An introduction to mmWave connectors for 5G wireless networks
The 5G mobile network standard is now being widely deployed. Engineers can take steps to ensure that their designs achieve maximum performance with regard to link range, link reliability and signal protection.
5G New Radio (5G NR) encompasses the FR1 and FR2 frequency bands. FR1 bands are 6 GHz and below. The International Telecommunications Union (ITU) describes FR2 as bands in frequency segments between 24.25 GHz and 71 GHz, although the standards body, 3GPP, puts the upper limit at 52.6 GHz. Others consider it to go up to 100 GHz. No matter the interpretation, FR2 bands are universally referred to as being in the mmWave spectrum.
5G NR and future 6G mmWave access networks are needed to achieve the highest bandwidth and lowest latency performance. The available contiguous bandwidth is 10 times greater than in FR1 bands, and a typical latency target for technology developers and network operators is 1 ms or less under best-case conditions.
Unfortunately, at mmWave frequencies, signals are susceptible to absorption and attenuation from buildings, foliage and even rainfall, so the operating range of radios at these frequencies is limited. The effect is particularly problematic in dense urban environments–exactly where mmWave systems promise to deliver their greatest value. Here, the operating range of a rmmWave radio is often under half a mile.
Rather than make the radios and repeaters more powerful, which would increase both capital and operating costs, antenna arrays capable of beamforming are used to compensate for the losses. Analog or digital phasing techniques are applied to multi-antenna arrays, resulting in radio signals that can dynamically focus on network users, whether fixed or mobile. High-resolution beam steering may support 30,000 directions or more, sometimes with sub-220 ns beam switching speeds.
Because hundreds or even thousands of individual elements make up an antenna array, high-density RF interconnect is needed, and the technical challenges are considerable. At 71 GHz, the signal wavelength is just 4 mm, so even a tiny unshielded length of conductor is a potential source of unwanted emissions and an effective receptor for electromagnetic interference (EMI).
Densely packed cables and connectors also make systems susceptible to crosstalk from capacitive and inductive coupling, and the tiny dimensions involved at mmWave frequencies increase the risk of impedance mismatches due to conductor misalignment or the slightest interconnect contamination. Added to this, expansion and contraction caused by changes in environmental conditions create physical movement in interconnect. There may even be changes in the electrical characteristics of materials used in cables and connectors. Remember, 5G NR radios are usually located outdoors on masts or buildings and exposed to the elements.
Maintaining a good impedance match over the broad frequency range of mmWave bands is another important consideration to minimize voltage standing wave ratio (VSWR) and attenuation.
Design trade-offs in connectors for mmWave applications
The characteristic impedance of an RF connector must match the impedance of the transmission line (typically, but not always, 50 ohms) to ensure minimal signal reflection and loss. For a coaxial cable or connector, the diameter of the central conductor, the dielectric properties of the conductor’s insulation, and the inner diameter of the outer conductor determine its characteristic impedance.
At high frequencies, the skin effect causes most current to flow on the surface of the central conductor. A larger diameter means greater surface area, potentially reducing resistance and minimizing signal loss. However, while larger diameters provide more surface area, the skin depth (the depth at which the current density falls to 1/e of its surface value) decreases with frequency, so the benefits of a larger diameter reduce at higher frequencies.
Thicker central conductors reduce resistive losses due to their larger cross-section. Also, thicker conductors dissipate heat better, reducing thermal damage risks and supporting stable performance under high-power conditions. On the other hand, a smaller diameter central conductor means a smaller gap between the inner and outer conductors, which can help minimize dielectric losses by reducing the volume of dielectric material that the signal interacts with. What’s more, small-diameter conductors are better at suppressing higher-order transmission modes. This prevents signal distortion by maintaining a single transverse electromagnetic mode (TEM) of transmission up to the maximum frequency of operation. In addition, smaller conductors reduce the parasitic inductance and capacitance which can affect signal integrity. This is particularly important for high-frequency signals.
After determining the optimum conductor diameter, dielectric material and connector housing, the next challenge for connector manufacturers is the precision manufacturing of these tiny components. The connectors must be mechanically robust enough for 5G NR applications and able to maintain stable and predictable electrical performance in wide-ranging and changeable environmental conditions.
Some examples of mmWave connector designs
Some connectors that work well in 5G NR mmWave applications have been available for many years. The industry-standard (IEC 61169-35) K-type 2.92 mm internal diameter connectors are an example. These were launched by Mario Maury 50 years ago and then re-introduced by Wiltron in 1984 as the K connector. They are still popular today for systems operating at up to 46 GHz and can be threaded onto cheaper SMA and 3.5 mm connectors. Their central conductor pins are 1.27 mm in diameter.
However, compact 5G NR radios and repeaters benefit from smaller connector designs. Precision RF connectors and cable assemblies suitable for frequencies between 18 GHz and 110 GHz are now available from several of the industry leaders.
Some examples of GHz RF connectors with cable assemblies are shown above. [Source: Samtec]
For 5G applications, SMA connectors are typically used up to 26.5 GHz. SMPM types are attractive due to their more compact form factor, availability in single-port and multi-port ganged versions with pitches down to 3.56 mm, and low SWR (<1.4:1) to minimize signal reflections and subsequent losses. SMPM connectors work well at up to 65 GHz.
For operation above 65 GHz, IEEE 2870-2007 and IEEE 6119-32 are the most prominent connector industry standards. They include electrical and mechanical specifications for operation at up to 110 GHz. Connector interfaces down to 1 mm are found for highest frequency applications, and while connector family names seem not to be shared between manufacturers, all connectors are designed for use with industry-standard cables.
In anticipation of the ever-growing demand for network bandwidth, manufacturers are now developing precision RF connectors down to just 0.8 mm internal diameter that will work at up to 140 GHz.
In general, threaded coaxial compression connectors are becoming more popular. They typically have two mounting screws to fix them to printed circuit boards. Tiny versions with internal diameters of 1.35 mm have center pins diameters as small as 254 µm and require careful alignment when being mounted. Misalignment in any direction will result in impedance changes and increased VSWR. Precision connector manufacturers sometimes provide visual alignment guides machined into connector housings to mitigate this problem without making the assembly process unnecessarily complicated.
How a configuration tool can simplify RF cable assembly design
With such a high degree of precision needed for mmWave connectors and cables, many OEMs and systems integrators opt for buying ready-made cable assemblies. By doing so, they can improve the quality and reliability of system interconnect, save time and often reduce costs too. To simplify the design and procurement process for RF cable assembles, Avnet partnered with Molex to design and host an online configuration tool. Engineers select the connectors they need at each end of the assembly along with the length and type of cable.
Users select one of 25 connector types, including most of those mentioned above, for side A of the assembly from a drop-down menu. They then select, where available, the gender, orientation, body material and plating, ingress protection, and reverse polarity options.
The next step is to specify the cable length, which can be anything from 100 mm to 9999 mm, and the cable type. There are 35 cable types in the drop-down menu. To simplify selection, only those compatible with the selected connector are highlighted.
Next, users choose the connector for side B of the assembly. The configuration tool also offers the opportunity to enter the operating frequency range over which the cable assembly needs to operate and the maximum VSWR or return loss value that can be tolerated. The maximum insertion loss at a given frequency and passive intermodulation distortion (PIM) figures in dBm or dBc can be added to this optional data.
Finally, there is a box into which users can enter requirements for labeling, shrink tubing, or other electrical or power characteristics needed. After entering this information, which takes just a few minutes at most, users can request a quote, a 3D drawing or a product sample.
Avnet offers mmWave RF connectors from the following suppliers:
