Determination of the spectral density of plasma waves from the cross‐power density spectrum is shown to be simpler than the alternative determination from the cross‐correlation function. A device for measuring the cross‐power spectral density is described in detail. A model with a bandwidth of 30 MHz has been constructed using integrated circuits, and applied to the study of ion acoustic wave propagation on a plasma positive column. Since it is an analog device, it should be possible to extend the bandwidth for operation into the gigahertz range. Only a fixed delay is required to measure the quadrature component of the cross‐power spectral density. These features are in contrast to other correlators and cross‐power spectrum analyzers described to date, which are either limited to the low megahertz range, or require a variable delay line.