The present invention is directed to an apparatus and methodology for performing spurious-free dynamic range (SFDR) measurements on an RF circuit, such as a mixer, using a single analog input port. The present invention is designed for use when access to the intermediate frequency (IF) port in a radio frequency (RF) front-end circuit is not available, when the traditional two-port method for making an SFDR measurement is inadequate. Passing the analog input through a directional coupler between the RF combiner and the mixer facilitates the performance of the traditional third order intermodulation (IMD) test. Key differences between the single-port and traditional two-port setups are considered and examined, and experimental data obtained using the single-port setup is compared to data obtained using the traditional two-port set-up for different mixer models. Comparison of similar results yields confirmation and a calibration to account for the additional losses introduced by the directional coupler.