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RF circuit design examples and some frequently encountered problems

Many people know RF circuit design, so do you know some of its precautions? In actual circuit design, you will encounter various strange problems, which requires you to accumulate experience through practice. The really practical trick is how to compromise these guidelines and laws when they cannot be implemented accurately 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 laminates, and wavelengths and standing waves. Careful planning under the premise of a comprehensive grasp of various design principles is the guarantee of a successful first-time design. .

First, the common problems of RF circuit design

1. Interference between digital circuit modules and analog circuit modules

If analog circuits (RF) and digital circuits work separately, they may each work well. However, once the two are put together on the same board, using the same power supply, the entire system is likely to be unstable.

This is mainly because digital signals frequently swing between ground and positive supply (>3 V) and have extremely short periods, often on the order of nanoseconds. due to larger amplitudes and shorter switching times. As a result, these digital signals contain a large number of high-frequency components that are 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 can not be well separated from the RF signal. Weak radio frequency signals can be damaged, so that the performance of wireless devices will deteriorate, or even completely inoperable.

2. Noise interference of power supply

RF circuits are quite sensitive to power supply noise, especially to glitch voltages and other high frequency harmonics. Microcontrollers draw most of the current for a short period of time during each internal clock cycle, because modern microcontrollers are fabricated using CMOS processes. So, assuming a microcontroller is running with an internal clock frequency of 1MHz, it will draw current from the power supply at this frequency. If proper power supply decoupling is not taken, it will cause voltage glitches on the power line. If these voltage glitches 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. In the RF frequency band, even a very short ground wire can act like an Inductor. Roughly calculated, the inductance per millimeter of length is about 1 nH, and the inductive reactance of a 10 toni PCB line at 433 MHz is about 27Ω. Without a ground plane, most grounds would be long and the circuit would not have the characteristics it was designed for.

4. The radiation interference of the antenna to other analog circuit parts

In PCB circuit design, there are often other analog circuits on the board.

For example, many circuits have analog-to-digital converters (ADCs) or digital-to-analog converters (DACs). The high-frequency signal emitted by the antenna of the RF transmitter may reach the analog signal of the ADC. If the ADC input is not properly processed, the RF signal may self-excite within the ESD Diode at the ADC input. resulting in ADC bias.

2. Summary of five major experiences

1. Principles of RF circuit layout

When designing an 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, in short, keep the high-power RF transmitting circuit away from the low-power RF receiving circuit;

(2) Make sure that there is at least one whole ground in the high-power area on the PCB, preferably without vias. Of course, the larger the copper foil area, the better;

(3) Circuit and power supply decoupling is also extremely important;

(4) RF output usually needs to be far away from RF input;

(5) Sensitive analog signals should be kept away from high-speed digital signals and RF signals as much as possible;

2. Physical partition, electrical partition design partition

Can be broken down into physical partitions and electrical partitions. Physical partitions mainly involve issues such as component layout, orientation, and shielding; electrical partitions can continue to be decomposed into partitions for power distribution, RF traces, sensitive circuits and signals, and grounding.

1) We discuss the physical partition problem:

Component placement is the key to a good RF design, and the most effective technique is to first fix the components on the RF path and orient them to minimize the length of the RF path, keeping the input away from the output and as far away as possible. ground to separate high-power and low-power circuits. The most efficient board stacking method is to arrange the main ground plane (main ground) on the second layer below the surface layer, and run the RF lines on the surface layer as much as possible. Minimizing the size of vias on the RF path not only reduces path inductance, but also reduces void solder joints on the main ground and reduces the chance of RF energy leaking to other areas within the stack.

In physical space, linear circuits like multistage amplifiers are usually sufficient to isolate multiple RF zones from each other, but duplexers, mixers, and IF amplifiers/mixers always have multiple RF/IFs 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 spaced between them as much as possible:

The correct RF path is very important to the performance of the entire PCB, which is why component placement usually takes up most of the time in mobile phone PCB 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 end and baseband processing on the same side through a duplexer on the antenna of the device.

Some tricks are required to ensure that straight-through vias do not transfer RF energy from one side of the board to the other, and a common technique is to use blind vias on both sides. The detrimental effects of straight-through vias can be minimized by arranging the straight-through vias in an area on both sides of the PCB that is free from RF interference. Sometimes it is not possible to ensure sufficient isolation between multiple circuit blocks, in which case a metal shield must be considered to shield the RF energy in the RF area. The metal shield must be soldered to the ground and must be kept from the components. an appropriate distance, thus taking up valuable PCB board space.

It is very important to ensure the integrity of the shielding cover as much as possible. The digital signal line entering the metal shielding cover should go to the inner layer as much as possible, and it is better that the layer of PCB below the wiring layer is the ground layer. The RF signal line can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground gap, but as much ground as possible should be distributed around the gap, and the ground on different layers can be connected together through multiple vias .

3) Proper and effective chip power decoupling is also very important:

Many RF chips with integrated linear lines are very sensitive to noise from the power supply, typically requiring up to four capacitors and an isolation inductor per chip to ensure all power supply noise is filtered out. An integrated circuit or amplifier often has an open-drain output, so a pull-up inductor is required to provide a high-impedance RF load and a low-impedance DC source. The same principle applies to decoupling the supply on this inductor side.

Some chips require multiple power supplies to work, so you may need two or three sets of capacitors and inductors to decouple them separately, inductors are rarely close together in parallel, as this would create an air core transformer and induce interference with each other signal, so the distance between them should be at least the height of one of the devices, or they should be arranged at right angles to minimize their mutual inductance.

4) The principles of electrical partitioning are generally the same as physical partitioning, but include some other factors:

Certain parts of the phone operate at different voltages and are controlled by software to prolong battery life. This means the phone needs to run on multiple power sources, which creates more problems with isolation. Power is usually brought in at the connector and is immediately decoupled to filter out any noise from outside the board before being distributed through a set of switches or voltage regulators.

Most circuits on mobile phone PCBs have fairly small DC currents, so trace width is usually not an issue, however, a separate high-current trace as wide as possible must be run for the power supply of high power amplifiers to minimize transmission voltage drop .

To avoid too much current loss, multiple vias are needed to pass current from one layer to another. Additionally, if the high power amplifier is not sufficiently decoupled at its power supply pins, high power noise will radiate across the board and cause various problems. Grounding of high power amplifiers is critical and often requires a metal shield. In most cases, it is also critical to ensure that the RF output is kept 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 under any temperature and voltage conditions.

In fact, they can become unstable and add noise and intermodulation signals to the RF signal. If the RF signal lines have to be looped back from the input of the filter to the output, this can seriously damage the bandpass characteristics of the filter. In order to obtain good isolation between input and output, first, a ground must be placed around the filter, and secondly, a ground should be placed in the lower area 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 go through the filter as far away from the filter pins as possible.

Also, be careful with grounding everywhere on the board, or you will introduce a coupling channel. Sometimes single-ended or balanced RF signal lines can be chosen, and the same principles regarding cross-interference and EMC/EMI apply here. Balanced RF signal lines can reduce noise and cross-interference if they are routed correctly, but their impedance is usually high, and a reasonable line width should be maintained to obtain an impedance that matches the source, trace, and load. Actual wiring may There will be some difficulties.

A buffer can be used to improve isolation because it can split the same signal into two parts and use it to drive different circuits, especially if the LO may need a buffer to drive multiple mixers. When the mixer reaches common mode isolation at RF frequencies, it will not function properly. Buffers are good at isolating impedance changes at different frequencies so that circuits do not interfere with each other.

Buffers are a great help in the design, they can be placed right after the circuit that needs to be driven, so that the high power output traces are very short, because the input signal level of the buffer is relatively low, so they are not easy to be affected by other circuits on the board. circuit causing interference. Voltage Controlled Oscillators (VCOs) convert changing voltages to changing frequencies, a feature used for high-speed channel switching, but they also convert small amounts of noise on the control voltage into small frequency changes, which give RF signals add noise.

5) To ensure that no noise is added, the following aspects must be considered:

First, the desired bandwidth of the control line may range from DC to 2MHz, and filtering to remove noise in such a wide band is almost impossible; second, the VCO control line is usually part of a feedback loop that controls the frequency, which is in many Noise can be introduced anywhere. Therefore the VCO control lines must be handled with great care. Make sure that the ground below 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, make sure that the power supply of the VCO is adequately decoupled, since the RF output of the VCO tends to be 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 circuit (one for the transmitter and the other for the receiver) is related to the VCO, but also has its own characteristics.

Simply put, a resonant circuit is a parallel resonant circuit with a capacitive diode that helps set the VCO operating frequency and modulate speech or data onto an RF signal. All VCO design principles apply equally to resonant circuits. Resonant circuits are often very sensitive to noise due to their considerable number of components, wide distribution on the board, and typically operating at a very high RF frequency.

Signals are usually arranged on adjacent pins of the chip, but these signal pins need to work with relatively large inductors and capacitors, which in turn requires these inductors and capacitors to be located close together and connected back to on a noise-sensitive control loop. It is not easy to do this. The automatic gain control (AGC) amplifier is also a problem-prone place, and there will be an AGC amplifier in both the transmit and receive circuits. AGC amplifiers are usually effective at filtering out noise, but due to the cell phone’s ability to handle rapid changes in transmitted and received signal strength.

Therefore, the AGC circuit is required to have a relatively wide bandwidth, which makes the AGC amplifier on some critical circuits easy to introduce noise. Good analog circuit design techniques must be followed when designing AGC lines, 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. Also, good grounding is essential, and the power supply to the chip must be well decoupled. If you have to run a long wire at the input or output, it’s best at the output, which usually has a much lower impedance and is less prone to inductive noise.

Usually the higher the signal level, the easier it is to introduce noise into other circuits. Keeping digital circuits as far away from analog circuits as possible is a general principle in all PCB designs, and it also applies to RF PCB design.

The common analog ground is often as important as the ground used to shield and separate the signal lines, so careful planning, thoughtful component placement, and thorough placement* estimation are all important in the early stages of design. Likewise, RF should be Lines should be kept away from analog lines and some very critical digital signals. All RF traces, 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 route a layer of ground connected to the main ground along the RF traces between them. If not possible, make sure they are criss-crossed to minimize capacitive coupling, and run as many grounds as possible around each RF trace and connect them to the main ground. Additionally, minimizing the distance between parallel RF traces minimizes inductive coupling. Isolation is best when a solid, monolithic ground plane is placed directly under the first layer of the surface, although other approaches can also work with careful design.

On each layer of the PCB board, lay as many grounds as possible and connect them to the main ground. Place the traces as close together as possible to increase the number of pads on the internal signal and power distribution layers, and adjust the traces so that you can route ground connection vias to isolated pads on the surface. Free grounds on various layers of the PCB should be avoided as they can pick up or inject noise like a small antenna. In most cases, if you can’t connect them to the main ground, then you’re better off removing them.

3. When designing mobile phone PCB boards, several aspects should be paid attention to

1) Handling of power supply and ground wire:

Even if the wiring in the entire PCB board is well completed, the interference caused by the lack of thoughtful consideration of the power supply and the ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of the power and ground wires should be taken seriously, and the noise interference generated by the power and ground wires should be minimized to ensure the quality of the products. Every engineer who is engaged in the design of Electronic products understands the reasons for the noise between the ground wire and the power wire. Now only the reduced noise suppression is expressed:

(1) It is well known to add a decoupling capacitor between the power supply and the ground wire.

(2) Try to widen the width of the power and ground wires, preferably the ground wire is wider than the power wire. The thin width can reach 0.05~0.07mm, and the power cord 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, a ground network can be used (the ground of the analog circuit cannot be used in this way)

(3) Use a large-area copper layer as a ground wire, and connect the unused places on the printed board to the ground as a ground wire. Or make a multi-layer board, power supply, ground wire each occupy one layer.

2) Common ground processing of digital circuits and analog circuits

There are many PCBs now that are no longer a single function circuit (digital or analog), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground wire. The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be kept as far away from sensitive analog circuit devices as possible. For ground lines, the entire PCB has only one node to the outside world.

Therefore, the problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and the analog ground are actually separated inside the board, and they are not connected to each other, but at the interface between the PCB and the outside world (such as plugs, etc.) . The digital ground is a little shorted to the analog ground, note that there is only one connection point. There are also different grounds on the PCB, which are determined by the system design.

3) The signal line is arranged on the electrical (ground) layer

When wiring a multi-layer printed board, since there are not many wires left in the signal wire layer, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To resolve this contradiction, it is possible to consider wiring on the electrical (ground) layer. The power plane should be considered first, followed by the ground plane. Because it is best to preserve the integrity of the formation.

4) Handling of connecting legs in large area conductors

In a large area of ​​grounding (electricity), the legs of commonly used components are connected to it, and the handling of the connecting legs needs to be comprehensively considered. There are some bad hidden dangers in the welding assembly of components, such as: ① Welding requires high-power heaters; ② It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process needs, a cross-shaped pad is made, which is called heat shield, commonly known as thermal pad. Sex is greatly reduced. The electrical (ground) leg of a multilayer board is treated the same way.

5) The role of the network system in wiring

In many CAD systems, wiring is determined by the network system. If the grid is too dense, although the number of channels is increased, the step is too small, and the amount of data in the image field is too large, which must have higher requirements on the storage space of the equipment, and also affect the computing speed of computer electronic products. great influence. And some vias are invalid, such as those occupied by pads of component legs or occupied by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution 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 to 0.1 inches (2.54 mm) or less than an integral multiple of 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 corners of the transmission line should be at a 45° angle to reduce return loss.

(2) High-performance insulating circuit boards whose insulation constant values ​​are strictly controlled by level should be used. This approach facilitates efficient management of electromagnetic fields between insulating materials and adjacent wiring.

(3) It is necessary to improve the PCB design specifications for high-precision etching. Consider specifying a total error of +/- 0.0007 inches in line width, managing undercuts and cross-sections of wiring shapes, and specifying wiring sidewall plating conditions. Overall management of wiring (conductor) geometry and coating surface is important to address skin effect issues associated with microwave frequencies and to achieve these specifications.

(4) There is a tap inductance in the protruding leads, and components with leads should be avoided. In high frequency environments, it is best to use surface mount components.

(5) For signal vias, avoid using the via processing (pth) process on the sensitive board, because this process will cause lead inductance at the via.

(6) To provide a rich ground plane. Molded vias are used to connect these ground planes to prevent the effects of 3D electromagnetic fields on the board.

(7) To choose electroless nickel plating or immersion gold plating process, do not use HASL method for electroplating.

(8) The solder resist layer prevents the flow of solder paste. However, covering the entire board surface with solder mask material will result in large variations in the electromagnetic energy in the microstrip design due to thickness uncertainty and unknown insulating properties. Generally, a solder dam is used as the electromagnetic field of the solder mask.

In this case, we manage the transition between microstrip and coax. In a coaxial cable, the ground planes are interwoven in a ring and evenly spaced. In microstrip, the ground plane is below the active line. This introduces certain edge effects that need to be understood, predicted and taken into account at design time. Of course, this mismatch also results in 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, so that electronic equipment can 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 wire is mainly caused by the inductance component of the printed wire, the inductance of the printed wire should be minimized. 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. Clock traces, signal lines of row drivers or bus drivers often carry large transient currents and traces should be kept as short as possible. For discrete component circuits, when the width of the printed wire is about 1.5mm, it can fully meet the requirements; for integrated circuits, the width of the printed wire can be selected between 0.2 and 1.0 mm.

2) Adopt the correct wiring strategy: The use of equal wiring can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase. If the layout allows, it is best to use a well-shaped mesh wiring structure. The specific method is one side of the printed board. Route horizontally, route longitudinally on the other side, and then connect with metallized vias at the cross holes.

3) In order to suppress the crosstalk between the conductors of the printed board, the long-distance and equal wiring should be avoided as far as possible when designing the wiring: the distance between the lines should be widened as much as possible, and the signal line and the ground line and the power line should be as far away as possible. cross. Setting a grounded trace 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 the printed wires, the following points should also be paid attention to when wiring the printed circuit board:

(1) Minimize the discontinuity of the printed conductors, for example, the width of the conductors should not be abruptly changed, and the corners of the conductors should be greater than 90 degrees. Ring routing is prohibited.

(2) The lead of the clock signal is most likely to generate electromagnetic radiation interference. The wiring 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 right 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 right next to the least important address lead because the latter often carry high frequency currents.

(5) When arranging high-speed, medium-speed and low-speed logic circuits on the printed board, the devices should be arranged as shown in Figure 1.

5) Suppress reflection interference

In order to suppress the reflection interference that appears at the end of the printed line, except for special needs, the length of the printed line should be shortened as much as possible and slow circuits should be used. If necessary, terminal matching can be added, that is, a matching resistor with the same resistance value is added at the end of the transmission line to the ground and the power supply end. According to experience, for the generally faster TTL circuit, the terminal matching measures should be adopted when the printed lines are longer than 10cm. The resistance value of the matching resistor should be determined according to the output drive current and the maximum value of the sink current of the integrated circuit.

6) The differential signal line routing strategy is adopted in the circuit board design process

Differential signal pairs that are routed very close to each other are also tightly coupled to each other. This mutual coupling reduces EMI emissions. Usually (with some exceptions) differential signals are also high-speed signals, so high-speed design rules usually apply. This is especially true for the wiring of differential signals, especially when designing signal lines for transmission lines. This means that we must design the routing of the signal lines 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 pair, we want the two PCB lines in the differential pair to be exactly the same.

This means that, in practice, every effort should be made to ensure that the PCB traces in a differential pair have exactly the same impedance and that the traces are of the same length. Differential PCB traces are usually always routed in pairs, and the distance between them remains constant anywhere along the direction of the pair. Typically, differential pairs are placed and routed as close together as possible. The above are some precautions for RF circuit design, I hope to help you.

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