Apparatus to measure the correlation between two wideband (10–100MHz) noise signals has been developed for the stellar intensity interferometer at the Narrabri observatory. This instrument is used to measure the angular diameters of stars in the range 0.01–0.0001″ of arc, with an accuracy within about 5%. To achieve this precision with the low signal/noise ratio of an intensity interferometer, it is necessary to integrate and average the data for periods up to 100h, which may involve observations spread over several weeks. The paper describes the novel techniques which have been used to obtain the necessary efficiency and stability in the electronic correlator.