Sub-harmonic mixers are the core element of terahertz room temperature, high spectral resolution heterodyne receivers for planetary science, and remote sensing. Here, terahertz sub-harmonic mixer up to 400 GHz using discrete Schottky diode is presented. Measured performance is in agreement with results from the linear and nonlinear co-simulations, and this methodology shows its practicability for the discrete planar GaAs Schottky diode-based terahertz core circuit design.