A method and apparatus for frequency down-converting a radio frequency (RF) signal to baseband in two steps using the two quadrature components of a single local oscillator. In the first stage, the RF signal is mixed with both components of the local oscillator signal (LO) in a sub-harmonic mixer to produce an intermediate frequency (IF) output signal having a frequency component at twice the local oscillator frequency minus the RF frequency, i.e., (2.times.LO)-RF=IF. In the second stage, the IF signal is split into two and supplied to a quadrature mixer that mixes the IF signal with the I component of the same local oscillator signal and separately mixes the IF signal with the Q component of the same local oscillator signal to produce an output signal comprising an I component and a separate Q component at baseband.