High-speed connector design — Part 1

High-speed digital connectors have the same requirements as any other rugged connector — for example, they must meet specifications for shock, force, insertions and vibration. There are, however, additional requirements that must be addressed in order to ensure proper performance for high-speed applications.

With gigabit data rates through connectors now commonplace, the parameters that impact high-speed digital performance must be understood by both connector manufacturers and connector users. This is the first in a series of three articles that aim to help readers better understand the critical concepts and parameters that must be considered for high-speed connector design.

Introduction

Breakdown of the old order

For low-speed signals, the connector and cable can be adequately modelled as a small resistor. This resistor will accurately represent the loss that is created due to the length and diameter of the path. As speeds approach the high-speed regime (generally 100 Mbps+), a small resistor will no longer accurately model the electrical performance. Being able to understand and accurately predict the performance will require a paradigm shift in how electrical signals are viewed.

Paradigm shift

Electrical signals are actually electromagnetic waves that traverse down a signal path. At low speed, the electromagnetic waves can be simplified by using circuit theory — the wave can be modelled as a voltage across/current through the path, with an instantaneous transfer rate. This is modelled with the simple resistor discussed above. This model, however, breaks down at high speed, and understanding this requires a new way of thinking about electrical signals.

Fluid flow analogy

High-speed signals must be viewed as waves. A simplified understanding of this signal-as-a-wave concept can be obtained by using a fluid flow analogy. 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.

What is impedance?

In its most basic definition, impedance is the ratio of the voltage to the current of a signal path. Like the diameter of the pipe in the fluid flow analogy above, the impedance of a signal path is defined by the cross-sectional geometry at any point along the path. This is an important point that bears repeating — impedance is specific to each point along a signal path. An ideal signal path maintains a constant impedance — like a constant diameter of a pipe — throughout the path. The optimal impedance is defined by each specific application, but the most common impedance is 100 Ω.

Analog and digital specifications

We live in a digital world. From phones to tablets to automobiles, digital electronics are everywhere. Our world is so strongly shaped by digital electronics that analog electronics are often considered a thing of the past. This perception, however, is untrue.

Every piece of digital information was at some point converted from an analog signal. The relationship between digital and analog signals must be understood in order to interpret many of the latest high-speed connector specifications. Since there is no standard method for specifying connector high-speed performance, some connectors are specified as analog frequencies (MHz/GHz), while other connectors are specified as digital data rates (Mbps/Gbps). This often leads to much confusion among those who are trying to procure the proper connector for their application.

Determining frequency specifications

Maximum frequency specifications are determined from insertion loss measurements. Insertion loss measures the amount of a signal that transmits through a path across all critical frequencies, typically expressed in decibels (dB) (see example in Figure 2). Specifications are determined by the maximum frequency that can pass a signal with a predetermined amount of loss, typically between -3 and -8 dB. For example, using -3 dB as the threshold, the measurement shown in Figure 2 yields a maximum frequency of 2.4 GHz.

Figure 2.

Deriving data rate specifications

Maximum data rate specifications are derived from insertion loss measurements. Data rate specifications cannot be explicitly measured, so deriving data rate specifications requires approximations. For most applications, specifications are approximated by multiplying the maximum frequency by a factor of two. The doubled frequency is based on the fact that there are two digital bits in one analog period, and assumes the resulting signal will look like a sine wave (Figure 3, red).

Figure 3.

Performance varies based on application

Specifications are best used as first order approximations. Creating a specification requires the manufacturer to make assumptions about the end application. There are four main application-specific variables that must be understood:

1. Path topology. Specifications must assume a particular path. Some specifications represent a cable/connector assembly, while others depict a mated connector pair without a cable.
2. Maximum allowable loss. Specifications must assume a specific amount of loss through the assembly, but this loss value likely differs from the loss required for a specification application. With allowable loss varying from less than 1 dB of loss all the way up to 20 dB, the actual loss requirement will have a significant impact on the maximum data rate.
3. Actual cable length. Specifications must assume one specific cable length, but the length used will likely differ from the actual length. Shorter cable lengths will increase the maximum data rate, while longer cable rates will decrease the rate.
4. Expected output waveform. Most specifications assume a somewhat rounded output waveform (Figure 3, red), but some applications have more sensitive circuitry that require an output that more accurately represents a square wave (Figure 3, grey). For these waveforms, the maximum data rate is often derived by multiplying the frequency by a factor smaller than one.

Since each connector manufacturer specifies performance differently, it is important that the both the specification values and the characterisation methodologies are both understood and scrutinised. Some manufacturers use conservative specification methods, while others use aggressive. A higher specification value does not necessarily mean a higher performing connector.

Top image credit: ©Albert Lozano-Nieto/Dollar Photo Club