This paper deals with a simple and low-cost test setup capable to measure the impedances needed for designing a FET resistive mixer. The proposed method uses an external signal generator along with one port S-parameters measurement and signal flow theory to determine the impedances at the RF, LO and IF frequencies. A packaged GaN FET is used to design a high linearity FET resistive mixer suitable to down-convert a 2.4 GHz LTE signal to a 0.1 GHz IF signal and the experimental results show a conversion loss of 6.9 dB and ACLR better than 45 dBc.