| 摘要 |
A modulation circuit utilized in a mobile radio communication transmitter includes an amplifier for amplifying a modulation signal and a modulator having a voltage-controlled oscillator for generating a frequency modulation signal corresponding to the modulation signal. An amplifier controller applies a first predetermined constant direct current voltage V1 to a power source of the amplifier when the frequency of the modulation signal is less than a predetermined frequency f3, and for a second predetermined constant direct current voltage V2 to a power source of the amplifier when the frequency of the modulation signal is higher than the frequency f3. Thus, when the frequency of the modulation signal is less than the frequency f3, the amplitude of the modulation signal which is added to the voltage-controlled oscillator decreases by a predetermined value compared with the case where the frequency of the modulation signal is higher than the frequency f3. By using this modulator circuit, a lock-up time of a phase-locked loop can be accelerated and a maximum frequency deviation of the frequency modulation signal can be flat within the transmission band.
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