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"Which 50 Ohm is better?"

Visit to the Customer

Recently, I received requests for help from several prominent and respected customers. One case involved a card with a SATA Rev 3.0 interface, which means a data transfer rate of 6 Gbit/s, passing from card to card, with a total length of about 20 cm. In another case, the author was invited to a meeting to examine a PCIe Gen 2 signal, which means a data transfer rate of 5 Gbit/s.


Questions asked during the visit:

Asking what seemed to be the problem, the customer's answer was "Signal Integrity problem." The second question was: What PCB trace width was used to route the High-Speed signals? In the first case, the answer was 4mil, and in the second, 4.5mil. So...what caused the Signal Integrity problem?


The Skin Effect:


Reminder:

It is a known phenomenon that when a sinusoidal wave is transmitted at a high frequency, the signal is attenuated due to the Skin Effect, which forces the signal to propagate along the surface of the medium. The higher the frequency, the more severe and complex the "pushing" of the signal to the edges, and the smaller the "penetration depth" of the signal into the medium material.

Based on research and simulations conducted in Field Solvers, it can be seen that this phenomenon begins to occur already at relatively low frequencies of 10 MHz.

The "contribution" of Controlled Impedance to the Skin Effect problem:

The signals were routed at 4 mil width due to the need to treat Signal Integrity by Controlled Impedance to achieve a characteristic impedance of 50 Ohm.


Understanding the problem and its solution:

  1. Figure 1 shows a trace routed on a PCB of FR4 material, with er=4.3 and a length of 3 inches.

  2. In Figure 2 we see a trace of the same material, with the same length. The difference between the two figures is in the Trace width.

  3. In Figure 1 it is 4.5 mil wide and in Figure 2 it is 8 mil wide.

  4. Figure 3 shows the comparison of attenuation between the two signals. It can be seen that there is a difference of 0.25 dB at 10 GHz for a 3-inch Trace. The attenuation will continue to increase as the length of the routed line on the PCB increases.

  5. The wider the conductor width, the less impact the Skin Effect will have on attenuation, so in fast interfaces, it is essntial to check carefully whether the conductor width is appropriate for the materials and the interface with which you have to work. It is highly desirable to perform simulations to reduce the risk in the design.


Figure 1 - 4.5mil width trace



Figure 2 - 8mil width trace


Figure 3  - Comparison of S12 between 4.5 and 8mil wide traces


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