the signal line very carefully to ensure that the characteristic impedance of the signal line is continuous and constant throughout the signal line.
During the layout and routing process of the differential line pair, we hope that the two PCB lines in the differential line pair are exactly the same. This means that in practical applications, we should try our best to ensure that the PCB lines in the differential line pair have exactly the same impedance and the length of the wiring is exactly the same. Differential PCB lines are usually always routed in pairs, and the distance between them remains constant at any position along the direction of the pair. Typically, the layout and routing of differential pairs is always as close as possible.
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