or interconnect system will work as designed and that signals will not be degraded as they pass through an interconnect. A conductor that has some non-uniformity will also have some impedance variations. This creates reflections along the length of the conductor, and the magnitude of these reflections can be described in terms of return loss. This type of return loss is called structural return loss. The impedance variations along the length of the conductor are compared to the conductor’s rated or desired impedance when calculating structural return loss. Structural return loss is common in poorly manufactured coaxial cables, where variations in impedance along the cable length cause signal reflections for a wave travelling along the cable. These impedance variations can arise from variations in conductor thickness, spacing between the shielding, or both. It is therefore vital that only quality cables are used to avoid structural return loss. In a PCB, a number of design problems can produce the same effect. Variations in trace geometry due to length tuning methods, vias along interconnects, and inconsistent return path/ trace spacing due to warpage create impedance discontinuities along the length of a transmission line. If an interconnect is not designed correctly, incident signals reflect back to the source from these impedance discontinuities. In general, impedance discontinuities and structural return loss will arise if signal paths are not properly designed. Impedance discontinuities
may be capacitive or inductive, depending on the geometric variations along a conductor, producing larger structural return loss in specific frequency ranges. To further combat structural return loss high quality RF coaxial connectors are required to continue the superior signal path. A poorly designed connector will cause the same issues with continuity of signal path, resulting in an increase in return loss. A well-designed RF coaxial connector made with the correct materials for the application will ensure the signal path is uninterrupted from the first connector, through the PCB and/or cable, to the last connector, and on to the next device. When developing a new RF connector design for use on a PCB, the launch has to be considered right from the start. The transition from an air dielectric and a printed circuit substrate can create problems that cannot be fixed with a board redesign later; they must be mitigated at the start. Now that the signal is on the board, transitioning onto an uninterrupted transmission line is vital. Close cooperation with the board manufacturer is critical at this stage. Among the most commonly used interfaces for high-speed applications are MCX, MMCX, and SMA. These subminiature connector series offer precision design, high frequency capabilities, and good signal integrity. Their compact size and large number mean that most high-speed applications can be catered to and large numbers of connectors can be installed in a compact area.
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