Abstract: This is an apparatus for and a method of measuring the time coherence of high frequency signals fed into different points along a high frequency transmission line regardless of where the input signal enters the line or what the propagation delay of the line is.This describes a method and apparatus for measuring multiple unknown signals from different signal sources coupled into different points on a transmission line, which measurement is independent of the physical distances between the signal sources and the measurement device and of the propagation delay. In essence, any transmission medium positioned between the signal source and the measurement device becomes effectively transparent. This means that there is minimal signal loss and no phase change or propagation delay change due to physical position is realized. Thus, significant advantages are realized from the present invention.