High-speed connector design — Part 2

Impedance is a critical parameter in determining the performance of high-speed applications.

In Part 1 of this series of articles, we used the analogy of a pipe diameter to relate impedance to electrical performance. Just as optimal fluid flow is achieved with a pipe with a constant diameter, optimal electrical performance through a high-speed path is achieved with a constant impedance at every point along the path. But why is impedance so important?

The importance of impedance

Revisiting the fluid flow analogy

High-speed signals should be viewed as waves. As waves, travelling high-speed electrical signals are analogous to fluid travelling through a pipe. As a wave travels through a pipe, a portion of the wave will reflect back every time the pipe diameter changes. Thus, optimal fluid flow is achieved with a pipe that has a constant diameter (Figure 1a). If the pipe diameter is constantly changing (Figure 1b), large portions of the wave will reflect and the efficiency of the pipe will decrease.

Figure 1.

The performance of the pipe is analogous to the performance of a high-speed signal path in a cable/connector assembly, with the critical parameter in a signal path being impedance instead of diameter.

Why is impedance so important?

The impedance of the path is critical because any time the path impedance deviates from the system impedance, a portion of the signal will reflect back to the source and therefore will not reach its destination. The magnitude of the reflection, or discontinuity, will be dictated by two variables:

1. The physical length of the impedance mismatch, and
2. How far the impedance differs from the specified system impedance.

In order to understand this better, we will look at four examples. In these examples, we will assume a system impedance of 100 Ω and a cable with a connector on each end, similar to what is shown in Figure 2:

• Example 1: 100 Ω connectors with 100 Ω cable. Performance: Excellent. The impedance is matched through the entire path.
• Example 2: 70 Ω connectors with 100 Ω cable. Performance: Good. The length of the impedance mismatch (only through the length of the connector) is small enough that it doesn’t have a significant impact on the performance.
• Example 3: 100 Ω connectors with 70 Ω cable. Performance: Poor. The length of the impedance mismatch (through the entire cable) is large, which yields poor performance.
• Example 4: 40 Ω connectors with 100 Ω cable. Performance: Poor. Although the length of the impedance mismatch is small, the magnitude of the mismatch is large enough to yield poor performance.

Impedance and data rate

Of course, impedance isn’t always important. For low-speed signals (less than 100 Mbps/MHz), the impedance of the connectors and the cable is not likely to be an issue. However, as speeds increase, the impedance becomes more important. As a general rule, the impedance of the cables is important for signals above 100 Mbps/MHz, and the impedance of the connector becomes important for signals above 1 Gbps/GHz.

Determining cable and connector impedance

As data transfer rates continue to increase, the impedance of the cable and the connector become increasingly important. In order to ensure that designs adequately address this, we must understand the factors that impact impedance and what can be done to optimise our designs.

Calculating impedance

Unfortunately, impedance is very difficult to calculate. In fact, it is nearly impossible to calculate without a high-powered electromagnetic field solver. Due to this complexity, it is often helpful to simply understand the implications of specific design changes on impedance. This can help us make the necessary design changes to increase or decrease the impedance of our current design.

The impedance of any path is determined by the cross-sectional geometry at any point in the path. For any path where the cross-section changes, the impedance will have some variation. In most cable/connector assemblies, this occurs in the connector. It is relatively easy to keep the cross-section of a shielded, twisted pair cable constant. However, it is very difficult, if not impossible, to keep the cross-section constant as the path transitions from the cable to pins to a circuit board.

Equation for impedance

Impedance (Z) is proportional to inductance (L) and inversely proportional to capacitance (C) (see Figure 3). In order to understand this equation, it is necessary to have a general understanding of inductance and capacitance.

Figure 3.

Inductance is the ability to store magnetic charge, and it is determined by the size of the circuit loop. The loop size is determined by the size of the conductors (length/width) as well as the distance between the conductors. Inductance increases as the length of the loop increases and decreases as the width of the loop increases.

Capacitance is the ability to store electric charge. Capacitance increases as the size of the conductors increases and decreases as spacing between conductors increases. Capacitance is also proportional to the dielectric constant, a material constant of the insulating plastic that is typically provided on the datasheet of the insulator.

Impedance in cables and connectors

Several design parameters impact impedance. As the spacing between conductors increases, the inductance increases and the capacitance decreases. Both of these factors will cause the impedance to increase. For cables, the impedance increases as spacing between wires increases. In connectors, the impedance increases as the spacing between the pins increases.

As the diameter of the signal conductors increases, the inductance decreases and the capacitance increases. These both cause the impedance to decrease.

The dielectric constant of the insulating material also impacts impedance. However, since dielectric constant only affects capacitance, not inductance, the impact of dielectric constant on impedance is less profound than diameter and spacing. Impedance has an inverse relationship with dielectric constant: as the dielectric constant of the insulating material increases, impedance decreases.

Finally, impedance has no relationship to length. Since length increases inductance and capacitance with the same proportion, length has no impact on impedance. This is why impedance is a function of cross-sectional geometry and can be determined at any point along a path.

Conclusion

Impedance is an important parameter for all high-speed designs. It is critical that designs are optimised to provide a matched impedance throughout the entire path.

Top image caption: Figure 2.