Modulation circuit for wireless installation

A modulation circuit includes a band restriction filter 2, an attenuator 3, a modulator 4, a reference voltage generator 8, a comparing reference voltage generator 9, a voltage comparator 10 and an attenuator controller 7. The band restriction filter 2 receives modulation data. The attenuator 3 is connected to the band restriction filter 2 for adjusting its attenuation in response to an attenuator control signal. The reference voltage generator 8 is connected to the attenuator 3 for providing a reference voltage independent of a temperature and a power supply voltage to the attenuator. The comparing reference voltage generator 9 is connected to the reference voltage generator 8 for providing a comparing reference voltage independent of the temperature and the power supply voltage. The modulator 4 is connected to the attenuator 3. The voltage comparator 10 is connected to the attenuator 3 and the comparing reference voltage generator 9 for comparing the comparing reference voltage with a voltage of an output of the attenuator 3 and generating a comparison signal based on the comparison. The attenuator controller 7 is connected to the attenuator 3 and the voltage comparator 10 for generating the attenuator control signal in response to the comparison signal.