Abstract:
A noise eliminator comprises: a noise detection unit for detecting the pulsed noise mixed in a reception signal, and detecting the period of occurrence of the pulsed noise; a detuning frequency detection unit for detecting an adjacent interference signal superimposed on the reception signal, and detecting a detuning frequency between the frequency of the adjacent interference signal and the frequency of a desired signal; a gate control unit for calculating a period approximated to the period of occurrence of the pulsed noise as a gate period, the gate period being an integer multiple of the period equivalent to a reciprocal of the detuning frequency; and a gate unit for interrupting the pulsed noise mixed in the reception signal during the gate period, thereby outputting a reception signal of which pulsed noise is eliminated. Thus, the noise eliminator can reduce the effect of spurious signal ascribable to an adjacent interference signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a noise eliminator for eliminating noise mixed in a signal to be processed inside a receiver.  
         [0002]     The present application claims priority from Japanese Patent Application No. 2004-7961, the disclosure of which is incorporated herein by reference.  
         [0003]     In general, it is technically important for receivers to perform signal processing based on noise-eliminated reception signals.  
         [0004]     To give a concrete example, an in-car radio receiver to be mounted on an automobile is provided with a noise eliminator for eliminating pulsed noise such as ignition noise mixed in an intermediate frequency signal (IF signal), so that signal processing is performed based on the noise-reduced IF signal (see Japanese Patent Application Laid-Open No. Hei 6-112853; hereinafter, referred to as patent document).  
         [0005]      FIG. 7  is a block diagram showing the schematic configuration of a receiver to which the noise eliminator shown in  FIG. 1  of the patent document is applied.  FIGS. 8A  to  8 G are waveform charts for explaining the operation of the noise eliminator.  
         [0006]     In  FIG. 7 , the noise eliminator comprises a gate circuit and a noise detection circuit which are connected with the output of a tuner.  
         [0007]     When an IF signal SIF 1  in which such pulsed noise as shown in  FIG. 8A  is mixed is input to the noise detection circuit, the noise detection circuit detects the periods τ of occurrence of the pulsed noise, and outputs a gate control signal GT of rectangular waveform as shown in  FIG. 8B . Next, according to the gate control signal GT, the gate circuit interrupts the passing of the pulsed noise mixed in the IF signal SIF 1  during the periods τ of occurrence, whereby an IF signal SIF 2  of which the pulsed noise is eliminated is output and supplied to an IF filter.  
         [0008]     Then, while the IF signal SIF 2  passes through the IF filter having a predetermined passband, a desired signal SIF 3  is extracted and supplied to a detector. A detection signal detected by the detector is output to a demodulation circuit and the like.  
         [0009]     Now, in the conventional noise eliminator disclosed in the foregoing patent document, the pulsed noise included in the IF signal SIF 1  is interrupted in the periods τ of occurrence of the pulsed noise based on the gate control signal GT. The output IF signal SIF 2  is thus demodulated by the interruption characteristic.  
         [0010]     More specifically, suppose that A(f) is the frequency spectrum of the IF signal SIF 1  having no pulsed noise mixed therein, B(f) is the frequency spectrum of the interruption characteristic mentioned above, and C(f) is the frequency spectrum of the IF signal SIF 2  of which the pulsed noise is eliminated, output from the noise eliminator. The frequency spectrum C(f) of the IF signal SIF 2  ideally coincides with the frequency spectrum A(f), whereas it actually is one expressed as the product of the frequency spectra A(f) and B(f), or A(f) B(f). This can cause harmonic distortion and the like in the IF signal SIF 2  due to the effect of the interruption characteristic.  
         [0011]     Moreover, suppose the case where an adjacent interference signal is superimposed on and pulsed noise is mixed in the IF signal SIF 1  output from the tuner. Here, the gate circuit interrupts the pulsed noise, the adjacent interference signal modulated by the foregoing interruption characteristic appears as a spurious signal in the IF signal SIF 2 .  
         [0012]     For example, when an adjacent interference signal having a frequency Fu which is different from the frequency (in other words, intermediate frequency) Fd of the desired signal by a frequency ΔF is superimposed on the IF signal SIF 1 , the IF signal SIF 1  exhibits such a frequency spectrum as shown in  FIG. 8E . The foregoing interruption characteristic has a frequency spectrum which varies with the time width τ and period T of the gate control signal GT as parameters as shown in  FIG. 8F .  
         [0013]     Consequently, the IF signal SIF 2  output from the gate circuit has such a frequency spectrum as shown in  FIG. 8G , which is one expressed as the product of the frequency spectrum of the IF signal SIF 1  shown in  FIG. 8E , having the adjacent interference signal superimposed thereon, and the frequency spectrum of the interruption characteristic shown in  FIG. 8F .  
         [0014]     Then, as shown in  FIG. 8G , the frequency spectrum of the desired signal and the frequency spectrum of the spurious signal ascribable to the adjacent interference signal overlap with each other at the frequency Fd. This produces the problem that even if the IF signal SIF 2  of which the pulsed noise is eliminated is passed through the IF filter having its passband set to the intermediate frequency Fd, the desired signal is extracted with spurious signals mixed therein, which deteriorates the selectivity of the desired signal in the IF filter.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention has been achieved in view of the foregoing conventional problems. It is thus an object of the present invention to provide a noise eliminator which reduces the effect of a spurious signal ascribable to an adjacent interference signal, for example, and eliminates pulsed noise mixed in the reception signal with higher reliability.  
         [0016]     According to a first aspect of the present invention, a noise eliminator for eliminating pulsed noise mixed in a reception signal, comprises: a noise detection unit for detecting the pulsed noise mixed in the reception signal, and detecting a period of occurrence of the pulsed noise; a detuning frequency detection unit for detecting an adjacent interference signal superimposed on the reception signal, and detecting a detuning frequency between the frequency of the adjacent interference signal and the frequency of a desired signal; a gate control unit for calculating a period approximated to the period of occurrence of the pulsed noise as a gate period, the gate period being an integer multiple of a period equivalent to a reciprocal of the detuning frequency; and a gate unit for interrupting the pulsed noise mixed in the reception signal during the gate period, thereby outputting the reception signal of which pulsed noise is eliminated.  
         [0017]     According to a second aspect of the present invention, in the noise eliminator according to the first aspect, the detuning frequency detection unit comprises: an oscillator for outputting an alternating signal having the same frequency as that of the desired signal; and a multiplier for mixing the alternating signal and the reception signal to detect the detuning frequency.  
         [0018]     According to a third aspect of the present invention, in the noise eliminator according to the second aspect, the detuning frequency detection unit comprises: a sensing unit for sensing whether any adjacent interference signal is superimposed on the reception signal or not, based on a mixed signal of the alternating signal and the reception signal output from the multiplier; and a switching unit for making the gate control unit calculate a period approximated to the period of occurrence of the pulsed noise as the gate period, the gate period being an integer multiple of a period equivalent to the reciprocal of the detuning frequency when the sensing unit senses that the adjacent interference signal is superimposed, and making the gate control unit calculate a period approximated to the period of occurrence of the pulsed noise as the gate period, the gate period being an integer multiple of a period equivalent to the reciprocal of the same frequency as that of the desired signal when the sensing unit senses that the adjacent interference signal is not superimposed.  
         [0019]     According to a fourth aspect of the present invention, in the noise eliminator according to the third aspect, the sensing unit comprises: an AM detector for subjecting the mixed signal to AM detection; and a comparator for detecting whether the adjacent interference signal is superimposed on the reception signal or not, based on the amplitude of a signal output from the AM detector.  
         [0020]     According to a fifth aspect of the present invention, a noise eliminator for eliminating pulsed noise mixed in an RF signal, comprises: a noise detection unit for detecting the pulsed noise mixed in the RF signal, and detecting the period of occurrence of the pulsed noise; a detuning frequency detection unit for detecting an adjacent interference signal superimposed on the RF signal, and detecting a detuning frequency between the frequency of the adjacent interference signal and the frequency of a desired signal; a gate control unit for calculating a period approximated to the period of occurrence of the pulsed noise as a gate period, the gate period being an integer multiple of a period equivalent to the reciprocal of the detuning frequency; and a gate unit for stopping supply of a local oscillation signal to an RF multiplier for mixing the local oscillation signal and the RF signal, during the gate period.  
         [0021]     According to a sixth aspect of the present invention, in the noise eliminator according to the fifth aspect, the detuning frequency detection unit comprises: a first multiplier for mixing the local oscillation signal and a signal having the same frequency as an intermediate frequency; and a second multiplier for mixing the RF signal and an output of the first multiplier to detect the detuning frequency between the frequency of the adjacent interference signal superimposed on the RF signal and the frequency of the desired signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     These and other objects and advantages of the present invention will become clear from the following description with reference to the accompanying drawings, wherein:  
         [0023]      FIG. 1A  is a block diagram showing the configuration of a noise eliminator according to an embodiment of the invention, and  FIGS. 1B  to  1 E are charts for explaining the operation of the noise eliminator;  
         [0024]      FIG. 2  is a block diagram showing the configuration of the noise eliminator according to a more specific practical example of the embodiment shown in  FIGS. 1A  to  1 E;  
         [0025]      FIGS. 3A  to  3 G are charts for explaining the operation of the noise eliminator shown in  FIG. 2 ;  
         [0026]      FIG. 4  is a block diagram showing the configuration of the noise eliminator according to a second practical example;  
         [0027]      FIGS. 5A  to  5 C are charts for explaining the operation of the noise eliminator shown in  FIG. 4 ;  
         [0028]      FIG. 6  is a block diagram showing the configuration of the noise eliminator according to a third practical example;  
         [0029]      FIG. 7  is a diagram showing the configuration of a conventional noise eliminator; and  
         [0030]      FIGS. 8A  to  8 G are charts for explaining the operation of the conventional noise eliminator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     For one of preferred embodiments of the present invention, a noise eliminator to be incorporated in an in-car radio receiver or the like will be described with reference to  FIGS. 1A  to  1 E.  
         [0032]      FIG. 1A  is a block diagram showing the configuration of the noise eliminator according to the present embodiment.  FIGS. 1B  to  1 E are waveform charts for explaining the operation of the noise eliminator according to the present embodiment.  
         [0033]     In  FIG. 1A , this noise eliminator  1  comprises a gate unit  2 , a noise detection unit  3 , and a detuning frequency detection unit  4  which receive the reception signal output from an internal tuner (not shown) of the receiver, or an IF signal S 1 , and a gate control unit  5 .  
         [0034]     The noise detection unit  3  detects pulsed noise such as ignition noise and thunderbolt noise when any is mixed in the IF signal S 1 , and outputs a noise detection signal S 3  for indicating the periods τ of occurrence of the pulsed noise.  
         [0035]     The detuning frequency detection unit  4  detects the frequency Fu of an adjacent interference signal superimposed on the IF signal S 1 . The detuning frequency detection unit  4  also detects a frequency difference (hereinafter, referred to as “detuning frequency”) ΔF between the frequency Fu and the frequency Fd of a desired signal, and outputs a detuning frequency detection signal S 4  for indicating the detuning frequency ΔF.  
         [0036]     The gate control unit  5  receives the noise detection signal S 3  and the detuning frequency detection signal S 4 , and calculates periods (m/ΔF) which are m (integer) times the reciprocal of the detuning frequency ΔF, or (1/ΔF), and are the closest to the periods τ of occurrence of the pulsed noise, respectively. The gate control unit  5  then outputs a gate control signal S 5  of rectangular waveform, having gate periods τs which extend from the times t of occurrence of the respective noise pulses to when the periods (m/ΔF) have elapsed.  
         [0037]     The gate unit  2  interrupts the passing of the pulsed noise mixed in the IF signal S 1  during the gate periods τs indicated by the gate control signal S 5 , and lets the IF signal S 1  pass during periods other than the gate periods τs. The gate unit  2  thus outputs an IF signal S 2  of which pulsed noise is eliminated.  
         [0038]     Next, description will be given of the operation of the noise eliminator  1  having the foregoing configuration.  
         [0039]     For example, when an adjacent interference signal is superimposed on the IF signal S 1  and relatively periodic pulsed noise such as ignition noise is mixed in as shown in  FIG. 1B , the noise detection unit  3  detects the time t of occurrence and the period τ of occurrence of the noise pulse by pulse, and outputs a noise detection signal S 3  of rectangular waveform as shown in  FIG. 1C .  
         [0040]     Moreover, the detuning frequency detection unit  4  detects the frequency Fu of the adjacent interference signal, detects the detuning frequency ΔF, or the frequency difference between the detected frequency Fu and the frequency Fd of the desired signal, and outputs the detuning frequency detection signal S 4 .  
         [0041]     Then, the gate control unit  5  calculates the period (m/ΔF), which is m (integer) times the reciprocal of the detuning frequency ΔF, or (1/ΔF), and is the closest to the period τ, with respect to each pulse of the noise. The gate control unit  5  thus outputs the gate control signal S 5  of rectangular waveform in which the gate period τs is set at the elapsed time of the period (m/ΔF) since the time t of occurrence of the noise pulse by pulse.  
         [0042]     Suppose, for example, that the three pulses of noise shown in  FIG. 1C  have periods τ of occurrence of 1 msec, 2 msec, and 3 msec in order of elapsed time, respectively. For the gate period τs for eliminating the pulsed noise of 1 msec, the gate control unit  5  calculates a period (m/ΔF) which is m (integer) times the reciprocal of the detuning frequency ΔF and is the closest to 1 msec. For the gate period τs for eliminating the pulsed noise of 2 msec, the gate control unit  5  then calculates a period (m/ΔF) which is m (integer) times the reciprocal of the detuning frequency ΔF and is the closest to 2 msec. For the gate period τs for eliminating the pulsed noise of 3 msec, the gate control unit  5  then calculates a period (m/ΔF) which is m (integer) times the reciprocal of the detuning frequency ΔF and is the closest to 3 msec. The gate control unit  5  thus outputs the gate control signal S 5  of rectangular waveform as shown in  FIG. 1D .  
         [0043]     Then, the gate unit  2  interrupts the passing of the pulsed noise mixed in the IF signal S 1  during the individual gate periods τs indicated by the gate control signal S 5 , and lets the IF signal S 1  pass during periods other than the gate periods τs. The gate unit  2  thus outputs the IF signal S 2  of which pulsed noise is eliminated.  
         [0044]     This noise eliminator  1  provides the following effects.  
         [0045]     When the IF signal S 1  on which an adjacent interference signal is superimposed and in which pulsed noise is mixed is input to the gate unit  2 , the gate unit  2  interrupts the pulsed noise during the gate periods τs for elimination. As shown in  FIG. 1E , the IF signal S 2  output from the gate unit  2  has a frequency spectrum expressed as the product of the frequency spectrum of the interruption characteristic in the gate periods τs and the frequency spectrum of the IF signal S 1  on which the adjacent interference signal is superimposed.  
         [0046]     More specifically, the frequency spectrum of the interruption characteristic is the same as the Fourier transform of the gate control signal S 2  of rectangular waveform shown in  FIG. 1D , exhibiting the harmonic characteristic of large attenuations at frequencies m (integer) times the frequency of 1/τs, i.e., 1/τs, 2/τs, 3/τs, . . . . Meanwhile, the frequency spectrum of the IF signal S 1  on which the adjacent interference signal is superimposed includes those of the desired wave having the frequency Fd and the adjacent interference signal having the frequency Fu. Consequently, the frequency spectrum of the IF signal S 2  is expressed as the product of the frequency spectrum of the foregoing interruption characteristic and the frequency spectrum of the IF signal S 1  on which the adjacent interference signal is superimposed, or as shown in  FIG. 1E .  
         [0047]     Then, as shown in  FIG. 1E , the spurious signal ascribable to the adjacent interference signal occurs at the frequency Fu while the desired signal occurs at a frequency of {Fu−(1/τs)}. In addition, the frequency spectrum of the harmonics included in the IF signal S 2 , resulting from the interruption characteristic, makes a significant attenuation at the frequency of {Fu−(1/τs)}.  
         [0048]     Consequently, the desired signal in the IF signal S 2  is no longer susceptible to the spurious signal ascribable to the adjacent interference signal and the harmonics ascribable to the interruption characteristic. When the IF signal S 2  is passed through the IF filter, the desired signal containing no pulsed noise or spurious signals can be extracted and supplied to a detector or the like without deteriorating the selectivity of the IF filter.  
         [0049]     That is, since the frequency {Fu−(1/τs)} of the desired signal shown in  FIG. 1E  is different from the frequency Fu of the adjacent interference signal by the detuning frequency ΔF, it coincides with the passband Fd of the IF filter provided in the receiver. As a result, even if any filter or the like having a special pass frequency band for extracting the desired signal in the IF signal S 2  is not connected to the subsequent stage of this noise eliminator  1 , it is possible to extract the desired signal containing no pulsed noise or spurious signals from the IF signal S 2  and supply it to the detector or the like with no deterioration in selectivity by simply connecting an ordinary IF filter provided in the receiver.  
       PRACTICAL EXAMPLE 1  
       [0050]     Next, a more specific practical example of the foregoing embodiment will be described with reference to FIGS.  2  to  3 G.  FIG. 2  is a block diagram showing the configuration of a receiver which is provided with the noise eliminator of this practical example.  FIGS. 3A  to  3 G are waveform charts for explaining the operation of the noise eliminator. In  FIG. 2 , parts identical or equivalent to those of  FIG. 1A  are designated by the same reference numerals.  
         [0051]     Initially, the configuration of the receiver will be overviewed with reference to  FIG. 2 . An RF multiplier  8  is connected to the output of an RF amplifier  6  which is connected with a reception antenna ANT. Then, the RF multiplier  8  mixes an RF signal output from the RF amplifier  6  and a local oscillation signal output from a local oscillator  7  to output a frequency-converted IF signal S 1 .  
         [0052]     An IF filter  10 , an IF amplifier  11 , and a detector  12  are connected in series with the output of a gate circuit  2  to be described later.  
         [0053]     Next, the configuration of the noise eliminator according to this practical example will be described in comparison with the noise eliminator shown in  FIG. 1A .  
         [0054]     This noise eliminator  1  comprises a gate circuit  2  corresponding to the gate unit  2  shown in  FIG. 1A , a noise detection circuit  3  corresponding to the noise detection unit  3  shown in  FIG. 1A , a detuning frequency detection circuit  4  corresponding to the detuning frequency detection unit  4  shown in  FIG. 1A , and a D flip-flop  5  corresponding to the gate control unit  5  shown in  FIG. 1A . Moreover, the detuning frequency detection circuit  4  comprises an IF multiplier  4   a , an IF oscillator  4   b , a high-pass filter  4   c , and a limiter amplifier  4   d.    
         [0055]     A delay circuit  9  has a predetermined delay time equal to the internal delay time of the noise detection circuit  3 , the detuning frequency detection circuit  4 , and the D flip-flop (hereinafter, referred to as “DFF”)  5 . The delay circuit  9  delays the IF signal S 1  output from the RF multiplier  8  and supplies the resultant to the gate circuit  2 , thereby adjusting the timing of pulsed noise elimination processing in the gate circuit  2  to be described later.  
         [0056]     The noise detection circuit  3  detects the periods τ of occurrence of pulsed noise mixed in the IF signal S 1 , and outputs a noise detection signal S 3  of rectangular waveform which turns to “H” in logic during the periods τ of occurrence alone.  
         [0057]     To be more specific, the noise detection circuit  3  comprises an amplitude detector, a high-pass filter, a low-pass filter, an amplifier, and a comparator which are not shown. The amplitude detector detects the IF signal S 1 . The high-pass filter extracts noise included in the output signal of the amplitude detector, in the range of frequencies higher than that of the desired signal. The low-pass filter smoothens the noise of higher frequencies in the output signal of the amplitude detector, thereby generating a smoothened signal near direct current. The amplifier amplifies the smoothened signal at a predetermined gain. The comparator compares the amplitude of the amplified smoothened signal and that of the output signal of the high-pass filter. Then, the comparator detects the periods in which the amplitude of the output signal of the high-pass filter exceeds that of the amplified smoothened signal as the periods τ of occurrence of the pulsed noise, and outputs a noise detection signal S 3 .  
         [0058]     The IF oscillator  4   b  is made of an oscillator for outputting an alternating signal CK having the same frequency as the intermediate frequency.  
         [0059]     The IF multiplier  4   a  is made of a multiplier. It multiplies (mixes) the IF signal S 1  and the alternating signal CK to generate and output a mixed signal SIF which is the IF signal S 1  frequency-converted based on the alternating signal CK.  
         [0060]     For example, an IF signal S 1  having an adjacent interference signal superimposed thereon is input to the IF multiplier  4   a , the IF multiplier  4   a  frequency-converts the foregoing desired signal included in the IF signal S 1  and the adjacent interference signal into the baseband frequency and the detuning frequency ΔF, respectively. Then, the frequency-converted mixed signal SIF is supplied to the high-pass filter  4   c.    
         [0061]     The high-pass filter  4   c  is made of a high-pass filter which passes signal components in the range of frequencies higher than that of the baseband desired signal included in the mixed signal SIF.  
         [0062]     The limiter amplifier  4   d  limits the amplitude of the signal passed through the high-pass filter  4   c , thereby outputting a wave-shaped binary signal, or a detuning frequency detection signal S 4 .  
         [0063]     When the foregoing mixed signal SIF including the frequency-converted adjacent interference signals is input to the high-pass filter  4   c , the high-pass filter  4   c  passes and outputs the adjacent interference signal having the same frequency as the detuning frequency ΔF described above. In addition, the limiter amplifier  4   d  limits the amplitude of the adjacent interference signal, thereby outputting the detuning frequency detection signal S 4  of rectangular waveform which makes logic inversions at periods equivalent to the reciprocal of the detuning frequency ΔF, or (1/ΔF).  
         [0064]     The DFF  5  receives the noise detection signal S 3  and the detuning frequency detection signal S 4  at its input terminal D and clock input terminal CP, respectively, and outputs a gate control signal S 5  from its output terminal Q.  
         [0065]     To be more specific, suppose that the detuning frequency detection signal S 4  which makes logic inversions at the foregoing periods of (1/ΔF) is input to the DFF  5  during the foregoing periods. τ of occurrence in which the noise detection signal S 3  is “H” in logic. Then, the DFF  5  outputs the gate control signal S 5  of rectangular waveform which remains “H” in logic during the periods τs which are m (integer) times the period of (1/ΔF) and is longer than and the closest to the foregoing periods τ of occurrence (i.e., during the gate periods).  
         [0066]     The gate circuit  2  is made of a switch element such as an analog switch. In the gate periods τs where the gate control signal S 5  is “H” in logic, the switch element interrupts the passing of an IF signal Sa supplied from the delay circuit  9 . In the periods other than the gate periods τs, i.e., while the gate control signal S 5  is “L” in logic, the switch element passes the IF signal Sa.  
         [0067]     Consequently, when the IF signal Sa having pulsed noise mixed therein is input to the gate circuit  2 , the gate circuit  2  interrupts the passing of the pulsed noise according to the gate control signal S 5  during the gate periods τs which are in synchronization with the periods τ of occurrence of the pulsed noise. As a result, the gate circuit  2  outputs the IF signal  2  of which pulse noise is eliminated, to the IF filter  10 .  
         [0068]     Now, an example of operation of the noise eliminator  1  according to this practical example having the foregoing configuration will be described with reference to  FIGS. 3A  to  3 G.  
         [0069]     Suppose, for example, that the RF multiplier  8  outputs such an IF signal S 1  as shown in  FIG. 3A  on which an adjacent interference signal is superimposed and in which pulsed noise is mixed. The noise detection circuit  3  detects the time of occurrence and the period τ of occurrence of the noise pulse by pulse, generates the noise detection signal S 3  of rectangular waveform as shown in  FIG. 3B , and supplies it to the input terminal D of the DFF  5 .  
         [0070]     The IF multiplier  4   a  mixes the IF signal S 1  and the alternating signal CK output from the IF oscillator  4   b  to output the frequency-converted mixed signal SIF. The mixed signal SIF is passed through the high-pass filter  4   c  and the limiter amplifier  4   d . As a result, the detuning frequency detection signal S 4  of rectangular waveform as shown in  FIG. 3C  is generated and input to the clock input terminal CP of the DFF  5 .  
         [0071]     Then, based on the noise detection signal S 3  and the detuning frequency detection signal S 4 , the DFF  5  generates the gate control signal S 5  as shown in  FIG. 3D  and supplies it to the gate circuit  2  through the output terminal Q.  
         [0072]     More specifically, the DFF  5  receives the noise detection signal S 3  of rectangular waveform, which turns to “H” in logic during the period τ of occurrence of each pulse of the noise mixed in the IF signal S 1 , and the detuning frequency detection signal S 4 , which repeats logic inversions at periods equivalent to the reciprocal of the detuning frequency ΔF, or (1/ΔF). This is equivalent to so-called delay processing on the noise detection signal S 3  based on the detuning frequency detection signal S 4 . As a result, the DFF  5  outputs the gate control signal S 5  of rectangular waveform which remains “H” in logic during the periods τs which are m (integer) times the period (1/ΔF) and are longer than and the closest to the foregoing periods τ of occurrence (i.e., gate periods).  
         [0073]     Next, the gate circuit  2  interrupts the passing of the pulsed noise mixed in the IF signal Sa supplied through the delay circuit  9  during the individual gate periods τs indicated by the gate control signal S 5 , and lets the IF signal Sa pass during periods other than the gate periods τs. The gate circuit  2  thus outputs the IF signal S 2  of which pulsed noise is eliminated.  
         [0074]     Then, when the IF signal S 2  is input to the IF filter  10  having the passband set at the frequency of the desired signal (in other words, intermediate frequency), a desired signal Sb included in the IF signal S 2  is extracted. The IF amplifier  11  then amplifies the extracted desired signal Sb into a signal Sc, which is input to the detector  12 . The detector  12  outputs a detection signal Sd.  
         [0075]     The noise eliminator  1  of this practical example provides the following effects.  
         [0076]     Initially, suppose the case where pulsed noise occurring relatively periodically, such as ignition noise, is mixed in the IF signal S 1 , and an adjacent interference signal having a frequency Fu different from the frequency Fd of the desired signal by the detuning frequency ΔF is superimposed on the IF signal S 1  as shown in  FIG. 3A . Here,  FIG. 3E  shows the frequency spectrum of the mixed signal SIF output from the IF multiplier  4   a .  FIG. 3F  shows the frequency spectrum of the interruption characteristic when the gate circuit  2  interrupts pulsed noise according to the gate control signal S 5 .  
         [0077]     That is, in the frequency spectrum of the mixed signal SIF, the desired signal and the adjacent interference signal occur at the positions of the baseband frequency and the detuning frequency ΔF, respectively. The frequency spectrum of the interruption characteristic varies with the period T and the gate period τs of occurrence of the pulsed noise as parameters, and attenuates significantly at frequencies n (integer) times the reciprocal of the gate period τs, or (n/τs)  
         [0078]     Consequently, the IF signal S 2  of which pulsed noise is eliminated, output from the gate circuit  2 , has the frequency spectrum expressed as the product of the frequency spectrum of the mixed signal SIF and the frequency spectrum of the interruption characteristic as shown in  FIG. 3G . Here, the spurious signal resulting from the adjacent interference signal included in the IF signal S 2  occurs at the frequency Fu. The desired signal included in the IF signal S 1 , having the frequency of Fd, appears in the IF signal S 2  at the position of the frequency {Fu−(1/τs)}. Moreover, the frequency spectrum of the harmonics included in the IF signal S 2 , resulting from the interruption characteristic, attenuates significantly at the frequency {Fu−(1/τs)}.  
         [0079]     Thus, the desired signal in the IF signal S 2  is no longer susceptible to the harmonics and spurious signals ascribable to the adjacent interference signal and the interruption characteristic.  
         [0080]     As above, the gate periods τs are determined as periods which are m (integer) times the period equivalent to the reciprocal of the detuning frequency ΔF, or (1/ΔF), and are longer than and the closest to the respective periods τ of occurrence of the pulsed noise. These gate periods τs are used to approximate the periods τ of occurrence of the pulsed noise. As a result, it is possible to adjust the frequency spectrum of the IF signal S 2  so that the desired signal shown in  FIG. 3E  occurs in accordance with the frequency (1/τs) at which the harmonics of the interruption characteristic shown in  FIG. 3F  attenuate significantly.  
         [0081]     Thus, when the IF signal S 2  is supplied to the IF filter  10 , it is possible to extract the desired signal containing no pulsed noise or spurious signals from the IF signal S 2  and supply it to the detector  12  and the like without a deterioration in selectivity.  
         [0082]     Moreover, since the frequency {Fu−(1/τs)} of the desired signal shown in  FIG. 3G  is different from the frequency Fu of the adjacent interference signal by the detuning frequency ΔF, it coincides with the passband Fd of the IF filter  10  provided in the receiver. This eliminates the need to provide an IF filter having a special pass frequency band for the sake of extracting the desired signal in the IF signal S 2 . It is possible to extract the desired signal containing no pulsed noise or spurious signals from the IF signal S 2  and supply it to the detector or the like with no deterioration in selectivity by simply connecting the gate circuit  2  with the ordinary IF filter  10  provided in the receiver.  
       PRACTICAL EXAMPLE 2  
       [0083]     Next, the noise eliminator  1  according to a second practical example will be described with reference to FIGS.  4  to  5 C.  FIG. 4  is a block diagram showing the configuration of a receiver which is provided with the noise eliminator of this practical example.  FIGS. 5A  to  5 C are waveform charts for explaining the operation of the noise eliminator. In  FIG. 4 , parts identical or equivalent to those of  FIGS. 1A and 2  are designated by the same reference numerals.  
         [0084]     In  FIG. 4 , this noise eliminator  1  comprises a gate circuit  2 , a noise detection circuit  3 , a detuning frequency detection circuit  4 , a DFF  5 , and a delay circuit  9 . The detuning frequency detection circuit  4  comprises an IF multiplier  4   a , an IF oscillator  4   b , a high-pass filter  4   c , a limiter amplifier  4   d , an AM detector  4   e , a comparator  4   f , and a switching circuit  4   g . The AM detector  4   e  and the comparator  4   f  function as sensing means for sensing if any adjacent interference signal is superimposed on an IF signal S 1 .  
         [0085]     That is, in terms of configuration, this noise eliminator  1  is different from the noise eliminator shown in  FIG. 2  in that the detuning frequency detection circuit  4  is provided with the AM detector  4   e , the comparator  4   f , and the switching circuit  4   g.    
         [0086]     Here, the switching circuit  4   g  is made of an analog multiplexer or analog switch of two-input one-output type, which makes switching operations in accordance with a switch control signal Sg from the comparator  4   f . One input terminal a is connected to the limiter amplifier  4   d , and the other input terminal b to the IF oscillator  4   b . The output terminal c is connected to the clock input terminal CP of the DFF  5 . When the switching circuit  4   g  is switched to the input terminal a, the detuning frequency detection signal S 4  which makes logic inversions at periods (1/ΔF), output from the limiter amplifier  4   d , is transferred to the clock input terminal CP of the DFF  5 . When the switching circuit  4   g  is switched to the input terminal b, the alternating signal CK having the same frequency as the intermediate frequency, output from the IF oscillator  4   b , is transferred to the clock input terminal CP of the DFF  5 .  
         [0087]     The AM detector  4   e  subjects an adjacent interference signal Se output from the high-pass filter  4   c  to AM detection, and outputs the resulting AM detection signal Sf to the comparator  4   f.    
         [0088]     More specifically, the IF multiplier  4   a  mixes the IF signal S 1  having the adjacent interference signal superimposed thereon and the alternating signal CK, and outputs the mixed signal SIF including the frequency-converted adjacent interference signal to the high-pass filter  4   c . Then, the adjacent interference signal Se passed through the high-pass filter  4   c  is input to the AM detector  4   e . The AM detector  4   e  subjects this adjacent interference signal Se to AM detection, thereby outputting the AM detection signal Sf to the comparator  4   f.    
         [0089]     The comparator  4   f  compares the amplitude of the AM detection signal Sf with a predetermined threshold, and outputs the result of comparison as the switch control signal Sg. If the amplitude of the AM detection signal Sf is greater than the threshold, the switching circuit  4   g  is switched to the input terminal a. If the amplitude of the AM detection signal Sf is smaller than the threshold, the switching circuit  4   g  is switched to the input terminal b.  
         [0090]     Now, an example of operation of the noise eliminator  1  having the foregoing configuration will be described with reference to  FIGS. 3A  to  3 C.  
         [0091]     Suppose, for example, that the RF multiplier  8  outputs such an IF signal S 1  as shown in  FIG. 3A  on which an adjacent interference signal is superimposed and in which pulsed noise is mixed. The noise detection circuit  3  detects the time of occurrence and the period τ of occurrence of the noise pulse by pulse, generates the noise detection signal S 3  of rectangular waveform as shown in  FIG. 3B , and supplies it to the input terminal D of the DFF  5 .  
         [0092]     The IF multiplier  4   a  mixes the IF signal S 1  and the alternating signal CK output from the IF oscillator  4   b  to output the frequency-converted mixed signal SIF. The mixed signal SIF is passed through the high-pass filter  4   c  and the limiter amplifier  4   d . As a result, the detuning frequency detection signal S 4  of rectangular waveform as shown in  FIG. 3C  is generated and input to the one input terminal a of the switching circuit  4   g.    
         [0093]     The AM detector  4   e  subjects the adjacent interference signal Se to AM detection, thereby outputting the AM detection signal Sf. The comparator  4   f  generates the switch control signal Sg from the AM detection signal Sf, so that the switching circuit  4   g  is switched to the input terminal a. Consequently, the detuning frequency detection signal S 4  is input to the clock input terminal CP of the DFF  5  through the input terminal a of the switching circuit  4   g.    
         [0094]     Then, the DFF  5  receives the noise detection signal S 3  and the detuning frequency detection signal S 4  which repeats logic inversions at periods equivalent to the reciprocal of the detuning frequency ΔF, or (1/ΔF). As a result, the DFF  5  generates the gate control signal S 5  of rectangular waveform which remains “H” in logic during the periods τs which are m (integer) times the period (1/ΔF) and are longer than and the closest to the foregoing periods τ of occurrence of the pulsed noise (i.e., the gate periods). The gate control signal S 5  is supplied to the gate circuit  2 .  
         [0095]     Next, the gate circuit  2  interrupts the passing of the pulsed noise mixed in the IF signal Sa supplied through the delay circuit  9  during the individual gate periods τs indicated by the gate control signal S 5 , and lets the IF signal Sa pass during periods other than the gate periods τs. The gate circuit  2  thus outputs the IF signal S 2  of which pulsed noise is eliminated.  
         [0096]     Up to this point, description has been given of the operation for situations where the RF multiplier  8  outputs the IF signal S 1  on which an adjacent interference signal is superimposed and in which pulsed noise is mixed. When the RF multiplier  8  outputs an IF signal S 1  on which no adjacent interference signal is superimposed but in which pulsed noise is mixed, the noise eliminator  1  makes the following operation.  
         [0097]     Initially, the noise detection circuit  3  detects the time of occurrence and the period τ of occurrence of the noise pulse by pulse, generates the noise detection signal S 3  of rectangular waveform as shown in  FIG. 3B , and supplies it to the input terminal D of the DFF  5 .  
         [0098]     The IF multiplier  4   a  mixes the IF signal S 1  and the alternating signal CK output from the IF oscillator  4   b  to output the frequency-converted mixed signal SIF. Note that since no adjacent interference signal is superimposed on the IF signal S 1 , the mixed signal SIF does not contain any adjacent interference signal. Thus, even when the mixed signal SIF is passed through the high-pass filter  4   c  and the limiter amplifier  4   d , such a detuning frequency detection signal S 4  as shown in  FIG. 3C  is not supplied to the input terminal a of the switching circuit  4   g.    
         [0099]     In addition, the signal Se input from the high-pass filter  4   c  to the AM detector  4   e  does not contain any adjacent interference signal, either, so that the AM detection signal Sf output from the AM detector  4   e  becomes smaller in amplitude. The comparator  4   f  compares the AM detection signal SF and the threshold, and thus outputs the switch control signal Sg for switching the switching circuit  4   g  to the input terminal b.  
         [0100]     Consequently, when no adjacent interference signal is superimposed on the IF signal S 1 , the alternating signal CK output from the IF oscillator  4   b  is input to the clock input terminal CP of the DFF  5  through the input terminal b of the switching circuit  4   g.    
         [0101]     Receiving the noise detection signal S 3  and the alternating signal CK, the DFF  5  then generates the gate control signal S 5  of rectangular waveform which remains “H” in logic during the periods τs which are integer multiples of the period equivalent to the reciprocal of the frequency of the alternating signal CK and are longer than and the closest to the foregoing periods τ of occurrence of the pulsed noise (i.e., the gate periods). The gate control signal S 5  is supplied to the gate circuit  2 .  
         [0102]     Next, the gate circuit  2  interrupts the passing of the pulsed noise mixed in the IF signal Sa supplied through the delay circuit  9  during the individual gate periods τs indicated by the gate control signal S 5 , and lets the IF signal Sa pass during periods other than the gate periods τs. The gate circuit  2  thus outputs the IF signal S 2  of which pulsed noise is eliminated.  
         [0103]     As described above, in the noise eliminator  1  shown  FIG. 4 , the AM detector  4   e  and the comparator  4   f  detect whether or not any adjacent interference signal is superimposed on the IF signal S 1 . If any adjacent interference signal is superimposed on the IF signal S 1 , the gate circuit  2  interrupts the pulsed noise by using periods which are m (integer) times the period (1/ΔF) equivalent to the reciprocal of the detuning frequency ΔF and are longer than and the closest to respective periods τ of occurrence of the pulsed noise as the gate periods τs. If no adjacent interference signal is superimposed on the IF signal S 1 , the gate circuit  2  interrupts the pulsed noise by using periods which are integral multiples of the period equivalent to the reciprocal of the frequency (intermediate frequency) of the alternating signal CK and are longer than and the closest to respective periods τ of occurrence of the pulsed noise.  
         [0104]     The noise eliminator  1  according to this practical example provides the following effects.  
         [0105]     First, as described above, when pulsed noise is mixed in the IF signal S 1  having no adjacent interference signal superimposed thereon, the AM detector  4   e  and the comparator  4   f  detect that no adjacent interference signal is superimposed on the IF signal S 1 . Meanwhile, the noise detection circuit  3  detects the periods τ of occurrence of the pulsed noise. Consequently, the noise detection signal S 3  indicating the periods τ of occurrence is supplied to the input terminal D of the DFF  5 , and the alternating signal CK having the same frequency as the intermediate frequency is supplied to the clock input terminal CP through the switching circuit  4   g . Then, the DFF  5  makes the gate circuit  2  interrupt the pulsed noise by using the periods which are integer multiples of the period equivalent to the reciprocal of the frequency of the alternating signal CK and are longer than and the closest to the respective periods τ of occurrence of the pulsed noise as the gate periods τs.  
         [0106]     Here, as shown in the frequency spectrum of  FIG. 5A , the alternating signal CK occurs at the position of the intermediate frequency. As shown in  FIG. 5B , the frequency spectrum of the interruption characteristic when the gate circuit  2  interrupts the pulsed noise makes large attenuations at frequencies which are integer multiples of the frequency (1/τs). Then, the IF signal S 2  of which pulsed noise is eliminated, output from the gate circuit  2 , exhibits the frequency spectrum expressed as the product of the frequency spectrum of the alternating signal CK and the frequency spectrum of the interruption characteristic as shown in  FIG. 5C .  
         [0107]     Then, as is evident from  FIG. 5C , the harmonics ascribable to the interruption characteristic attenuate significantly at a frequency N (integer) times the frequency (1/τs), and the desired signal occurs right at the frequency of attenuation of the harmonics.  
         [0108]     This makes the desired signal in the IF signal S 2  insusceptible to the harmonics ascribable to the interruption characteristic. When the IF signal S 2  is supplied to the IF filter  10 , it is possible to extract the desired signal containing no pulsed noise from the IF signal S 2  and supply it to the detector  12  and the like.  
         [0109]     Second, when pulsed noise is mixed in and an adjacent interference signal superimposed on the IF signal S 1 , the AM detector  4   e  and the comparator  4   f  detect that the adjacent interference signal is superimposed on the IF signal S 1 . Meanwhile, the noise detection circuit  3  detects the periods τ of occurrence of the pulsed noise. Consequently, the noise detection signal S 3  indicating the periods τ of occurrence is supplied to the input terminal D of the DFF  5 , and the detuning frequency signal S 4  is supplied to the clock input terminal CP through the switching circuit  4   g . Then, the DFF  5  determines periods which are m (integer) times the period (1/ΔF) equivalent to the reciprocal of the detuning frequency ΔF and are longer than and the closest to the respective periods τ of occurrence of the pulsed noise as the gate periods τs. The DFF  5  eliminates the pulsed noise by controls the gate circuit  2  with the gate control signal S 5  having the gate periods τs approximated to the periods τ of occurrence of the pulsed noise.  
         [0110]     Consequently, the IF signal S 2  output from the gate circuit  2  exhibits the same frequency spectrum as shown in  FIG. 3G , so that the desired signal in the IF signal S 2  becomes insusceptible to the harmonics and spurious signals ascribable to the adjacent interference signal and the interruption characteristic. When the IF signal S 2  is supplied to the IF filter  10 , it is possible to extract the desired signal containing no pulsed noise or spurious signals from the IF signal S 2  and supply it to the detector  12  and the like without a deterioration in selectivity.  
         [0111]     As above, according to the noise eliminator  1  of this practical example, it is possible to eliminate pulsed noise in both cases that the pulsed noise is mixed in the IF signal S 1  having no adjacent interference signal superimposed thereon, and that the pulsed noise is mixed in and an adjacent interference signal is superimposed on the IF signal S 1 . It is also possible to extract the desired signal from the IF signal S 2  and supply it to the detector  12  and the like without a deterioration in selectivity.  
         [0112]     Furthermore, in the case of eliminating the pulsed noise mixed in the IF signal S 1  having no adjacent interference signal superimposed thereon, the gate circuit  2  interrupts the pulsed noise by using the periods that are integer multiples of the period equivalent to the reciprocal of the frequency of the alternating signal CK and are longer than and the closest to the periods τ of occurrence of the pulsed noise as the gate periods τs. The frequency at which the harmonics of the interruption characteristic shown in  FIG. 5C  attenuate significantly can thus be matched with the frequency of the desired signal. It is therefore possible to extract the desired signal in a favorable manner and supply it to the detector and the like without affecting the interruption characteristic in eliminating the pulsed noise.  
       PRACTICAL EXAMPLE 3  
       [0113]     Next, the noise eliminator  1  according to a third practical example will be described with reference to  FIG. 6 .  FIG. 6  is a block diagram showing the configuration of a receiver which is provided with the noise eliminator of this practical example. In  FIG. 6 , parts identical or equivalent to those of  FIGS. 1A, 2 , and  4  are designated by the same reference numerals.  
         [0114]     In  FIG. 6 , this noise eliminator  1  comprises a gate circuit  2 , a noise detection circuit  3 , a detuning frequency detection circuit  4 , a DFF  5 , and a delay circuit  9 . The gate circuit  2  is interposed between a local oscillator  7  and an RF multiplier  8 . The delay circuit  9  is interposed between an RF amplifier  6  and the RF multiplier  8 . Moreover, the detuning frequency detection circuit  4  comprises an IF oscillator  4   b , first and second multipliers  4   h  and  4   i , a high-pass filter  4   c , and a limiter amplifier  4   d.    
         [0115]     The gate circuit  2  is made of an analog switch or the like which turns on during gate periods τs and turns off during periods other than the gate periods τs according to a gate control signal S 5  supplied from the DFF  5 . During the gate periods τs, a local oscillation signal LO output from the local oscillator  7  is supplied to the RF multiplier  8 . In the periods other than the gate periods τs, the supply of the local oscillation signal LO to the RF multiplier  8  is stopped.  
         [0116]     The delay circuit  9  is provided for the sake of timing adjustment, as in the first and second practical examples. It delays a reception signal output from the RF amplifier  6 , or an RF signal SRF, by a predetermined time and supplies the resultant to the RF multiplier  8 .  
         [0117]     Consequently, the RF multiplier  8  mixes the RF signal SRF supplied through the delay circuit  9  and the local oscillation signal LO supplied through the gate circuit  2 , thereby frequency-converting the RF signal SRF of radio frequency into an intermediate frequency signal S 2 , and supplies the resultant to an IF filter  10 .  
         [0118]     The noise detection circuit  3  detects pulsed noise mixed in the RF signal SRF, and supplies an input terminal D of the DFF  5  with a noise detection signal S 3  of rectangular waveform which turns to “H” in level during the periods τ of occurrence of the pulses.  
         [0119]     The IF oscillator  4   b  outputs the alternating signal CK having the same frequency as the intermediate frequency, and supplies it to the first multiplier  4   h.    
         [0120]     The first multiplier  4   h  is made of a multiplier. It multiplies (mixes) the local oscillation signal LO from the local oscillator  7  and the alternating signal CK to generate a signal (hereinafter, referred to as “first mixed signal”) Sm 1 , and supplies it to the second multiplier  4   i.    
         [0121]     The second multiplier  4   i  is made of a multiplexer. It multiplies (mixes) the RF signal SRF and the first mixed signal Sm 1  to generate a signal (hereinafter, referred to as “second mixed signal”) Sm 2 , and supplies it to the high-pass filter  4   c.    
         [0122]     Now, when the first multiplier  4   h  multiplies (mixes) the local oscillation signal LO and the alternating signal CK, the first mixed signal Sm 1  is generated with the frequency Fd of the desired signal. When the second multiplier  4   i  multiplies (mixes) the RF signal SRF and the first mixed signal Sm 1 , a signal having the frequency difference (Fu−Fd) between the frequency Fu of the adjacent interference signal superimposed on the RF signal SRF and the frequency Fd of the desired signal, or the detuning frequency ΔF, appears in the second mixed signal Sm 2 .  
         [0123]     The high-pass filter  4   c  has a cutoff frequency for passing the signal having the foregoing detuning frequency ΔF, included in the second mixed signal Sm 2 . The high-pass filter  4   c  supplies the passed signal having the detuning frequency ΔF to the limiter amplifier  4   d.    
         [0124]     The limiter amplifier  4   d  limits the amplitude of the signal passed through the high-pass filter  4   c , having the foregoing detuning frequency ΔF, and thereby outputs a wave-shaped binary signal, or a detuning frequency detection signal S 4 . More specifically, the limiter amplifier  4   d generates a detuning frequency detection signal S 4  which repeats logic inversions at periods equivalent to the reciprocal of the detuning frequency ΔF, or (1/ΔF), and supplies it to the clock input terminal CP of the DFF  5 .  
         [0125]     The DFF  5  receives the noise detection signal S 3  indicating the periods τ of occurrence of the pulsed noise, output from the noise detection circuit  3 , and the detuning frequency detection signal S 4  from the limiter amplifier  4 . The DFF  5  then performs so-called delay processing on the noise detection signal S 3  based on the detuning frequency detection signal S 4 . As a result, the DFF  5  outputs the gate control signal S 5  of rectangular waveform which remains “H” in logic during the periods τs which are m (integer) times the periods (1/ΔF) equivalent to the reciprocal of the detuning frequency ΔF and are longer than and the closest to the foregoing periods τ of occurrence. (i.e., gate periods).  
         [0126]     Consequently, in the gate periods τs approximated to the periods τ of occurrence of the pulsed noise mixed in the RF signal SRF, the DFF  5  turns off the gate circuit  2  to stop the supply of the local oscillation signal LO to the RF multiplier  8 . On the other hand, in the periods other than the gate periods τs, the DFF  5  turns on the gate circuit  2  so that the local oscillation signal LO is supplied to the RF multiplier  8 .  
         [0127]     According to the noise eliminator  1  of this practical example having the foregoing configuration, when an adjacent interference signal is superimposed on and pulsed noise is mixed in the RF signal SRF, the noise detection circuit  3  detects the periods τ of occurrence of the pulsed noise and outputs the noise detection signal S 3 . The detuning frequency detection circuit  4  detects the detuning frequency ΔF and outputs the detuning frequency detection signal S 4 . The DFF  5  outputs the gate control signal S 5  for turning off the gate circuit  2  during the gate periods τs approximated to the periods τ of occurrence of the pulsed noise.  
         [0128]     Consequently, when the gate circuit  2  is turned off during the gate periods τs, the RF multiplier  8  stops mixing the local oscillation signal LO and the pulsed noise which is mixed in the RF signal SRF supplied through the delay circuit  9 , and outputs the IF signal S 2  of which pulsed noise is eliminated.  
         [0129]     In addition, the gate periods τs are ones m (integer) times the period (1/ΔF) equivalent to the reciprocal of the detuning frequency ΔF and are longer than and the closest to the foregoing periods τof occurrence. This results in a coincidence between the frequency of the desired signal in the IF signal S 2  and the frequency at which a large attenuation occurs in the frequency spectrum of the interruption characteristic when the gate circuit  2  is turned off to interrupt the supply of the local oscillation signal LO to the RF multiplier  8 . Consequently, the desired signal in the IF signal S 2  is no longer susceptible to the spurious signal ascribable to the adjacent interference signal and the harmonics ascribable to the interruption characteristic of the gate circuit  2 . When the IF signal S 2  is passed through the IF filter, the desired signal containing no pulsed noise or spurious signals can thus be extracted and supplied to the detector or the like without deteriorating the selectivity of the IF filter  10 .  
         [0130]     While there has been described what are at present considered to be preferred embodiments of the present invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.