Patent Publication Number: US-2018048500-A1

Title: Demodulator

Description:
The contents of the following Japanese patent application are incorporated herein by reference:
         NO. 2015-170720 filed in JP on Aug. 31, 2015.   NO. PCT/JP2016/074414 filed on Aug. 22, 2016.       

    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a demodulator. 
     2. Related Art 
     One of modulation schemes for modulating a signal such as an audio signal to a radio wave is frequency modulation (FM modulation) for changing a frequency of a carrier wave in accordance with amplitude of a signal wave. A signal that is FM modulated (FM signal) is demodulated in the following steps: in a demodulator, passing a received signal through an AD converter to convert the signal to a digital signal; subsequently passing the signal through a decimation filter to lower a sampling rate; and then passing the signal through a demodulator (for example, refer to Non-Patent Document 1). 
     In particular, for a demodulator equipped in a vehicle, it is necessary to remove pulse noises mixed into a radio wave in demodulation of an FM signal. That is because a vehicle is equipped with electric windows, electric mirrors, an ignition apparatus and the like, and a hybrid vehicle is further equipped with a large-capacity power source, coils and the like, and these components generate pulse noises. For example, radio equipment described in Patent Document 1 passes a signal via an AD converter through a bandpass filter to limit a frequency band, passes the signal through a noise blanker to remove noises, and then passes the signal through a demodulator for demodulation. Also, for example, a receiver described in Patent Document 2 passes a signal via an AD converter through a wave detector for demodulation, and subsequently passes the signal through a noise gate to remove noises.
         Patent Document 1: Japanese Patent Application Publication No. 2006-50016   Patent Document 2: Japanese Patent Application Publication No. 2012-191337   Non-Patent Document 1: Digital Design Technology No. 1, CQ Publishing Co., Ltd., 2009, p. 115       

     However, when the radio equipment as described in Patent Document 1 passes a radio wave with pulse noises through a bandpass filter, a filter delay occurs so that a width (i.e., a time width) of pulse noises included in the radio wave is extended in accordance with the number of taps which configure the filter. Therefore, when pulse noises are removed from a filtered signal, an extended zone is subject to removal, which leads to a problem of large-scale degradation of a signal wave. Note that the similar problem occurs for use of, not only a bandpass filter, but also a decimation filter described in Non-Patent Document 1. Also, the similar problem also occurs when the receiver as described in Patent Document 2 removes noises from a demodulated signal because a signal with noises is filtered prior to noise removal. 
     SUMMARY 
     (Item 1) 
     A demodulator may include an AD conversion section that performs analogue-to-digital conversion on a received signal. 
     The demodulator may include a noise removal section that is connected to a back side of the AD conversion section to detect and remove a noise from an input signal. 
     The demodulator may include a first decimation filter that is connected to a back side of the noise removal section to reduce a data rate of an input signal. 
     The demodulator may include a demodulation section that is connected to a back side of the first decimation filter to demodulate an input signal. 
     (Item 2) 
     The demodulator may further include a second decimation filter that is connected to a back side of the AD conversion section and a front side of the noise removal section to reduce a data rate of an input signal. 
     (Item 3) 
     The demodulator may further include a third decimation filter that is connected to a back side of the demodulation section to reduce a data rate of an input signal. 
     (Item 4) 
     The noise removal section may include a zone detection section that detects a replacement target zone to be replaced in an input signal. 
     The noise removal section may include a replacement section that replaces a signal of the replacement target zone in an input signal with a replacement target signal. 
     (Item 5) 
     The zone detection section may include a high-pass filter that allows an input signal to pass therethrough. 
     The zone detection section may include a comparison section that detects the replacement target zone based on a result of comparing a signal from the high-pass filter with a reference value. 
     (Item 6) 
     The replacement section may include a low-pass filter that allows an input signal to pass therethrough and a signal passing through the low-pass filter is used as the replacement target signal. 
     (Item 7) 
     The AD conversion section may include an orthogonal frequency converter that converts a signal based on the received signal that is FM modulated to an I signal and a Q signal orthogonal to each other. 
     The AD conversion section may include an I-side AD converter that performs analogue-to-digital conversion on the I signal. 
     The AD conversion section may include a Q-side AD converter that performs analogue-to-digital conversion on the Q signal. 
     (Item 8) 
     The AD conversion section may include an AD converter that performs analogue-to-digital conversion on a signal based on the received signal that is FM modulated. 
     The AD conversion section may include an orthogonal frequency converter that converts an output of the AD converter to an I signal and a Q signal orthogonal to each other. 
     The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a configuration of a demodulator according to the present embodiment. 
         FIG. 2  shows the configuration of the noise removal section. 
         FIG. 3A  shows one example of an input signal to the noise removal section. 
         FIG. 3B  shows one example of an output of the filter (HPF). 
         FIG. 3C  shows one example of an output of the calculator (ABS). 
         FIG. 3D  shows one example of a generation result of a reference value by the reference value generator. 
         FIG. 3E  shows one example of a comparison result by the comparator. 
         FIG. 3F  shows one example of an output of the pulse stretcher. 
         FIG. 3G  shows one example of an output of the filter (LPF). 
         FIG. 3H  shows one example of a result of noise processing (an output of the replacer) by the noise removal section. 
         FIG. 4  shows one example of a result of noise processing by the noise removal section according to the variant configuration. 
         FIG. 5  shows one example of an output (lower side) when a signal with a pulse noise (upper side) is passed through the decimation filter. 
         FIG. 6  shows the configuration of the demodulator according to the first variant example. 
         FIG. 7  shows the configuration of the demodulator according to the second variant example. 
         FIG. 8  shows the configuration the demodulator according to the third variant example. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, the present invention is described through the embodiments of the invention. However, the following embodiments are not to limit the claimed invention. Also, all of combinations of features described in the embodiments are not necessarily required for the solution of the invention. 
       FIG. 1  shows the configuration of a demodulator  100  according to the present embodiment. The demodulator  100  is an apparatus that demodulates a signal modulated to a radio wave, and is designed to provide a demodulator less subject to degradation of a signal wave even when noise are removed using a noise blanker or other noise removal means. Note that in the present embodiment, it is assumed that the radio wave is FM modulated to V FM =C sin (ω c t+m ∫ s dt) by changing a frequency of a carrier wave V c =C sin (ω c t) in accordance with amplitude of a signal wave V s . Here, ω c =2πf c  with a frequency of the carrier wave is f c , and m is a constant. 
     The demodulator  100  includes an AD conversion section  10 , a noise removal section  40 , a first filter section  50 , a demodulation section  60 , and a second filter section  70 . 
     The AD conversion section  10  converts, for example, a received signal RF in an analogue form, obtained by receiving a radio wave V FM  through an antenna, to a digital signal. The AD conversion section  10  includes an orthogonal frequency converter  20 , and AD converters (ADCs)  32  and  34 . 
     The orthogonal frequency converter  20  is a converter that converts the received signal RF to an I signal and a Q signal orthogonal to each other, and includes a local transmitter  26  and mixers  22  and  24 . The local transmitter  26  generates two orthogonal local signals cos (ω c t) and sin (ω c t) having a frequency f c  and orthogonal to each other, and outputs the signals to the mixers  22  and  24 , respectively. The mixer  22  mixes the received signal RF with the orthogonal signal sin (ω c t) (i.e., multiplication) to generate the I signal (I=V FM  sin (ω c t)), and outputs the signal to the AD converter  32  after it passes the signal through a filter (not shown) for removing unnecessary components. The mixer  24  multiplies the received signal RF by the orthogonal signal cos (ω c t) to generate the Q signal (Q=V FM  cos (ω c t)), and outputs the signal to the AD converter  34  after it passes the signal through a filter (not shown) for removing unnecessary components. 
     The AD converters  32  and  34  are connected to the mixers  22  and  24  to convert the I signal and the Q signal, input from the mixers  22  and  24 , to digital signals, and output the signals to the filter section  40 , respectively. Sampling rates of the AD converters  32  and  34  are sufficiently higher than an output frequency of the demodulation section  60 , for example, twice or more to approximately 200 times. Thus, the AD converters  32  and  34  over sample input signals. 
     The noise removal section  40  is connected to a back side of the AD conversion section  10  to detect noises included in the I signal and the Q signal in a digital form input from the AD conversion section  10 , and remove at least a portion of the detected noises. The detailed configuration of the noise removal section  40  is described below. 
     The first filter section  50  is connected to a back side of the noise removal section  40  to reduce data rates (i.e., down samples) of the I signal and the Q signal input from the noise removal section  40  from which noises are removed. The first filter section  50  includes decimation filters  52  and  56  and sampling frequency converters  54  and  58 . 
     The decimation filters  52  and  56  receive the I signal and the Q signal from the noise removal section  40 , cut portions of a high frequency band thereof, and output the signals to the sampling frequency converters  54  and  58 , respectively. A low-pass filter can be used as the decimation filters  52  and  56 . A cutoff frequency can be determined in accordance with down sampling rates of the sampling frequency converters  54  and  58  as appropriate. 
     The sampling frequency converters  54  and  58  are connected to the decimation filters  52  and  56  to down sample (convert or decimate sampling frequencies of) the I and Q signals, input from the filters  52  and  56 , of which portions of the high frequency band are cut, respectively. The down sampling rates of the sampling frequency converters  54  and  58  are, for example, one half or less. 
     The first filter section  50  removes a component out of the band from the I signal and the Q signal by down sampling the I signal and the Q signal, and outputs the signals only with a signal wave component within the band to the demodulation section  60 . Here, the decimation filters  52  and  56  cut portions of the high frequency band of the I signal and the Q signal, prior to down sampling by the sampling frequency converters  54  and  58 , which can prevent aliasing due to down sampling. 
     The demodulation section  60  is connected to a back side of the first filter section  50  to demodulate the received signal RF using the I signal and the Q signal input from the filter section  50 . The demodulation section  60  includes a wave detector  64  of an arc-tangent wave detection scheme. The wave detector  64  calculates an arc-tangent value θ=tan −1  (Q/I) using the I signal and the Q signal, and further calculates a differential value thereof with a time differential dθ/dt or a difference. The wave detection result by the wave detector  64  is output to the second filter section  70 . 
     The second filter section  70  is connected to a back side of the demodulation section  60  to down sample the wave detection result input from the demodulation section  60 . The second filter section  70  includes a decimation filter  72  and a sampling frequency converter  74 . 
     The decimation filter  72  receives the wave detection result of the demodulation section  60 , cuts a portion of the high frequency band thereof, and outputs the result to the sampling frequency converter  74 . A low-pass filter can be used as the decimation filter  72 . A cutoff frequency can be determined in accordance with down sampling rates of the sampling frequency converter  74  as appropriate. 
     The sampling frequency converter  74  is connected to the decimation filter  72  to down sample (converts or decimates a sampling frequency of) the wave detection result, input from the decimation filter  72 , of which a portion of the high frequency band is cut. The down sampling rate of the sampling frequency converter  74  is, for example, one half or less. 
     Here, in down sampling by the second filter section  70 , the wave detection result is passed through the sampling frequency converter  74  via the decimation filter  72 , which can prevent aliasing due to down sampling. 
     Note that, by using the first filter section  50  and the second filter section  70  in combination, signals that are over sampled by the AD converters  32  and  34  are reduced to correspond to a determined sampling rate. Accordingly, the down sampling rates of the first filter section  50  and the second filter section  70  are determined such that an inverse of the product thereof is equal to the sampling rates of the AD converters  32  and  34 , respectively. For example, with respect to approximately 20 times the sampling rates of the AD converters  32  and  34 , an inverse of the product of the respective down sampling rates of the first filter section  50  and the second filter section  70  is approximately one twentieth. Accordingly, for example, when signals that are over sampled only by the first filter section  50  are reduced to correspond to a determined sampling rate, the second filter section  70  is not necessarily provided. 
       FIG. 2  shows the configuration of the noise removal section  40 . In the present embodiment, a noise blanker is used as the noise removal section  40 . The noise removal section  40  includes a zone detection section  40   a  and a replacement section  40   b.    
     The zone detection section  40   a  compares a signal input to the noise removal section  40  (here, the I signal or the Q signal input from the AD conversion section  10  and simply referred to as an input signal) with a reference value, and based on the result, detects a replacement target zone (a so-called blanking zone) to be replaced in the input signal. The zone detection section  40   a  includes a filter  41 , a calculator (ABS)  43 , a comparator  44 , and a pulse stretcher  45 . The filter  41  includes a high-pass filter (HPF), cuts a portion of a low frequency band of the input signal using the high-pass filter, and outputs the signal to the calculator  43 . The calculator (ABS)  43  is connected to the filter  41  to calculate an absolute value of a signal input from the filter  41  and output the result to the comparator  44 . The comparator  44  is connected to the calculator  43  to compare a signal input from the calculator  43  with the reference value, and output the result to the pulse stretcher  45 . The pulse stretcher  45  is connected to the comparator  44  to stretch a time width of a pulse signal included in a signal input from the comparator  44 , and output the signal to the replacer  48  included in the replacement section  40   b , as a replacement target zone signal indicating the replacement target zone. 
     Note that the zone detection section  40   a  may further include a reference value generator (not shown) that generates the reference value used by the comparator  44 . The reference value generator generates, as one example, the reference value by detecting a peak of an output signal of the calculator  43  and adding an offset thereto. Here, in peak detection, a steep peak structure included in the output signal of the calculator  43  is detected. In a transient response to the peak structure, a small time constant is applied to the rise while a large time constant is applied to the decay, which results in a signal generated with a peak structure suppressed with respect to the output signal. Note that the offset is to be determined as appropriate in accordance with a level of a noise to be removed. 
     The replacement section  40   b  replaces a signal (input signal) of the replacement target zone in a signal input to the noise removal section  40  with a replacement target signal. The replacement section  40   b  includes delay circuits  46   a  and  46   b , a filter  47 , and a replacer  48 . The delay circuit  46   a  delays an input signal and outputs the signal to the delay circuit  46   b  (and the filter  47 ). The delay circuit  46   b  further delays an input signal that is delayed by the delay circuit  46   a , and outputs the signal to the replacer  48 . The input signal is input to the replacer  48  by the delay circuits  46   a  and  46   b  in accordance with a timing at which the replacement target zone signal is input from the zone detection section  40   a  to the replacer  48 . The filter  47  includes a low-pass filter (LPF), cuts a portion of the high frequency band of the input signal via the delay circuit  46   a  using the low-pass filter to generate the replacement target signal, and outputs the signal to the replacer  48 . Here, a delay time of the delay circuit  46   b  is set equal to a delay time of the filter  47 . This allows the filter  47  to input the replacement target signal to the replacer  48  in accordance with a timing at which the input signal is input to the replacer  48  via the delay circuits  46   a  and  46   b . The replacer  48  replaces the input signal input from the delay circuit  46   b  with the replacement target signal generated by the filter  47 , when the replacement target zone signal input from the zone detection section  40   a  is a logic high, that is, when the replacement target zone signal indicates the replacement target zone. 
       FIG. 3A  to  FIG. 3H  show results of a series of processes by the noise removal section  40 .  FIG. 3A  to  FIG. 3H  show signals obtained by each process (i.e., signal strengths are plotted on the vertical axis while times are on the horizontal axis), respectively. 
       FIG. 3A  shows one example of a signal (input signal) input to the noise removal section  40 . The input signal is to include two noises of a spiked shape having large amplitude on signal components of a carrier wave oscillating sinusoidally at frequency f c . 
       FIG. 3B  shows an output of the filter  41 . The input signal ( FIG. 3A ) is passed through a high-pass filter (HPF) included in the filter  41  so that signal components included in the input signal which fall within the low frequency band are suppressed and the two noises are clearly extracted. 
       FIG. 3C  shows an output of the calculator (ABS)  43 . An absolute value of the output of the filter  41  ( FIG. 3B ) is generated by the calculator  43 . 
       FIG. 3D  shows a generation result of the reference value by the reference value generator (not shown). The reference value generator generates the reference value by detecting a peak of an output signal of the calculator  43  ( FIG. 3C ) and adding an offset to the result. The reference value is generated in this manner, so that the reference value can be determined appropriately for a largely fluctuating input signal. Note that when fluctuation of the input signal is small enough to be ignored, the reference value may be determined constant. 
       FIG. 3E  shows a comparison result, by the comparator  44 , of the output signal of the calculator  43  ( FIG. 3C ) with the reference value ( FIG. 3D ). When the output signal is higher than the reference value, a logic high pulse is generated. In this example, two pulses are generated to correspond to two noises. 
       FIG. 3F  shows an output of the pulse stretcher  45 , that is, the replacement target zone signal. By the pulse stretcher  45 , time widths of the two pulses included in the output of the comparator  44  ( FIG. 3E ) are stretched back and forth. 
       FIG. 3G  shows an output of the filter  47 , that is, the replacement target signal. The input signal ( FIG. 3A ) is passed through the low-pass filter (LPF) included in the filter  47  (via the delay circuit  46   a ) so that two noises included in the input signal which fall within the high frequency band are suppressed. 
       FIG. 311  shows an output of the replacer  48 , that is, the noise removal section  40 . The replacement target zone signal ( FIG. 3F ) input, by the replacer  48 , from the zone detection section  40   a  triggers the input signal input from the delay circuit  46   b  ( FIG. 3A ) to be replaced with the replacement target signal passing through the filter  47  ( FIG. 3G ), in the replacement target zone indicated by the replacement target zone signal. 
     Note that the demodulator  100  of the present embodiment removes noises by replacing the input signal with the replacement target signal generated by passing the input signal through the filter  47 . Alternatively, the input signal may also be replaced with a value of a blank signal or an input signal immediately before the replacement target zone. In such a variant configuration, the replacer  48  (and the filter  47 ) may also be, for example, a D-type flip flop (not shown) which is triggered by the replacement target zone signal to hold the input signal input via the delay circuit  46   a.    
       FIG. 4  shows a result of noise processing by the noise removal section according to the variant configuration. The input signal ( FIG. 3A ) is output with signal values, in the replacement target zone indicated by the replacement target zone signal ( FIG. 3F ), held to be a value before the zone by the D-type flip flop (not shown). 
       FIG. 5  shows one example of an output (lower side) when a signal with a pulse noise (upper side) is passed through the decimation filters  52  and  54 . The signal includes a pulse noise having large amplitude from 0.02 to 0.03 milliseconds. When this signal is input to the decimation filters  52  and  54 , in accordance with the number (in this example, about 100) of taps which configure the filter, a width of the noise is to be extended (in this example, about 10 times). Accordingly, if the first filter section  50  is connected to a front side of the noise removal section  40 , a width of the noise included in the received signal RF is stretched by the decimation filters  52  and  54  included in the first filter section  50 , a broad replacement target zone is detected by the noise removal section  40  in order to remove the noise, and the received signal is replaced with the replacement target signal in the broad zone, which results in original signal components largely degraded. On the other hand, in the demodulator  100  of the present embodiment, the first filter section  50  is connected to a back side of the noise removal section  40 , which avoids such degradation of signal components. 
     In addition, the noise removal section  40  is connected to a back side of the AD conversion section  10 . Here, the received signal RF is over sampled by the AD converters  32  and  34  so that the replacement target zone detected by the noise removal section  40  becomes narrow and the received signal RF is replaced with the replacement target signal in the narrow zone, which can minimize degradation of signal components. 
     Note that, the AD converters  32  and  34  included in the AD conversion section  10  over sample the received signal RF, and accordingly, three or more filter sections may be provided, and in particular, two or more filter sections may also be provided at a front side of the demodulation section  60  for down sampling. 
       FIG. 6  shows the configuration of the demodulator  110  according to the first variant example. The demodulator  110  includes the AD conversion section  10 , a third filter section  80 , the noise removal section  40 , the first filter section  50 , the demodulation section  60 , and the second filter section  70 . The demodulator  110  has the same configuration as the demodulator  100  described above, except for the third filter section  80  connected to a back side of the AD conversion section  10  and a front side of the noise removal section  40 . Hereinafter, only the third filter section  80  is described. 
     The third filter section  80  is connected between the AD conversion section  10  and the noise removal section  40  to down sample the I signal and the Q signal input from the AD conversion section  10 , and output the signals to the noise removal section  40 . The third filter section  80  includes decimation filters  82  and  86  and sampling frequency converters  84  and  88 . 
     The decimation filters  82  and  86  receive the I signal and the Q signal from the AD conversion section  10 , cut portions of the high frequency band thereof, and output the signals to the sampling frequency converters  84  and  88 , respectively. A low-pass filter can be used as the decimation filters  82  and  86 . A cutoff frequency can be determined in accordance with down sampling rates of the sampling frequency converters  84  and  88  as appropriate. 
     The sampling frequency converters  84  and  88  are connected to the decimation filters  82  and  86  to down sample (convert or decimate sampling frequencies of) the I and Q signals, input from the filters  82  and  86 , of which portions of the high frequency band are cut, respectively. The down sampling rates of the sampling frequency converters  84  and  88  are, for example, one half or less. 
     In addition, one or more filter sections may also be connected serially to at least one of the first filter section  50 , the second filter section  70 , and the third filter section  80 . 
     In this manner, three or more filter sections are provided, and in particular, two or more filter sections are provided at a front side of the demodulation section  60  for down sampling in combination, which allows a simple configuration of the filter section to down sample the over sampled I and Q signals. In such a case, the noise removal section  40  is desired to be connected to front sides of all of the filter sections, and at a front side of the demodulation section  60 , is at least connected to a front portion of at least one of the filter sections. This allows the demodulation section  60  to demodulate a signal wave, after the decimation filter included in the filter section connected to a back portion of the noise removal section  40  cuts harmonic noise components which may be taken in due to noise blank by the noise removal section  40 . 
     Note that the AD converters  32  and  34  included in the AD conversion section  10  may also be replaced with one AD converter. 
       FIG. 7  shows the configuration of the demodulator  120  according to the second variant example. The demodulator  120  includes an AD conversion section  140 , the noise removal section  40 , the first filter section  50 , the demodulation section  60 , and the second filter section  70 . Note that each constituent other than the AD conversion section  140  has the same configuration as that of the demodulator  100  described above. Hereinafter, only the AD conversion section  140  is described. 
     The AD conversion section  140  includes an AD converter (ADC)  36  and the orthogonal frequency converter  20 . Note that the configuration of the orthogonal frequency converter  20  is the same as the one described above. The AD converter (ADC)  36  is connected to a front side of the orthogonal frequency converter  20  to convert the received signal RF in an analogue form to a digital signal, and output the signal to the mixers  22  and  24  included in the orthogonal frequency converter  20 , respectively. A sampling rate of the AD converter  36  can be determined in a similar manner to those of the AD converters  32  and  34 . 
     Note that the AD conversion section  140  may further include a mixer (not shown) connected to a front side of the AD converter  36 . The mixer mixes the received signal RF in an analogue form with an output of a local oscillator (not shown) to lower a frequency to a frequency a bit higher than DC (referred to as a DC frequency), for example, to approximately hundreds of kHz. A signal generated in such a manner is referred to as a Low-IF signal. The AD conversion section  140  passes the Low-IF signal through the AD converter  36  to convert it to a digital signal, passes the converted Low-IF signal through the orthogonal frequency converter  20  to generate the I signal and the Q signal at the DC frequency. 
     In the demodulator  120 , the noise removal section  40  is also connected to front sides of the first filter section  50  and the second filter section  70 . This prevents a signal of which a noise width included in the received signal RF is stretched by the decimation filter from being input to the noise removal section  40 , which prevents degradation of signal components due to noise blank over a broad replaced zone. 
     Note that the demodulator  100  described above generates the I signal and the Q signal from the received signal RF, and after passing them through the noise removal section  40  and the first filter section  50 , demodulates the signals by the wave detector  64  included in the demodulation section  60 . Alternatively, it may also use a Hilbert converter to further pass the received signal RF through the Hilbert converter after passing the signal through the noise removal section  40  and the first filter section  50 , and subsequently demodulate the signal by the wave detector  64 . 
       FIG. 8  shows the configuration of the demodulator  130  according to the third variant example. The demodulator  130  includes the AD converter (ADC)  36 , the noise removal section  40 , a first filter section  150 , a demodulation section  160 , and the second filter section  70 . 
     The AD converter (ADC)  36  converts the received signal RF in an analogue form to a digital signal and outputs the signal to the noise removal section  40 . A sampling rate of the AD converter  36  can be determined in a similar manner to those of the AD converters  32  and  34 . 
     The noise removal section  40  is connected to a back side of the AD converter  36  to process the received signal RF input from the AD converter  36  to remove noises, and output the signal to the first filter section  150 . 
     The first filter section  150  is connected to the noise removal section  40  to down sample the received signal RF from which noises are removed and output the signal to the demodulation section  160 . The first filter section  150  includes one decimation filter  52  and one sampling frequency converter  54 . The decimation filter  52  and the sampling frequency converter  54  are configured in a similar manner to those of the demodulator  100  described above. 
     The demodulation section  160  demodulates the received signal RF using Hilbert conversion. The demodulation section  160  includes a Hilbert converter  62  and a wave detector  64 . The Hilbert converter  62  delays a phase of the received signal RF input from the first filter section  150  by 90 degrees and inputs the signal to the wave detector  64 . The wave detector  64  performs wave detection by arc-tangent wave detection, using the received signal RF input from the first filter section  150  and the delayed signal input from the Hilbert converter  62 . The wave detection result is output to the second filter section  70 . 
     The second filter section  70  is connected to a back side of the demodulation section  160  to down sample the wave detection result input from the demodulation section  160  and output the signal as the demodulated signal. The second filter section  70  is configured in a similar manner to that of the second filter section  70  of the demodulator  100  described above. 
     The demodulation section  160  demodulates the received signal RF using Hilbert conversion so that the noise removal section  40  and the first filter section  50  of a two-channel configuration in the demodulator  100  can be replaced with the noise removal section  40  and the first filter section  150  of a one-channel configuration in the demodulator  130  according to the present variant example. 
     Note that, although the demodulator  100  according to the present embodiment and the demodulators  110 ,  120 , and  130  according to the variant examples are described as demodulators that demodulate the FM modulated radio wave, they are not limited thereto, but may also be demodulators that demodulate an AM modulated, PM modulated, FSK modulated, ASK modulated, or PSK modulated radio wave. 
     While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. 
     The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order. 
     As can be understood clearly from the description above, the demodulator can be realized according to an (one) embodiment of the present invention.