by Bryan Micelli, Director of Engineering, Delta Electronics Manufacturing Corp.
Measurement precision and accuracy often relies not only on the quality of the test equipment but also on the interconnections between a device under test (DUT) and the test equipment. Especially as newer applications such as automotive radars and Fifth Generation (5G) wireless cellular networks are extending well into the millimeter-wave frequency range, test setups must be carefully configured and maintained, down to the coaxial interconnections. Trying to make quick interconnections between different coaxial connector types to save time during repetitive measurements can lead to degraded insertion loss and VSWR in the measurements and unnecessary wear or damage to the coaxial connectors and adapters, especially when trying to make different connectors that might look similar in appearance fit through an adapter. But color coding can make it easy to make the right match: using color to identify coaxial adapter ports for 1.0 mm, 1.85 mm, 2.4 mm, and 2.92 mm male and female millimeter-wave coaxial connectors with bandwidths as wide as DC to 110 GHz. These unique color-coded coaxial adapters provide the electrical and mechanical performance to not only extend the lifetimes of the connectors using the adapters but also improve the impedance match for better measurements. Find out more about these color-coded adapters and how they can speed and simplify measurements on a growing number of millimeter-wave applications, including for 5G and in autonomous vehicles.
Whether in test systems or in the field for commercial or military electronic systems, engineers often have concerns about the quality of a coaxial interconnection. If not fitted properly or with optimum torque for mechanical conditions that eliminate air gaps and provide a smooth impedance transition, interconnections between coaxial connectors and adapters can result in excessive signal reflections and losses, especially at higher frequencies. Even when different connectors are mechanically compatible, they may have different frequency limits, with the lower-frequency component setting the upper-frequency limit for the interconnection.
When time is of the essence, such as in high-volume production measurements, efficient and practical measurements may require rapid and effective coaxial interconnections between test gear and DUTs, sometimes leading to the connection of mismatched connector pairs or connectors and adapters. It may not be possible to constantly avoid mismatched coaxial interconnections, especially in high-volume measurement applications, given the almost microscopic size of the connector dimensions. The minute dimensions of coaxial connectors and adapters designed for millimeter-wave frequencies makes them especially susceptible to damage if mishandled, whether connectors are on DUTs or the test equipment.
Connector interfaces are critical to the accuracy and repeatability of millimeter-wave component measurements, such as characterization of antenna radiation patterns for 5G networks. Connectors and adapters with the same characteristic impedance, such as 50 Ω, will yield minimal reflections and minimal return loss when properly impedance matched (Figure 1). But different torque values applied to different coaxial interfaces of the same types of coaxial interconnections will result in deviations from a nominal 50-Ω test system impedance and variations in the measurement results. Even soil or dirt on the contact points of a millimeter-wave connector/adapter assembly can result in degraded performance at frequencies of 28 GHz and above. Coaxial interconnection consistency grows in importance with increasing frequency as a factor in achieving consistent measurement results.
In particular, in measurements relying on a determination of a DUT’s phase performance or phase stability with such variables such as time, temperature, humidity, or even flexure in a coaxial cable, a less than optimum coaxial connector/adapter interface can result in measurement errors and poor repeatability from measurement to measurement (Figure 2). Newer high-frequency instruments such as vector network analyzers (VNAs) operating to 110 GHz and beyond are quite sensitive to impedance mismatches between coaxial connectors and adapters. Poor impedance matches at the connector/adapter interface will cause higher reflections and high return loss (high VSWR) at the signal source. This in turn can result in amplitude and phase measurement errors of ±1 dB and ±5° or more, respectively, with lack of repeatability from DUT to DUT. For phase-sensitive applications, such as modulated communications systems or millimeter-wave radars, measurement results can vary widely across many DUTs.
Color Makes a Difference
Coaxial connector adapters have represented longtime solutions for mating connector pairs not originally meant for forming an interconnection, such as SMA and Type N connectors or even in-series connectors such as male SMA to male SMA connectors. When well designed they provide an excellent match in impedance between the connector and adapter interfaces with low insertion loss and return loss across a frequency range of interest. The mechanical design, a machining process, and manufacturing tolerances of the adapters contribute a great deal to their effectiveness, along with their material composition.
The plating materials and thicknesses used on coaxial connectors and adapters can have a great deal to do with interconnection longevity, and how many interconnection cycles are possible without degradation in measurement performance and accuracy. Many higher-frequency coaxial connectors are based on construction with an air dielectric to minimize losses at high frequencies. Some lower-frequency coaxial connectors use a polytetrafluoroethylene (PTFE) dielectric, which can impact the characteristic impedance of a connector and adapter depending on air gaps in the material when fitted into the connector housing. The type of coaxial connector and adapter material and plating, such as brass or stainless steel, and any variations in the thickness of those materials can also affect connector and adapter performance, especially at millimeter-wave frequencies (Figure 1).
Color-coded millimeter-wave adapters are built with durable materials, including corrosion-resistant stainless steel, meant to contribute to the longevity and repeatability of millimeter-wave coaxial interconnections in measurement and other high-frequency applications. They use a color-based identification scheme developed by the IEEE for connectors through DC to 110 GHz and higher (see Table 1).
The color-coded adapters enable interconnections of different types (between-series) of coaxial connectors as well as of the same type (in-series) coaxial connections at millimeter-wave frequencies (Figure 3). They fit combinations of 2.92 mm, 2.4 mm connectors, 1.85 mm connectors, and 1.00 mm connectors and can be used to connect a 2.92 mm plug (male) to 1.85 mm plug (male), a 2.92 mm plug to 2.92 mm plug, a 2.4 mm plug to 2.4 mm plug, a 1.85 mm plug (male) to 1.0-mm jack (female), and a 1.85-mm jack to 1.85 mm jack. The different colors allow a test engineer to check on the compatibility of a connector/adapter interconnection based on connector type and/or operating frequency range, as required by an application. Each adapter meets high quality standards according to the International Electrotechnical Commission (IEC), including IEC 61169-31 for 1.0 mm connectors, IEC 61169-32 for 1.85 mm connectors, and IEC 61169-35 for 2.92 mm connectors.