The double edge of standing waves
A standing wave occurs when the reflected signal is in phase with the source. When it happens in a transmission line, it is normally unwanted because it results in emitted radiation. However, it is exactly that radiation that maximizes the signal’s strength in an antenna. Because of that, standing waves are welcome at the very end of the signal path.
This can create something of a dichotomy for RF engineers. A standing wave is caused by the signal being reflected on itself and can occur at any interconnection point along the transmission path. The potential for a standing wave to occur is related to the frequency of the signal and the length of the path.
The dependence on frequency and path length means it is intimately related to how well the impedance at each interconnect along the signal path is matched. The closer the impedance match, the less reflection created.
Most of the time, designers can avoid generating a standing wave by making sure the impedance of the transmission line is matched at each interconnect. The last stop on that line will be the antenna and, here, the standing wave needs to be welcomed.
To achieve this, engineers need to pay particularly close attention to the quality of the connectors used in the signal path. Unfortunately, most RF signal paths will feature multiple interconnects. The connectors used at these points will determine how well the impedance is matched up to the point where it meets the antenna.
At higher frequencies, ensuring matched impedances becomes more critical. Unfortunately for engineering teams, those frequencies continue to go up. This is in response to the demand for higher data bandwidths in general, but also because of developments in cellular communications and automotive engineering.
One of the big drivers here is 5G, but WiFi is another, as it reaches into the 6 GHz frequency range. Vehicles are using more RF-based technologies for ADAS. Radar and lidar all operate at GHz frequencies and are becoming more important as we move closer toward autonomous driving.
Developing for 5G
Beamforming is one of the key RF technologies used in 5G and it relies on multiple-in multiple-out technology (MIMO). As the name suggests, a MIMO configuration uses multiple antennas. Avnet has worked with Otava, Samtec and Xilinx to help develop a beamforming evaluation kit based on Otava’s beamforming IC, the OTBF103. The IC is a mmWave wideband time-division-duplexing beamformer solution with eight transmit channels and eight receive channels.
The evaluation kit includes a MicroZed system-on-module based on the Zynq 7010 SoC from Xilinx. It also includes a GUI and can be controlled over USB or Ethernet by a PC. It operates from 24 GHz to 40 GHz, covering a number of 5G bands.
A key part of the kit is the way the RF signals are routed to and from the board. Otava chose Samtec’s 2.4 mm compression mount connectors for the board. RF engineers from both companies worked together to optimize the IC-to-PCB and PCB-to-connector design.
Let’s be precise with RF
The SMPM family of precision RF connectors from Samtec provide connectivity for GHz signals, in 5G and other high-frequency applications.
Working with RF signals at these frequencies requires precision. In fact, Samtec has developed a range of precision RF connectors that meet this need. Many variables define “precision” in this context. Examples includes the material selection in the PCB, the dielectric and the connector itself plus mechanical tolerances imposed in the differing manufacturing processes.
Samtec’s precision RF cable-to-board and board-level connectors include a range of off-the-shelf solutions for microwave and millimeter wave applications from 18 GHz to 110 GHz. This includes the SMPM product line with a maximum voltage standing wave ratio (VSWR) of 1.40 and a frequency range of DC to 65 GHz. These micro-miniature connectors can be used in high-frequency applications where push-on mating is needed, which could include applications where physical space is limited.
Multi-port ganged cable assemblies with eight- and 10-port configurations are also available in the form of the GC47 and GPPC series. The GPPB series is a multi-port board-to-board system, while the SMPM is a cable-to-board single-port solution which comprises the RF047-A, RF086 and RF232C series.
The push-on format of the SMPM products deliver quick-attach blind mating. Misalignment between mating parts can lead to RF leakage, but the push-on design compensates for misalignment, minimizing leakage.
Rapid development for implementing 5G
The XRF16 system-on-module from Avnet is also aimed at teams developing 5G systems. This SoM is based on the Zynq UltraScale+ RF SoC from Xilinx. This powerful SoC can support applications including 5G MIMO, radar, beamforming and medical imaging.
The RF connectors used on the XRF16 are the IJ5 series 4.00 mm IsoRate 50 Ohm high-isolation RF Jack Strip. The IsoRate RF connectors and cables are around half the cost of traditional RF connectors but offer virtually the same performance.
Connections between the SOM and carrier board are implemented with Samtec’s 0.80 mm SEARAY high-speed high-density Open-Pin-Field Array connector. These reduce the board space required by around 50% but offer the same performance as similar arrays from Samtec. This connector can operate at up to 17.5 GHz, or 35 Gbps, and is part of Samtec’s 28+ Gbps solution. It can accommodate up to 500 I/Os in 7 mm and 10 mm stack heights.
The precision of GHz RF
Impedance matching is a major design consideration in RF systems. With signal frequencies in the GHz, the potential impact of unintended standing waves is significant. Samtec has a wide portfolio of precision RF connector solutions to address this design challenge.