Ultra-utility 70 Q&A high frequency PCB circuit design

1. How to choose PCB board? Choosing a PCB board requires balancing design requirements, mass production feasibility, and cost. Design requirements involve both electrical and mechanical aspects. Material selection is especially critical when designing very high-speed PCBs (above GHz). For example, FR-4 material may have significant dielectric loss at several GHz frequencies, affecting signal attenuation. It's important to consider the dielectric constant and loss at the intended operating frequency. 2. How to avoid high-frequency interference? The main idea is to minimize electromagnetic interference from high-frequency signals, known as crosstalk. This can be achieved by keeping high-speed signals away from analog signals or adding ground guard/shunt traces near analog lines. Also, digital ground noise should be carefully managed to prevent it from affecting the analog ground. 3. How to solve signal integrity problems in high-speed design? Signal integrity primarily relates to impedance matching. Factors influencing this include the source architecture, output impedance, trace characteristic impedance, load characteristics, and trace topology. The solution involves termination and adjusting the trace layout. 4. How is the differential wiring method implemented? When routing differential pairs, two key points must be considered: the lengths of the two lines should be as equal as possible, and the spacing between them (determined by differential impedance) should remain consistent, meaning they should be parallel. There are two common methods: side-by-side or top-by-side routing, with side-by-side being more typical. 5. How to implement differential wiring for a clock signal line with only one output? Differential routing requires both the source and receiver to support differential signals. Therefore, it cannot be used for a clock signal with only one output. 6. Can I add a matching resistor between the differential pairs at the receiving end? Yes, a matching resistor is typically added between the differential pair at the receiving end, with a value equal to the differential impedance, which improves signal quality. 7. Why is the wiring of the differential pair close and parallel? Differential pairs should be closely and evenly spaced to maintain consistent differential impedance. Proper spacing ensures signal integrity and timing, while excessive proximity can cause inconsistent impedance and degrade signal performance. 8. How to deal with some theoretical conflicts in the actual wiring? It is generally correct to isolate analog and digital sections. Signal traces should not cross moats, and the return current path for power and signals should be minimized. Crystal oscillators, being analog circuits, require careful placement to avoid noise interference. High-speed wiring and EMI requirements often conflict, but using techniques like routing and PCB stacking can help reduce EMI without compromising signal integrity. 9. How to solve the contradiction between manual wiring and automatic wiring of high-speed signals? Modern autorouters have constraints on winding methods and via counts. However, their capabilities vary. Manual adjustment depends on the router’s performance, such as line pushing and hole drilling. Choosing a router with strong winding engine capabilities is crucial. 10. About the test coupon. Test coupons are used to measure the characteristic impedance of a PCB using a TDR. They should match the controlled lines in width and spacing. The grounding point during measurement is critical to reduce inductance. 11. In high-speed PCB design, the blank area of the signal layer can be coated with copper, and how should the copper of multiple signal layers be distributed on the ground and the power supply? Copper is usually grounded in empty areas. When applying copper near high-speed signals, ensure it doesn't affect the trace impedance. Care must also be taken to avoid disrupting the layer's characteristic impedance structure. 12. Is it possible to calculate the characteristic impedance using the microstrip line model on the signal line above the power plane? Can the signal between the power supply and the ground plane be calculated using the stripline model? Yes, both power and ground planes serve as reference planes. For example, a four-layer board with a top layer over a power layer uses a microstrip model. 13. Automatically generate test points through high-density printed boards through software. Under normal circumstances, can it meet the test requirements for mass production? Whether software-generated test points meet test requirements depends on whether the test point specifications align with the equipment. If the wiring is too dense, manual adjustments may be necessary. 14. Does adding test points affect the quality of high-speed signals? Adding test points can affect signal quality, depending on how they are added and the signal speed. Test points may introduce small capacitances or branches, which can impact high-speed signals. Simulation helps assess the impact. 15. How many PCBs form the system, and how should the ground wires between the boards be connected? When connecting signals or power between PCBs, the number of ground pins should be sufficient to reduce impedance and noise. Analyzing the entire current loop helps control the current path and minimize interference. 16. Can you introduce some foreign technical books and data on high-speed PCB design? High-speed PCB design involves complex considerations, such as GHz frequencies and 40-layer boards. Advances in chip technology have increased demand for blind/buried vias and build-up processes. 17. Two characteristic impedance equations that are often referred to: For microstrip: Z = 87 / sqrt(Er + 1.41) * ln(5.98H / (0.8W + T)) For stripline: Z = 60 / sqrt(Er) * ln(4H / (0.67π(T + 0.8W))) 18. Can the ground wire be added in the middle of the differential signal line? No, adding a ground wire disrupts the coupling effect of differential signals, reducing noise immunity and flux cancellation. 19. Does the rigid flexible board design require special design software and specifications? Where can I undertake this type of circuit board processing in China? Flexible PCBs can be designed with standard software, but manufacturers have specific minimum line widths and apertures. Online searches for "FPC" can find suitable manufacturers. 20. What are the principles for properly selecting the point at which the PCB and the case are grounded? Use chassis ground to provide a low-impedance return path. Connecting the PCB ground to the chassis near high-frequency devices minimizes loop area and reduces radiation. 21. The circuit board DEBUG should start from those aspects? In digital circuits, first verify power supplies, clock signals, and reset signals. Then follow system operation principles and bus protocols for debugging. 22. In the case of fixed circuit board size, if the design needs to accommodate more functions, it is often necessary to increase the PCB trace density, but this may lead to increased mutual interference of the traces, while the traces are too thin and the impedance Can't be reduced, ask experts to introduce techniques in high-speed (>100MHz) high-density PCB design? Crosstalk is a major concern in high-speed, high-density designs. Control trace spacing, use appropriate termination, avoid overlapping traces, and use differential termination to mitigate effects. 23. The filtering at the analog power supply is often done with an LC circuit. But why is LC sometimes worse than RC filtering? LC filtering is effective for high-frequency noise, but RC filtering is simpler and cheaper. However, RC filtering consumes energy and may not be efficient enough for certain applications. 24. The choice of inductors when filtering, what is the method of capacitance value? Capacitor value depends on ripple noise tolerance and ESR/ESL. Larger capacitors reduce ripple but may increase cost. Consider inductor values to avoid slow response to sudden current changes. 25. How to achieve EMC requirements as much as possible without causing too much cost pressure? Use slower slew rates, place high-frequency components away from connectors, ensure impedance matching, and use decoupling capacitors. Ground segmentation and guard traces can also help. 26. When there are multiple digital/analog function blocks in a PCB board, the conventional practice is to separate the digital/analog grounds. What are the reasons? Separating grounds prevents digital noise from affecting analog signals. If not separated, digital noise can interfere with analog circuits even without crossing. 27. Another method is to ensure that the digital/analog separate layout, and the digital/analog signal traces do not cross each other, the entire PCB board is not divided, and the digital/analog ground is connected to the ground plane. What is the truth? Digital and analog traces should not cross to avoid noise coupling. Return currents from digital signals can interfere with analog circuits if not properly managed. 28. How to consider impedance matching when designing high-speed PCB design schematics? Impedance matching is crucial in high-speed design. Trace width, layer position, and material all affect impedance. Simulations may not account for discontinuities, so terminators like series resistors are often used. 29. Where can I provide a more accurate IBIS model library? IBIS models depend on chip manufacturer data. Accurate models are essential for simulation, and manufacturers are the best source for up-to-date information. 30. In high-speed PCB design, should designers consider the rules of EMC and EMI from those aspects? EMC/EMI design should consider both radiated and conducted emissions. Layout, component placement, and grounding are critical to reduce interference and improve signal integrity. 31. How to choose EDA tools? EDA tools vary in features and cost. PADS and Cadence offer good price-performance ratios. Beginners may use integrated environments provided by PLD manufacturers. 32. Please recommend an EDA software suitable for high speed signal processing and transmission. PADS is suitable for conventional design, while Cadence and Mentor offer advanced solutions for high-speed and mixed-signal designs. 33. Interpretation of the meaning of each layer of the PCB Topoverlay: Top silkscreen or component legend. Bottomoverlay: Same for bottom layer. Multi-layer pads appear on all relevant layers. 34. 2G above high-frequency PCB design, routing, typesetting, what should be paid attention to? RF circuits require specialized tools and parametric models. Mentor’s BoardStation has dedicated RF modules for such designs. 35. 2G or higher high-frequency PCB design, what rules should be followed in the design of microstrip? Microstrip design requires 3D field analysis tools to extract transmission line parameters. 36. For a full digital signal PCB, there is an 80MHz clock source on the board. In addition to the use of wire mesh (grounding), what circuit should be used to protect it in order to ensure sufficient driving capability? Use a clock driver chip to distribute the clock signal to multiple loads, ensuring proper signal edge rate and delay. 37. If a separate clock signal board is used, what kind of interface is generally used to ensure that the transmission of the clock signal is less affected? Use differential signaling, such as LVDS, for long-distance clock transmission to reduce interference. 38, 27M, SDRAM clock line (80M-90M), the second and third harmonics of these clock lines are just in the VHF band, and the interference is very high after the high frequency input from the receiving end. In addition to shortening the line length, what better way? Modify the signal duty cycle or use source-side series matching to suppress reflections. 39. What is the topology of the trace? Topology refers to the routing order of multi-port connections, affecting signal integrity and timing. 40. How to adjust the topology of the trace to improve signal integrity? Different signal types (unidirectional, bidirectional) require different topologies. Pre-simulation helps determine the best approach. 41, how to reduce the EMI problem by arranging the laminate? Use tightly coupled ground and power layers to suppress common-mode interference and reduce coupling area. 42. Why do you want to lay copper? Copper plating improves EMC, meets process requirements, enhances signal integrity, and aids heat dissipation. 43. In a system, including dsp and pld, what problems should I pay attention to when wiring? Consider signal speed and transmission line delay. Multiple DSPs and clocks require careful routing to avoid timing issues. 44. In addition to protel tool wiring, are there other good tools? Other tools include MENTOR’s WG2000, EN2000, Cadence Allegro, and Zuken’s CADSTAR, each with unique strengths. 45. What is the “signal return path”? The return path is the shortest route for the return current, typically along the ground or power plane. 46, how to perform SI analysis on the connector? Use IBIS models or SPICE models for connectors. Multi-board simulation software can help analyze distribution parameters. 47. What are the methods of termination? Termination includes series and parallel resistors, AC, and Schottky diode matching, depending on the signal type. 48. What is the factor of termination (matching)? Matching depends on buffer characteristics, topology, signal level, and power consumption. 49. What are the rules for using termination (matching)? Match to ensure signal integrity and timing. Reference books like “High Speed Digital Design” provide detailed guidance. 50. Can I simulate the logic function of the device using the IBIS model of the device? If not, how do you perform board level and system level simulation of the circuit? IBIS models are behavioral and not suitable for functional simulation. Use SPICE models for that purpose. 51. In the system where digital and analog coexist, there are 2 kinds of processing methods, one is to separate the digital ground and the analog ground. For example, in the ground layer, the digital ground is an independent piece, and the analog ground is independent. The single point is copper or FB magnetic. The beads are connected, and the power supply is not separated; the other is that the analog power supply and the digital power supply are connected separately by FB, and the ground is uniformly. May I ask Mr. Li, are the two methods the same effect? Both methods aim to reduce interference. The choice depends on signal return paths and system requirements. 52. Safety issues: What is the specific meaning of FCC and EMC? FCC is the Federal Communications Commission, and EMC is electromagnetic compatibility. Standards define testing methods and requirements. 53. What is differential wiring? Differential wiring uses two signals with opposite polarities to transmit data, relying on their difference. Maintaining parallelism and consistent spacing is crucial. 54. What are the PCB simulation software? Common tools include ICX, SignalVision, HyperLynx, and HSPICE, used for signal integrity analysis. 55. How does the PCB simulation software perform LAYOUT simulation? High-speed designs often use multi-layer boards with dedicated power and ground layers to improve signal integrity. 56. How to deal with the layout and wiring to ensure the stability of signals above 50M High-speed signal routing requires minimizing transmission line effects. Keep traces short and ensure proper impedance matching. 57. The RF part of the outdoor unit, the IF part, and even the low-frequency circuit part that monitors the outdoor unit are often deployed on the same PCB. What are the requirements on the material of such a PCB? How to prevent interference between RF, IF and even low frequency circuits? Hybrid designs need careful layout, shielding, and high-quality materials. RF circuits often use substrates with high Q values to reduce losses. 58. For the RF part, the IF part and the low frequency circuit part are deployed on the same PCB. What solution does the mentor have? Mentor provides RF design modules and interfaces with EESOFT for simulation, making hybrid circuit design more efficient. 59. What is the product structure of Mentor? Mentor offers WG and Enterprise series for PCB design. 60. How does Mentor's PCB design software support BGA, PGA, COB and other packaging? Mentor’s AutoActive RE supports arbitrary angle routing and high-speed signal routing, including differential pairs. 61、Mentor 的PCB 设计软件对差分线队的处理又如何？ Mentor 软件在定义好差分对属性后，两根差分对可以一起走线，严格保证差分对线宽，间距和长度差，遇到障碍可以自动分开，在换层时可以选择过孔方式。 62、在一块12 层PCb 板上，有三个电源层2.2v，3.3v,5v，将三个电源各作在一层，地线该如何处理？ 一般说来，三个电源分别做在三层，对信号质量比较好。因为不大可能出现信号跨平面层分割现象。跨分割是影响信号质量很关键的一个因素，而仿真软件一般都忽略了它。对于电源层和地层，对高频信号来说都是等效的。在实际中，除了考虑信号质量外，电源平面耦合( 利用相邻地平面降低电源平面交流阻抗)，层迭对称，都是需要考虑的因素。 63、PCB 在出厂时如何检查是否达到了设计工艺要求？ 很多PCB 厂家在PCB 加工完成出厂前，都要经过加电的网络通断测试，以确保所有联线正确。同时，越来越多的厂家也采用x 光测试，检查蚀刻或层压时的一些故障。对于贴片加工后的成品板，一般采用ICT测试检查，这需要在PCB 设计时添加ICT 测试点。如果出现问题，也可以通过一种特殊的X 光检查设备排除是否加工原因造成故障。 64、“机构的防护”是不是机壳的防护？ 是的。机壳要尽量严密，少用或不用导电材料，尽可能接地。 65、在芯片选择的时候是否也需要考虑芯片本身的esd 问题？ 不论是双层板还是多层板，都应尽量增大地的面积。在选择芯片时要考虑芯片本身的ESD 特性，这些在芯片说明中一般都有提到，而且即使不同厂家的同一种芯片性能也会有所不同。设计时多加注意，考虑的全面一点，做出电路板的性能也会得到一定的保证。但ESD 的问题仍然可能出现，因此机构的防护对ESD 的防护也是相当重要的。 66、在做pcb 板的时候，为了减小干扰，地线是否应该构成闭和形式？ 在做PCB 板的时候，一般来讲都要减小回路面积，以便减少干扰，布地线的时候，也不应布成闭合形式，而是布成树枝状较好，还有就是要尽可能增大地的面积。 67、如果仿真器用一个电源，pcb 板用一个电源，这两个电源的地是否应该连在一起？ 如果可以采用分离电源当然较好，因为如此电源间不易产生干扰，但大部分设备是有具体要求的。既然仿真器和PCB 板用的是两个电源，按我的想法是不该将其共地的。 68、一个电路由几块pcb 板构成，他们是否应该共地？ 一个电路由几块PCB 构成，多半是要求共地的，因为在一个电路中用几个电源毕竟是不太实际的。但如果你有具体的条件，可以用不同电源当然干扰会小些。 69、设计一个手持产品，带LCD，外壳为金属。测试ESD 时，无法通过ICE-1000-4-2 的测试，CONTACT 只能通过1100V，AIR 可以通过6000V。ESD 耦合测试时，水平只能可以通过3000V，垂直可以通过4000V 测试。CPU 主频为33MHZ。有什么方法可以通过ESD 测试？ 手持产品又是金属外壳，ESD 的问题一定比较明显，LCD 也恐怕会出现较多的不良现象。如果没办法改变现有的金属材质，则建议在机构内部加上防电材料，加强PCB 的地，同时想办法让LCD 接地。当然，如何操作要看具体情况。 70、设计一个含有DSP，PLD 的系统，该从那些方面考虑ESD？ 就一般的系统来讲，主要应考虑人体直接接触的部分，在电路上以及机构上进行适当的保护。至于ESD 会对系统造成多大的影响，那还要依不同情况而定。干燥的环境下，ESD 现象会比较严重，较敏感精细的系统，ESD 的影响也会相对明显。虽然大的系统有时ESD 影响并不明显，但设计时还是要多加注意，尽量防患于未然。

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