Bench Talk

PAM4 Signaling and the Role of Connectors in Next-Gen Capabilities

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Innovation takes two forms. Some developments are truly ahead of their time, released in anticipation of a demand. The first tablet computers are a prime example, meeting the need for small and portable computing power even before the widespread adoption of the touchscreen that has made them so prevalent today. In contrast, other developments follow an urgent need. In the world of connectors, USB Type-C was created as a solution to the complex landscape that computer users faced, with its wide variation in charging cables and data connectors.

The interconnect industry is responding to similar challenges today, with the relentless demand for high-speed communications. These include the widespread adoption of 5G communications, the Internet of Things (IoT), and, more recently, the enormous growth in artificial intelligence (AI) workloads. This surge places great demands on interconnect infrastructure, pushing networks and hardware to their limits.

This blog examines how a shift in signaling enables higher data rates for the latest AI and high-performance computing applications. It also explores the role high-speed connectors play in preserving signal integrity at increased data rates.

Two Becomes Four (or More)

To meet the demand for high-speed devices, hardware developers are constantly expanding the capabilities of the devices themselves, improving their speed and allowing them to send a greater number of signals per second. This is an effective solution, but there is another solution that does not require an increase in transmission rate. 

Modern communications systems transmit information as the difference between two voltages. The receiver recognizes the difference in voltage and interprets it as a series of ones and zeros, each transition representing one bit of data. This use of two voltage levels is known as non-return-to-zero (NRZ) signaling. Therefore, the key task of a receiver is to recognize the difference between voltages.

However, if the receiver can distinguish between more than two voltage levels, more information can be sent per cycle. With four levels instead of two, data capacity can be doubled without increasing the frequency of the signal. This technology is called Pulse Amplitude Modulation 4-level (PAM4), which has become common in high-speed communications. PAM4 signaling allows the transmission of two bits per cycle, which enables developers to double a system's throughput.

Signal and Noise

PAM4 is an ideal tool for achieving increased data rates, but it comes with challenges. The easiest way to illustrate the limitations of PAM4 signaling is to compare the eye diagrams for each technology. Figures 1 and 2 show the NRZ and PAM4 signals in terms of the voltages that reach the receiver. In Figure 1, the two horizontal lines represent the voltage levels. The curved sections show the voltage changing between the two levels, and the empty space in the center (i.e., the eye) shows a clear definition between the two levels.

Figure 1: In NRZ signaling, voltage switches between two voltage levels, creating an empty space, or eye, in the center. (Source: Author/Mouser Electronics)

The PAM4 eye diagram (Figure 2) is similar, but it shows four voltages instead of two. As in the NRZ diagram, the gaps in the diagram show the clarity of the data, often known as signal integrity (SI). However, the smaller eyes of PAM4 make them more sensitive to electromagnetic interference (EMI), the radiation that is emitted by almost all electronic devices.

Figure 2: PAM4 signaling creates smaller, stacked eyes between the four voltage levels. (Source: Author/Mouser Electronics)

PAM4 modulation is most commonly used for high symbol rates, where other effects also impact SI. Crosstalk, reflections, and insertion loss become more pronounced at higher frequencies. Any discontinuity in impedance along the path can cause reflections that significantly degrade the signal-to-noise ratio. At high data rates, even small losses can further compromise SI.

PAM4 signals are also highly susceptible to jitter. In an ideal PAM4 or NRZ application, the transitions between voltage levels occur at perfectly regular intervals. Interference and imperfections in the transmission path cause those transitions to shift slightly forward or backward in time, increasing the bit error rate (BER)—the measure of accuracy in the data received. At today's increased data speeds, even a low BER can result in millions of errors per second.

Choosing the Right Connector

The signal chain, represented by the complete system of cables and connectors between transmitter and receiver, therefore plays a critical role in preserving SI, and different technologies are available depending upon the application. For long-distance transmission between racks or across data hauls, optical fiber remains the ideal medium. Fiber's low attenuation and immunity to EMI make it the right choice for high data rates over long distances. The additional complexity and cost of signal conversion between electrical and optical technologies are justified when protecting SI over these longer distances.

Some applications require data to be transmitted over shorter distances, including top-of-rack to middle-of-row installations or in high-performance computing. The distances involved are shorter but still too long for passive copper cables to preserve SI. These cables use powered electronics inside the cable assembly to maintain the signal quality over distances and data rates that passive copper alone cannot handle reliably. As a result, the cables require external power to operate, which adds cost and complexity. This extra power also adds heat, a constant concern for modern data center architects.

Connections over short distances (e.g., between boards or within a chassis) can be transmitted over passive copper channels. Without the need for complex optical systems or external power, these board-to-board solutions can be more compact and energy efficient. However, not all connectors are suitable for this role at very high speeds.

Molex Mirror Mezz connectors are board-to-board connectors engineered for the mezzanine applications common in today’s data center and high-performance computing systems. Mirror Mezz connectors are small enough to leave room for other key components, deliver a high differential pair count, and maintain extremely low crosstalk levels. With an exceptionally high density of 107 to 115 differential pairs (DP)/in² and 56Gbps PAM4 performance, Mirror Mezz was designed to handle the data demands of even the most demanding applications.

Mirror Mezz uses a hermaphroditic interface, a self-mating feature that reduces the number of parts required and simplifies the bill of materials. The connectors are compatible with flex cables to increase board-to-board distances without sacrificing performance.

Molex Mirror Mezz Pro supports data rates up to 112 Gbps PAM4, making it ideal for mainstream AI and cloud applications. With an ultra-high density of up to 115 differential pairs (DP)/in², it delivers strong performance in a compact footprint. Its low insertion loss and crosstalk characteristics help maintain signal integrity in high-speed systems operating at 56Gbps NRZ or 112Gbps PAM4.

Conclusion

Designing high-speed signaling means simultaneously balancing competing demands like faster data rates, lower signal loss, less interference, and minimal power use. PAM4 helps to deliver the data rates required by modern workloads. For short-reach, high-density environments, connectors like the Molex Mirror Mezz family provide superior performance and mechanical reliability to help designers meet today’s challenges while preparing for the next generation of signaling technologies.

Author

David Pike is well known across the interconnect industry for his passion and general geekiness. His online name is Connector Geek.