An amplitude of a carrier signal at an output of an impedance matching block is measured as a first amplitude value. A value of the signal amplitude at the output of the impedance matching block is calculated as a second amplitude value that a signal resulting from amplitude modulation with said modulation factor from said carrier signal should assume, said carrying signal having an amplitude with the first amplitude value. A setting of a transmitter is changed to decrease the carrier signal amplitude at the output of the impedance matching block. An amplitude of a new carrier signal at the output of the impedance matching block is measured as a new amplitude value. The transmitter setting keeps changing so many times until the new amplitude value is equal to or lower than said second amplitude value or within a predetermined tolerance range around said second amplitude value. Parameters of the last new transmitter setting are stored as the parameters of the transmitter setting, by means of which the transmitter will generate a transmitted signal being amplitude-modulated in the specified manner. The setting of the amplitude modulation with the specified modulation factor is carried out automatically. The designing of the transmitter circuit is simplified.