Down-converters using Schottky-barrier diodes

The analysis of noise in down-converters, in which signal and image ports are equally terminated, is extended to include the effect of a nonlinear junction capacitance. The theory is applied to Schottky-barrier diodes, which are represented by a constant resistance in series with a parallel combination of a nonlinear conductance and nonlinear capacitance pumped in phase. Using numerical optimization procedures, the minimum overall receiver noise figure, the associated conversion loss, and the required optimum external terminations are calculated and compared with experimental results. It is shown that in a resistive mixer with equal termination of signal and image ports, the addition of a nonlinear capacitance can lower the conversion loss substantially; but the overall noise figure remains nearly invariant. A conversion loss as low as 3.5 dB was measured (3.3 dB calculated) for a silicon Schottky-barrier diode operated at 3 GHz, driven by 0.1 mW LO power, and without dc bias. The overall noise figure, including an IF amplifier noise figure of 1.5 dB, however, stays at 5.7 dB (5.5 dB calculated).