This thesis describes a portable special-purpose digital instrument which uses digital-computer processes to provide decimal readouts of the autocorrelation function (and hence the mean square value too) of a fluctuating voltage, and also of the measuring time involved. The mathematical concepts set out in a paper by R. Kitai and F. Diest¹ are extended to provide the basic equations for the measurement of correlation function. The design and construction processes using integrated circuits are described in detail. The time difference between two samples can be varied at will from 10 μseconds downwards, with no lower limit. The measurement commences on a start signal and runs for a programmed time. Readouts are available immediately after the end of the measuring time. The upper frequency limit is several KHz. There are no low-frequency limits. Some familiar signals were measured and the results confirm that the system works satisfactorily. / Thesis / Master of Engineering (ME)