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 50-ohm coaxial test cable to a 50-ohm 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 9 GHz 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. |S-21| > -3 dB such as in the DPI standard [1], but that one also needs to specify a maximum value for the reflection coefficient (e.g. |S-11| < -18 dB) to limit the frequency ripple of the transmitted power during an actual test.