Frequency modulation circuit, transmission circuit, and communication apparatus
View Patent ↗A bandpass type delta sigma modulation section performs delta sigma modulation on an inputted modulation signal such that quantization noise is reduced in a frequency band which requires low noise. A low pass filter removes a noise component in a high frequency region from the signal on which the delta sigma modulation has been performed. A frequency modulation circuit reduces noise in the frequency band which requires low noise with the bandpass type delta sigma modulation section and the low pass filter, and reduces noise in the vicinity of a direct current component DC with a feedback comparison section and a loop filter.
1. A frequency modulation circuit for frequency modulating and outputting an output frequency modulation signal, the frequency modulation circuit comprising:
a bandpass type delta sigma modulation section operable to receive an input modulation signal and perform delta sigma modulation on the received input modulation signal, such that quantization noise is suppressed in a frequency band which requires low noise, and output a first resultant signal;
a feedback comparison section operable to receive the same input modulation signal, compare the output frequency modulation signal with the received input modulation signal, control a frequency of the received input modulation signal according to a result of the comparison, and output a frequency-controlled modulation signal;
a loop filter operable to suppress a high-frequency component of the frequency-controlled modulation signal and output a second resultant signal; and
a voltage controlled oscillator operable to perform frequency-modulation by controlling an oscillatory frequency according to a combination of the second resultant signal outputted from the loop filter and the first resultant signal outputted from the bandpass type delta sigma modulation section, and output the output frequency modulation signal.
2. The frequency modulation circuit according to claim 1 , further comprising a compensating filter operable to filter the input modulation signal and output the input modulation signal on which filtering by the compensating filter has been performed to the bandpass type delta sigma modulation section to compensate for output characteristics of the bandpass type delta sigma modulation section.
3. The frequency modulation circuit according to claim 1 , wherein
the feedback comparison section includes:
a frequency digital converter operable to convert a frequency of the output frequency modulation signal outputted by the voltage controlled oscillator into a digital value based on a predetermined manner; and
an arithmetic section operable to add or subtract the digital value converted by the frequency digital converter to or from the input modulation signal.
4. The frequency modulation circuit according to claim 2 , wherein
the feedback comparison section includes:
a frequency digital converter operable to convert a frequency of the output frequency modulation signal outputted by the voltage controlled oscillator into a digital value based on a predetermined manner; and
an arithmetic section operable to add or subtract the digital value converted by the frequency digital converter to or from the input modulation signal.
5. The frequency modulation circuit according to claim 3 , further comprising a lowpass type delta sigma modulation section operable to perform delta sigma modulation with signal transfer characteristics of lowpass type on the second resultant signal outputted from the loop filter, and output to the voltage controlled oscillator the second resultant signal on which the delta sigma modulation has been performed.
6. The frequency modulation circuit according to claim 4 , further comprising a lowpass type delta sigma modulation section operable to perform delta sigma modulation with signal transfer characteristics of lowpass type on the second resultant signal outputted from the loop filter, and output to the voltage controlled oscillator the second resultant signal on which the delta sigma modulation has been performed.
7. The frequency modulation circuit according to claim 5 , further comprising a second arithmetic section operable to control a center frequency of the input modulation signal by adding or subtracting a constant to or from the input modulation signal, and output to the arithmetic section the controlled input modulation signal.
8. The frequency modulation circuit according to claim 6 , further comprising a second arithmetic section operable to control a signal a center frequency of the input modulation signal by adding or subtracting a constant to or from the input modulation signal, and output to the arithmetic section the controlled input modulation signal.
9. The frequency modulation circuit according to claim 1 , further comprising a lowpass type delta sigma modulation section operable to perform delta sigma modulation with signal transfer characteristics of lowpass type on the input modulation signal, and output to the feedback comparison section the input modulation signal on which the delta sigma modulation has been performed, wherein
the feedback comparison section includes:
a frequency divider operable to frequency-divide the output frequency modulation signal outputted by the voltage controlled oscillator with the input modulation signal on which the delta sigma modulation has been performed by the lowpass type delta sigma modulation section; and
a comparison section operable to compare a predetermined reference signal with the frequency-divided output frequency modulation signal from the frequency divider, and control the oscillatory frequency of the voltage controlled oscillator such that the frequency-divided output frequency modulation signal and the predetermined reference signal are synchronized.
10. The frequency modulation circuit according to claim 2 , further comprising a lowpass type delta sigma modulation section operable to perform delta sigma modulation with signal transfer characteristics of lowpass type on the input modulation signal, and output to the feedback comparison section the input modulation signal on which the delta sigma modulation has been performed, wherein
the feedback comparison section includes:
a frequency divider operable to frequency-divide the output frequency modulation signal outputted by the voltage controlled oscillator with the input modulation signal on which the delta sigma modulation has been performed by the lowpass type delta sigma modulation section; and
a comparison section operable to compare a predetermined reference signal with the frequency-divided output frequency modulation signal from the frequency divider, and control the oscillatory frequency of the voltage controlled oscillator such that the frequency-divided output frequency modulation signal and the predetermined reference signal are synchronized.