Abstract: A frequency source (1) in, for example, a remote unit in a mobile communications system, is controlled to maintain a stable frequency signal. In normal operation, the frequency source (1) is frequency locked to an external reference frequency (10). A temperature detecting device (2) monitors the temperature of the frequency source, and information relating to temperature is stored in a storage device (7) together with information relating to control signals (6) applied to the frequency source (1). In the absence of the reference frequency (10), the temperature of the frequency source (1) is detected and the stored information is used to generate a control signal (6) to control the output frequency of the frequency source (1) in accordance with the detected temperature.

Abstract: A spread-spectrum transmission system wherein a plurality of spread-spectrum signals are frequency division multiplexed by offsetting the center or carrier frequencies of the spread-spectrum signals by a fraction of the spectral-line spacing of the signals. The signals are generated by modulating a carrier with a pseudo-noise (PN) code signal. At a receiver, demultiplexing is carried out by mixing (42) the received multiplexed signal with a plurality of local oscillator frequencies (43) which are offset from each other in the same manner as the centre or carrier frequencies of the multiplexed spread-spectrum signals. The mixed signals are then further mixed (45) with generated PN code signals (47) corresponding to those used to generate the multiplexed spread-spectrum signals. The epoch of each generated PN code signal is then advanced or retarded in response to a respective epoch control signal (48) generated by a respective controller (46).