Nonlinearity of Anti-Parallel Schottky Diodes For Mixer Applications

Nonlinearity of Anti-Parallel Schottky Diodes For Mixer Applications

Abstract

This work investigates the nature of 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 and this property makes it very attractive at millimeter-wave frequencies. The sources of nonlinearity in a metal-semiconductor Schottky junction are the junction resistance caused by the thermionic emission of electrons across the barrier and the junction capacitance resulting from the charge seperation across the barrier. The mixing terms of utmost importance in an anti-parallel diode pair mixer are the wanted fundamental frequency converted product, the unwanted third-order frequency converted product and the undesirable second-order local oscillator breakthruough. This work aims to discover which; among the nonlinear sources is the primary contributor in the generation of unwanted third-order products. An anti-parallel diode pair has an anti-symmetrical current-voltage characteristic about the origin and a symmetrical capacitance-voltage characteristic about the Y-axis. In other words the current in the diode pair is an odd function of applied voltage whereas the capacitance is an even function of applied voltage. When both nonlinearities are described by a polynomial model extracted from measurements, the resistive nonlinearity contains only odd power terms and the capacitive nonlinearity is a sum of even power terms. The nonlinear resistance with only the odd power terms will be the dominant of the two in generating the unwanted third-order products where as the capacitive nonlinearity will be the primary contributor of local oscillator breakthrough. This work supports such an assertion by means of analysis, simulation and measurement. The results demonstrate the dominance of nonlinear resistance over junction capacitance in the generation of third-order products in an anti-parallel diode pair mixer. Understanding the nonlinearities involved also enables us to optimise the physical structure and geometry of the diode for good mixer performance.