Two types of phase sensitive detection systems which have been used up to 130 Mc are described. The first utilizes a wideband amplifier and a single phase detector to provide a system which is continuously tuneable between 1 and 130 Mc. The second uses two phase detectors in quadrature to obtain the vector representation of a wave, and extends by three orders of magnitude the frequency range of previously available apparatus. Below 100 Mc, the equivalent input noise of these systems is less than 5×10−8V rms.