Closed-loop quadrature converter

Circuits, methods, and apparatus that can provide in-phase and quadrature LO signals having an accurate phase relationship and matching amplitudes. An example can provide a quadrature modulator having phase-correction circuitry and amplitude-correction circuitry. The quadrature modulator can include a quadrature hybrid to receive an LO signal from a phase-locked loop or other clock source. The quadrature hybrid can provide in-phase and quadrature LO signals to phase-correction circuitry, which can accurately align a difference in the phase of the in-phase and quadrature signals to 90 degrees. The amplitudes of the resulting signals can be adjusted to have a desired amplitude using amplitude-correction circuitry. The amplitude-and-phase-corrected LO signals can be used to modulate in-phase and quadrature components of a baseband signal. The modulated products can be combined, gained, and filtered as necessary for wireless transmission, device testing, or other purpose.