Modulation circuit having frequency sensitive power supply voltage

A modulation circuit including an amplifier for amplifying a modulation signal having a frequency band from a first frequency f.sub.1 to a second frequency f.sub.2, and a modulator having a voltage-controlled oscillator, for generating a frequency modulation signal corresponding to the modulation signal;

• comprising
• an amplifier control means for supplying a first predetermined direct current voltage V.sub.1 as a power source voltage of the amplifier when a frequency of the modulation signal is detected to be lower than a predetermined third frequency f.sub.3 which is mid way between the first frequency f.sub.1 and the second frequency f.sub.2 ;
• and for supplying a second predetermined direct current voltage V.sub.2, being higher than the first constant predetermined direct current voltage V.sub.1, as a power source voltage of the amplifier when a frequency of the modulation signal is detected to be higher than the predetermined third frequency f.sub.3 ;
• said amplifier control means including a frequency/voltage converter, a comparator for comparing an output of the frequency/voltage converter with a reference voltage, and a power supply circuit being voltage controlled by the output of the comparator;
• whereby, when the frequency of the modulation signal is lower than the predetermined third frequency f.sub.3, an amplitude of the modulation signal which is supplied to the voltage-controlled oscillator is lower by a predetermined amount than that of when the frequency of the modulation signal is higher than the third frequency f.sub.3.

A modulation circuit including an amplifier for amplifying a modulation signal having a frequency band from a first frequency f.sub.1 to a second frequency f.sub.2, and a modulator having a voltage-controlled oscillator, for generating a frequency modulation signal corresponding to the modulation signal;

• comprising
• an amplifier control means for supplying a first predetermined direct current voltage V.sub.1 as a power source voltage of the amplifier when a frequency of the modulation signal is detected to be lower than a predetermined third frequency f.sub.3 which is mid way between the first frequency f.sub.1 and the second frequency f.sub.2 ;
• and for supplying a second predetermined direct current voltage V.sub.2, being higher than the first constant predetermined direct current voltage V.sub.1, as a power source voltage of the amplifier when a frequency of the modulation signal is detected to be higher than the predetermined third frequency f.sub.3 ;
• whereby, when the frequency of the modulation signal is lower than the predetermined third frequency f.sub.3, an amplitude of the modulation signal which is supplied to the voltage-controlled oscillator is lower by a predetermined amount than that of when the frequency of the modulation signal is higher than the third frequency f.sub.3 ;
• and
• an amplitude detection switch control means for applying a predetermined direct third voltage V.sub.3 being higher than the second voltage V.sub.2 as a power source of the amplifier notwithstanding the frequency of the modulation signal when it is detected that the amplitude of the modulation signal is less than a set value.

A modulation circuit including an amplifier for amplifying a modulation signal having a frequency band from a first frequency f.sub.1 to a second frequency f.sub.2, and a modulator having a voltage-controlled oscillator, for generating a frequency modulation signal corresponding to the modulation signal;

• comprising
• an amplifier control means for supplying a first predetermined direct current voltage V.sub.1 as a power source voltage of the amplifier when a frequency of the modulation signal is detected to be lower than a predetermined third frequency f.sub.3 which is mid way between the first frequency f.sub.1 and the second frequency f.sub.2 ;
• and for supplying a second predetermined direct current voltage V.sub.2, being higher than the first constant predetermined direct current voltage V.sub.1, as a power source voltage of the amplifier when a frequency of the modulation signal is detected to be higher than the predetermined third frequency f.sub.3 ;
• whereby, when the frequency of the modulation signal is lower than the predetermined third frequency f.sub.3, an amplitude of the modulation signal which is supplied to the voltage-controlled oscillator is lower by a predetermined amount than that of when the frequency of the modulation signal is higher than the third frequency f.sub.3 ;
• and
• said amplifier having a parallel circuit being a pair of series circuits of diodes and resistors, the modulation signal is supplied to one of the connecting points between the diode and the resistor, the output signal is obtained from another connecting point between the other diode and the other resistor, and the direct current first voltage V.sub.1 or the second voltage V.sub.2 is applied to a connecting point between the two diodes as a control voltage.