A signal-generating apparatus comprising: a quadrature modulation block comprising: a first digital-to-analog converter for receiving digital signals corresponding to an in-phase waveform component; a second digital-to-analog converter for receiving digital signals corresponding to a quadrature waveform component; a first anti-aliasing low-pass filter receiving an output of the first digital-to-analog converter; a second anti-aliasing low-pass filter receiving an output of the second digital-to-analog converter; a first mixer (111; 313) receiving an output of the first anti-aliasing low-pass filter; a second mixer receiving an output of the second anti-aliasing low-pass filter; a 90° splitter having an input, an in-phase output for mixing at the first mixer, and a quadrature output for mixing at the second mixer; a first frequency synthesizer (121; 401) having an output to the input of the 90° splitter; a summing unit for summing an output of the first mixer with an output of the second mixer; and a third mixer receiving an output of the summing unit of the quadrature modulation block; a second frequency synthesizer (123; 407) having an output to the third mixer; and a switch having an output selectable between the output of the summing unit and an output of the third mixer. a quadrature modulation block comprising: a first digital-to-analog converter for receiving digital signals corresponding to an in-phase waveform component; a second digital-to-analog converter for receiving digital signals corresponding to a quadrature waveform component; a first anti-aliasing low-pass filter receiving an output of the first digital-to-analog converter; a second anti-aliasing low-pass filter receiving an output of the second digital-to-analog converter; a first mixer (111; 313) receiving an output of the first anti-aliasing low-pass filter; a second mixer receiving an output of the second anti-aliasing low-pass filter; a 90° splitter having an input, an in-phase output for mixing at the first mixer, and a quadrature output for mixing at the second mixer; a first frequency synthesizer (121; 401) having an output to the input of the 90° splitter; a summing unit for summing an output of the first mixer with an output of the second mixer; and a first digital-to-analog converter for receiving digital signals corresponding to an in-phase waveform component; a second digital-to-analog converter for receiving digital signals corresponding to a quadrature waveform component; a first anti-aliasing low-pass filter receiving an output of the first digital-to-analog converter; a second anti-aliasing low-pass filter receiving an output of the second digital-to-analog converter; a first mixer (111; 313) receiving an output of the first anti-aliasing low-pass filter; a second mixer receiving an output of the second anti-aliasing low-pass filter; a 90° splitter having an input, an in-phase output for mixing at the first mixer, and a quadrature output for mixing at the second mixer; a first frequency synthesizer (121; 401) having an output to the input of the 90° splitter; a summing unit for summing an output of the first mixer with an output of the second mixer; and a third mixer receiving an output of the summing unit of the quadrature modulation block; a second frequency synthesizer (123; 407) having an output to the third mixer; and a switch having an output selectable between the output of the summing unit and an output of the third mixer.