Apparatus for correcting frequency in a coherent receiver

Abstract

Apparatus for correcting the frequency in a coherent receiver associated with a radio communication installation, e.g. a mobile telephone installation for transmitting digital information in time-division multiplex. The receiver includes a controllable local oscillator (7) for generating a mixing signal for converting the frequency of a received signal, and unit (4,8,9) for calculating a frequency error and generating a control signal (f') to the oscillator (7). Each new value of the control signal (f') is dependent on a previous value and on the calculated frequency error, and is utilized for correcting the frequency of said mixing signal. At least the major part of a frequency error in the receiver is thus eliminated. A possibly remaining part of a required frequency correction is carried out by a frequency change in the baseband. This is achieved with the aid of a unit (11) included in the receiver, and adapted to perform regularly repeated phase shifts of a signal (I,Q) in the baseband by a value (V.sub.r), which is changed for each received modulation symbol, e.g. a data bit, and which is responsive to said remaining part of the required frequency correction.