Electronic system architectures need faster data rates with higher-level modulation schemes in more compact form factors. This complicates printed circuit board (pc board) layout as designers work to minimize transmission line losses and reduce susceptibility to noise, reflections, and crosstalk to maintain signal integrity and meet maximum bit error rate (BER) requirements. Also, electrical or optical multi-lane signals between ICs or board-to-board require minimized signal skew, especially in differential signal pairs.
One way to address these needs that allows the use of standard board substrates to avoid higher costs is to use high-speed cable assemblies instead of relying solely on pc board traces. These assemblies use single-ended and differential configurations, advanced materials, and techniques that provide excellent signal integrity, and support high-density, multi-lane signal paths in copper or optical fiber. Some implementations feature operating rates up to 64 gigabits per second (Gbps).
This article discusses what is driving the need for higher speed and how it is being addressed. It then introduces high-speed cable assemblies from Samtec and describes their capabilities and use.
The need for speed
The world is hungry for faster communications. Applications like 5G and 6G cellular, artificial intelligence (AI), quantum computing, and ‘Big Data’ drive new system architectures and demand higher bandwidths at faster transmission rates while shrinking device and system size. These developing technologies require interconnects that can provide the highest signal integrity and maintain high signal-to-noise ratios (SNRs) in the presence of noise, crosstalk, reflections, electromagnetic interference, and other losses and sources of interference.
Higher speeds have necessitated changes in connection technology. First, single-ended signal transmission, where data is carried by a single wire referenced to a return path (often referred to as ‘ground’), is being supplanted by differential signal connections where two wires carry data signals 180˚ out of phase. Differential signaling improves the SNR by suppressing noise common to the two conductors (common mode noise). Second, data encoding is moving from single bit per clock cycle, non-return to zero (NRZ) encoding to multiple bits per clock cycle, such as pulse amplitude modulation 4 level (PAM4), which encodes four distinct levels or two bits per clock cycle (Figure 1).
PAM4 packs two bits of data into each clock cycle using four levels encoded as 00, 01, 10, or 11. This doubles the data rate for a fixed clock rate but decreases SNR due to the smaller amplitude variations between data states. PAM4 signaling, therefore, requires a higher level of signal integrity.
Characterizing transmission line performance
Whether printed circuit runs or cables, transmission line performance is usually characterized in the frequency domain by scattering parameters (s-parameters). S-parameters describe a device’s properties based on the electrical behavior observed at the inputs and outputs without knowing the specific components inside the device. Several figures of merit (FoMs), based on measured s-parameters, are used to describe two-port devices such as cables. The most used FoMs are:
- Insertion loss: The attenuation experienced by a signal propagating from the input to the output of a cable, expressed in decibels (dB) (an ideal transmission line has an insertion loss of 0 dB)
- Return loss: The loss (in dB) due to signal reflections resulting from an impedance mismatch at the output
- Crosstalk: A measure (in dB) of unwanted signals coupled into the transmission line due to adjacent wiring
Other FoMs of interest are the transmission line’s propagation delay and time skew. Propagation delay is the time delay of a signal propagating through a transmission line. Time skew is the time difference between signals on two or more transmission lines.
Transmission line options
It is challenging to cost-effectively meet the FoM requirements of high-frequency, multi-lane configurations of modern data communications standards using traditional PC board substrate design approaches. To address this, Samtec Inc. has developed high-speed cable assemblies using its proprietary Eye Speed micro coax and twinax cables, which are notable for their low loss and excellent signal integrity. These cables, incorporated into multi-lane cable assemblies, offer superior performance due to their unique construction (Figure 2).
Eye Speed coaxial cables are available with center stranded 26 to 28 American wire gauge (AWG) conductors. This coaxial cable construction results in high flexibility, light weight, and small size, which are especially important for longer runs.
The dielectric is formed as a solid extrusion of low dielectric constant, air foamed, fluorinated ethylene propylene (FEP). Foaming creates air intrusions, resulting in high signal velocity. This cable family offers a choice of metallic serve, tape, or braided shields for improved signal integrity.
The Eye Speed twinax cable construction uses 28 to 36 AWG silver-plated copper conductors. Larger wire sizes provide lower insertion losses, while smaller wires offer greater flexibility. Co-extruding the dielectric improves signal integrity and bandwidth, enabling 28 to 112 Gbps rates. The compact design results in tight coupling between the signal conductors and smaller spacing for a smaller pitch within the cable assembly. The insertion loss for 0.25 meters (m) of Eye Speed twinax for data clocked at 14 gigahertz (GHz) (56 Gbps PAM4) is in the range of -1 to -2.2 dB, depending on the wire diameter. The timing skew between conductors in the twinax cable is less than 3.5 picoseconds (ps) per meter. Both types of cable support Samtec’s Flyover technology.
What is Flyover technology?
Samtec’s Flyover technology uses the high bandwidth and low loss of Eye Speed cable assemblies to replace onboard bus structures, significantly reducing losses (Figure 3).
By requiring fewer board layers, Flyover technology simplifies board layouts for data rates above 28 Gbps. It also allows the use of less expensive pc board materials.
Samtec cable assemblies
There is a wide range of Eye Speed micro coax and twinax cable assembly options. They are available as high-density arrays and offer features like integral ground planes, hermaphroditic connectors, strain relief, and various connecting and latching options.
For example, the ARC6-16-06.0-LU-LD-2-1 is a slim, plug-to-plug, direct-attach cable assembly with 16 signal pairs that is 6 inches (in.) (152.4 millimeters (mm)) long and supports 64 Gbps PAM4 signaling (Figure 4).
This assembly comprises 16 ultra-low-skew twinax cables in a high-density, two-row design broken out into 32 contacts with a pitch of 0.025 in. (0.635 mm). The contacts are directly soldered to the twinax conductors for optimal signal integrity. The cables are 100 ohm (Ω) differential using 34 AWG wire and are available in 8 and 24-pair configurations. They have an operating temperature range of -40°C to +125°C.
The ERCD-020-12-00-TEU-TED-1-B is a card-edge-to-card-edge cable assembly comprising two rows of twenty, single-ended, 50 Ω coaxial cables with a 40 contact connector (Figure 5). The cable length is 12 in. (305 mm).
The coaxial lines use 34 AWG center conductors arranged as a ribbon cable. The connector pitch is 0.0315 in. (0.80 mm). These cables are rated to handle 14 Gbps signals. The connectors use a squeeze latch locking mechanism to ensure positive mating. Optionally, the assembly is available with 10 to 60 cables per row with a variety of latching mechanisms. All operate over a temperature range of -25°C to +105°C.
The HLCD-20-40-00-TR-TR-2 cable assembly uses two rows of ten, 50 Ω, single-ended cables with a length of 40 in. (1.02 m). It provides forty contacts with a contact pitch of 0.0197 in. (0.5 mm) (Figure 6).
Hermaphroditic connectors have pins and sockets that can be mated together with the same connector. They are used in applications where contact polarization is not required, such as bidirectional data pairs.
The HLCD-20-40.00-TR-TR-2 offers a choice of standard or extended operating temperature ranges of -25°C to +105°C or -40°C to +125°C, respectively.
The HQDP-020-12-00-TTL-TEU-5-B cable assembly uses dual rows of 100 Ω, 30 AWG twinax cables. It measures 12 in. (305 mm) long, has 20 cables, uses a plug-to-card-edge connector, and is rated for operation at 14 Gbps (Figure 7).
This family offers options of 20, 40, or 60 cables and a variety of surface and edge-mount connectors, and have a 0.020 in. (0.5 mm) connector pitch.
Conclusion
Higher data rates continue pushing designers to seek innovative ways to ensure signal integrity. Working with Samtec allows them to rise above the constraints of classical multi-lane pc board signaling buses, and take advantage of a wide range of high-performance, flexible, and cost-effective cable assemblies that meet or exceed the specifications of today’s communication applications.