Data is transmitted in a signal comprising a data signal D.sub.m at a frequency within a range B of discrete frequency tones D.sub.0 to D.sub.m-1 representing a predetermined range of digital values, and a pilot signal (36, 37). The signal is received by a circuit comprising a transformer (44 to 54) which transforms the received signal into a set of frequency samples of the pilot signal and a set of frequency samples of the data signal. The samples are input to a correlator (58) which correlates the set of frequency samples of the pilot signal with the set of frequency samples of the data signal at each of the discrete frequency tones in order to identify the discrete frequency tone to which the frequency of the data signal corresponds. In this way, the digital value represented by the data signal can be identified. The circuit is suitable for use in a radio pager for receiving data signal transmissions from a distant source which may be transmitted from a satellite.