Ultra-utility 70 Q&A high frequency PCB circuit design
1. How to choose PCB board?
Choosing a suitable PCB involves balancing design requirements, mass production feasibility, and cost. Design requirements include both electrical and mechanical aspects. This is especially important when designing very high-speed PCBs (above GHz). For example, FR-4 material has significant dielectric loss at several GHz frequencies, which can affect signal attenuation. It's crucial to consider the dielectric constant and dielectric loss at the designed frequency for proper performance.
2. How to avoid high-frequency interference?
The main idea is to minimize electromagnetic interference from high-frequency signals, known as crosstalk. You can increase the distance between high-speed and analog signals or add ground guard/shunt traces near the analog lines. Also, be cautious about digital ground noise affecting the analog ground.
3. How to solve signal integrity issues in high-speed designs?
Signal integrity mainly depends on impedance matching. Factors like source architecture, output impedance, trace characteristic impedance, load characteristics, and trace topology affect this. Solutions include termination and adjusting the trace topology.
4. How is the differential wiring method implemented?
When routing differential pairs, ensure the two lines are as long as possible and maintain consistent spacing (determined by differential impedance) to remain parallel. There are two common methods: side-by-side or top-by-side. Side-by-side is more commonly 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 applied to a single-output clock signal.
6. Can I add a matching resistor between the differential pairs at the receiving end?
Yes, a matching resistor is usually added between the differential pairs at the receiving end. Its value should match the differential impedance, improving signal quality.
7. Why is the wiring of the differential pair close and parallel?
Proper proximity and parallelism ensure consistent differential impedance, which is critical for differential pair design. If the lines are too close, the differential impedance may vary, affecting signal integrity and timing.
8. How to deal with theoretical conflicts during actual wiring?
It is generally correct to isolate analog and digital partitions. Avoid having signal traces cross the moat, and keep the return current path small. The crystal oscillator is an analog circuit, so it should be placed close to the chip to reduce interference.
9. How to resolve conflicts between manual and automatic routing for high-speed signals?
Most autorouters have constraints to control routing methods and via count. However, EDA tools sometimes lack sufficient constraints. Choosing a router with a strong winding engine helps achieve better results.
10. About test coupons.
Test coupons measure the PCB’s characteristic impedance using TDR. The line width and spacing must match the controlled lines. Grounding points are crucial to reduce inductance.
11. In high-speed PCB design, can copper be coated on the blank areas of the signal layer, and how should copper be distributed across multiple signal layers?
Generally, copper is grounded in the blank areas. Be careful not to place copper too close to high-speed signals, as it may reduce the characteristic impedance.
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 the power and ground planes should be considered reference planes. For example, in a four-layer board, the top layer uses a microstrip model with the power plane as a reference.
13. Can software automatically generate test points on high-density boards to meet mass production testing needs?
Whether the software-generated test points meet the requirements depends on the test equipment specifications. If the wiring is too dense, it may be difficult to add test points automatically.
14. Does adding test points affect the quality of high-speed signals?
Adding test points may affect signal quality, depending on how they are added and the signal speed. Test points without vias or DIP pins can act as capacitors or branches, potentially impacting signal integrity.
15. How many PCBs form the system, and how should the ground wires between the boards be connected?
When connecting power or signals between PCBs, ensure that the number of ground pins is sufficient to reduce impedance and noise. Analyze the current loop to manage the flow of current effectively.
16. Can you introduce some foreign technical books and data on high-speed PCB design?
High-speed PCB design includes applications in communication networks and calculators. These require advanced design techniques such as blind/buried vias and build-up processes. Manufacturers offer these capabilities.
17. Two characteristic impedance equations often referred to:
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 the ground wire be added in the middle of the differential signal line?
No, adding a ground wire in the middle of a differential pair disrupts the coupling effect, reducing noise immunity and other benefits.
19. Does rigid-flex board design require special design software and specifications? Where can I find such manufacturers in China?
Flexible PCBs can be designed with standard PCB software. Manufacturers provide specific guidelines for minimum line width, spacing, and aperture. Online searches can help find FPC manufacturers.
20. What are the principles for selecting the grounding point between the PCB and the case?
Use chassis ground to provide a low-impedance path for return current. Connect the PCB ground plane to the chassis near high-frequency devices to minimize loop area and reduce radiation.
21. What aspects should be considered when debugging a circuit board?
In digital circuits, verify power supplies, clock signals, and reset signals first. Then follow the system operation principle and bus protocol for further debugging.
22. What techniques are recommended for high-speed, high-density PCB design (>100MHz)?
Crosstalk is a major concern. Control trace spacing, use appropriate termination, avoid overlapping adjacent layers, and use differential termination to improve signal integrity.
23. Why is LC filtering sometimes worse than RC filtering?
LC filtering effectiveness depends on the selected frequency and inductance value. If the inductance is too small, RC filtering may be more effective, though it consumes more power.
24. How to choose inductor values when filtering?
Consider the noise frequency and the capacitor value based on ripple noise tolerance. The inductor value affects the current response, and the capacitor’s ESR/ESL also plays a role.
25. How to achieve EMC requirements without excessive cost?
Use slower slew rate devices, avoid placing high-frequency components near external connectors, and ensure proper impedance matching. Add decoupling capacitors and segment the ground plane near connectors.
26. Why separate digital and analog grounds on a PCB?
Digital circuits generate noise that can interfere with analog signals. Separating grounds reduces this interference, but careful layout is required to avoid signal integrity issues.
27. Can digital and analog signals be routed without separating the ground?
Avoid crossing digital and analog signals to prevent noise from affecting analog circuits. Ensure the ground plane remains intact for stable return paths.
28. How to consider impedance matching in high-speed PCB design?
Impedance matching is essential. Trace width, layer configuration, and material properties all affect characteristic impedance. Avoid discontinuities to ensure signal integrity.
29. Where can I get more accurate IBIS models?
IBIS models are provided by chip manufacturers. They are based on SPICE models and reflect the actual device behavior. Accurate models are crucial for reliable simulation.
30. Should designers consider EMC and EMI rules in high-speed PCB design?
EMC/EMI design should address both radiated and conducted emissions. Place high-frequency components away from connectors, use inner layers for high-speed signals, and ensure proper grounding.
31. How to choose EDA tools?
For thermal analysis, avoid using it unless necessary. PADS or Cadence are good choices for most applications. Beginners can use PLD manufacturer tools for simpler designs.
32. What EDA software is suitable for high-speed signal processing?
PADS is good for conventional designs, while Cadence and Mentor offer advanced solutions for high-speed and mixed-signal circuits.
33. What do the layers of a PCB mean?
Topoverlay is the top silkscreen, showing component labels. Bottomoverlay is similar for the bottom layer. Multi-layer pads appear on all layers if defined correctly.
34. What should be considered in high-frequency PCB design above 2G?
RF circuits require 3D field analysis tools. Use parametric devices and specialized copper shapes for accurate design.
35. What rules apply to microstrip design in high-frequency PCBs?
Use 3D field analysis tools to extract transmission line parameters. All rules should be specified in the field extraction tool.
36. How to protect a clock signal on a digital PCB?
Use a clock driver chip to ensure sufficient driving capability. Consider signal edge rates and clock delay in the timing calculation.
37. What interface is used for a separate clock signal board?
Use differential signaling for long-distance clock transmission. LVDS can meet drive requirements if the clock is not too fast.
38. How to reduce harmonic interference on high-frequency clock lines?
Modify the signal duty cycle to eliminate even harmonics. Use source-side series matching to suppress reflections.
39. What is the trace topology?
Trace topology refers to the routing order for multi-port connections. It affects signal integrity and timing.
40. How to adjust trace topology for better signal integrity?
Different signal types (unidirectional, bidirectional, etc.) require different topologies. Pre-simulation helps determine the best approach.
41. How to reduce EMI by arranging the laminate?
Provide the shortest return path for signals and reduce coupling areas. A tightly coupled ground plane helps suppress common-mode interference.
42. Why lay copper on a PCB?
Copper improves EMC, ensures plating quality, provides a return path for high-frequency signals, and helps with heat dissipation.
43. What should be considered when wiring a system with DSP and PLD?
Check the signal rate and wire length. High-speed signals may require attention to signal integrity and timing.
44. Are there other good wiring tools besides Protel?
Mentor, Cadence, and Zuken offer advanced tools with various strengths. Choose based on your specific needs.
45. What is the signal return path?
The return path is the path through which the signal returns to the source. It is crucial for signal integrity and electromagnetic compatibility.
46. How to perform SI analysis on a connector?
Use IBIS or SPICE models for connectors. Multi-board simulation tools like Hyperlynx can help analyze distribution parameters.
47. What are the methods of termination?
Termination includes series and parallel resistors, as well as AC and Schottky diode matching. Choose based on the signal type and system requirements.
48. What factors determine termination (matching)?
Matching depends on buffer characteristics, signal level, and system power consumption. Consider the signal duty cycle and timing requirements.
49. What are the rules for using termination?
Ensure signal integrity and timing by choosing the right termination method. Refer to industry guides and textbooks for detailed strategies.
50. Can IBIS models simulate logic functions?
No, IBIS models are behavioral and cannot be used for functional simulation. SPICE models are needed for that purpose.
51. Are the two methods of handling analog and digital ground the same?
Both aim to prevent interference. The choice depends on whether the signal return path is affected and how much interference occurs.
52. What do FCC and EMC stand for?
FCC is the Federal Communications Commission, and EMC is Electromagnetic Compatibility. Standards define testing procedures and requirements.
53. What is differential wiring?
Differential signals use two opposite polarity signals to transmit data. Maintain parallelism and consistent line width and spacing.
54. What are PCB simulation software?
Tools like ICX, Hyperlynx, and HSPICE are used for signal integrity analysis. They help improve signal quality and reduce design complexity.
55. How does PCB simulation software perform layout simulation?
High-speed designs often use multi-layer boards with dedicated power and ground layers to improve signal quality.
56. How to ensure signal stability above 50MHz?
Reduce transmission line effects by keeping high-speed traces short. Different signal types require different approaches for signal integrity.
57. What are the requirements for materials in hybrid PCBs with RF, IF, and low-frequency circuits?
Use high-Q substrates for RF circuits. Separate RF and digital areas, and use shielding to reduce interference.
58. What solution does Mentor offer for hybrid PCBs?
Mentor provides dedicated RF design modules and tools for simulation and analysis. These help accelerate hybrid circuit design.
59. What is the product structure of Mentor?
Mentor offers WG and Enterprise series tools for PCB design. These cater to different design needs and complexities.
60. How does Mentor’s PCB design software support BGA, PGA, COB packaging?
Autoactive RE supports BGA and COB devices with features like push-through via and REROUTE. It handles high-speed routing and differential pairs effectively.
61. How does Mentor handle differential pairs?
Define differential pair attributes, and the software routes them together, ensuring consistent width, spacing, and length. It automatically separates when encountering obstacles.
62. How to handle ground planes with three power layers on a 12-layer PCB?
Keep each power layer separate for better signal integrity. Consider power plane coupling and layer symmetry for optimal performance.
63. How is the PCB checked for manufacturing compliance?
Manufacturers use network tests, X-ray inspection, and ICT testing to ensure quality. Add test points during design for easier verification.
64. Is "mechanical protection" the same as "case protection"?
Yes. The case should be as sealed as possible, using non-conductive materials and grounding where possible.
65. Should ESD be considered when selecting chips?
Yes, ensure the PCB has a large ground area. Check chip ESD specifications and consider mechanical protection for additional safety.
66. Should ground lines be closed loops to reduce interference?
No, open ground lines in a tree-like structure are better. Increase the ground area to reduce interference.
67. Should the power supplies of the simulator and PCB be connected?
If possible, use isolated power supplies to avoid interference. Otherwise, avoid connecting their grounds.
68. Should multiple PCBs in a system share a common ground?
Generally, yes, as using multiple power supplies is impractical. However, using different power supplies can reduce interference.
69. How to pass ESD testing for a metal-cased handheld device with LCD?
Add anti-static materials inside the case, reinforce the PCB ground, and ensure the LCD is grounded. Adjust design based on specific conditions.
70. What considerations are needed for ESD in a system with DSP and PLD?
Focus on protecting parts that users touch. Use proper circuit and mechanical protection. ESD impact varies with environment and system sensitivity.
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