Key Characteristics in Selecting Low-Frequency RF Cables and Connectors

Technical Overview: Selecting low-frequency RF cables and connectors requires balancing impedance, frequency, signal integrity, physical constraints, and manufacturing needs. This article examines 50 and 75 Ω interconnect options for applications up to 18 GHz and explains how cable construction, connector type, mounting method, and assembly requirements guide component selection. (Overview courtesy of ChatGPT)

Radio frequency (RF) technology supports applications across wireless communications, industrial, medical, automotive, and military systems. Many of these applications operate from 1 GHz to 18 GHz, a range covered by Samtec’s low-frequency RF product portfolio. Selecting RF hardware requires evaluating electrical and RF performance alongside physical requirements, including cable and connector compatibility, while accounting for manufacturability, reliability, and cost.

The importance of impedance

RF cables, connectors, and other transmission components require matching characteristic impedances to minimize signal reflections at the interfaces between connected media.

The voltage standing wave ratio (VSWR), measured over a range of frequencies, indicates the impedance match between components. A VSWR of 1.0 at a given frequency indicates a perfect match; the match degrades as the VSWR increases. Another important characteristic is low insertion loss, which minimizes signal attenuation.

A characteristic impedance of 50 Ω is common in wireless data transmission and in test applications, while 75 Ω is found in high-resolution video transmissions and broadcasting. This includes serial digital interface (SDI) signals, especially at the 12 Gbps data rate found in 12G-SDI video technology.

The fast signal transitions of 12G-SDI contain substantial high-frequency spectral content, making 75 Ω controlled-impedance coaxial interconnects important for preserving signal integrity. For example, Samtec’s BNC7T series of bayonet-style connectors (Figure 1) is designed for 12G-SDI video applications, offering 75 Ω impedance matching and performance up to 12 GHz. The BNC7T series uses gold-plated contacts on both the jacks and plugs, with the plugs securely mating with a quarter twist.

Figure 1: Samtec BNC7T connectors have a 75 Ω characteristic impedance and up to 12 GHz performance for 12G-SDI video transmission. (Image source: Samtec)

Cable capabilities

For 12G-SDI applications, BNC7T plugs work with a 75 Ω RG-6-style cable, such as Belden 4694R, which is frequency tested up to 12 GHz. The cable has an 18 AWG solid bare copper conductor, gas-injected high-density polyethylene insulation, dual foil shielding, tinned copper braiding, and a PVC jacket.

The Belden 4694R cable is optimized for 12G-SDI applications, but BNC7T series connectors also work with other 75 Ω cables, such as Beldon’s RG-6/U-style 1694A, which handles 6G-SDI applications up to 6 GHz.

For short-distance signal transmission up to 4 GHz in 75 Ω applications, RG 179 cables or cable assemblies such as Samtec’s RF179-72SP1-74SP3-0600 (Figure 2) provide another option. It is only 600 mm long with a 10.2 mm bend radius.

Figure 2: Samtec RF179 cable assemblies use flexible RG-179 75 Ω cable and are available with BNC-to-MCX (shown here) or BNC-to-DIN connectors for operation up to 4 GHz over short distances. (Image source: Samtec)

RG-179 cables pack their 28 AWG silver-plated copper conductors, polytetrafluoroethylene (PTFE) insulation, silver-plated copper braid, and fluorinated ethylene propylene (FEP) jacket into a 2.54 mm diameter.

RG-316 and RG-174-type cables are commonly used for 50 Ω low-frequency RF applications. RG-174 type cables are constructed with a 24 AWG bare copper conductor, a polyethylene dielectric, a tinned copper shield, and a PVC jacket. They are available with MCX, SMA, BNC, TNC, and ganged terminations, or with MMCX and N-type terminations as in the RF174-03RP1-06BJ2-0152 cable assembly (Figure 3).

Figure 3: RF174 50 Ω cable assemblies for RF up to 6 GHz use RG-174 coaxial cable. Termination options include MMCX and N-type connectors. (Image source: Samtec)

RG-174 type cables can handle frequencies up to 6 GHz, work at temperatures from -20°C to +80°C, and have a bend radius of 25.4 mm.

RG-316-type cable assemblies such as the RF316-01SP1-04SP3-0305 (Figure 4) provide greater flexibility and a wider temperature range of -55°C to +165°C.

Figure 4: RF316 50 Ω cable assemblies use RG-316 coaxial cable and terminations such as N-type, MCX, TNC, BNC, and SMA connectors. (Image source: Samtec)

This assembly features a 305 mm long RG-316 coaxial cable with a 24 AWG SCCS conductor, FEP dielectric, silver-plated copper shield, and FEP jacket, terminated with a BNC connector on one end and an SMA connector on the other. RG-316 cables have a 12.80 mm bend radius and work at frequencies up to 6 GHz.

Connector characteristics

Cable terminations and board connectors must also be selected based on an application’s requirements. Connectors must have the right frequency capability and characteristic impedance, and they must be a good fit in terms of size, mating method, connector density, and durability.

For example, the BNC-style connectors common in 12G-SDI video applications are also used in RF applications at both 50 Ω and 75 Ω because bayonet-style connectors are easy to mate and they resist vibration. BNC7T connectors have mating faces from 9.65 mm in diameter to 17.50 mm across a hexagonal cross-section.

For more space-efficient connections, HDBNC series connectors (Figure 5) can be used. Designed for 75 Ω, 12 GHz applications like video broadcast, where many connections to a PC board (PCB) are required in a tight space, HDBNC connectors enable a four-fold increase in connection density over BNC connectors with outer diameters of 9.65 mm.

Figure 5: HDBNC series 75 Ω bayonet-style connectors support operating frequencies up to 12 GHz, have up to four times the connection density, and are 20% lighter. (Image source: Samtec)

Ganged connectors can also boost space efficiency on PCBs. For instance, GRF1 series micro-miniature ganged connectors (Figure 6) connect up to eight independent RG-316 cable paths on a 5.00 mm pitch and support operating frequencies up to 5.31 GHz. Optional threaded inserts add to the connectors’ mechanical robustness in rugged environments.

Figure 6: GRF1 series 50 Ω micro-miniature ganged connectors support up to eight RG-316 cables on a 5.00 mm pitch and operate up to 5.31 GHz. (Image source: Samtec)

SMA connectors also save space due to their subminiature size. These screw-type connectors can mate with 2.92 mm and 3.50 mm connectors and have outer diameters ranging from 7.00 to 11.00 mm. With the ability to support 18 GHz operating frequencies and up to 26.5 GHz with precision RF designs, 50 Ω SMA connectors are known for their non-magnetic options for space and medical applications.

Small connectors with non-magnetic options are also available in the MCX and MMCX series of snap-on connectors, both with 50 and 75 Ω versions that support operating frequencies up to 6 GHz. MCX mini board-mount connectors have a 5.88 mm square by 6.00 mm housing surrounding a 3.45 mm diameter mating surface, while MMCX micro-mini board-mount connector housings with the same mating surface diameter are 3.45 mm square by 5.00 mm.

Managing manufacturing

Most low-frequency RF connectors, whether they are board-mount connectors or cable terminations, are available in both straight and right-angle styles. Many also have automation-friendly options, such as being supplied pre-sorted in trays or in tape-and-reel setups. BNC and HDBNC connectors have variants that are mechanically balanced for automated pick-and-place equipment, eliminating the need for a separate placement fixture.

Samtec board-mount plugs and jacks offer several options for securing them to the PCB. Edge mounting, for instance, is a common layout for coaxial cable connectors in which electrical leads from the connector are soldered to a controlled-impedance copper transmission line along the PCB perimeter. Through-hole technology creates both an electrical and a robust physical connection by soldering connector leads that protrude through holes in the PCB. With surface-mount technology (SMT), electrical contacts on the underside of the connector align with solder pads on the PCB.

Conclusion

Successful RF interconnect design depends on matching the cable and connector to the application’s impedance, frequency, mechanical, and assembly requirements. Samtec’s low-frequency RF portfolio offers a range of 50 Ω and 75 Ω solutions to support these needs across applications up to 18 GHz.