Method and apparatus for nulling local oscillator feedthrough

摘要

A method and apparatus for nulling local oscillator feedthrough, suitable for use in superheterodyne receivers, is disclosed. A local oscillator (10) produces a signal (VLO) that is split by a signal splitter (12) into two signals (V₁ and V₂). V₁ is mixed with an input signal (Vs) by a mixer (30), producing a mixed signal (Vm). V₂ bypasses the mixer (30) by way of a feedforward circuit (38) and is modified by a polarity and amplitude control circuit (16) and a low-pass filter and phase control circuit (18). The output of the feedforward circuit (38) is a nullifying signal (V₂′). V₂′ is combined with an amplifier mixed output signal (Vm′) in a signal combiner (34) to produce a combined signal (Vc) that contains a minimized local oscillator feedthrough component (V₁′ component). Digital-to-analog converters (20) and (22) convert control data produced by a microprocessor (24) into analog control signals (Va an Vp) for controlling the polarity and amplitude control circuit (16) and the low-pass filter and phase control circuit (18). The polarity, amplitude and phase of V₂′ substantially nullifies the V₁′ components contained in Vm.