The present invention relates generally to the field of frequency modulation, and in particular to dual port frequency modulators. The present invention provides a frequency modulator comprising a phase lock loop circuit (108) for receiving and modulating a carrier signal according to the low frequency component of a modulating signal, the phase lock loop circuit comprising a voltage controlled oscillator (118) for outputting a modulated carrier signal and a loop filter (116) for outputting a steering voltage to the VCO, the VCO having a tank circuit (120) comprising a voltage controlled capacitance (VAR1). The frequency modulator also comprises an external voltage controlled capacitance (122) which is arranged to modulate its capacitance according to a high frequency component of the modulating signal, the second voltage controlled capacitance being coupled to the tank circuit. The phase lock loop circuit (108) is further arranged to modulate the steering voltage to the VCO (118) with the high frequency component of the modulating signal.