An electronic measuring system for extending the effective measurement input frequency range of an electronic measuring instrument includes an electronic measuring instrument and a frequency downconverting subsystem, separate from the electronic measuring instrument, having one or more cascaded (i.e., series-connected) downconverting frequency extending units (FEU-Ds). Each FEU-D of the frequency downconverting subsystem includes a downconverting circuit (e.g., a block downconverter) enclosed within a housing. The frequency downconverting subsystem operates to downconvert a test signal from a device under test (DUT) to a downconverted test signal having a frequency within the permissible input frequency range of the measuring instrument, thereby extending the effective measurement input frequency range of the electronic measuring instrument.