This paper presents an extension of the ANC ("Analogue Network of Converters")-based method, which is an original Design-for-Test (DfT) technique associated to a dedicated test algorithm to characterize the harmonic components of a set of embedded converters using only digital test resources. The ANC-based method was primar- ily developed under the assumption that the harmonics' phase is proportional to the input phase. This assumption is not valid for all converter architectures, where filtering effects may affect the harmonics' phase. The improved ANC-based method is able to calculate the magnitude of the harmonic components with unknown phase. The fundamental principle of this improved version of the ANC-based method is the same, but further mathematical developments have been established using a model independent from the harmonics' phase. The simulation results and the experiments show an excellent agreement between the values measured using the method and the values measured with a usual test setup, for the THD and SFDR parameters. Simulations were carried out considering both random phases and realistic phase delays such as the ones induced by a low pass filter.