Technology Update by John D. Summerville

Coaxial interconnects (connectors and cable assemblies) are sometimes considered merely mechanical interfaces between radio frequency (RF) and microwave components. They are selected by packaging engineers for mechanical convenience rather than by microwave and reliability engineers for performance and long-term design stability. Treating a coaxial connector as a commodity in today's competitive market however, is not without risk.

In critical applications, coaxial interconnects must be treated like an RF or microwave component, and their effect on performance and long-term reliability must be considered. This is particularly important in systems that are exposed to severe environments and operate over large instantaneous bandwidths, where the frequency-dependent voltage standing wave ratio (VSWR) of the interconnect medium can cause unacceptable variations in composite peak-to-peak insertion loss.

Many coaxial connectors are currently manufactured to de facto military specifications such as MIL-PRF-39012 and MIL-STD-348, which have loose mechanical requirements and place little emphasis on microwave performance, thermal integrity or long-term reliability. To achieve optimum performance, a coaxial connector must have appropriate transitional compensation in the housing, contact and dielectric, and it must be machined to tight tolerances.

Often, the effect that the method of contact captivation can have on the performance and reliability of a coaxial connector or cable assembly is overlooked. Precision connectors and adapters use epoxy to captivate the housing, dielectric and contact without putting the dielectric under pressure. To minimize RF leakage through the epoxy cross-hole, it can be backfilled with a silver-loaded epoxy. Assemblies using precision connectors matched to high-performance cable provide stable low-insertion loss, low VSWR interconnects and high reliability.

The small investment in premium connectors and cable assemblies can reduce the cost of components and systems by improving performance, increasing yield and shortening tuning time. Delivered units tend to stay delivered. This is important because returns are major logistics drivers and increase the user's cost of ownership over the lifetime of a fielded system.

A safe design guideline is to use precision connectors and cable assemblies in critical applications above 4 GHz. Consider the SMA, which is the most common miniature connector in use today. Extended range devices that are typically mode free to 26.5 GHz are a cost-effective choice to improve performance, enhance reliability, and reduce total program lifetime cost and downtime.