This paper presents a technique for performing high frequency measurements with a scanned electrostatic force microscope. The technique is used to non-invasively measure the local voltage waveform on the interconnects of an operating integrated circuit. The signal voltage at a point on the circuit is extracted by nulling the electrostatic force interaction between a small probe and the test point. A heterodyne approach is used where a high frequency sampling signal, modulated at the lower probe resonance, is applied to the probe and the nonlinear force interaction between die probe and circuit mixes the signals. This allows the measurement of signal frequencies which are much greater than the mechanical response of the probing mechanism. Accurate measurements can be performed without complex calibration or probe positioning requirements, and the technique is not sensitive to dc offset effects. The ability to measure passivated circuits is discussed.