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Bench Talk


Bench Talk for Design Engineers | The Official Blog of Mouser Electronics

Avoiding Connector Reflections in mmWave Applications Robert Huntley

5G Enabling New Beginnings

As the adoption of 5G—the latest generation of cellular communications—gains momentum, the race is on to push out 5G communications infrastructure. Cellular operators are busy deploying infrastructure and have already commenced their marketing initiatives to entice us to upgrade our smartphone contracts and handsets so we can benefit from the significantly improved data rates. Unlike the previous generation change from 3G to 4G, the communications architecture of 5G is not an iterative upgrade. 5G encompasses first-time frequencies in the 24GHz to 40GHz millimeter wave (mmWave) spectrum in addition to coexisting with multi-radio communication networks in the licensed and unlicensed sub-6GHz bands.

Making mmWave Work for 5G

Achieving 5G’s significantly higher data rates, which are forecasted to be a minimum of four times faster than 4G requires the high-bandwidth mmWave spectrum. However, using these much higher frequencies introduces some technical and operational challenges designers must address. A key consideration is that the maximum possible range a signal can send decreases as the frequency increases due to higher propagation losses. This is one of the reasons why a mmWave 5G deployment requires many more base stations than that of 4G. Making 5G mmWave commercially viable using an optimal number of mmWave base stations at the same time as ensuring that mobile handsets receive a sufficiently strong signal led RF engineers to implement beamforming of the mmWave signals. When it comes to designing the massive multiple-in multiple-out (MIMO) antennas, the higher frequency means the transmit/receive elements are much smaller than 4G, enabling the multiple mmWave antenna elements required for a beamforming array to be physically small. Beamforming, also termed beam steering, uses a combination of analog phase shifters and digital control techniques to dynamically concentrate output power into a single lobe to achieve the maximum signal-to-noise ratio and bit error rates for any one signal path.

mmWave Interconnect Challenges

When it comes to the design of the infrastructure equipment, one of the aspects of mmWave RF development is that for frequencies in the order of 30GHz and above the materials used for a product’s PCB substrate can introduce signal losses and unwanted propagation influences. Ideally, the substrate’s dielectric constant (Dk) needs to be low. This led the industry to adopt thinner PCB sizes and different substrate materials, such as polytetrafluoroethylene (PTFE) laminate. Making a coaxial connection between a stripline board and an antenna has traditionally used solderless compression-type connectors. However, as frequencies go higher and the substrates become thinner and softer, the compressive force on the PCB can compact the substrate, producing a capacitive effect that can cause reflections—which in turn negatively impacts the Voltage Standing Wave Ratio (VSWR)—resulting in lower link performance and decreased transmitter efficiencies.

The Amphenol SV Solution

Rather than using a solid mating connector surface, the Amphenol SV Microwave LiteTouch series of solderless PCB connectors use a rounded bead-contact spring pin assembly to minimize the transmission of mating torques to the main assembly (Figure 1).

Figure 1: On the left is a traditional solderless compressive force connector showing the deflection of the PCB substrate. On the right is the solderless Amphenol SV Microwave LiteTouch connector, which exerts no deflection or compressive forces on the PCB assembly. (Source: Amphenol SV Microwave)

The screw-mounted LiteTouch series is designed for 2.92mm, 2.4mm, and 1.85mm connectors. An SMA version is available too. Designed for 50Ω impedance, the 2.92mm connector is rated up to 40GHz, the 2.4mm up to 50GHz, and the 1.85mm to a maximum frequency of 67GHz. The SMA connector is suitable for use up to 26.5GHz. In addition to the board-mounted version, a PCB edge launch series is available too.

Figure 2 illustrates the impact on VSWR reflections using a standard compressive connector can have above 30GHz—see the red trace. By comparison, the blue plot shows a minimal increase in reflections when using an Amphenol SV Microwave LiteTouch connector.

Figure 2: VSWR comparison against frequency between a standard compression connection and an Amphenol SV Microwave LiteTouch connector from 0GHz to 40GHz. (Source: Amphenol SV Microwave)

In addition to use in 5G infrastructure equipment such as antennas, front-end modules, and beamformers, designers can use the Amphenol SV Microwave LiteTouch connector series, a variety of RF and high-speed digital test and measurement equipment, RF pallets, and development and prototyping boards.

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