We introduce a new concept for combined probing of topography and local AC fields, and report preliminary results of tip fabrication and scale-model measurements. The scanning force microscope (SFM) has been recently used to measure AC fields using the nonlinear response of the cantilever to voltage differences between the sample and scanning tip. By contrast, we extend the concept of near-field scanning optical microscopy (NSOM) down in frequency by employing a non-cutoff coaxial micromachined waveguide/tip, maintaining the topographic and tip-sample distance control capabilities of the SFM. Because this instrument will be used to measure both AC fields and topography with nanometer-level localization and better than nanosecond temporal resolution, we call it a nanoscilloscope.