A method for modulating an output voltage of a transmitter circuit, comprising a voltage controlled oscillator with a modulation input, a digital/analog converter and an antenna circuit, the method comprising sending an output signal from the voltage controlled oscillator directly to the antenna circuit and directly modulating a frequency of the output signal of the voltage controlled oscillator, by applying an output signal of the D/A converter to the modulation input of the voltage-controllers oscillator, wherein a crystal oscillator circuit associated with the voltage controlled oscillator is capacitively loaded for direct modulation of the frequency of the voltage controlled oscillator.