The focus of this work is the testing of RF signal paths, specially mixers. The presented technique is based on spectral analysis and subsampling. It enables the partitioning of the RF signal path, making it easier to locate faulty stages. A synchronization scheme is proposed in order to enable the direct sampling of the high frequency signal, thus avoiding the use of an extra anti-alias filter. Furthermore, as the technique is done mainly digitally, it is suitable to be implemented by a common external digital tester, or even in a BIST scheme for an SoC environment. Theoretical background and experimental results are provided in order to evaluate the feasibility of the method.