RF Board Connector: Footprint Optimization For Optimal Return Loss Performance

The continuous demand for high data rates (56 Gbps and beyond) and mmWave frequencies (up to 90 GHz) necessitates that RF board connectors, such as the MXPM and MXPM90, function as an "electrically transparent" component within the test setup. Using a generic connector footprint can be disastrous for performance, causing unwanted reflections, resonances, and a significant reduction in bandwidth.

To achieve the full specified performance, the PCB footprint must be custom-optimized for the specific board configuration, stack-up, and transmission line type (e.g., GCPW). This optimization requires electromagnetic (EM) 3D-field simulation to carefully manage the transition where the coaxial TEM wave converts to the board’s planar mode.

Key techniques focus on mitigating parasitic capacitance at this critical transition point. This includes implementing a ground cutout underneath the signal pad on internal ground planes. Additionally, a compensation line is used on the signal layer to provide a gradual, optimized impedance transition, counteracting the mismatch caused by common PCB manufacturing tolerances.

Implementing this optimized design dramatically improves performance. Simulations show that non-optimized Return Loss (RL) performance, which can fall below 10 dB at high frequencies, is corrected to better than 18 dB across the full 70 GHz or 90 GHz operating range, validating the necessity of a custom footprint for every high-speed design.