The design, construction, and performance of a 100‐MHz noise spectrometer are described. Basically a 100‐MHz bridge, this system uses the properties of a double balanced mixer to enable the experimenter to determine spectral densities of fluctuations in the real or imaginary parts of a sample impedance, as well as the real part of the cross‐spectral density. Calibration circuits and techniques are described. The results of measurements on commercial 50‐&OHgr; terminators demonstrate fluctuations of both the real and the imaginary parts of the impedance and partial correlation between them.