Frequency Extension up to 9 GHz of the Coaxial-to-Microstrip Transition on a 4-Layer IC-level EMC Test Board

Standardised IC-level EMC test boards need to be equipped with coaxial connectors to connect them to the EMC/RF test equipment. An optimisation of the footprints of a common through-hole SMA coaxial connector (connecting a $\mathbf{5 0}-\Omega$ coaxial test cable to a $50-\Omega$ microstrip line on a 4-layer FR4 test board) was carried out. In this way, we were able to extend the useful frequency range from about 2 GHz for a typical transition to at least $\mathbf{9} \mathbf{G H z}$ without any need to switch to RF-grade connectors (e.g., edge-fed or SMD), high-frequency PCB materials (e.g., PTFE), or more complex transmission lines (e.g., grounded coplanar lines). It was found that in order to ensure a good connector performance, it does not suffice to specify a minimum value for the transmission coefficient of the injection path (e.g. $\left|S_{21}\right|\gt-3 \mathrm{~dB}$ such as in the DPI standard [1], but that one also needs to specify a maximum value for the reflection coefficient (e.g. $\left|S_{11}\right|\lt-18 \mathbf{d B}$) to limit the frequency ripple of the transmitted power during an actual test.