A time-discrete harmonic rejection mixer circuit comprising: a parallel set of N unit cells that quantize a mixer transconductance into N + 1 selectable levels, each level selectable by application of a corresponding digital control word to the set of unit cells; a mixer input port coupled to an input of the set of unit cells through a source termination conductance; and a mixer output port coupled to an output of the set of unit cells through a load termination conductance; a quantization function representing the transconductance levels between said input and said output in dependence of digital control words; the conductance values of said source and load termination conductances resulting in said quantization function being non-linear and being configured so that a subset of the quantization levels better match a set of nominal transconductance points corresponding to a desired sinusoidal Local Oscillator waveform than would be achieved with a linear quantization of said mixer transconductance; and a digital sequencer configured to store or otherwise generate a sequence of digital control words corresponding to said subset of quantization levels, and further configured to apply said sequence of digital control words to the set of unit cells at a timing corresponding to a frequency of the sinusoidal LO waveform; wherein said sequence of digital control words corresponding to said subset of the quantization levels comprises a first sequence representing a first phase offset for the desired sinusoidal LO waveform, and wherein the digital sequencer is configured to store or generate one or more additional sequences representing one or more additional phase offsets, and to select one of said sequences as indicated by a phase control signal input to said digital sequencer. a parallel set of N unit cells that quantize a mixer transconductance into N + 1 selectable levels, each level selectable by application of a corresponding digital control word to the set of unit cells; a mixer input port coupled to an input of the set of unit cells through a source termination conductance; and a mixer output port coupled to an output of the set of unit cells through a load termination conductance; a quantization function representing the transconductance levels between said input and said output in dependence of digital control words; the conductance values of said source and load termination conductances resulting in said quantization function being non-linear and being configured so that a subset of the quantization levels better match a set of nominal transconductance points corresponding to a desired sinusoidal Local Oscillator waveform than would be achieved with a linear quantization of said mixer transconductance; and a digital sequencer configured to store or otherwise generate a sequence of digital control words corresponding to said subset of quantization levels, and further configured to apply said sequence of digital control words to the set of unit cells at a timing corresponding to a frequency of the sinusoidal LO waveform; wherein said sequence of digital control words corresponding to said subset of the quantization levels comprises a first sequence representing a first phase offset for the desired sinusoidal LO waveform, and wherein the digital sequencer is configured to store or generate one or more additional sequences representing one or more additional phase offsets, and to select one of said sequences as indicated by a phase control signal input to said digital sequencer.