Apparatus for generating an output dependent upon the impedance or at least one component of the impedance of a device, the apparatus comprising: a load component having a known impedance or at least one component thereof for connection in series with the device to allow for the measurement of a voltage drop across the load component; a generator arrangement (22,23,27,28) for applying a signal having a voltage to the series connected load component and device; a measurement arrangement (23,29,30,31) adapted to measure a first voltage one side of said load component, and one of a second voltage on the other side of said load component or a difference voltage comprising the voltage difference across said load component; and a signal processing arrangement for processing the measurements to generate an output dependent upon the impedance or at least one component of the impedance of the device, wherein said processing arrangement is adapted to: determine a first parameter indicative of the complex amplitude of the first voltage on a first side of said load component connected to said device, and a second parameter indicative of the complex amplitude of said difference voltage or a calculated difference voltage comprising the difference between the first and second voltages; multiply each of the first and second determined parameters by the complex conjugate of the second determined parameter to generate third and fourth parameters respectively; and compare said third and fourth parameters to generate an output or compare one or more components or derivatives of the third parameter and said fourth parameter to generate said output. a load component having a known impedance or at least one component thereof for connection in series with the device to allow for the measurement of a voltage drop across the load component; a generator arrangement (22,23,27,28) for applying a signal having a voltage to the series connected load component and device; a measurement arrangement (23,29,30,31) adapted to measure a first voltage one side of said load component, and one of a second voltage on the other side of said load component or a difference voltage comprising the voltage difference across said load component; and a signal processing arrangement for processing the measurements to generate an output dependent upon the impedance or at least one component of the impedance of the device, wherein said processing arrangement is adapted to: determine a first parameter indicative of the complex amplitude of the first voltage on a first side of said load component connected to said device, and a second parameter indicative of the complex amplitude of said difference voltage or a calculated difference voltage comprising the difference between the first and second voltages; multiply each of the first and second determined parameters by the complex conjugate of the second determined parameter to generate third and fourth parameters respectively; and compare said third and fourth parameters to generate an output or compare one or more components or derivatives of the third parameter and said fourth parameter to generate said output. determine a first parameter indicative of the complex amplitude of the first voltage on a first side of said load component connected to said device, and a second parameter indicative of the complex amplitude of said difference voltage or a calculated difference voltage comprising the difference between the first and second voltages; multiply each of the first and second determined parameters by the complex conjugate of the second determined parameter to generate third and fourth parameters respectively; and compare said third and fourth parameters to generate an output or compare one or more components or derivatives of the third parameter and said fourth parameter to generate said output.