In order to operate in a satisfactory matter, a mixer needs to have well controlled amplitude, phase response, and group delay. Constant group delay is especially important in wireless and satellite communications systems and in applications involving high target resolution in surveillance systems. When mixers are used to measure group delay in frequency converters, some setbacks can occur if the local oscillator isn’t accessible. As device integration and miniaturization continues to increase, local oscillators are becoming less and less accessible. This application note explains how one of Rohde & Schwarz’s vector network analyzers enables absolute group delay on inaccessible local oscillators without the need of a calibration mixer.