66 common problems in high frequency PCB circuit design
With the rapid advancement of electronic technology and the widespread application of wireless communication in various fields, high frequency, high speed, and high density have become key trends in modern electronics. High-frequency signal transmission and high-speed digitization are pushing PCBs toward micro-holes, buried/blind vias, fine lines, and uniform thinning of dielectric layers. High-frequency, high-speed, and high-density multilayer PCB design has emerged as a critical research area. Based on years of experience in hardware design, this article summarizes some design techniques and precautions for high-frequency circuits to provide guidance.
1. How to choose a PCB board?
Choosing the right PCB material involves balancing design requirements, mass production feasibility, and cost. Design requirements include both electrical and mechanical aspects. At very high frequencies (above GHz), material selection becomes crucial. For example, FR-4 may not be suitable due to its significant dielectric loss at several GHz, which can affect signal attenuation. It's important to consider the dielectric constant and dielectric loss at the designed frequency.
2. How to avoid high-frequency interference?
The main idea is to minimize electromagnetic field interference from high-frequency signals, known as crosstalk. This can be achieved by increasing the distance between high-speed and analog signals or adding ground guard/shunt traces near analog signals. Also, digital ground noise should be carefully managed to avoid affecting the analog ground.
3. How to solve signal integrity issues in high-speed design?
Signal integrity is primarily about impedance matching. Factors such as source architecture, output impedance, trace characteristic impedance, load characteristics, and trace topology influence it. Solutions include termination and adjusting the trace topology.
4. How to implement differential wiring?
For differential pairs, two key points are maintaining equal length and consistent spacing between the two lines. They should be parallel. There are two common methods: side-by-side or top-by-side. Side-by-side is more frequently used.
5. How to implement differential wiring for a clock signal with only one output?
Differential routing requires both the source and receiver to support differential signals. Therefore, it cannot be used for a single-output clock signal.
6. Can a matching resistor be added between differential pairs at the receiving end?
Yes, a matching resistor is typically added at the receiving end, with a value equal to the differential impedance, improving signal quality.
7. Why should differential pairs be close and parallel?
Proper proximity ensures the correct differential impedance, while parallelism maintains consistency. If the lines are too close, the differential impedance may vary, affecting signal integrity and timing delay.
8. How to deal with theoretical conflicts during actual wiring?
It’s generally best to isolate analog and digital sections. Avoid having signal traces cross a "moat" and ensure that the return current path for power and signals remains small.
9. How to resolve conflicts between manual and automatic wiring for high-speed signals?
Most autorouters have constraints for winding methods and via count. The capabilities of EDA tools vary, so choosing a router with a strong winding engine is essential.
10. About test coupons.
Test coupons are used to measure PCB characteristic impedance using TDR. The trace width and spacing on the coupon should match those of the controlled lines. The grounding point location is crucial for accurate measurements.
11. Can copper be applied in the blank areas of a signal layer, and how should copper be distributed on multiple signal layers?
Copper is usually grounded in blank areas. However, care must be taken when placing copper near high-speed signals to avoid reducing the trace's characteristic impedance.
12. Can the microstrip line model be used for a signal line above a power plane?
Yes, both the power and ground planes act as reference planes. For example, in a four-layer board, the top layer's characteristic impedance is modeled as a microstrip line with the power plane as the reference.
13. Can software automatically generate test points for high-density boards?
Whether the generated test points meet the requirements depends on their specifications and the board's density. Manual input may be necessary in dense areas.
14. Does adding test points affect high-speed signal quality?
Adding test points can introduce small capacitors or branches, which may impact signal integrity. The effect depends on the signal speed and edge rate, and simulations can help assess it.
15. How to connect ground wires between PCBs in a system?
At each interface, sufficient ground pins should be allocated to reduce impedance and noise. Analyzing the current loop and controlling the ground connection helps manage current flow.
16. Are there foreign technical books on high-speed PCB design?
High-speed PCB applications span communications and computing. With working frequencies reaching GHz and up to 40 layers, the demand for blind/buried vias and build-up processes is increasing.
17. Two commonly used characteristic impedance equations:
Microstrip Z = {87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)]
Stripline Z = [60/sqrt(Er)]ln{4H/[0.67Ï€(T+0.8W)]}
18. Can a ground wire be added in the middle of a differential pair?
No, adding a ground wire disrupts the coupling benefits of differential signals, such as noise immunity and flux cancellation.
19. Does rigid-flex board design require special software and specifications?
Flexible PCBs can be designed using standard software, but manufacturers may have specific restrictions on line widths, spacing, and apertures. Online queries for FPC manufacturers can help find suitable options.
20. What are the principles for selecting PCB and case grounding points?
Use chassis ground to provide a low-impedance path for return currents. Connect the PCB ground to the chassis near high-frequency devices to minimize loop area and reduce radiation.
21. What aspects should be considered during PCB debugging?
Start by verifying power supply voltages, clock frequencies, and reset signals. Then follow the system operation principle and bus protocol for further debugging.
22. What techniques are used in high-speed, high-density PCB design?
Crosstalk is a major concern. Control trace impedance, spacing, and use appropriate termination methods. Avoid overlapping adjacent traces and consider using blind/buried vias.
23. Why is LC filtering sometimes worse than RC filtering?
LC filtering effectiveness depends on the selected frequency band and inductance value. If the inductance is too small, RC may be more effective. However, RC consumes more power.
24. How to choose inductor and capacitor values when filtering?
Consider the noise frequency and the ability to handle sudden current changes. Larger inductors may slow down current flow, while larger capacitors reduce ripple noise.
25. How to meet EMC requirements without excessive costs?
Use slower slew rate devices, place high-frequency components away from connectors, and ensure proper impedance matching. Add decoupling capacitors and segment the ground plane near connectors.
26. Why separate digital and analog grounds?
Digital circuits generate noise that can interfere with analog signals. Separating grounds prevents this, especially when analog and digital areas are close.
27. Is it acceptable to not divide the PCB into digital and analog sections if traces don’t cross?
If digital and analog traces do not cross, it might be possible, but careful attention to return paths is still needed.
28. How to consider impedance matching in high-speed PCB design?
Impedance matching is crucial. Trace width, layer position, and materials all affect impedance. Simulations can help identify discontinuities, and series resistors can mitigate them.
29. Where to get accurate IBIS models?
IBIS models provided by chip manufacturers are the most accurate, as they reflect the actual device process. Inaccurate models should be reported for improvement.
30. What aspects should designers consider for EMC and EMI in high-speed PCBs?
Both radiated and conducted emissions must be addressed. Place clock generators away from connectors, route high-speed signals internally, and use decoupling capacitors to reduce noise.
31. How to choose EDA tools?
PADS and Cadence offer good performance-to-price ratios. Beginners can use PLD manufacturer tools for simpler designs.
32. Which EDA software is suitable for high-speed signal processing?
PADS is widely used, while Cadence and Mentor offer advanced solutions for complex designs.
33. What do the various layers of a PCB mean?
Topoverlay includes component labels, while bottomoverlay shows similar information. Multi-layer boards allow pads and vias to appear across all layers.
34. What should be considered in 2G+ high-frequency PCB design?
RF circuit design requires 3D field analysis tools. Layout and routing should be coordinated with the schematic, and parametric devices are needed for accurate simulation.
35. What rules apply to microstrip design in 2G+ high-frequency PCBs?
Use 3D field analysis tools to extract transmission line parameters. All design rules should be specified in these tools.
36. How to protect an 80MHz clock signal?
Use a clock driver chip to distribute the signal. Ensure the driver matches the load and meets edge rate requirements.
37. What interface is used for a separate clock signal board?
For long-distance transmission, use differential signaling like LVDS. This reduces interference and improves signal integrity.
38. How to reduce harmonic interference in high-speed clocks?
Adjust the signal duty cycle to eliminate even harmonics. Use source-side series matching to suppress reflections.
39. What is trace topology?
Trace topology refers to the routing order for multi-port networks. It affects signal integrity and timing.
40. How to adjust trace topology for better signal integrity?
Topology choice depends on signal type and circuit complexity. Pre-simulation helps determine the best approach.
41. How to reduce EMI through laminate design?
Provide the shortest return path for signals and reduce coupling areas. A tightly coupled ground and power layer suppresses common-mode interference.
42. Why lay copper on a PCB?
Copper provides shielding, improves plating, and ensures a complete return path for high-frequency signals. It also aids in heat dissipation.
43. What to consider when wiring a system with DSP and PLD?
Check the signal rate relative to the transmission line delay. Multiple DSPs and clocks may affect signal quality and timing.
44. Are there other PCB wiring tools besides Protel?
Yes, tools like Mentor WG2000, Cadence Allegro, and Zuken Cadstar offer advanced features and are widely used.
45. What is a signal return path?
The return path is the shortest route for the signal current back to the source, typically along the ground or power plane.
46. How to perform SI analysis on a connector?
Use IBIS 3.2 models or SPICE models for special connectors. Multi-board simulation tools like HyperLynx can also help.
47. What are the methods of termination?
Common methods include series and parallel resistors, pull-up/pull-down, and Schottky diodes.
48. What factors influence termination?
Buffer characteristics, topology, signal level, and power consumption all play a role in determining the best termination method.
49. What are the rules for using termination?
Ensure signal integrity by matching impedances and controlling signal timing. Refer to industry resources like "High Speed Digital Design."
50. Can IBIS models simulate logic functions?
No, IBIS models are behavioral and cannot be used for functional simulation. SPICE models are required for this purpose.
51. Are the two methods of separating analog and digital grounds equivalent?
In principle, yes, as power and ground are treated similarly at high frequencies. However, segmentation may affect signal return paths and EM compatibility.
52. What do FCC and EMC stand for?
FCC is a U.S. regulatory body, while EMC refers to electromagnetic compatibility standards.
53. What is differential wiring?
Differential signaling uses two opposite polarity signals to transmit data, relying on their difference. Maintaining parallelism and consistent spacing is crucial.
54. What PCB simulation software is available?
Tools like ICX, SignalVision, HyperLynx, and HSPICE are commonly used for signal integrity analysis.
55. How does PCB simulation software perform layout simulation?
Multi-layer boards with dedicated power and ground layers are typically used to improve signal quality and reduce wiring complexity.
56. How to ensure signal stability above 50MHz?
Minimize transmission line effects by keeping high-speed signals short. Different signal types require different quality assurance methods.
57. What are the requirements for PCB materials in hybrid circuits?
Hybrid designs face challenges in preventing interference. RF circuits are often isolated on separate boards with shielding and specialized materials.
58. What solutions does Mentor offer for hybrid RF and digital circuits?
Mentor’s tools include dedicated RF modules and bidirectional interfaces with EESOFT for analysis and simulation. This accelerates hybrid circuit design.
59. How to handle three power layers on a 12-layer PCB?
Separate the power layers for better signal integrity. Consider power plane coupling and layer symmetry for optimal performance.
60. How to check if a PCB meets design requirements?
Manufacturers use network testing and X-ray inspection to verify connectivity and detect faults. ICT testing is also used for final verification.
61. Should ESD be considered when selecting chips?
Yes, ESD protection is important. Chip datasheets often mention ESD characteristics, and proper layout and shielding enhance reliability.
62. Should ground be closed in a PCB to reduce interference?
Avoid closed loops; instead, use a tree-like structure. Increasing the ground area helps reduce interference.
63. Should the simulator and PCB power sources share a ground?
It’s better to keep them separate to avoid interference. However, practical constraints may require shared grounding.
64. Should multiple PCBs in a system share a ground?
Generally, yes, unless specific conditions allow for separate power supplies. Shared grounding simplifies design.
65. How to pass ESD tests for a metal-cased handheld device?
Improve internal shielding, strengthen the PCB ground, and ensure the LCD is grounded. Adjusting the design can help achieve compliance.
66. What considerations are needed for ESD in a system with DSP and PLD?
Focus on protecting contact points and use appropriate shielding. ESD effects vary depending on environmental conditions and system sensitivity.
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