The invention provides for a mobile radio communications device having a first crystal oscillator for providing a first master clock frequency for a timebase of a first communication system, a second crystal oscillator for providing a second master clock frequency for a timebase for a second communication system, a third oscillator for providing a relatively low frequency clock signal within the device, and means for calibrating each of the first and second master clock signals to the third oscillator clock signal, means for determining the ratio of the first and second master clocks on the basis of respective measurements of the first and second master clocks to the relatively low frequency clock signal.