| 摘要 |
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 (VAR 1 ). 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.
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