High-speed connector design — Part 3

Impedance is one of the most important aspects of a high-speed connector design that needs to be accounted for.

In Part 2 of this series of articles, we discussed the importance of impedance as well as how various design changes impact impedance. This final instalment will discuss how impedance is measured and how impedance practically impacts the design of high-speed connectors.

Measuring impedance through a connector

How is impedance measured?

Impedance is measured using a method called TDR, or time-domain reflectometry. A time-domain reflectometer measures the characteristic impedance through a cable/connector assembly and is able to detect the locations and magnitudes of all impedance discontinuities in the path. An example of a TDR plot is shown in Figure 1.

Figure 1.

What is time-domain reflectometry?

Time-domain is simply a way to describe analysis that is done with respect to time. One can tell that a TDR measurement is in the time-domain because the parameter on the x-axis of the plot is time.

Reflectometry refers to the method by which the impedance is measured: an incident signal is transmitted through a path and any reflected signal that returns to the source is measured. If the impedance of the signal path is matched to the system impedance along the entire path, then no reflections will occur. However, if the impedance of any part of the path deviates from the system impedance, a portion of the signal will reflect back to the source. The time it takes for the reflection to reach the source will determine the location of the discontinuity, and the magnitude of the reflection will determine the impedance of the path at that location.

Translating time to distance

In order to establish the physical location of an impedance discontinuity that is displayed on a TDR plot, the time units on the x-axis must be translated into distance. Distance is translated in two steps:

1. Determine the time (t) by dividing the time on the TDR plot when the discontinuity occurs by two. The time must be halved because the time on a TDR plot is round-trip time.
2. Use the equation in Figure 2 to determine distance. This provides the location of the discontinuity in inches, measured from the input of the measurement.

Figure 2.

A connector TDR example

Now’s let’s consider an example. In Figure 3, an image of a connector was overlaid on the TDR plot shown in Figure 1, and six numbered regions were added. The plot, in conjunction with the image of the connector, can be used to determine the impedance through different parts of the connector. We learn the following about the impedance of each region:

1. The cable has a 100Ω impedance.
2. The cable-connector transition has a high impedance.
3. The first part of the connector has a low impedance.
4. The second part of the connector has a lower impedance.
5. The impedance increases as the signal exits the connector.
6. The cable on the other end of the connector has a 100 Ω impedance.

Figure 3.

TDR is thus an extremely helpful tool that can be used to design, optimise and troubleshoot any high-speed connector design.

Optimising the impedance in connectors

There are three goals of connector design that require careful attention in order to ensure that the impedance is optimised:

1. Use a controlled impedance cable.
2. Minimise the wire-pin transition.
3. Optimise the pin-to-pin spacing.

Use a controlled-impedance cable

A controlled-impedance cable is a requirement for any high-speed application. Wire manufacturers achieve controlled-impedance pairs by closely managing the wire-to-wire spacing and by adding an individual shield for each controlled-impedance twisted pair. The shield reduces the impedance by increasing capacitance and helps maintain a tighter impedance tolerance by providing a constant spacing between the signal and the ground. Although applications with other target impedance requirements exist, the target impedance for the vast majority of high-speed applications is 100 Ω.

Once a controlled-impedance cable is used, a typical TDR plot will be similar to what is shown in Figure 4. The measurement of the cable (#1) shows that the impedance of the cable is 100 Ω. The plot also shows the other two primary impedance mismatches that need to be addressed — the high impedance caused by the wire-pin transition (#2) and the low impedance caused by the pin-to-pin spacing in the connector (#3).

Figure 4.

Minimise wire-pin transition mismatch

The wire-pin transition creates an impedance discontinuity that occurs in nearly every connector. The discontinuity can be minimised, but it can rarely be removed. An example of a wire-pin transition is shown in Figure 5. The mismatch occurs for three reasons:

1. The transition requires that a portion of the shield on the twisted pair be removed.
2. The wire spacing changes in order to connect to the pins.
3. The diameter of the wires is often smaller than the diameter of the connector pins.

Figure 5.

Since any change in cross-sectional geometry will likely change the impedance, all three of these factors will impact the impedance. Although it is often impossible to completely remove this discontinuity, reducing the length of the discontinuity will minimise its impact. By using advanced manufacturing techniques, the length of the unshielded region of the wires can typically be reduced to between 0.1″ and 0.4″ (depending on the connector). Once implemented, the performance improves significantly.

Modify pin-to-pin spacing

For many applications, using a controlled-impedance cable and minimising the wire-pin transition mismatch will be sufficient. But for sensitive applications and higher gigabit data rates, it may be necessary to further improve the performance. For these cases, the next step is to optimise the pin-to-pin spacing. The challenge with this design change is that it will likely require custom parts, adding cost to the design. For this reason, optimising the spacing is only recommended for applications that require the improved performance.

Figure 4 revealed that the differential impedance through the connector is low, which is common for most connectors. Because of this, the spacing needs to increase in order to achieve an impedance match. Extensive simulation and measurements have determined that the optimal spacing typically increases the standard pitch by approximately 50%. For example, the ideal spacing for a nano connector, typically spaced at 25 mm, is approximately 37.5 mm.

Designing a high-speed connector

By addressing the issues discussed above, a connector will be well on its way to successfully passing the high-speed signals that are required in many of today’s applications.