When you're deep in the trenches of RF design, especially when working with signals that hum at GHz frequencies, the humble coax connector and its accompanying cable assembly become surprisingly critical players. They're the unsung heroes, the vital arteries carrying precious signals between circuit boards, sub-assemblies, and enclosures. You need them to be robust, to hold their electrical integrity, and to simply work without introducing unwanted noise or signal degradation.
For years, connectors like the venerable BNC, with its familiar bayonet lock, have served us well. They're sturdy, easy to connect and disconnect, and have been around since the mid-20th century. However, as our designs get smaller, faster, and push into higher frequency realms, these older designs start to show their limitations. Their physical bulk can be a problem in compact systems, and more importantly, their performance at higher frequencies – say, beyond 4 GHz – can lead to unacceptable signal loss. It’s like trying to push a large truck through a narrow alley; it just doesn't fit anymore.
This is where the newer generation of RF connectors steps in. Engineers today have a fantastic array of options, each designed to tackle the challenges of modern, high-frequency, miniaturized designs. We're talking about connector families like SMA, SMB, SMP, MMCX, and MCX. What's common among most of these is their standard 50-ohm impedance, a crucial factor for maintaining signal integrity. But the real magic lies in the combinations of bandwidth, size, and the ability to work with finer coaxial cables. And importantly, they offer a much smaller physical footprint, often in the 5mm range compared to the BNC's 17mm.
It's worth taking a moment to clarify some terms, as they can get a bit fuzzy in casual conversation. A 'connector' is the actual mating and unmating device. 'Cable' refers to the coaxial line itself – the conductor, dielectric, shield, and jacket. But what we often mean when we say 'cable' is actually a 'cable assembly' – the cable with connectors attached at one or both ends. For clarity, we'll stick to these precise definitions here.
While connectors themselves are passive – they don't amplify or process signals – their impact is profound. An ideal connector would be invisible electrically, offering perfect mechanical connection and disconnection without any resistance or impedance mismatches. As frequencies climb into the GHz range, achieving this ideal becomes exponentially harder. The mechanical precision required in design, manufacturing, and assembly is immense.
Let's dive into some of these popular GHz-range connector families:
SMA (SubMiniature version A)
These are workhorses for many high-frequency applications. Their threaded coupling mechanism provides excellent mechanical stability, crucial for environments with vibration. The design also enhances axial force and torque resistance. With a gold-plated center contact, they offer good electrical performance and a respectable mating cycle count of around 500.
A good example is the Würth Elektronik 60312242114510, an SMA jack designed for board edge use and end-launch configurations. It features a front-mounted nut and lock washer, making it easy to secure to a panel for added rigidity. Electrically, it boasts a VSWR below 1.2 and an insertion loss (IL) of less than 0.14 dB from DC to 12.4 GHz, extending to 18 GHz with slightly higher VSWR and IL figures.
SMB (SubMiniature version B)
Smaller than SMA, SMB connectors use a snap-on coupling, making them quick to connect and disconnect. They're ideal for miniaturized designs and offer broadband capabilities up to 4 GHz. You'll find them in various configurations, including PCB sockets for through-hole and surface mounting, as well as edge-launch and cable connectors.
Consider the 61611002121501, an SMB connector with a right-angle, through-hole, solderable jack. It offers a VSWR of 1.5 and an insertion loss below 0.2 dB, with a similar 500-mating cycle rating as SMA.
SMP (SubMiniature version Push-on)
Pushing the frequency envelope, SMP connectors can operate up to 40 GHz. They utilize a slide-on or snap-on mechanism and come in three interface types, offering different levels of retention force and mating cycles. The 'full detent' provides maximum retention and vibration resistance (100 cycles), 'limited detent' offers medium to low retention (500 cycles), and the 'smooth bore' has the lowest retention but the highest mating cycles (1000 cycles).
The 60114202122305 is an example of an SMP smooth bore, edge-launch connector designed for PCBs up to 1.2mm thick. Its specified VSWR is 1.5, with an insertion loss of 0.42 dB from DC to 12 GHz.
MCX (Micro Coaxial)
These connectors offer a quick and convenient snap-on coupling for applications up to 6 GHz. They comply with the IEC 61169-36 standard. The MCX series is known for its compact size and ease of use.
The 60612202111308 is a surface-mount, edge-launch MCX jack for PCBs up to 1.6mm thick. It presents a VSWR of 1.3 and an insertion loss of 0.25 dB within its operating range, with a 500-mating cycle rating.
MMCX (Micro-Miniature Coaxial)
As the name suggests, MMCX connectors are about 30% smaller than MCX, making them perfect for ultra-miniature designs. They also feature a snap-on coupling and adhere to the IEC 61169-36 standard.
Take the 66046011210320 MMCX plug, a free-hanging, crimp-style connector designed for cables like RG174, RG316, and RG188. It operates up to 6 GHz, with a VSWR of 1.3 and an insertion loss of 0.3 dB.
Adapters and the Curious Case of Reverse Polarity
Given the variety, it's no surprise that adapters are essential for interconnecting different connector series. Würth Elektronik offers a comprehensive range to bridge these gaps, allowing seamless transitions between SMA, SMB, MCX, and MMCX, for instance.
Then there's the slightly perplexing 'Reverse Polarity' (RP) connector. Standard connectors have a male pin in the plug and a female socket in the jack. RP connectors flip this. This quirk originated decades ago with early Wi-Fi routers. The FCC, concerned about users extending the range of these devices and causing interference, mandated RP connectors on some wireless equipment. The idea was to make it harder to connect standard external antennas or amplifiers. Ironically, RP-terminated cable assemblies quickly became the standard for external Wi-Fi antennas, making them compatible with those very routers.
An example is the 63012042124504, a panel-mount, through-hole solderable RP-SMA jack, offering a VSWR of 1.2 up to 12.4 GHz and 1.4 from 12.4 GHz to 18 GHz, with corresponding insertion losses.
