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Understanding and Choosing GHz-Range Coaxial Connectors and Cable Assemblies

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文章创建人 Bill Schwebe...

Original Title:Understanding and Choosing GHz-Range Coaxial Connectors and Cable Assemblies

  Radio frequency (RF) connectors and their completed coaxial (coax) cable assemblies provide essential signal pathways between circuit boards, subassemblies, and chassis. An appropriate connector will at least provide the required minimum electrical performance and mechanical ruggedness. However, the RF connector families which have served for many years, ­including the bayonet-attachment BNC connector, are no longer adequate due to their physical bulk and performance limits.

  To meet the many challenges of today’s designs, engineers can choose from among many specific types available in several major families, each offering some combination of higher bandwidth, less bulk, and the use of thinner coaxial cables. These connectors are available in a wide variety of pc board termination styles as well as cable termination types to meet the many classes of physical installation priorities. Designers must therefore first select the appropriate connector family to meet the design requirements and then the style within that family.

  This article will look at five widely used gigahertz (GHz) range, RF connector families. It will also look at the closely related issue of complete cable assemblies terminated with the chosen connector, using components from the various families from Würth Elektronik.

  The basics of RF connectors

  It’s important to clarify connector-related terminology. A “connector” is the metal termination which can be mated and unmated as needed, while the “cable” is the coaxial wire consisting of an inner copper conductor, spacing dielectric, outer shield, and insulation to which the connector is attached. A “cable assembly” is the combination of a cable with a connector at one or both ends. However, the term “cable” is often used in place of “cable assembly” in casual conversation, and the actual meaning is usually clear from the context. We will use these terms in their strict sense in this article.

  While connectors are passive components and don’t provide any signal processing or enhancement, they are essential elements in almost any product design. The “ideal” connector offers critical mechanical attributes such as convenient mating and unmating, mechanical and electrical integrity, and it should be electrically invisible with no DC ohmic resistance or RF impedance discontinuities. The challenges of designing, manufacturing, and using connectors increase with operating frequency. As their required operating frequency extends into the RF domain, into and above the gigahertz (GHz) range, their mechanical construction by necessity becomes increasingly precise, with many critical performance attributes and parameters.

  Classic connectors such as the BNC (Bayonet Neil-Concelman), offered in 50 Ω and 75 Ω versions (the latter for video and TV), have been widely used since the 1950s and are still in use (Figure 1). This locking connector features a one-third turn, quick-connect/disconnect action via a “bayonet” system. Although the frequency response is formally rated to 4 GHz, the connector’s losses increase to often unacceptable levels at higher frequencies. Physically, it is not a good fit for today’s compact, densely packed designs due to its relatively large size and the large minimum bending radius of a complete cable assembly.


Image of BNC connector includes a bayonet body lock

  Figure 1: The BNC connector includes a bayonet body lock and has been widely used since its development in the early 1950s, but it is not a good electrical or mechanical fit for many of today’s high-frequency, space-constrained applications. A male plug is usually used with cable assembles (left); a female jack (right), for use on instrument panels. (Image source: Wikipedia; Pinterest)

  Newer families for new applications

  Many industry-standard connector families are available which are more effective for higher frequency, more compact applications. Among the most popular are the SMA, SMB, SMP, MMX, and MMCX families, all with the standard 50 Ω RF impedance. Each offers a different combination of electrical and mechanical characteristics. In contrast to the 17 millimeter (mm) diameter of the BNC connector, these connectors have a much smaller diameter in the range of 5 mm.

  This article will look at a single connector member in each of these families. However, within each family, there are many members with nearly identical electrical specifications, but very different mechanical configurations and arrangements. These include printed circuit board (pc board) versions with a right angle body or a straight body, and with surface mount, through-hole, or end launch termination; rear-mounted bulkhead types; and panel mounted versions with solder cup, flat tab, or round post connection. There are also different arrangements for mating connectors that go on the end of the cable, such as straight and right angle variations.

  Having so many options within a given connector type is good for designers as it increases the likelihood that there is one available off-the-shelf that has a specific form factor that is well-suited to the product design and constraints. This means there will be little or no change required to the mechanical design priorities of the product. Now, a closer look at these five families:

  •SMA: Subminiature SMA series coaxial connectors are designed with threaded coupling technology to ensure high mechanical stability in the face of intense vibration (Figure 2). The connector’s captivated center contact and insulator increase axial force and torque. The thick gold plating on the center contact contributes to enhanced electrical performance and up to 500 mating cycles.


Image of subminiature SMA series connectors

  Figure 2: Subminiature SMA series connectors use a threaded coupling for enhanced mechanical integrity in the face of intense vibration. (Image source: Würth Elektronik)

  A good example of this type of connector is Würth Elektronik’s 60312242114510, a DC to 10 GHz SMA connector jack with a female socket (Figure 3). It is designed for board edge use and end launch orientation. This panel mount solder connector also comes with a front-side nut and lock washer to facilitate bulkhead (panel) attachment for additional rigidity in the end product.


Image of Würth Elektronik 60312242114510 DC to 10 GHz SMA connector jack

  Figure 3: The 60312242114510 DC to 10 GHz SMA connector jack with a female socket includes a front-side nut and associated lock washer for additional mechanical integrity when mounted through a panel or bulkhead (all dimensions in millimeters). (Image source: Würth Elektronik)

  Key RF specifications include voltage standing wave ratio (VSWR) under 1.2 and insertion loss (IL) of less than 0.14 decibels (dB) from DC to 12.4 GHz, with corresponding VSWR and IL numbers of 1.4 and 0.2 dB from 12.4 to 18 GHz.

  •SMB: Connectors in the SMB series are designed for snap-on coupling with broadband capability from DC up to 4 GHz. They are smaller than SMA series connectors and thus are well suited for circuit miniaturization. Among the available SMB connectors are pc board receptacles for through-hole and surface mount, as well as edge card and cable connectors for plugs and jacks (Figure 4).


Image of SMB connectors are snap-on devices that are smaller than the SMA connectors

  Figure 4: SMB connectors are snap-on devices that are smaller than the SMA connectors and not threaded; they are also available in a range of configurations. (Image source: Würth Elektronik)

  An example of an SMB connector is the 61611002121501, a male pin, right angle, through-hole, solder connector jack, with a VSWR of 1.5 and an insertion loss under 0.2 dB (Figure 5). Like the SMA device, it is also rated at 500 mating cycles.


Image of Würth Elektronik 61611002121501 SMB connector is a snap-on right angle unit

  Figure 5: The 61611002121501 SMB connector is a snap-on right angle unit designed for through-hole board attachment and soldering, which is smaller than the SMA unit but has comparable specifications. (Image source: Würth Elektronik)

  •SMP series: These miniature connectors with both slide-in and snap-on features can be used in applications up to 40 GHz. They are available with three interface types: full “dent” with maximum retention for high vibrant resistance (100 cycles); limited dent with medium to low retention (500 cycles); and smooth bore (1000 cycles) with the lowest retention achieved via sliding contacts for modular systems and applications (Figure 6).


Image of connectors in the SMP series offer a variety of retention ratings

  Figure 6: Connectors in the SMP series offer a variety of retention ratings, including limited dent for medium to low retention (left) and 500 cycle rating; and smooth bore (right) with the lowest retention but double the number of cycles. (Image source: Würth Elektronik)

  One of the connectors in this series is the 60114202122305, a surface mount, edge card connector with an extended solder leg for circuit boards having up to a 1.2 mm maximum thickness (Figure 7). It is specified to have a VSWR of 1.5 and an insertion loss of 0.42 dB from DC to 12 GHz.


Image of Würth Elektronik 60114202122305 smooth bore, edge card connector

  Figure 7: The 60114202122305 is a smooth bore, edge card connector in the SMP series that is rated to 12 GHz. (Image source: Würth Elektronik)

  •MCX series: Connectors in the MCX (Micro Coaxial) series feature a snap-on coupling mechanism for fast and convenient connection and are intended for operation from DC to 6 GHz (Figure 8). These connectors are compatible with IEC 61169-36, “Radio-frequency connectors - Part 36: Microminiature r.f. connectors with snap-on coupling - Characteristic impedance 50 Ω (Type MCX)”.


Image of Würth Elektronik MCX connector series is an even smaller snap-on family of connectors

  Figure 8: The MCX connector series is an even smaller snap-on family of connectors that are compatible with IEC 61169-36. (Image source: Würth Elektronik)

  The 60612202111308 is a surface mount, edge launch jack in the MCX series, suitable for boards up to 1.6 mm thick. It has a VSWR of 1.3 and insertion loss of 0.25 dB over that range and is rated for 500 cycles.


Image of Würth Elektronik MCX series 60612202111308 surface mount, edge launch jack

  Figure 9: The MCX series 60612202111308 surface mount, edge launch jack has an insertion loss of just 0.25 dB to 6 GHz. (Image source: Würth Elektronik)

  •MMCX series: These connectors are approximately 30% smaller compared to the MCX connectors and are suitable for applications with ultra-small design requirements (Figure 10). They have a snap-on coupling mechanism for fast and easy connection and also meet IEC 61169-36.


Image of the small Würth Elektronik MMCX series

  Figure 10: Connectors in the MMCX series are approximately 30% smaller than those in the MCX series and exhibit comparable RF performance. (Image source: Würth Elektronik)

  As an example, the 66046011210320 MMCX plug is a male pin, “free-hanging” (in-line), crimp connector in the MMCX family (Figure 11). This 6 GHz connector works with RG174, RG316, and RG188 coaxial cables, and features a VSWR of 1.3 and insertion loss of 0.3 dB.


Image of Würth Elektronik 66046011210320 MMCX plug

  Figure 11: The 66046011210320 MMCX plug is designed to be crimped onto a cable such as the RG174, RG316, and RG188 coaxial types. (Image source: Würth Elektronik)

  Specialty connectors, adapters round out the families

  Given the wide range of connectors in use, it is inevitable that there would be a need for adapters to enable interconnection between one family and another. Würth Elektronik offers several complete series of adapters that support transitions from one connector type and gender to another, such as from SMA plugs and jacks to the other connector plug and jack series (Figure 12).


Image of many available SMA plug and jack adapters (click to enlarge)

  Figure 12: Shown are the many available SMA plug and jack adapters that provide a seamless transition to SMB, MCX, and MMCX family connectors of various types. (Image source: Würth Elektronik)

  There’s another special connector type that can confuse designers at first: the reverse polarity (RP) connector. The standard connector configuration is to have a male (pin) center contact in the plug, and a corresponding female (receptacle) in the jack. But in the US, Federal Communications Commission (FCC) regulations mandate reverse gender “polarity” in some unique cases.

  The situation dates back several decades when wireless Wi-Fi routers for consumer use were introduced. They were designed for limited range using a small antenna having a connector at its base which screwed directly into the Wi-Fi unit’s antenna connection, and thus with no ability to relocate it. However, the FCC was concerned that end-users would attempt to boost the device’s range with add-on amplifiers and/or external antennas, causing Wi-Fi band interference. Their “solution” was to attempt to prevent easy connection of such add-ons by mandating the use of RP connectors on these wireless devices (which often used SMA connectors) to make them incompatible with standard add-ons (Figure 13).


Image of RP SMA plug and jack connectors

  Figure 13: RP SMA plug and jack connectors have the opposite center conductor gender compared to conventional SMA connectors; (left to right) standard SMA male connector, SMA standard female connector, RP-SMA female connector, RP-SMA male connector. (Image source: Wikipedia)

  Within a short time, however, cable assemblies terminated with RP connector pairs became widely available and were standard add-ons for devices such as external, relocatable Wi-Fi antennas (Figure 14).


Image of external Wi-Fi antenna

  Figure 14: This external Wi-Fi antenna can be moved around to find an optimal location and is connector compatible with the antenna interface on the Wi-Fi router due to its RP-SMA connector. (Image source: Amazon)


Image of reverse polarity (RP) connectors

  Figure 15: Reverse polarity (RP) connectors are available in range of circuit board styles as well as cable termination configurations. (Image source: Würth Elektronik)

  One available RP-SMA jack connector is the panel mount, through-hole solder 63012042124504 (Figure 16). This connector features a VSWR of 1.2 from DC to 12.4 GHz, and 1.4 from 12.4 to 18 GHz, while the insertion loss in those two ranges is 0.14 dB and 0.2 dB, respectively.


Image of Würth Elektronik 63012042124504 is a reverse polarity SMA connector

  Figure 16: The 63012042124504 is a reverse polarity SMA connector designed for through-hole mounting and soldering. (Image source: Würth Elektronik)

  Cables and assemblies complete the connections

  Connectors alone are only part of the RF signal path scenario; their plugs are usually fitted to standard coaxial cables such as RG174, RG316, and RG188, among others. Although all are 50 Ω cables for RF work (75 Ω cables and connectors are available for video systems), they differ in frequency range, attenuation, diameter, dielectric type, phase characteristics, power handling, minimum bend radius, external jacketing, and other mechanical and electrical attributes (Figure 17).


Graph of attenuation versus frequency for some common standard coaxial cables

  Figure 17: Designers can choose among a wide array of 50 Ω coaxial cables, differing in many electrical and mechanical characteristics. Shown is the attenuation versus frequency—an important specification— for some common standard coaxial cables. (Image source: Würth Elektronik)

  Designers must also decide whether to make their own coaxial cable assemblies or buy them already fabricated—the classic “make versus buy” question. It is possible to terminate these coaxial cables with the selected connectors as needed—the “make” option—but doing so is a challenge which takes skill, practice, time, suitable crimping tools, and other tooling in many cases.

  Further, these completed cable assemblies need more than just a simple continuity test; they also need to be checked for RF performance factors such as bandwidth and flatness, impedance discontinuities, loss, and phase shift, to cite just a few factors. These electrical tests take time and require sophisticated measurement equipment, and the assemblies need mechanical ruggedness added via strain relief.

  Fortunately, cable assemblies are available in many lengths as standard, stocked items for the most common cable and connector types. They also come in custom lengths and connector pairings with fairly short delivery times. Consider, for example, the Würth 65503503530505, a 12 inch/305 mm cable assembly with a straight SMA male plug on each end, using RG-316 coaxial cable (0.102 in/2.59 mm outside diameter), with heat shrink tubing added over the connector/cable junctions for strain relief and ruggedness (Figure 18).


Image of Würth Elektronik 65503503530505 is a standard 12-inch coaxial cable assembly

  Figure 18: The 65503503530505 is a standard 12-inch coaxial cable assembly using RG-316 cable with straight SMA male plugs on each end; note the strain relief between connector and cable. (Image source: Würth Elektronik)

  The datasheet for this cable assembly includes comprehensive mechanical and material details and dimensions, as well as guaranteed specifications for VSWR (1.3) and insertion loss (1.2 dB) from DC to 6 GHz. There is also a chart showing attenuation versus frequency per 100 feet, so users can quickly determine the attenuation for this or any chosen length of cable assembly style (Figure 19).


Image of attenuation versus frequency for the Würth Elektronik 65503503530505 cable assembly

  Figure 19: Shown is the attenuation versus frequency for the 65503503530505 cable assembly. (Image source: Würth Elektronik)

  The wide range of vendor-supplied cable assemblies is not limited to having the same connector type at each end, but can instead also directly address interconnect and transition issues as well. For example, the 65530260515303 is a short (6 inch/152 mm) cable assembly using RG-174 cable with an RP-SMA bulkhead male jack on one end and a straight MMCX male jack on the other (Figure 20).


Image of the Würth Elektronik 65530260515303 assembly uses RG-174 cable

  Figure 20: Cable assemblies can also be used as transitions between different connector families; the 65530260515303 assembly, for example, uses RG-174 cable and has an RP-SMA bulkhead male jack on one end and a straight MMCX male jack on the other. (Image source: Würth Elektronik)

  There’s one more thing to keep in mind with these connectors and their cable assemblies: they are small and sometimes difficult to handle when tightening or loosening their threaded body. At the same time, they need to be torqued to a specified value: too little torque and they may not make reliable contact; too much and their threads may be stressed and deformed, causing their number of mating/unmating cycles to be reduced. For this reason, Würth Elektronik offers the 6006330101 WR-Tool, a small torque wrench for all WR-SMA connectors (Figure 21).


Image of Würth Elektronik 6006330101 WR-Tool ensures that the SMA connector threaded body

  Figure 21: The 6006330101 WR-Tool ensures that the SMA connector threaded body is properly and consistently torqued, which is often challenging given the small size of the SMA body. (Image source: Würth Elektronik)

  The use of this tool ensures that the applied connector torque is at the specified level, thus ensuring proper contact mating, maximizing reliability, and consistent performance.


  Designers of RF circuits and systems with frequencies extending into the gigahertz range have a choice of connectors with different sizes, body styles, gender arrangements, and other critical parameters. By selecting a connector with appropriate electrical and mechanical specifications, and torquing it correctly, the challenges of ensuring reliable, consistent, low-loss signal paths between circuits, subcircuits, and systems are minimized.


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