This work investigates the nature of the nonlinearities in an anti-parallel combination of Schottky diodes, which is often used as a frequency converting device or a mixer. An anti-parallel Schottky diode pair mixer requires only half the local oscillator frequency. The mixing terms of utmost importance are, the wanted fundamental frequency converted product, the unwanted third-order frequency converted product and the breakthrough of the local oscillator signal. This work aims to discover how the nonlinearities inside the mixer can affect the generation of unwanted products. Frequency upconversion measurements of an anti-parallel diode pair mixer, fabricated on WIN semiconductor's 0.15μm low-noise process have been used to extract a polynomial model of the mixer nonlinearities. The polynomial function representing nonlinear resistance contains only odd-power terms and a second polynomial function encapsulating capacitance and its asymmetry comprises of both even and odd-power terms. The coefficients of the model are extracted by fitting to measured amplitudes of the third-order mixing products and the local oscillator breakthrough over a range of the local oscillator drive powers, commonly encountered in practise. The model so extracted is validated by comparing its predictions with measured amplitudes of wanted and unwanted frequency products, as functions of local oscillator power at various local oscillator frequencies. The form of the polynomial model indicates the dominance of nonlinear resistance in the generation of the unwanted third-order mixing products. It also points to asymmetry of the capacitance-voltage characteristic as a possible reason behind unwanted local oscillator breakthrough in anti-parallel diode pair mixers.