A single channel microwave interferometer has been developed for a tokamak with an improved detection that uses an heterodyne configuration and is based on a differential quadrature phase concept. With the new detection very sensitive and accurate direct phase measurements are obtained due to the elimination of the spurious variations of the detected signals resulting from the nonideal behavior of the output mixers. As a result, the line integrated density along the vertical central chord of the vacuum chamber can be determined during the whole plasma discharge. Fast density variations occurring both at the formation phase of the plasma and during sawtooth cycles could also be measured. ©1995 American Institute of Physics.