The aim of this project is to create a test fixture that is both mechanically robust (therefore based on 1.6mm PCB) and enables accurate S parameter measurement. The latter requires a ground plane spacing less than the 1.6mm of a double sided PCB. A four layer build has been chosen that uses the standard RF prototype build from Eurocircuits based on outer layers of I-Tera substrate and an inner core of FR4 prepreg. This gives better transmission line impedance control than purely FR4 without the cost of expensive PTFE based laminates.
The SMA edge launchers interfaces with a transmission line that uses the full 1.6mm groundplane spacing. The transmission line is then reduced in groundplane spacing in two steps as internal groundplane layers are introduced under the line, all the while maintaining 50 ohms by adjusting track width and top side ground spacing.
Each section of transmission line was optimized using the TNT EM modelling tool. The transitions were optimized using the Sonnet Lite EM simulator. The simulation design files are included in this project as attachments.