FREQUENCY SYNTHESISER CIRCUIT

The invention relates to frequency synthesiser circuits, and in particular to frequency synthesiser circuits characterised by a small channel spacing. Embodiments disclosed include a frequency synthesiser circuit (100) for a radio receiver, the circuit comprising: a digitally controlled oscillator (118) configured to generate an output signal (128) with an output frequency on application of an oscillator enable signal (126); a delay module (160; 210) configured to delay an input reference signal (142) to generate a delayed reference signal (144; 244); and a duty cycle module (150) configured to modulate the oscillator enable signal based on a period of an input reference signal (142) and the delay of the delayed reference signal (144), such that a ratio between the output frequency and the frequency of the input reference signal (142) is a non-integer.