Amelioration of frequency errors and/or their effects

Abstract

A satellite positioning subsystem includes a frequency converter that applies a downshift in frequency to a received signal that should contain a satellite signal that has been spread and modulated on a carrier signal. A non-crystal oscillator produces an output signal whose frequency acts as a controlling reference for the frequency converter. A crystal oscillator is coupled to a controller that develops a control signal for controlling the frequency of the output signal of the non-crystal oscillator. A processor intermittently corrects the crystal oscillator such that its output frequency experiences jumps. A filter filters the control signal to limit the rate of change of frequency that the control signal demands of the output signal of the non-crystal oscillator such that a phase rotation of π cannot occur in the output signal of the non-crystal oscillator in the time that would be taken to despread a sample of the satellite signal.

1 . A radio communications device comprising: a satellite positioning subsystem arranged to determine the device's location based on satellite signals and comprising a frequency converter arranged to apply a downshift in frequency to a received signal that should contain a satellite signal that has been spread and modulated on a carrier signal; a non-crystal oscillator arranged to produce an output signal whose frequency acts as a controlling reference for the frequency converter; a crystal oscillator; a controller arranged to develop, from the output signal of the non-crystal oscillator and an output signal of the crystal oscillator, a control signal for controlling the frequency of the output signal of the non-crystal oscillator; a processor arranged to intermittently correct the crystal oscillator such that its output frequency experiences jumps; and a filter arranged to filter the control signal to limit the rate of change of frequency that the control signal demands of the output signal of the non-crystal oscillator to the extent that a phase rotation of it cannot occur in the output signal of the non-crystal oscillator in the time that would be taken to despread a sample of the satellite signal. 
     
     
         2 . A radio communications device according to  claim 1 , wherein the satellite positioning subsystem is arranged to take account of a jump in the output frequency of the crystal oscillator by discarding one or more calculations affected by the jump. 
     
     
         3 . A radio communications device according to  claim 1 , wherein the satellite positioning subsystem is arranged to take account of a jump in the output frequency of the crystal oscillator by limiting one or more calculations to a partial result reached prior to the processing of the jump. 
     
     
         4 . A radio communications device according to  claim 1 , wherein the satellite positioning subsystem is arranged to take account of a jump in the output frequency of the crystal oscillator by correcting one or more calculations that have been affected by the jump. 
     
     
         5 . A radio communications device according to  claim 4 , wherein the satellite positioning subsystem is arranged to recover a frequency downshifted satellite signal and the satellite positioning subsystem is arranged to take account of a jump in the output frequency of the crystal oscillator by applying to that part of the frequency downshifted satellite signal that is affected by the jump a frequency shift that corrects for the jump. 
     
     
         6 . A radio communications device according to  claim 1 , further comprising a radio communications subsystem arranged to receive or send and receive signals via an air interface, wherein the crystal oscillator is arranged to act as a controlling reference for the radio communication subsystem, the processor is arranged to derive from signals received via the radio communication subsystem control information for the intermittent correction of the crystal oscillator.