This document describes the features and functions of the HMC129 GaAs MMIC double-balanced mixer. It is ideal for applications such as microwave and VSAT radios, test equipment, military electronic warfare, ECM, C3I, and space telecommunications. The chip is a miniature double-balanced mixer that can be used as an upconverter or downconverter in the 4 to 8 GHz frequency range. It can be integrated directly into hybrid MICs without DC bias or external baluns, providing an extremely compact mixer. The mixer can operate over a wide LO drive input range of +9 to +15 dBm and performs well as a bi-phase modulator or demodulator.