C/N ratio detection circuit and signal receiving circuit

1. A signal receiving circuit comprising:

a bandpass filter that receives an input signal through an antenna and selectively outputs a signal of a predetermined bandwidth in the input signal;

a demodulator that demodulates the signal outputted from the bandpass filter;

a carrier to noise (C/N) ratio detection circuit that calculates a C/N ratio of the signal outputted from the bandpass filter, the C/N ratio detection circuit, which in operation operates as:

a voltage detector that measures an input voltage of the signal output from the bandpass filter;

an averaging section that calculates an average of a number of voltages from the voltage detector over a predetermined time;

a time variation range calculator that calculates a first time variation range of the voltages over the predetermined time; and

a C/N ratio calculator that calculates the C/N ratio of the signal by using based on the average and a second time variation range; and

an audio signal processor that applies an audio signal process to the demodulated signal outputted from the demodulator, and outputs an audio signal generated by the audio signal process to a speaker, in accordance with the C/N ratio inputted from the carrier to noise (C/N) ratio detection circuit, wherein:

the carrier to noise (C/N) ratio detection circuit makes a determination to correct the first time variation range such that the first time variation range becomes a noise level reference value set in accordance with the bandwidth of the bandpass filter, and outputs the corrected first time variation range as the second time variation range, when the first time variation range is less than the noise level reference value, and

the carrier to noise (C/N) ratio detection circuit makes a determination to not correct the first time variation range, and outputs the first time variation range as the second time variation range, when the first time variation range is greater than or equal to the noise level reference value.

2. The signal receiving circuit according to claim 1 , wherein

the voltage detector is electrically connected to an output terminal of a bandpass filter and measures a voltage of a signal output from the bandpass filter as the input voltage, and

the C/N ratio detection circuit further comprises

a determination section that compares the input voltage with the C/N ratio to determine whether a bandwidth of the bandpass filter is narrower than an occupied bandwidth of the signal, and

a switch that outputs the input voltage when the determination section determines that the bandwidth of the bandpass filter is narrower than the occupied bandwidth of the signal and that outputs the C/N ratio when the determination section determines that the bandwidth of the bandpass filter is not narrower than the occupied bandwidth of the signal.

3. The signal receiving circuit according to claim 1 , wherein the C/N ratio detection circuit further comprises:

a determination section that determines whether a difference between the input voltage and the C/N ratio is equal to or less than a threshold; and

a switch that outputs the input voltage when the difference is more than the threshold and that outputs the C/N ratio when the threshold is equal to or less than the threshold.

4. The signal receiving circuit according to claim 1 , wherein the C/N ratio detection circuit further comprises:

a blend section that outputs the C/N ratio when the C/N ratio is equal to or less than a first threshold and that outputs the input voltage or an interpolated value obtained by interpolating the C/N ratio and the input voltage when the C/N ratio is more than the first threshold.

5. The signal receiving circuit according to claim 4 , wherein

the blend section outputs the input voltage when the C/N ratio is more than the first threshold and the input voltage is equal to or more than a second threshold, the second threshold being more than the first threshold, and outputs the interpolated value when the C/N ratio is more than the first threshold and the input voltage is less than the second threshold.

6. The signal receiving circuit according to claim 1 , wherein

the C/N ratio calculator calculates, as the C/N ratio, a ratio of a square of the average to a product of a square of the time variation range and a constant.