Disclosed is an apparatus and methodology for characterization of small devices. On-chip subtraction of parasitic effects including coupling capacitive effects is provided by way of a rat-race employing a pair of gaps. A device or material being tested is positioned in a test position gap and an output signal is extracted from the rat-race at a position displaced along the rat-race between the test position gap and the other gap to provide subtractive cancellation of any parasitic effects associated with the rat-race and especially associated with the test position gap.