The key goal of this paper consists of a theoretical and an experimental performances analysis of a photonic microwave mixer used as a signal down- and up-converter based on cross-gain modulation in a semiconductor optical amplifier (SOA). We give, by a small-signal analysis approach, the analytical equations of the output optical power carrying the down- and up-converted signal. Simplified formulas are developed, relating the mixing conversion gain to the modeling parameters of the SOA. The efficiency of the photonic microwave mixer based on an SOA has been evaluated in terms of mixing conversion gain, third-order input intercept point, and electrical phase noise. In addition, a subcarrier modulated by a QPSK, a 16 quadratic-amplitude modulation (QAM) and a 64 QAM at a data rate of 270 ksymb/s has been down converted from 1 GHz to 100 MHz with a low error vector magnitude of about 3.5%.