High-Speed Interface Layout Guidelines - TI

High-Speed Interface Layout Guidelines (Rev. J) - Texas Instruments

Per Gemini - Google AI

There are actually two main reasons why differential signals are not always preferred for routing on the outer layers of a printed circuit board (PCB), and it has to do with the type of transmission line formed:

Microstrip vs. Stripline:

Outer layers create microstrip transmission lines, where the signal trace has a reference plane on one side (usually a power plane) and air on the other.

Differential signals rely on a consistent and symmetrical environment for optimal performance. Microstrip, due to its air exposure, introduces an asymmetry that can affect the common-mode vs. differential-mode propagation characteristics.

Controlled Impedance:

Precise control over the impedance of the transmission line is crucial for high-speed differential signals. Microstrip's impedance is more sensitive to variations in manufacturing tolerances compared to stripline.

However, there are some advantages to routing differential signals on outer layers:

Slightly Faster Signal Propagation:

Outer layers have a lower effective dielectric constant due to the presence of air. This can lead to marginally faster signal travel compared to inner layer routing.

Easier Access for Controlled-Impedance Routing Techniques:

The outer layers are more accessible for adding solder masks or prepreg layers to adjust the trace width and spacing, allowing for finer control over impedance.

In conclusion, the decision to route differential signals on outer layers depends on the specific application's priorities. For high-performance applications where signal integrity is paramount, stripline routing on inner layers with a controlled and symmetrical environment is usually preferred. However, for lower-speed applications where manufacturability and ease of access are important, outer layer routing with microstrip can be a viable option, considering the trade-offs mentioned above.

That's it !

Copper Tolerances

Tolerance | Bay Area Circuits

Microwave and RF Absorbing Materials

ARC Technologies, a Hexcel Company, is the leading supplier of microwave and RF absorbing materials for commercial and defense applications.

RF Interference Control | Hexcel

Wave-X™ Near Field Absorbers | Hexcel

RF Interference Control | Hexcel

ARC 0.030" 13446 Datasheet

ARC 0.020"13444 Datasheet

Notes:

Aluminum doesn't block magnetic waves. Magnetic waves and electromagnetic waves, though related, are different. Aluminum is effective at blocking electromagnetic waves, especially at higher frequencies like radio and light waves. This is why aluminum foil is sometimes used to block radio signals.

For magnetic waves, however, aluminum acts like air because magnetic permeability of aluminum is very close to that of air. This means magnetic waves pass through aluminum just as easily as they pass through air.

If you need to block magnetic fields, you should use materials called ferromagnetic materials like steel or iron. These materials have a much higher magnetic permeability than air or aluminum, and they can significantly weaken magnetic fields.

That's it !

SIMBEOR® Electromagnetic Signal Integrity Software

SIMBEOR® Electromagnetic Signal Integrity Software

Hook-Up Wire

Remington - https://www.remingtonindustries.com/hook-up-wire/

DigiKey - 30STRPTFEKIT Remington Industries | Kits | DigiKey Marketplace

USB 2.0 Physical Layer Testing and Debugging

This App Note describes the Bandwidth Requirements of an Oscilloscope and Probes.

Understanding and Performing USB 2.0Physical Layer Testing and Debugging

Source Tektronix

Gemini - USB Diff Pairs

Source Google Gemini

USB Differential Pair Length Matching

In USB design, differential pair length matching is crucial for maintaining signal integrity. Here's a breakdown of what it is and why it matters:

Differential Pairs in USB:

• USB uses differential signaling where data is transmitted on two balanced lines (D+ and D-).
• The difference in voltage between these lines carries the signal, making it less susceptible to noise.

Importance of Length Matching:

• Ideally, both lines in a differential pair should be exactly the same length.
• Any mismatch in length creates skew, which is the time difference between the arrival of signals on the two lines.
• Excessive skew can degrade signal integrity, leading to errors or data corruption.

Length Matching Requirements:

• The allowed skew varies depending on the USB version:
  • USB 2.0 High-Speed: Maximum skew of 100 ps (picoseconds). This translates to a length difference of roughly 0.6 inches (15 mm).
  • SuperSpeed USB 3.0 and above: Even tighter tolerances, with a maximum skew in the range of 30-60 ps. This translates to a much smaller length mismatch allowed, typically around 5 mils (0.13 mm).

General Guidelines:

• Keep the differential pair as close together as possible throughout their routing.
• Use design tools with differential pair routing features to ensure matching.
• If possible, keep the total length of the pair short, especially for USB 3.0 and above.

Additional Considerations:

• The length matching requirement applies to the entire signal path, including the PCB traces and any cables attached.
• While length matching is important, maintaining the correct differential impedance (typically around 90 Ohms for USB) is equally crucial.

By following these guidelines for differential pair length matching, you can ensure reliable signal transmission in your USB designs.

That's it !