Connectors are essential components in radio frequency (RF) systems, especially in high-frequency applications like Gigahertz (GHz) 5G devices. Designers have a broad range of connectors to choose from, but there are a few key parameters to consider when narrowing down the choices. Six keys to selecting and integrating GHz connectors include physical size, frequency range capabilities, power handling, voltage standing wave ratio (VSWR) and return loss, passive intermodulation distortion (PIM), and controlling unwanted electromagnetic interference (EMI). A connector’s physical size, frequency range, and power handling capabilities are interrelated (Figure 1).
A few of the common connector types used in RF applications include:
BNC connectors are inexpensive, include a locking mechanism to prevent unintentional disconnection, and are available with characteristic impedances of 50 or 75 Ω. BNC connectors are typically rated for 500V and about 100W average power up to about 1 GHz; however, they are most commonly used at 500 MHz and below.
TNC connectors are threaded versions of a BNC connector. The threads provide a more solid connection that can withstand vibrations and handle higher frequencies and power levels than BNCs.
N connectors are rugged and inexpensive. Standard versions are rated for 11 GHz, with precision designs capable of operating to 18 GHz. Like BNC connectors, N connectors are available with characteristic impedances of 50 or 75 Ω.
SMA, sub-miniature type A, connectors are rated for up to 18 GHz, with precision designs rated up to 26.5 GHz. They can handle power levels between BNC and TNC connectors.
3.5mm connectors are precision designs similar to an SMA connector but with an air dielectric that supports frequencies up to 34 GHz. 3.5mm connectors tend to have lower power ratings compared with SMA connectors.
2.4mm connectors are rated to 50 GHz and are available in three grades; general-purpose, instrumentation, and metrology. They have limited power-handling capability.
2.92mm / K type connectors have similar performance to 2.4mm designs, but are limited to 40 GHz. They can be used at all K-band frequencies.
C-type connectors use a bayonet retention collar similar to BNCs but are designed to handle much higher power levels. Standard C-type connectors are rated for 50 Ω, but 75 Ω designs are also available. There are versions with threaded collars for a more secure connection, like TNC connectors.
7-16 DIN connectors have largely replaced C-type connectors, and can handle somewhat higher power levels. These connectors have an inner conductor with a diameter of 7 mm and an outer conductor with a diameter of 16 mm. They use an M29 x 1.5 threaded coupling nut.
EIA series coaxial connectors come in a variety of sizes, including EIA 7/8”, EIA 1 5/8”, EIA 3 1/8”, EIA 4 1/2”, and EIA 6 1/8”. These high-power connectors can be used with foam or air dielectric cables.
Return loss and VSWR
Return loss and VSWR are important metrics of RF interconnect system performance. The primary difference between return loss and VSWR is that return loss is a logarithmic measurement, making it useful when measuring very small reflections. In contrast, VSWR is a linear measurement and helps measure larger reflections.
In general, a return loss of 15 dB or better is considered an acceptable overall return loss for a cable and antenna system. Higher return loss is preferred. A 20 dB system return loss is very efficient since only 1% of the power is returned, and 99% is transmitted, while with a return loss of 10 dB, 10% of the power is returned, and only 90% is transmitted.
VSWR measures the amount of signal reflected by a connector; it’s the ratio of voltage applied to voltage reflected and is a major factor in the connector’s signal efficiency. VSWR displays the match of the system linearly. A perfect or ideal match in VSWR terms would be 1:1. A common VSWR for an RF connector is about 1.43 (15 dB), but can range as high as 2.0. The VSWR is frequency related, and for a given connector, it is usually higher at higher frequencies.
Connector geometry and materials play major roles in determining VSWR. Inline connectors typically have superior VSWR compared with right-angle connectors, particularly at frequencies approaching the limit of the connector. This is due to a two-part internal straight contact configuration that allows for easy assembly and low cost with internal impedance control. A recently-released high-frequency, right-angle SMA adapter replaces the straight contact configuration with an internal swept right-angle configuration with a two-piece concept. This incorporates all low-profile, compact size, and other physical advantages of standard right-angle connectors while maintaining electrical performance comparable with conventional straight connectors. In addition to an improved internal geometry, the high-performance right-angle SMA adapter features a gold-plated, brass body and gold-plated, beryllium copper contact.
Connector integration considerations
Through-hole connectors exhibit some insertion loss since the pins act like inductive/capacitive impedance discontinuities. An impedance mismatch caused by surface-mount pads on a connector is the primary source of return loss. Once a signal reaches a surface-mount connector pad with wider copper, the capacitance per length of the trace is larger, decreasing the trace’s characteristic impedance at the connector pad. This capacitive impedance discontinuity causes signal reflection, leading to connector return loss.
Adding a second small ground layer below the connector can help eliminate the problem with capacitive impedance discontinuity. (Figure 2). A cutout in the ground plane the same size as the connector pad and below the signal layer forces the trace to reference to the ground plane in the third layer. That results in higher capacitance and helps reduce the capacitive impedance discontinuity, which decreases the connector return loss.
Passive intermodulation (PIM) occurs when two signals present on a transmission line mix in a non-linear manner. This mixing creates additional frequency components that may fall within the uplink band, causing interference. There are three common sources of PIM:
Design PIM can be caused by passive components such as connectors. There is often a trade-off between lower cost, smaller size or lower system performance, and higher PIM levels when specifying connectors.
Assembly/Aging PIM results from improper installation or long-term weathering of connectors and other passive components. Higher quality and more expensive connectors can help address this PIM source.
Environmental PIM, also called ‘rusty bolt’ PIM, results from external environmental factors. Corrosion on antennas or structural elements are common sources of environmental PIM. Connectors rarely contribute to this source of intermodulation interference.
Managing EMI in 5G UE Devices
5G user equipment (UE) devices like handsets are challenging thickets of EMI sources with multiple RF subsystems, including GPS, Wi-Fi, Bluetooth, and various cellular connections, including mmWave 5G. For example, the 5G mmWave subsystem in a handset is located in close proximity to the CPU core and other sensitive devices raising electromagnetic compatibility (EMC) concerns. One potential solution is using microstrip and stripline miniature coaxial connectors combined with cable grounding and wiring management to provide a series of successively more effective EMI/EMC solutions.
A simple, low-cost approach is to implement a board-to-cable solution using a microstrip interconnect (Figure 3). This approach provides a micro-coaxial connection to a microstrip structure on the circuit board. It uses only two metal layers on the PCB, saving cost, but it may not suppress radiated EMI sufficiently at higher frequencies to comply with EMI/EMC regulations.
When the basic micro-coaxial connection to a microstrip does not meet the needed level of performance, the addition of an SMT grounding clip can help further reduce EMI by suppressing EMI-induced current on the cable shield. It can also help improve cable routing management on the PCB. The ground clip localizes EMI radiation to the area around the connection point, reduces it along the length of the microstrip line.
For applications that require higher performance, a 3-layer stripline transmission line structure can be used (Figure 4). The use of custom stamped connectors can further improve performance. The ground layers completely contain the signal conductor, providing a high level of shielding.
For highly sensitive applications needing the highest EMI shielding suppression, the addition of an SMT grounding clip will improve the performance of the RF-shielded stripline-to-micro-coaxial connector structure.
As discussed above, four successive improvements in EMI shielding effectiveness in 5G UE devices can be achieved using:
- Microstrip RF coaxial connector
- Microstrip RF coaxial connector with SMT cable ground clip
- Stripline RF coaxial connector
- Stripline RF coaxial connector with SMT cable ground clip
RF connectors are critical components to meet the link budget and overall performance demands in various 5G systems from base stations to UE devices. Designers have a wide variety of RF connector options that provide trade-offs in terms of physical size, frequency range capabilities, power handling, voltage standing wave ratio (VSWR) and return loss, and cost. In addition, successful integration of RF connectors needs to take into consideration PIM, and controlling unwanted EMI.
Design Consideration, Tips & Techniques for 5G Connector Solutions, i-PEX
Guide to RF Coaxial Connectors and Cables, AR RF/Microwave Instrumentation
High Performance SMA Adapter Improves VSWR Performance, Amphenol RF
RF Connectors for 5G, Pasternack
What Determines Connector Return Loss and Insertion Loss?, Cadence Design Systems
What Is The Difference Between Return Loss And VSWR?, Amphenol RF