In the world of electronics design, high-performance design has its own unique challenges.

1 The birth of high-speed design

In recent years, the increasing number of high-frequency signal designs has been closely linked to the steadily increasing performance of electronic systems.

As system performance improves, the challenges for PCB designers are increasing day by day: smaller chips, denser circuit board layout, and lower power consumption chip requirements.

With the rapid development of all technologies, we are at the core of high-speed design and need to consider its complexity and all factors.

2 review

PCB design has changed a lot over the past 30 years. In 1987, we thought 0.5 micron was the end of the technology, but today, 22nm has become the norm.

As shown in the figure below, the edge rate in 1985 promoted the increase in design complexity (typically 30 nanoseconds), and today the edge rate has become 1 nanosecond.

Changes in marginal rates over the past 30 years.

3 Technological progress is accompanied by various problems

The advancement of technology is always accompanied by a series of problems. As system performance increases and high-speed designs are adopted, some issues must be addressed in the design environment.

Below, we summarize the challenges faced:

Signal quality

IC manufacturers favor lower core voltages and higher operating frequencies, which results in sharply rising edge rates. Edge rates in unterminated designs will cause reflections and signal quality issues.

crosstalk

In high-speed signal designs, dense paths often lead to crosstalk—the phenomenon associated with electromagnetic coupling between traces on a PCB.

Crosstalk can be edge coupling of traces on the same layer or broadside coupling on adjacent layers.

The coupling is three-dimensional. Parallel paths and wide-side traces cause more crosstalk than side-by-side trace paths.

Broadside coupling (top) compared to edge coupling (bottom)

Fast edge rates in traditional designs can cause ringing on unterminated transmission lines, even when using the same frequency and trace length as before.

This essentially results in higher emissions, well in excess of the FCC/CISPR Class B limits for unterminated transmission lines.

Edge rate radiation at 10 nanoseconds (left) and 1 nanosecond (right).

4 Design Solutions

Signal and power integrity issues occur intermittently and are difficult to diagnose. Therefore, the best way is to find the root cause of the problem during the design process and eliminate it, rather than trying to solve it in the later stages and delaying production.

The stackup planning tool makes it easier to implement solutions to signal integrity issues in your design.

5 Circuit board stackup planning

The number one priority in high-speed design must be circuit board stackup. The substrate is the most important component of the assembly, and its specifications must be carefully planned to avoid discontinuous impedance, signal coupling, and excessive electromagnetic radiation.

When looking at the circuit board stackup for your next design, keep these tips and suggestions in mind:
All signal layers need to be adjacent and tightly coupled to an uninterrupted reference plane that creates a clear loop and eliminates broadside crosstalk.

The substrate of each signal layer is adjacent to the reference plane.

There are good planar capacitors to reduce AC impedance in high frequencies. The tightly coupled inner electrical layer plane reduces the AC impedance of the top layer and greatly reduces electromagnetic radiation.

Reducing the dielectric height significantly reduces crosstalk without impacting the available space on the board.

The substrate should be able to accommodate a range of different technologies. For example: 50/100 ohm digital, 40/80 ohm DDR4, 90 ohm USB.

6 Cabling and Workflow

With your stackup carefully planned, the next step is to focus on board routing. Based on design rules and careful configuration of your work area, you can route your board most efficiently and successfully.

These tips can help make your wiring easier and avoid unnecessary crosstalk, radiation, and signal quality problems:

Simplify the view to clearly see the split planes and current loops.

To do this, first determine which copper foil plane (ground or power) serves as the reference plane for each signal layer, and then open the signal layer and internal electrical layer planes to view them at the same time. This helps you more easily see the traces that split the plane.

Multiple signal layers (left), top and adjacent plane views (right)

If a digital signal must cross a power reference plane, you can place one or two decoupling capacitors (100nF) close to the signal. This provides a current loop between the two power supplies.

Avoid parallel routing and broadside routing, which can cause more crosstalk than side-by-side routing.

Unless you are using a synchronous bus, keep the parallel intervals as short as possible to reduce crosstalk. Leave room for signal groups so that their address and data spacing is three times the trace width.

Be careful when using combined microstrip layers on the top and bottom layers of the board. This can lead to crosstalk between traces on adjacent board layers, compromising signal integrity.

Routing the clock (or strobe) signal with the longest delay by signal group ensures that the data has been established before the clock is read.

Routing embedded signals between planes helps minimize radiation and provides ESD protection.

7 Signal clarity

In the future, the complexity of electronic design will undoubtedly continue to increase, which will bring a series of challenges to PCB designers that need to be solved. Ensuring the correct configuration of circuit board stackup, impedance, and current loops is the basis for design stability.

In the rapid development of modern electronic products, high-speed PCB proofing has become a key part of improving product performance. This article will introduce to you how to improve product performance through high-speed PCB proofing.

1. Understand the concept and significance of high-speed pcb prototyping
In electronic product design, high-speed PCB prototyping refers to the use of high-performance materials, precision processes and advanced equipment when designing and manufacturing PCB boards to meet high-speed signal transmission, anti-interference ability and stability requirements. Through high-speed PCB prototyping, signal transmission delay and signal distortion can be effectively reduced, and the reliability and stability of the product can be improved.

2. Choose suitable high-speed PCB materials
Choosing suitable high-speed PCB materials is an important step to ensure product performance improvement. Common high-speed PCB materials include FR-4, Rogers, PTFE, etc. According to the specific needs of the product, the dielectric constant, loss factor and thermal stability of the material are selected to meet the requirements of high-speed signal transmission.

3. Optimize PCB layout and routing
When performing high-speed PCB prototyping, reasonable layout and wiring design can significantly improve product performance. Try to shorten the signal transmission path as much as possible to reduce signal loss; avoid excessive plane layering to reduce signal crosstalk and interference. In addition, the ground wire and power wire should be properly set up to provide good ground potential and power supply stability.

4. Pay attention to the details in high-speed PCB proofing
When performing high-speed PCB prototyping, you also need to pay attention to some details to ensure the improvement of product performance. Choose the appropriate PCB manufacturer to ensure that it has advanced production equipment and rich experience; conduct strict process control, including controlling the accuracy of parameters such as board thickness, line width, line spacing; and conduct necessary testing and verification to ensure that the product The quality and performance meet expectations.

Using high-speed PCB proofing can significantly improve product performance, thereby achieving better user experience and market competitiveness. By choosing the right materials, optimizing layout and trace design, and paying attention to detail and process control, you can maximize product performance.

High-frequency microwave circuit board is a circuit board specially used for high-frequency microwave signal transmission, which has unique advantages and limitations. By understanding its advantages and disadvantages, we can better apply and optimize this circuit board and improve product quality and performance.

First, let us understand the advantages of high frequency microwave circuit boards. High-frequency microwave PCB circuit board has excellent high-frequency characteristics and can show lower signal loss and higher signal transmission efficiency in high-frequency microwave signal transmission. In addition, it also has good anti-interference performance and can effectively resist the impact of external interference on signal transmission. In addition, the manufacturing process of high-frequency microwave PCB circuit boards is relatively mature and can meet the needs of mass production.

However, high-frequency microwave circuit boards also have some limitations. First, its manufacturing process is relatively complex and requires highly precise processing and production equipment, so it is prone to process challenges during the production process. High-frequency microwave circuit boards have higher material requirements and require the use of specific high-frequency materials, which also increases the cost and manufacturing difficulty. In addition, high-frequency microwave PCB circuit boards need to consider more high-frequency characteristics in the design and layout, and the technical requirements for designers are also higher.

Understanding the advantages and disadvantages of high-frequency microwave PCB circuit boards is crucial for the electronics industry and communications field. Only by fully understanding its advantages and disadvantages can we better apply and optimize this circuit board and improve product quality and performance. Through targeted optimization and improvement, we can better meet the needs of high-frequency microwave signal transmission and provide more reliable technical support for development and innovation in the communications field.

1. Reduce the impact of temperature on circuit board stress

Since “temperature” is the main source of circuit board stress, as long as the temperature of the reflow oven is lowered or the speed of heating and cooling of the circuit board in the reflow oven is slowed down, the bending and warping of the board can be greatly reduced. occur. However, other side effects may occur, such as solder short circuits.

2. PCB uses high Tg board

Tg is the glass transition temperature, which is the temperature at which the material changes from glass to rubber. The lower the Tg value of the material, the faster the circuit board begins to soften after entering the reflow oven and becomes soft and rubbery. The time will also become longer, and the deformation of the circuit board will of course become more serious. Using a higher Tg board can increase its ability to withstand stress deformation, but the price of high Tg PCB boards is relatively high.

3. Increase the thickness of the circuit board

In order to make many electronic products thinner and lighter, the thickness of the circuit board has been reduced to 1.0mm, 0.8mm, or even 0.6mm. Such a thickness is really a bit difficult to keep the circuit board from deforming after passing through the reflow oven. It is difficult to force people to do so. It is recommended that if there is no requirement to be thin and light, the circuit board should be 1.6mm thick, which can greatly reduce the risk of board bending and deformation.

4. Reduce the size of the circuit board and reduce the number of panels

Since most reflow ovens use chains to drive the circuit board forward, circuit boards with larger PCB design sizes will be dented and deformed in the reflow oven due to their own weight, so try to treat the long side of the circuit board as the board edge. Putting it on the chain of the reflow furnace can reduce the dent deformation caused by the weight of the circuit board itself. This is also the reason for reducing the number of panels. That is to say, when passing through the furnace, try to use the narrow edge perpendicular to the furnace passing direction. Achieve the lowest amount of dent deformation.

5. Use oven tray jig

If the above methods are difficult to achieve, the last option is to use an oven tray to reduce the amount of deformation. The reason why the oven tray can reduce the bending of the board is because whether it is thermal expansion or cold contraction, the tray is expected to be able to fix the circuit board. When the temperature of the circuit board drops below the Tg value and begins to harden again, the original size can be maintained.

If the single-layer pallet cannot reduce the deformation of the circuit board, you must add a layer of cover and clamp the circuit board with the upper and lower pallets. This can greatly reduce the problem of deformation of the circuit board after passing through the reflow oven. However, these oven trays are quite expensive, and labor is required to place and recycle the trays.

Today’s PCB technology mainly changes according to the characteristics and requirements of electronic products. In recent years, electronic products have become increasingly multi-functional, sophisticated and comply with environmental regulations. Therefore, the precision of PCB is getting higher and higher, and its application of combining soft and hard boards will also increase.

PCB is the foundation of the information industry. From computers to portable electronic devices, circuit boards are found in almost all electronic and electrical products. With the development of communication technology, handheld wireless radio frequency circuit technology is used more and more widely. One of the biggest characteristics of these devices (such as mobile phones, wireless PDAs, etc.) is: first, they include almost all portable subsystems; second, they are small Miniaturization means that the density of components is very high, which makes the mutual interference of components (including SMD, SMC, bare chips, etc.) very prominent. Therefore, it has become a very important topic to design a perfect PCB for RF circuits and audio circuits to prevent and suppress electromagnetic interference and thereby improve electromagnetic compatibility.

Because the performance indicators of the same circuit and different PCB design structures will vary greatly. Especially as the audio functions of today’s handheld products continue to increase, more attention must be paid to the PCB layout of the audio circuit. Based on this, this article focuses on the ingenious design of the PCB of the RF circuit and audio circuit of the handheld product (that is, including component layout, component arrangement, wiring and Grounding and other techniques) for analysis and explanation.

1. Component layout

First, let’s talk about the general principles of layout: components should be arranged in the same direction as much as possible, and the phenomenon of poor welding can be reduced or even avoided by choosing the direction in which the PCB enters the tin melting system; as known from practice, there must be at least 0.5mm spacing between components to meet the requirements of the components. Regarding the soldering requirements of the device, if the space of the PCB board allows, the spacing between components should be as wide as possible. For double-sided panels, one side should generally be designed with SMD and SMC components, and the other side should be designed with discrete components.

1.1 Divide the PCB into digital areas and analog areas.

The first step in any PCB design is, of course, choosing the PCB placement of each component. We call this step “layout consideration”. Careful component placement can reduce signal interconnections, ground separation, noise coupling, and occupied circuit board area.

Electromagnetic compatibility requires that each circuit module PCB is designed to produce as little electromagnetic radiation as possible and to have a certain anti-electromagnetic interference capability. Therefore, the layout of components also directly affects the interference and anti-interference capabilities of the circuit itself, which is also directly related to performance of the designed circuit.

Therefore, when designing an RF circuit PCB, in addition to considering the layout of ordinary PCB designs, you must also consider how to reduce the mutual interference between various parts of the RF circuit, how to reduce the interference of the circuit itself to other circuits, and the circuit itself. anti-interference ability.

It is known from experience that the quality of the RF circuit effect not only depends on the performance indicators of the RF circuit board itself, but also largely depends on the interaction with the CPU processing board. Since RF circuits contain digital circuits and analog circuits, in order to prevent digital noise from interfering with sensitive analog circuits, they must be separated. Dividing the PCB into digital areas and analog areas can help improve the layout of such circuits, which is particularly important. important.

1.2 Need to prevent RF noise from coupling into audio circuits

Although the RF portion of handheld products is often treated as analog circuitry, a common concern in many designs is RF noise. It is necessary to prevent RF noise from coupling into the audio circuitry, because the RF noise is demodulated to produce audible noise. In order to solve this problem, the RF circuit and the audio circuit need to be separated as much as possible. After dividing the PCB into analog and digital, the component layout of the analog part needs to be considered. The component layout should make the path of the audio signal the shortest, and the audio amplifier should be placed as close as possible to the headphone jack and speaker to minimize the EMI radiation of the Class D audio amplifier and the coupling noise of the headphone signal. The analog audio signal source must be as close as possible to the input of the audio amplifier to minimize input coupling noise. All input leads act as an antenna for RF signals, and shortening the lead length helps reduce the antenna radiation effect in the corresponding frequency band.

2. Issues that should be paid attention to in component layout and application examples

2.1 Issues that should be paid attention to in layout:

Carefully analyze the circuit structure. Divide circuits into blocks (such as high-frequency amplification circuits, mixing circuits and demodulation circuits, etc.) to separate strong current signals and weak current signals as much as possible. After separating digital signal circuits and analog signal circuits, attention should also be paid to Circuits that perform the same function should be arranged within a certain range as much as possible to reduce the signal loop area; the filter network of each part of the circuit must be connected nearby, which can not only reduce radiation, but also reduce the chance of interference and improve the efficiency of the circuit. anti-interference ability.

Unit circuits are grouped according to their different sensitivity to electromagnetic compatibility during use. For components in the circuit that are susceptible to interference, the layout should also try to avoid interference sources (such as interference from the CPU on the data processing board, etc.).

2.2 Examples of the impact of component placement on audio signals

The impact of unreasonable component layout on audio signal quality

An unreasonable layout of audio components has two serious problems: one is that the audio amplifier is too far away from the audio signal source, because the leads pass near noisy digital circuits and switching circuits, thereby increasing the probability of noise coupling. Longer leads also enhance the RF antenna effect. If a mobile phone uses GSM technology, these antennas can pick up the GSM transmitted signal and feed it into the audio amplifier. Almost all amplifiers can demodulate the 217Hz envelope to some extent, producing noise at the output. In the worst case, the noise may completely drown out the audio signal. Shortening the length of the input lead can effectively reduce the noise coupled to the audio amplifier. The second audio amplifier is placed too far away from the speaker and headphone jack.

If the audio amplifier uses a Class D amplifier, longer headphone leads will increase the EMI radiation of the amplifier. This radiation may cause the device to fail testing standards set by local governments. Longer headphone and microphone leads also increase lead impedance, reducing the power the load can draw. Finally, because the components are so spread out, the wires between them will have to pass through other subsystems. This will not only increase the wiring difficulty of the audio part, but also increase the wiring difficulty of other subsystems.

The impact of unreasonable component layout on audio signal quality

Reasonable component layout improves audio signal quality

Arrangement of the same components, rearranged components can use space more efficiently and shorten lead lengths. Note that all audio circuits are distributed near the headphone jack and speakers, the audio input and output leads are much shorter than the above solution, and no audio circuits are placed in other areas of the PCB. This design can comprehensively reduce system noise, reduce RF interference, and simplify wiring.

Reasonable component layout indicates the improvement of audio signal quality

3. Wiring principles and techniques

After the layout of the components is basically completed, wiring can begin.

3.1 Basic principles of wiring

When the assembly density permits, try to use a low-density wiring design, and keep the signal traces as consistent in thickness as possible to facilitate impedance matching.

For RF circuits, unreasonable design of the direction, width, and line spacing of signal lines may cause cross-interference between signal transmission lines. The signal path has a very limited impact on audio output noise and distortion, which means there are very few compromises that need to be made to ensure performance. Audio amplifiers are usually powered directly from batteries and require considerable current. If you use long, thin power supply leads, the power supply ripple will be increased. Compared with short and wide leads, long and thin leads have greater impedance, and the current changes generated by the lead impedance are converted into voltage changes and fed into the device. To optimize performance, amplifier power supplies should use the shortest possible leads. Differential signaling should be used whenever possible. Differential inputs have higher noise rejection, allowing the differential receiver to reject common-mode noise on both the positive and negative signal lines. To take full advantage of a differential amplifier, it is important to keep the differential signal pairs the same length when routing them, so that they have the same impedance, and that they are as close to each other as possible so that they couple the same amount of noise. The amplifier’s differential inputs are very effective at rejecting noise from the system’s digital circuitry. In addition, the system power supply itself also has noise interference, so comprehensive considerations must be taken when designing the RF circuit PCB and the wiring must be reasonable.

3.2 Wiring skills

When wiring, all traces should be kept away from the frame of the PCB board (about 2mm) to avoid wire breakage or potential risk of wire breakage during PCB board production. The power lines should be as wide as possible to reduce loop resistance. At the same time, the direction of the power lines and ground wires should be consistent with the direction of data transmission to improve anti-interference capabilities. The signal lines should be as short as possible and minimize excessive interference. The number of holes; the shorter the connections between components, the better, in order to reduce distribution parameters and mutual electromagnetic interference; incompatible signal lines should be kept away from each other, and try to avoid parallel wiring, and on both sides of the forward direction The signal lines should be perpendicular to each other; when wiring, the angle of 135° should be used at addresses that require corners, and avoid turning at right angles.

4. Grounding

In the design of RF circuit PCB, the correct wiring of power lines and ground wires is particularly important. Reasonable design is the most important means to overcome electromagnetic interference. A considerable number of interference sources on PCB are generated through power and ground wires, among which the ground wire causes the largest noise interference. The main reason why ground wires easily cause electromagnetic interference is that there is impedance in the ground wire. When a current flows through the ground wire, a voltage will be generated on the ground wire, resulting in a ground loop current and causing loop interference in the ground wire. When multiple circuits share a ground wire, common impedance coupling is formed, resulting in so-called ground noise. Therefore, when routing the ground wire of the RF circuit PCB, you should do the following:

The circuit is divided into blocks. The radio frequency circuit can basically be divided into high-frequency amplification, mixing, demodulation, local oscillator and other parts. It is necessary to provide a common potential reference point for each circuit module PCB design, that is, the respective ground wires of each module circuit. , so that signals can be transmitted between different circuit module PCB designs. Then, it is summarized at the place where the RF circuit PCB is connected to the ground wire, that is, it is summarized in the main ground wire. Since there is only one reference point, there is no common impedance coupling, and thus there is no mutual interference problem.

The digital area and analog area should be isolated as much as possible, and the digital ground and analog ground should be separated, and finally connected to the power ground.

When space permits, it is best to isolate each module with a ground wire to prevent signal coupling effects between each other.

For audio circuits, grounding is critical to achieving the performance requirements of the audio system. There are two important considerations for grounding in any system: first, it is the return path for current flowing through the device, and second, it is the reference potential for digital and analog circuits. Here are tips that apply to all systems:

Establish a continuous ground plane for digital circuits. The digital current from the ground plane returns through the signal path, and the area of this loop should be kept to a minimum to reduce antenna effects and parasitic inductance. Make sure that all digital signal leads have corresponding ground paths. This layer should cover the same area as the digital signal leads, with as few breakpoints as possible. Breaks in the ground plane, including vias, cause ground current to flow through larger loops, thus producing greater radiation and noise.

Ensure ground galvanic isolation. The ground currents of digital circuits and analog circuits must be kept isolated to prevent digital current from interfering with analog circuits. To achieve this goal, the components need to be arranged correctly. If you place analog circuits in one area of the PCB and digital circuits in another area, the ground currents will be naturally isolated. It’s best to have separate PCB layering for analog circuits.

The analog circuit uses star grounding. Star grounding regards one point of the PCB as a common grounding point, and only this point is regarded as ground potential. In cellular phones, the battery ground terminal is usually used as a star grounding point. The current flowing into the ground plane will not automatically disappear. All ground currents will flow into this ground point. Audio amplifiers draw considerable current, which affects the circuit’s own ground reference and the ground reference of other systems. To solve this problem, it is best to provide a dedicated return loop that bridges the amplifier’s power ground and the headphone jack’s ground return. Note that these dedicated loops do not cross digital signal lines, as they will block digital return current.

Maximize the effect of bypass capacitors. Almost all devices require a bypass capacitor to provide transient current that the power supply cannot provide. These capacitors need to be placed as close as possible to the power pins to reduce parasitic inductance between the capacitor and the device pins, which reduces the effectiveness of the bypass capacitor.

5 Conclusion

The above well-designed PCB is a time-consuming and extremely challenging task, but the investment is indeed worth it. A good PCB layout can help reduce system noise, improve RF signal suppression capabilities, and reduce signal distortion. Good PCB design will also improve EMI performance, potentially requiring less shielding. If the PCB is unreasonable, avoidable problems will occur during the testing phase. If you take measures at this time, it may be too late, and it will be difficult to solve the problems you are facing. It will require more time and effort, and sometimes additional components will be added, increasing system cost and complexity.