Advances in Coaxial Connectors: Solder Versus Screw Mount
Coaxial connector performance is vital in RF applications. However, the connector’s role in system performance is often an afterthought in the design. Although seemingly a simple and straightforward component to specify, selecting an RF connector that can enhance and improve equipment performance can be an arduous task. Understanding the role of the RF connector and selecting the optimum solution can increase performance, reliability and durability of the system. This is particularly important for sensitive laboratory equipment and test & measurement (T&M) instruments. These instruments require high-performance RF connectors that deliver accurate, repeatable results. Because of today’s rapidly expanding communication technology platforms, modern test & measurement equipment requirements include significantly higher bandwidth, data rates and speed—all in a smaller package. Advancements in screw mount coaxial RF connectors have been essential to meeting these new, high-performance demands.
T&M Market Factors
Test and measurement equipment is critical to the development and production of electronic devices, particularly when it comes to RF technologies. Test and measurement solutions used wherever signals—from the audio range up to the highest microwave frequencies—must be generated or analyzed, whether in the lab or on the production floor. Test & measurement equipment is used to test these signals to determine RF characteristics in various electrical systems.
With the evolution of consumer desires that continuously demand smaller devices with more power (longer battery life) and functionality (video on demand), test & measurement equipment must keep pace in terms of handling more data and improving testing throughput. Test and evaluation boards can utilize as many as 128 coaxial connectors at one time, testing a wide range of performance characteristics and parameters at once.
In addition, with 3G technology moving to 4G/LTE to 5G now currently being defined, chip designers, user equipment manufacturers, infrastructure suppliers, network operators and test houses are introducing new innovations that require higher frequency testing to 50 GHz and higher.
T&M Application Requirements
The choice of connector type is critical and is defined by the application requirements, including frequency range, power handling, physical dimensions, etc. A primary consideration in test and measurement equipment is the number of connect/disconnect cycles that a connector pair can withstand while still performing as specified. RF and microwave connectors are precision-made parts, and can be easily damaged by mistreatment, ultimately reducing repeatability and accuracy of the test instruments.
To deliver the high speeds and bandwidth, a differential pair connectivity method is often utilized. Differential signaling is a technique by which information is electrically transmitted using two complementary signals. This method sends the same electrical signal as a differential pair of signals, each in its own conductor. These differential pairs need to be matched, as to not introduce a phase shift. This could introduce interference or signal degradation, which would deliver inaccurate measurements.
To reach the high speeds and throughput requirements in modern laboratory equipment and test & measurement instruments, up to 64 I/Os are commonly being tested at once. This means 128 connectors are attached to the test or evaluation board at once. As a result, sensitive laboratory equipment and test & measurement instruments are often subjected to space constraints.
Coaxial lines represent the most efficient method of transmitting signals from a source. Once used primarily for network cabling applications, coaxial cabling is now used in a wide range of applications including industrial, military, test & measurement, and more. Coaxial cabling is moderately inexpensive, lightweight, flexible, and simple to handle. Coaxial connectors ease mating, joining two components by simply screwing the plug and jack pairs together. This is much faster than soldering or hard-wiring connectors. In addition, the coupled connectors can be easily unscrewed when needed, simplifying infield repair and maintenance.
There are two primary ways to connect coaxial connectors to test and evaluation boards depending on the application demands; the end launch (horizontal) version and the vertical launch type.
Most edge launch, or edge mount, connectors require soldering. However, new high-speed compression screw mount end launch connectors eliminate soldering. These new compression screw mount edge launch connectors, like Hirose’s 8.4mm (0.331 in) wide model, feature a smaller pitch and offer significant board real estate savings. One of the smallest coaxial connector types in the industry, this end launch coaxial design allows more connectors to be mounted on a PCB board. This maximizes test & measurement throughput.
The vertical launch type connectors are typically a lower cost option that solves the challenge of getting to a mid-board location. High-frequency vertical launch connectors feature one-piece contacts for lower loss, non-solder compression screw mount termination.
Both the end launch and vertical launch are typically available in high performance versions, such as Hirose’s 2.92mm 40 GHz HK Series and 2.4mm 50 GHz H2.4 Series.
High-speed compression-mounted coaxial connectors also simplify the assembly process and provide several electrical and mechanical advantages over soldered versions. Coaxial connector designs inherently offer excellent EMC / RFI shielding and protection, ensuring interference does not affect performance. This is vital in test & measurement applications, where interference can compromise the test by introducing faulty data. Interconnects used in test & measurement applications must be matched without introducing measurement errors. The interconnects need to be “invisible” so the user actually tests the device under test (DUT), not the interconnects.
Traditional soldered through-hole terminations can introduce the potential for human error. Variations caused by soldering can cause signal reflection via the stub protruding through the board.
A screw mount design removes this human error variable and ensures a high reliability connector with excellent high-frequency performance and consistent assembly quality. The compression-mounted connectors feature a stub-less design with screw mounting to the PCB, and a center contact that mounts flush with the PCB. This eliminates the stub and solder, thus significantly reducing signal reflection. Coaxial connectors also optimize design flexibility as they can be used with PCB boards of various thicknesses. The solderless compression screw mounting not only reduces installation time, but also inspection time. In addition, the connector is reusable and can be attached and removed from boards for multiple uses, saving time and costs.
In its simplest form, a traditional coaxial cable consists of a core of copper wire surrounded by insulation, a braided metal shielding, and an outer cover. The metallic conductors are usually plated with brass, silver or gold.
Ruggedized, higher performance coaxial connectors are designed with stainless steel that meets MIL-STD-348B standards and are ideal for use with mmWave and high-speed digital evaluation boards. The precision machining from stainless steel provides robust designs that can withstand the strain from test cables while maintaining a 50 Ohm impedance over a broad bandwidth. In addition, these more reliable connectors are not plated, eliminating flaking and any potential deforming during mating/unmating. RF connector performance is dependent on tightly held mechanical tolerances, as any deformities can significantly affect signal, frequency and bandwidth performance.
The RF connector’s contribution to system performance is often overlooked, but cannot be ignored. The connector must be a primary design consideration to ensure the performance of critical equipment. The continually evolving demands of higher frequency applications like sensitive laboratory equipment and test & measurement instruments have led connector developers to push the envelope and achieve higher frequencies, smaller footprints, and increased performance. Connector manufacturers will continue to develop new products to keep up with demands and trends, including 1.85mm (67 GHz products) and 1.0mm (110 GHz) products.
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