- What issues should be considered when routing high-frequency signals?
- Impedance matching of signal lines;
- Spatial isolation from other signal lines;
- For digital high-frequency signals, differential lines are more effective.
- How to improve the electrical performance of a PCB with dense wiring and numerous holes?
- For low-frequency signals, vias are not a major concern. For high-frequency signals, minimize the use of vias. If there are many wires, consider using multilayer boards.
- Is it true that the more decoupling capacitors added to a board, the better?
- Decoupling capacitors should be added at appropriate locations with suitable values. For example, they are needed at the power supply ports of analog devices, and different capacitor values are used to filter out different frequencies of stray signals.
- What are the criteria for a good PCB?
- Rational layout, sufficient power line redundancy, high-frequency impedance control, and simple low-frequency routing.
- What is the difference in signal impact between through-holes and blind vias, and what are the application principles?
- Using blind vias or buried vias is an effective way to increase multilayer board density, reduce the number of layers and board size, and significantly decrease the number of plated through-holes.
- However, through-holes are easier to implement in terms of process and lower in cost, so they are generally used in designs.
- When dealing with analog-digital mixed systems, there are suggestions for power plane segmentation and ground plane copper pouring. There are also suggestions for both power and ground segmentation. But this would make the signal return path longer. How to choose the appropriate method in specific applications?
- If there are high-frequency signals (>20MHz) with long lengths and high quantities, at least two layers are needed for these analog high-frequency signals: one for signal lines and one for a large ground plane, with sufficient vias connecting the signal layer to the ground. The purpose is:
- To provide a complete transmission medium and impedance matching for analog signals;
- To isolate analog signals from other digital signals;
- To ensure a sufficiently small ground loop due to the many vias and the large ground plane.
- In a PCB with signal input connectors on the leftmost edge and an MCU on the rightmost side, should the voltage regulator be placed closer to the connectors (where the power IC’s 5V output travels a longer path to reach the MCU) or closer to the middle-right (where the power IC’s 5V output line to the MCU is shorter, but the input power line traverses a longer section of the PCB)? Or is there a better layout?
- First, are the signal input connectors for analog devices? If so, the power supply layout should not compromise the signal integrity of the analog section. Considerations include:
- Whether the voltage regulator provides a clean, low-ripple power supply; analog power supplies have high requirements;
- Whether the analog and MCU share the same power supply; in high-precision circuits, it is recommended to separate the analog and digital power supplies;
- Minimize the impact of digital power supply on analog circuits.
- In high-speed signal chains with multiple ASICs, should the analog and digital grounds be segmented or not? What are the guiding principles, and which method is more effective?
- There is no definitive answer. Generally, consult the chip’s datasheet. ADI’s datasheets for mixed-signal chips recommend grounding schemes, some recommending a common ground and some suggesting isolated grounds, depending on the chip design.
- When should line length equalization be considered? If equal-length lines are used, what is the maximum length difference allowed between two signal lines, and how is it calculated?
- Differential line calculation: If transmitting a sine signal, a length difference equal to half its transmission wavelength results in a 180-degree phase difference, completely canceling out the signals. Therefore, this length difference is the maximum. Similarly, the length difference between signal lines must be less than this value.
- In which situations is serpentine routing suitable in high-speed circuits, and what are its drawbacks? For differential routing, what if the two sets of signals are required to be orthogonal?
- Serpentine routing serves different purposes depending on the application:
- In computer boards, it primarily functions as a filtering inductor and impedance matcher, enhancing circuit immunity to interference. It is used in clock signals like PCI-Clk, AGPCIK, IDE, DIMM, etc.
- In general PCBs, it also serves as an inductor coil for radio antennas, e.g., in 2.4G walkie-talkies.
- For signals requiring strict equal length, serpentine routing in high-speed digital PCBs ensures signal delay differences stay within a range, ensuring data validity within the same cycle (delay differences exceeding one clock cycle may cause data to be read incorrectly from the next cycle).
- Drawbacks include increased distributed capacitance and inductance, potentially degrading signal quality. For differential routing requiring orthogonality, special care must be taken to maintain impedance matching and minimize interference.
- When designing a PCB, how to consider electromagnetic compatibility (EMC/EMI)? What specific aspects need to be considered, and what measures should be taken?
- EMI/EMC design must consider device placement, PCB stack-up, routing of critical connections, and component selection from the outset. For example, place clock generators away from external connectors, route high-speed signals on inner layers with impedance matching and continuous reference planes to reduce reflections, minimize the slew rate of signals to reduce high-frequency components, choose decoupling capacitors with appropriate frequency responses to reduce power plane noise, minimize loop impedance by ensuring small return paths for high-frequency currents, use split ground planes to control high-frequency noise, and select appropriate PCB-to-chassis grounding points.
- What precautions should be taken when designing transmission lines for RF broadband circuit PCBs? How should ground vias be arranged, and should impedance matching be designed internally or in collaboration with the PCB manufacturer?
- This involves multiple factors, such as PCB material parameters, the transmission line model based on these parameters, and device parameters. Impedance matching is typically designed based on manufacturer-provided information.
- In mixed analog and digital circuits, such as half being FPGA or microcontroller digital circuits and the other half being DAC and related amplifier analog circuits, with various power supply voltages, can a shared power supply be used for voltages common to both circuits? What are the techniques for routing and ferrite bead placement?
- Generally, this is not recommended due to complexity and debugging difficulties.
- When designing high-speed multilayer PCBs, what are the main criteria for selecting the footprints of components like resistors and capacitors? Which footprints are commonly used, and can you give some examples?
- 0402 is commonly used in mobile phones; 0603 is commonly used in modules for high-speed signals. The criterion is that smaller footprints have smaller parasitic parameters, though there can be significant high-frequency performance differences among the same footprints from different manufacturers. It is recommended to use high-frequency specialized components in critical locations.
- When designing double-sided PCBs, should signal lines or ground lines be routed first?
- This depends on the overall layout consideration.
- What are the most important considerations when designing high-speed multilayer PCBs? Can you provide detailed solutions to potential problems?
- The most important consideration is how signal lines, power lines, grounds, and control lines are allocated across layers. Generally, analog signals and analog grounds should be kept on separate layers, and power should also be on a separate layer.
- When should 2-layer, 4-layer, or 6-layer boards be used? Are there technical restrictions beyond size considerations? Should the decision be based on the CPU frequency or the interaction frequency with external devices?
- Multilayer boards provide a complete ground plane and more signal layers for routing. For CPUs controlling external memory devices, consider the interaction frequency. High frequencies require a complete ground plane, and signal lines should be kept equal in length.
- How to analyze the impact of PCB routing on analog signal transmission? How to distinguish if noise introduced during signal transmission is due to routing or op-amp components?
- It is difficult to distinguish, so PCB routing should be designed to minimize additional noise.
- For high-speed multilayer PCBs, what are appropriate line widths for power lines, ground lines, and signal lines? What are common settings, and can you provide an example for a working frequency of 300MHz?
- For 300MHz signals, impedance simulations are necessary to determine line widths and distances between lines and grounds. Power line widths depend on current. In mixed-signal PCBs, grounds are typically not represented by lines but by entire planes to minimize loop resistance and provide a complete plane under signal lines.
- How to achieve optimal thermal performance in layout?
- The main sources of heat in PCBs are electronic components, the PCB itself, and heat transferred from other parts. The primary heat source is components, followed by the PCB. The purpose of thermal design is to reduce component and PCB temperatures using methods like reducing heat generation and enhancing heat dissipation.
