PCB RF Connectors

Key Takeaways

• How high speeds change from the basic operations below the RF range.

• How RF connectors interface with the board and each other.

• Parameters for selection among standard RF connectors.

PCB RF connectors support high-frequency communications while preventing signal loss and interference

The demands for increased data throughput are essential to modern PCB design. It is common to see consumer devices operate well into the GHz range in several applications, and with this increased frequency comes challenges to signal and power integrity. Best practices for maintaining performance extend to the board's layout and even the substrate materials. Still, the PCB RF connectors facilitate these communications and bear the foremost operating burden. Connectors must be selected for several features – size, impedance, and frequency passband – that reflect the best fit for various RF applications. 

RF Connector Color Code (Sourced From IEEE 287)

Note the differences with the resistor color code; here, violet is closer to the lower end of the spectrum

The Coaxial Cable: Synonymous With PCB RF Connectors

RF is a frequency range starting from 20 kHz – the upper range of average human audible frequencies – up to 300 GHz at the edge of the infrared spectrum. Designers may often informally refer to “high-speed design” at frequencies in the GHz range that require more specialized layout practices and materials than standard boards – this also falls under RF. These changes respond to how AC operates at high frequencies:

• The energy associated with the E/M waves at this frequency can meaningfully radiate into space.

• As frequency increases, the depth to which current penetrates a conductor decreases, resulting in a greater current density at the conductor's surface.

• Capacitive reactance decreases with increasing frequency, which allows RF AC to flow through dielectrics as if it were a short; conversely, inductive reactance becomes like an open in the presence of turns or bends in the conductor.

• Stubs in transmission paths can reflect and destructively interfere, decreasing the integrity of the signal and its maximum power delivery.

Mitigating or accentuating RF signal features led to the creation of the coaxial cable. The term coaxial refers to the inner conductor and outer shielding running along the same geometric axis, i.e., the outer shielding forms a concentric ring around the conductor when viewed at a cross-section. Coaxial cables are more precise than standard shielded conductors. By carefully controlling the radii of the inner and outer conductor along with the dielectric between them, coaxial cables can create a targeted impedance to match the signal transmission for maximum power and minimum standing wave ratio. Additionally, coaxial cables can restrict EMI by either tying the shield to ground or confining differential signals to the conductors to prevent leakage or external influence.

Multi-board RF systems may require inter-board coaxial connections to facilitate signal transfer. Mechanically, these connectors can be one of three available interfaces:

• Parallel - Two boards joined by a connector normal to both, i.e., a vertical connector for each board.

• Coplanar - The boards are joined by a connector that runs parallel to the surfaces of the boards.

• Right-angle - Also known as an elbow connector, a right-angle connector joins two boards perpendicular to each other. These connectors can be machined at right angles or molded to form a curvature on the outer jacket (also known as swept). Swept right-angle connectors offer better performance at high frequencies because the smooth transition between the connector openings allows for continuous dielectric coverage, unlike in the discontinuous right-angle connector. 

Connector Mating and Selection Criteria

It’s worthwhile to mention the sex of connectors. While this terminology may be depreciated in some settings, male and female connectors are not inherently interchangeable with plugs and receptacles/jacks, respectively. The sex of the connector refers to the center contact. In contrast, the connector that contains the coupling device on the outer conductor is a plug (the jack/receptacle lacks the coupling device). Often, a male connector is a plug while a female connector is a jack/receptacle, but reverse polarity connectors exist that defy the convention. These are often used as an extra preventative step for faulty connections, but this can also be accomplished with reverse-threaded connectors.

Regardless of the preferred naming convention, PCB RF connectors occasionally require adapters to adjust the interface within the same connector family. There are three different styles:

• Barrel - Two male ends. 

• Bullet - Two female ends.

• Connector saver - An adapter containing a male and female end on either side. Connector savers act as sacrificial elements for more expensive test equipment connectors or those that experience elevated connect/disconnect cycles. When one of the adapter ends is damaged, the connector saver can be cheaply and easily replaced compared to any onboard connectors.

Interfamily connections may also pose an issue during mating or operation if a few guidelines aren’t heeded. The lowest maximum frequency between the two connector families should be the limiting performance condition for signal integrity purposes. Furthermore, many combinations that are capable of threading can still damage connectors. Exercise caution when mating connectors where one (or both) are expensive, especially relative to the other, which may indicate a tighter level of tolerancing. In these cases, also be aware that the receptacle/jack may be more prone to damage than the plug; therefore, always mate the plug of the more expensive connector to the receptacle/jack of the less expensive connector wherever possible.

Common RF Connector Families

Cadence Stays Up to Speed With RF Design

PCB RF connectors are an increasingly integral component of any high speed design to prevent loss, distortion, and EMI.  As products continue to push into higher echelons of the RF spectrum, the importance of connector suitability will only continue to grow. 

With Cadence’s PCB Design and Analysis Software, development teams can quickly model and simulate circuits to ascertain electromagnetic compatibility according to industry benchmarks. This data can be imported to the fast and powerful OrCAD PCB Designer for a seamless layout process.

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