Apparatus for selectively receiving carrier wave signals

An apparatus for receiving carrier wave signals includes a frequency converting portion operative to produce an intermediate frequency signal having a carrier frequency of an intermediate frequency, a frequency demodulator for frequency-demodulating the intermediate frequency signal with a demodulation characteristic having a center frequency substantially equal to the intermediate frequency. The apparatus uses, a DC level detector to produce a first level detection output signal having a predetermined level when a DC level of a demodulation output signal obtained from the frequency demodulator is not less than a predetermined level, and an intermediate frequency level detector operative to produce a second level detection output signal having a predetermined level when a frequency component of the intermediate frequency signal, which falls within a predetermined frequency range that includes the center frequency of the demodulation characteristic in the frequency demodulating portion, has a level not less than a predetermined level. A signal discriminating portion is used for discriminating between a desirable condition in which the input signal is a carrier wave signal desired, to be received and an undesirable condition in which the input signal based is an image frequency signal on the strength of the first and second level detection output signals under a condition in which the receiving frequency is successively changed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an apparatus for selectively receiving
 carrier wave signals, and more particularly, is directed to an improvement
 in a carrier wave signal receiving apparatus for selectively receiving
 frequency-modulated (FM) carrier wave signals, such as FM radio
 broadcasting signals, and obtaining a demodulated output signal based on
 the FM carrier wave signal received thereby.
 2. Description of the Prior Art
 In the field of super heterodyne receivers used for receiving FM carrier
 wave signals, such as FM radio broadcasting signals, there has been
 generalized employ a digital tuning system wherein, for example, a
 phase-locked loop (PLL) is utilized in place of an analog tuning system
 wherein a variable capacitor is used. In the super heterodyne receiver in
 which the digital tuning system is employed, the phase-locked loop is
 operative to set exactly various receiving frequencies, for example, under
 the control by a microcomputer, and manual adjustments to the receiving
 frequency by a user are not necessary.
 In the super heterodyne receiver employing the digital tuning system, an
 automatic scanning tuning operation is performed. Under a condition in
 which the automatic scanning tuning operation is carried out, a receiving
 frequency provided for receiving selectively FM carrier wave signals is
 successively changed at predetermined regular frequency intervals by the
 PLL. Then, when there is an FM carrier wave signal which tunes with the
 receiving frequency, the change in the receiving frequency is temporarily
 ceased to keep the FM carrier wave signal tuning therewith, so that each
 FM carrier wave signal is searched. During the automatic scanning tuning
 operation in which such a signal search as mentioned above is carried out,
 it is desired to avoid such a situation that an image frequency signal
 which is not any FM carrier wave signal intended to be received acts as an
 FM carrier wave signal so that a malfunction by which the change in the
 receiving frequency is temporarily ceased is caused.
 Accordingly, it has been proposed to cause an intermediate frequency signal
 which is obtained by frequency-converting the FM carrier wave signal to
 have a relatively high carrier frequency, for example, 10.7 MHz so that
 the image frequency signal is not located in a frequency range wherein the
 FM carrier wave signals expected to be selectively received exist and
 therefore cannot be detected in the automatic scanning tuning operation.
 In addition, it is also proposed recently to form an intermediate frequency
 circuit portion of the super heterodyne receiver used for receiving the FM
 carrier wave signals, which includes an intermediate frequency filter for
 causing the intermediate frequency signal to pass therethrough, into an
 integrated circuit (IC) on the basis of the development in integrating
 technology related to electronic circuits. That is, in the super
 heterodyne receiver used for receiving the FM carrier wave signals, the
 intermediate frequency circuit portion is constituted by means of using
 one or more IC chips.
 In such a case, the intermediate frequency filter contained in the IC chip
 is formed with resistive elements and capacitive elements which are
 materialized in the IC chip and therefore a passing frequency band of the
 intermediate frequency filter must be arranged to be located in relatively
 low frequency range. Therefore, in the intermediate frequency circuit
 portion including the intermediate frequency filter and comprising one or
 more IC chips, the intermediate frequency set therein is selected to be
 relatively low, for example, 150 kHz.
 In the case where the intermediate frequency is selected to be relatively
 low, such as 150 kHz, in the super heterodyne receiver used for receiving
 the FM carrier wave signals as described above, the image frequency signal
 is undesirably located in the frequency range wherein the FM carrier wave
 signals expected to be selectively received exist. Consequently, it is
 feared that when a selected FM carrier wave signal is desired to tune with
 the receiving frequency successively changed by the PLL in the automatic
 scanning tuning operation, such a malfunction that an image frequency
 signal which results from another FM carrier wave signal adjacent to the
 selected FM carrier wave signal acts as the selected FM carrier wave
 signal to tune undesirably with the receiving frequency is brought about.
 OBJECTS AND SUMMARY OF THE INVENTION
 Accordingly, it is an object of the present invention to provide an
 apparatus for selectively receiving carrier wave signals, by which FM
 carrier wave signals expected to be received are selectively received in
 accordance with a super heterodyne system and a demodulation output signal
 is obtained based on the received FM carrier wave signal, and which avoids
 the aforementioned disadvantages encountered with the prior art.
 Another object of the present invention is to provide an apparatus for
 selectively receiving carrier wave signals, by which FM carrier wave
 signals expected to be received are selectively received in accordance
 with a super heterodyne system and a demodulation output signal is
 obtained based on the received FM carrier wave signal, and which can avoid
 interferences brought about by image frequency signals in an automatic
 scanning tuning operation. This is accomplished with the use of a
 receiving frequency successively changed under a situation wherein an
 intermediate frequency which is a carrier frequency of an intermediate
 frequency signal obtained by frequency-converting the received FM carrier
 wave signal is selected to be relatively low.
 A further object of the present invention is to provide an apparatus for
 selectively receiving carrier wave signals, by which FM carrier wave
 signals expected to be received are selectively received in accordance
 with a super heterodyne system and a demodulation output signal is
 obtained based on the received FM carrier wave signal, and which can avoid
 interferences brought about by image frequency signals in an automatic
 scanning tuning operation. The apparatus uses a receiving frequency
 successively changed under a situation wherein an intermediate frequency
 circuit portion of the apparatus, which includes an intermediate frequency
 filter for causing an intermediate frequency signal obtained by
 frequency-converting the received FM carrier wave signal to pass
 therethrough, with said filter formed on an IC operating with a relatively
 low carrier frequency of the intermediate frequency signal.
 According to the present invention, there is provided an apparatus for
 receiving carrier wave signals, which comprises a frequency converting
 portion for frequency-converting an input signal so as to convert a
 carrier frequency of the input signal into a predetermined intermediate
 frequency with a local oscillation signal having its frequency determining
 variably a receiving frequency and to produce an intermediate frequency
 signal having its carrier frequency of the intermediate frequency, an
 intermediate frequency amplifying portion for amplifying the intermediate
 frequency signal, a frequency demodulating portion for
 frequency-demodulating the intermediate frequency signal obtained from the
 intermediate frequency amplifying portion with a demodulation
 characteristic having a center frequency substantially equal to the
 intermediate frequency. In addition, a direct current (DC) level detecting
 portion is used to produce a first level detection output signal which has
 a predetermined level when a DC level of a demodulation output signal
 obtained from the frequency demodulating portion is equal to or more than
 a predetermined level. An intermediate frequency level detecting portion
 produces a second level detection output signal which has a predetermined
 level when a frequency component of the intermediate frequency signal
 obtained from the intermediate frequency amplifying portion is within a
 predetermined frequency range including the center frequency of the
 demodulation characteristic in the frequency demodulating portion and, has
 its level equal to or more than a predetermined level. A signal
 discriminating portion for discriminating between a desirable condition in
 which the input signal is a carrier wave signal desired to be received and
 an undesirable condition in which the input signal is an image frequency
 signal on the strength of the first level detection output signal obtained
 from the DC level detecting portion and the second level detection output
 signal obtained from the intermediate frequency level detecting portion
 under a situation wherein the receiving frequency is successively changed.
 In an embodiment of apparatus for receiving carrier wave signals thus
 constituted according to the present invention, a discrimination result
 obtained in the signal discriminating portion is used for controlling the
 frequency of the local oscillation signal which determines variably the
 receiving frequency.
 In the apparatus for receiving carrier wave signals constituted as
 mentioned above according to the present invention, when an automatic
 scanning tuning operation is performed to change successively the
 receiving frequency under a situation wherein the intermediate frequency
 is predetermined to be relatively low so that the image frequency signal
 comes into a frequency range in which carrier wave signals expected to be
 received exist, the first detection output signal obtained from the DC
 level detecting portion has a different level during a condition whererin
 a receiving frequency which is set immediately before another receiving
 frequency with which the second level detection output signal obtained
 from the intermediate frequency level detecting portion has the
 predetermined level, when the input signal is the carrier wave signal
 desired to be received. This level is different from that of the
 demodulation output signal based on the input signal is obtained from the
 frequency demodulating portion and in a situation wherein the input signal
 is the image frequency signal and the demodulation output signal based on
 the image frequency signal is obtained from the frequency demodulating
 portion.
 For example, in the desirable condition in which the input signal is the
 carrier wave signal desired to be received and the demodulation output
 signal based on the input signal is obtained from the frequency
 demodulating portion, the first detection output signal obtained from the
 DC level detecting portion during the condition whererin the receiving
 frequency which is set immediately before another receiving frequency with
 which the second level detection output signal obtained from the
 intermediate frequency level detecting portion has the predetermined
 level, has a predetermined level. In the undesirable condition in which
 the input signal is the image frequency signal and the demodulation output
 signal based on the image frequency signal is obtained from the frequency
 demodulating portion, the first detection output signal obtained from the
 DC level detecting portion during the condition whererin the receiving
 frequency which is set immediately before another receiving frequency with
 which the second level detection output signal obtained from the
 intermediate frequency level detecting portion has the predetermined
 level, does not have the predetermined level.
 Accordingly, the signal discriminating portion is operative to discriminate
 between the desirable condition in which the input signal is the carrier
 wave signal desired to be received and the undesirable condition in which
 the input signal is the image frequency signal on the strength of such a
 relation as described above between the level of the first detection
 output signal obtained from the DC level detecting portion and the level
 of the second detection output signal obtained from the intermediate
 frequency level detecting portion when the automatic scanning tuning
 operation is performed to change successively the receiving frequency.
 As a result, for example, in the case where the frequency of the local
 oscillation signal which determines variably the receiving frequency is
 controlled by use of the discrimination result obtained in the signal
 discriminating portion, the change in the receiving frequency is
 temporarily ceased to keep the carrier wave signal desired to be received
 tuning therewith when the input signal is the carrier wave signal desired
 to be received. The change in the receiving frequency continues without
 being temporarily ceased when the input signal is the image frequency
 signal, so that interferences brought about by the image frequency signal
 are eliminated from the automatic scanning tuning operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 shows an embodiment of apparatus for receiving carrier wave signals
 according to the present invention. This embodiment is operative to
 receive an FM radio broadcasting signal as an FM carrier wave signal.
 Referring to FIG. 1, an input signal which includes an FM carrier wave
 signal SFM and comes through an antenna 11 is amplified by a radio
 frequency amplifier 12 and then supplied to frequency convertors 13 and
 14. The FM carrier wave signal SFM is an FM radio broadcasting signal
 having its carrier frequency which in contained in a frequency range of,
 for example, 76 to 108 MHz.
 In the embodiment shown in FIG. 1, a local oscillator 15 and a
 microcomputer 16 are provided. The local oscillator 15 is operative to
 produce an oscillation output signal SOC having a frequency of double as
 high as a frequency which is lower by a predetermined value, for example,
 150 kHz than a receiving frequency. The frequency of the oscillation
 output signal SOC is controlled by a control signal CCV supplied from the
 microcomputer 16. That is, in the embodiment shown in FIG. 1, under a
 condition in which the frequency of the oscillation output signal SOC is
 controlled by the control signal CCV supplied from the microcomputer 16,
 the receiving frequency is variably determined to be a frequency which is
 higher by 150 kHz than a half of the frequency of the oscillation output
 signal SOC obtained from the local oscillator 15.
 The oscillation output signal SOC obtained from the local oscillator 15 is
 supplied to a frequency divider 17 and subjected to a frequency dividing
 processing to be divided into halves (1/2) in frequency in the frequency
 divider 17. As a result of such a frequency dividing processing, a couple
 of local oscillation signals SOLO and SOLQ are obtained from the frequency
 divider 17. Each of the local oscillation signals SOLO and SOLQ has a
 frequency corresponding to a half of the frequency of the oscillation
 output signal SOC obtained from the local oscillator 15 and a mutual phase
 difference by 90 degrees between each other. The local oscillation signal
 SOLO is supplied to the frequency convertor 13 and the local oscillation
 signal SOLQ is supplied to the frequency convertor 14. This means that
 each of the local oscillation signals SOLO and SOLQ obtained from the
 frequency divider 17 is operative to determine with its frequency variably
 the receiving frequency in the embodiment shown in FIG. 1 to be a
 frequency higher by 150 kHz than the frequency of the local oscillation
 signal SOLO or SOLQ.
 An intermediate frequency signal SIFA which is obtained by
 frequency-converting the input signal supplied through the radio frequency
 amplifier 12 with the local oscillation signal SOLO is derived from the
 frequency convertor 13. Similarly, an intermediate frequency signal SIFB
 which is obtained by frequency-conversion to the input signal supplied
 through the radio frequency amplifier 12 with the local oscillation signal
 SOLQ is derived from the frequency convertor 14. Each of the intermediate
 frequency signals SIFA and SIFB thus obtained has its carrier frequency
 corresponding to a difference between the frequency of the local
 oscillation signal SOLO or SOLQ and the receiving frequency, namely, an
 intermediate frequency of 150 kHz.
 The intermediate frequency signal SIFA derived from the frequency convertor
 13 and the intermediate frequency signal SIFB derived from the frequency
 convertor 14 are supplied to a phase shifter 18 which comprises
 biquadratic phase shifting circuits. A couple of intermediate frequency
 signals SIFO and SIFQ based on the intermediate frequency signals SIFA and
 SIFB, respectively, are derived from the phase shifter 18 to be supplied
 to a signal adder 19. Each of the intermediate frequency signals SIFO and
 SIFQ has its carrier frequency corresponding to the intermediate frequency
 determined to be 150 kHz and a mutual phase difference by 90 degrees
 between each other. The frequency convertors 13 and 14 constitute an image
 frequency signal suppresser by which an image frequency signal is
 suppressed at level by, for example, about 40 dB.
 An intermediate frequency signal SIF which is obtained by adding the
 intermediate frequency signals SIFO and SIFQ to each other and has its
 carrier frequency corresponding to the intermediate frequency determined
 to be 150 kHz is derived through an intermediate frequency filter 20 from
 the signal adder 19. Accordingly, the frequency convertors 13 and 14, the
 local oscillator 15, the frequency divider 17, the phase shifter 18 and
 the signal adder 19 in the aggregate constitute a frequency converting
 portion for frequency-converting the input signal, which includes the FM
 carrier wave signal SFM and is supplied through the radio frequency
 amplifier 12, into the intermediate frequency signal SIF having the
 carrier frequency corresponding to the intermediate frequency determined
 to be 150 kHz. The intermediate frequency signal SIF obtained through the
 intermediate frequency filter 20 is supplied to an intermediate frequency
 amplifier 21 to be amplified with a predetermined gain therein.
 The intermediate frequency signal SIF amplified in the intermediate
 frequency amplifier 21 is supplied to a frequency demodulator 22. The
 frequency demodulator 22 is provided with a frequency-demodulation
 characteristic having a center frequency equal to the intermediate
 frequency determined to be 150 kHz. A frequency-demodulation output signal
 SA which is obtained by frequency-demodulation to the intermediate
 frequency signal SIF is derived from the frequency demodulator 22.
 The frequency-demodulation output signal SA obtained from the frequency
 demodulator 22 is led to a terminal 23 and supplied to a detuning detector
 24. Further, the frequency-demodulation output signal SA obtained from the
 frequency demodulator 22 is supplied to a series connection of a resistive
 element 25 and a capacitive element 26 connected between an output end of
 the frequency demodulator 22 and a reference potential point. A DC voltage
 VSA based on the frequency-demodulation output signal SA is obtained
 between the reference potential point and a connecting point between the
 resistive element 25 and the capacitive element 26 (across the capacitive
 element 26).
 The DC voltage VSA obtained across the capacitive element 26 based on the
 frequency-demodulation output signal SA is supplied to a comparison input
 terminal a level comparator 27. A reference DC voltage VRE which
 corresponds to a central level VC of the DC voltage VSA obtained across
 the capacitive element 26 is supplied from a voltage source 28 to a
 reference input terminal of the level comparator 27. A first level
 detection output signal VCO is obtained from the level comparator 27 as a
 result of level comparison of the DC voltage VSA to the reference DC
 voltage VRE in the level comparator 27. The first level detection output
 signal VCO has a predetermined level when the DC voltage VSA is equal to
 or higher than the reference DC voltage VRE, in other words, when the DC
 voltage level of the frequency-demodulation output signal SA obtained from
 the frequency demodulator 22 is equal to or higher than the central level
 VC of the DC voltage VSA which is a DC voltage level of the
 frequency-demodulation output signal SA obtained by frequency-demodulation
 to a frequency component of the intermediate frequency signal SIF which
 corresponds to the center frequency of the frequency-demodulation
 characteristic in the frequency demodulator 22. Accordingly, the level
 comparator 27 constitutes a DC level detector for producing the first
 level detection output signal VCO having the predetermined level when the
 DC voltage level of the frequency-demodulation output signal SA obtained
 from the frequency demodulator 22 is equal to or higher than the
 predetermined level. The first level detection output signal VCO obtained
 from the level comparator 27 is supplied to the microcomputer 16.
 The DC voltage VSA obtained across the capacitive element 26 based on the
 frequency-demodulation output signal SA is supplied also to the detuning
 detector 24. A detection output signal VCF which has a predetermined high
 level when the frequency-demodulation output signal SA obtained based on
 frequency components of the intermediate frequency signal SIF which belong
 to a predetermined frequency range with a center frequency corresponding
 to the intermediate frequency determined to be 150 kHz, for example, a
 frequency range from a frequency lower by 50 kHz than the intermediate
 frequency, namely, 100 kHz to a frequency higher by 50 kHz than the
 intermediate frequency, namely, 200 kHz, is obtained from the frequency
 demodulator 22, is derived from the detuning detector 24 to be supplied to
 one of input terminals of an AND circuit 29.
 The intermediate frequency signal SIF amplified in the intermediate
 frequency amplifier 21 is supplied also to a level detector 30. A
 detection output signal VSIF which has a predetermined high level when the
 intermediate frequency signal SIF from the intermediate frequency
 amplifier 21 has its level higher than a predetermined level, is derived
 from the level detector 30 to be supplied to the other of input terminals
 of the AND circuit 29. A second level detection output signal VDC is
 obtained from the AND circuit 29 as a result of level check of the
 detection output signal VCF and the detection output signal VSIF in the
 AND circuit 29. The second level detection output signal VDC has a
 predetermined level when the detection output signal VCF from the detuning
 detector 24 has the predetermined high level and the detection output
 signal VSIF from the level detector 30 has also the predetermined high
 level, that is, when the frequency components of the intermediate
 frequency signal SIF which belong to the predetermined frequency range
 with the center frequency corresponding to the intermediate frequency
 determined to be 150 kHz, namely, the center frequency of the
 frequency-demodulation characteristic in the frequency demodulator 22, for
 example, the frequency range from the frequency lower by 50 kHz than the
 intermediate frequency, namely, 100 kHz to the frequency higher by 50 kHz
 than the intermediate frequency, namely, 200 kHz, have their levels equal
 to or higher than the predetermined level. Accordingly, the AND circuit 29
 constitutes an intermediate frequency level detector for producing the
 second level detection output signal VDC having the predetermined level
 when the frequency components of the intermediate frequency signal SIF
 which belong to the predetermined frequency range with the center
 frequency corresponding to the center frequency of the
 frequency-demodulation characteristic in the frequency demodulator 22 have
 their levels equal to or higher than the predetermined level. The second
 level detection output signal VDC obtained from the AND circuit 29 is also
 supplied to the microcomputer 16.
 In the microcomputer 16, for example, when an input portion 31 is operated
 to supply the microcomputer 16 with a command signal SX which orders the
 microcomputer 16 to perform operations for the automatic scanning tuning
 operation, the control signal CCV is produced to be supplied to the local
 oscillator 15 and it is checked on the strength of the first and second
 level detection output signals VCO and VDC whether the input signal
 supplied from the radio frequency amplifier 12 to the frequency converters
 13 and 14 is the FM carrier wave signal SFM desired to be received or the
 image frequency signal. The control signal CCV is supplied to the local
 oscillator 15 for causing the receiving frequency in the embodiment shown
 in FIG. 1 to change successively at predetermined regular frequency
 intervals, for example, 100 kHz.
 The local oscillator 15 to which the control signal CCV is supplied is
 operative to change the frequency of the oscillation output signal SOC
 successively in response to the control signal CCV so that the frequency
 of each of the local oscillation signals SOLO and SOLQ obtained from the
 frequency divider 17 is increased successively at predetermined regular
 frequency intervals, for example, 100 kHz. With such changes in frequency
 of the local oscillation signals SOLO and SOLQ, the receiving frequency in
 the embodiment shown in FIG. 1, which is higher by 150 kHz than the
 frequency of each of the local oscillation signals SOLO and SOLQ is also
 increased successively at predetermined regular frequency intervals, for
 example, 100 kHz, from a predetermined lowest frequency, for example, 75
 kHz and thereby the automatic scanning tuning operation is carried out.
 Supposing that the input signal supplied through the radio frequency
 amplifier 12 to the frequency converters 13 and 14 is the FM carrier wave
 signal SFM having its carrier frequency of 90 MHz, when the frequency of
 each of the local oscillation signals SOLO and SOLQ is set to be 89.85 MHz
 so that the receiving frequency is set to be 90 MHz, the intermediate
 frequency signal SIF having its carrier frequency equal to the
 intermediate frequency determined to be 150 kHz which is produced based on
 the FM carrier wave signal SFM having its carrier frequency of 90 MHz and
 the local oscillation signals SOLO and SOLQ each having its frequency of
 89.85 MHz is obtained from the intermediate frequency amplifier 21. Then,
 when the frequency of each of the local oscillation signals SOLO and SOLQ
 is set to be 90.15 MHz so that the receiving frequency is set to be 90.3
 MHz, the intermediate frequency signal SIF having its carrier frequency
 equal to the intermediate frequency determined to be 150 kHz which is
 produced based on the FM carrier wave signal SFM having its carrier
 frequency of 90 MHz and the local oscillation signals SOLO and SOLQ each
 having its frequency of 90.15 MHz is obtained from the intermediate
 frequency amplifier 21.
 The intermediate frequency signal SIF thus obtained from the intermediate
 frequency amplifier 21 when the frequency of each of the local oscillation
 signals SOLO and SOLQ is set to be 90.15 MHz so that the receiving
 frequency is set to be 90.3 MHz, which appears in spite of the absence of
 the FM carrier wave signal SFM having its carrier frequency of 90.3 MHz,
 is produced based on an image frequency signal resulting from the
 existence of the FM carrier wave signal SFM having its carrier frequency
 of 90 MHz.
 When the automatic scanning tuning operation is carried out with the
 receiving frequency which is successively increased at predetermined
 regular frequency intervals, for example, 100 kHz, from the predetermined
 lowest frequency, for example, 75 kHz, the DC voltage VSA, which is
 obtained across the capacitive element 26 based on the
 frequency-demodulation output signal SA to have the central level VC
 corresponding to the reference DC voltage VRE, varies at level in such a
 manner as shown in FIG. 2A in response to the successive increases in the
 receiving frequency at regular frequency intervals of 100 kHz. Therefore,
 as shown in FIG. 2B, the first level detection output signal VCO obtained
 from the level comparator 27 has the predetermined level (.alpha.) only
 when the level of the DC voltage VSA is equal to or higher than the
 central level VC. Further, as shown in FIG. 2C, the second level detection
 output signal VDC obtained from the AND circuit 29 has the predetermined
 level(.beta.) based on the intermediate frequency signal SIF obtained from
 the intermediate frequency amplifier 21 in response to the FM carrier wave
 signal SFM having its carrier frequency of 90 MHz when the receiving
 frequency is set to be 90 MHz and also has the predetermined level
 (.beta.) based on the intermediate frequency signal SIF obtained from the
 intermediate frequency amplifier 21 in response to the image frequency
 signal having equivalently its carrier frequency of 90.3 MHz when the
 receiving frequency is set to be 90.3 MHz.
 Under such a situation, in the microcomputer 16, a part of the second level
 detection output signal VDC obtained from the AND circuit 29, which has
 the predetermined level .beta. and is obtained based on the FM carrier
 wave signal SFM having its carrier frequency of 90 MHz when the receiving
 frequency is set to be 90 MHz (this part will be referred to as a part
 TU), and a part of the second level detection output signal VDC obtained
 from the AND circuit 29, which has the predetermined level .beta. and is
 obtained based on an image frequency signal having equivalently its
 carrier frequency of 90.3 MHz when the receiving frequency is set to be
 90.3 MHz (this part will be referred to as a part ER), are distinguished
 from each other and thereby the input signal supplied through the radio
 frequency amplifier 12 to the frequency convertors 13 and 14 is
 discriminated between the FM carrier wave signal SFM desired to be
 received and the image frequency signal.
 The discrimination on the input signal between the FM carrier wave signal
 SFM desired to be received and the image frequency signal is carried out
 based on such circumstances that, under a situation wherein the part TU of
 the second level detection output signal VDC is obtained with the
 receiving frequency set to be 90 MHz, the first level detection output
 signal VCO obtained from the level comparator 27 has the predetermined
 level .alpha. when the receiving frequency is set to be 98.9 MHz which is
 immediately before 90 MHz, as shown in FIG. 2B, and, under a situation
 wherein the part ER of the second level detection output signal VDC is
 obtained with the receiving frequency set to be 90.3 MHz, the first level
 detection output signal VCO obtained from the level comparator 27 does not
 have the predetermined level .alpha. when the receiving frequency is set
 to be 90.2 MHz which is immediately before 90.3 MHz, as shown in FIG. 2B.
 As a result of the discrimination mentioned above, it is recognized that
 the input signal supplied through the radio frequency amplifier 12 to the
 frequency convertors 13 and 14 is the FM carrier wave signal SFM desired
 to be received when the part TU of the second level detection output
 signal VDC is detected in the microcomputer 16 and it is also recognized
 that the input signal supplied through the radio frequency amplifier 12 to
 the frequency convertors 13 and 14 is the image frequency signal when the
 part ER of the second level detection output signal VDC is detected in the
 microcomputer 16.
 As described above, the microcomputer 16 constitutes a signal
 discriminating portion for discriminating between a desirable condition in
 which the input signal supplied through the radio frequency amplifier 12
 to the frequency convertors 13 and 14 is the FM carrier wave signal SFM
 desired to be received and an undesirable condition wherein the input
 signal supplied through the radio frequency amplifier 12 to the frequency
 convertors 13 and 14 is the image frequency signal on the strength of the
 first level detection output signal VCO obtained from the level comparator
 27 and the second level detection output signal VDC obtained from the AND
 circuit 29 under a situation wherein the automatic scanning tuning
 operation is carried out with the receiving frequency which is
 successively increased at predetermined regular frequency intervals, for
 example, 100 kHz.
 Then, the microcomputer 16 is operative to cause the control signal CCV
 supplied to the local oscillator 15 to cease the change in the frequency
 of the oscillation output signal SOC when it is recognized that the input
 signal supplied through the radio frequency amplifier 12 to the frequency
 convertors 13 and 14 is the FM carrier wave signal SFM desired to be
 received, and to cause also the control signal CCV supplied to the local
 oscillator 15 to maintain the change in the frequency of the oscillation
 output signal SOC when it is recognized that the input signal supplied
 through the radio frequency amplifier 12 to the frequency convertors 13
 and 14 is the image frequency signal. Accordingly, when the input signal
 supplied through the radio frequency amplifier 12 to the frequency
 convertors 13 and 14 is the FM carrier wave signal SFM desired to be
 received, the receiving frequency on that occasion is kept without being
 changed so that the FM carrier wave signal SFM is continuously received.
 While, when the input signal supplied through the radio frequency
 amplifier 12 to the frequency convertors 13 and 14 is the image frequency
 signal, the receiving frequency is maintained to be successively changed
 so that the image frequency signal cannot be received and thereby
 interferences brought about by the image frequency signal are eliminated
 from the automatic scanning tuning operation.