JP4466679B2 - Receiving machine - Google Patents

Description

  The present invention relates to a receiver mainly used in wireless communication devices such as a cordless remote controller, a pager, a cordless telephone, and a mobile phone, and more particularly to a receiver using an image rejection mixer in order to omit or simplify a reception filter.

  A conventional receiver will be described with reference to the drawings. FIG. 7 is a block diagram showing a configuration of a conventional receiver.

  In FIG. 7, 1 is a first local signal source, 5 is a first mixer, 7 is arithmetic means, 9 is carrier sense means, 10 is determination means, 11 is demodulation means, 12 is a channel selection filter, and 13 is 2 A frequency divider, 14 is a receiving amplifier, 15 is an antenna, and 16 is a phase shift means. The receiver of FIG. 7 has a single superheterodyne configuration, but employs an image rejection mixer in order to omit the reception filter directly under the antenna. The image rejection mixer prevents interference due to an image frequency signal.

  Here, in the image rejection mixer, the received signal and the signal of the first local signal source 1, that is, the local signal are input to the pair of the first mixer 5 and mixed with each other. Here, the received signal is divided into two and input to the pair of first mixers 5 respectively. On the other hand, the signal of the first local signal source 1 is frequency-divided by ½ with a frequency divider by 2 and two signals having a relative phase difference of 90 ° are generated. The two signals are input to the pair of first mixers 5 respectively. By using the divide-by-2, the relative phase difference can be accurately set to 90 °.

  Then, the respective outputs of the pair of first mixers 5 are input to the phase shift means 16 to relatively change the phase by 90 °, and then input to the calculation means 7 to perform addition or subtraction with each other. At this time, the signal of the desired frequency is in phase and the phase of the image frequency is opposite to each other, so that the image frequency can be suppressed. The image rejection mixer is configured as described above.

  Next, the output of the calculation means 7 is input to the demodulation means 11 after unnecessary components are removed by the channel selection filter 12, and the received signal is demodulated.

  However, the conventional receiver has a problem in that, when a relatively large image frequency signal is input, the reception sensitivity of the desired channel deteriorates, so-called sensitivity suppression by the image signal occurs. This is a problem that occurs because the amount of suppression of the image frequency signal of the image rejection mixer is insufficient. In a general image rejection mixer, the suppression amount of the image frequency signal that can be stably realized is about 30 dB, which is smaller than the suppression amount when a general reception filter is used. This has been an obstacle to obtaining a highly reliable receiver.

In order to solve the above-described conventional problem, the receiver of the present invention includes a first local signal source, a modulation unit, a mixer, an inverse modulation unit, a channel selection filter, and a demodulation unit, and the modulation unit. Modulates the first local signal source, mixes the output of the first local signal source and the received signal with the mixer, and the inverse modulation means cancels the modulation component of the output of the mixer. Is demodulated by inputting the output of the inverse modulation means to the demodulation means via the channel selection filter.

  Even when an image frequency signal is received, the image signal component can be reduced by the operation of modulation and inverse modulation, so that the sensitivity suppression characteristic can be improved.

  The receiver of the present invention comprises a first local signal source, a modulation means, a mixer, an inverse modulation means, a channel selection filter, and a demodulation means, and the modulation means modulates the first local signal source. Then, the output of the first local signal source and the received signal are mixed by the mixer, and the inverse modulation means inversely modulates the output of the mixer so as to cancel the modulation component of the output of the mixer, and the inverse modulation The output of the means is input to the demodulating means via the channel selection filter and demodulated.

  The invention according to claim 1 comprises a first local signal source, a modulating means, a mixer, an inverse modulating means, a channel selection filter, and a demodulating means, and the modulating means includes the first local signal source. Modulating, mixing the output of the first local signal source and the received signal by the mixer, the inverse modulation means inversely modulates the output of the mixer so as to cancel the modulation component of the output of the mixer, and It is a receiver that performs demodulation by inputting the output of the modulation means to the demodulation means via the channel selection filter.

  The sensitivity suppression characteristic with respect to the image frequency signal can be improved and the image frequency signal cannot be demodulated, so that it is not erroneously demodulated.

  According to a second aspect of the present invention, the modulating means is a receiver that performs frequency modulation. The frequency modulation can be easily obtained by changing the oscillation frequency of the first local signal source, and the circuit can be simplified.

  The invention according to claim 3 is a receiver in which the modulation means performs phase modulation. The phase modulation can be configured using a quadrature modulator, and the modulation can be performed accurately. Therefore, the reverse demodulation can be performed accurately with the same configuration, so that the modulation component can be canceled with high accuracy.

A first local signal source; a modulation means; a mixer; an inverse modulation means; a channel selection filter; a carrier sense means; a reception level detection means; and a determination means. The local signal source is modulated, the output of the first local signal source and the received signal are mixed by the mixer, and the inverse modulation means inversely modulates the output of the mixer so as to cancel the modulation component of the output of the mixer The output of the inverse modulation means is input to the channel selection filter, the reception level detection means detects the output level of the channel selection filter, and the signal level of the channel selection filter output is detected by the carrier sense means. , the determination unit before the outputs and the carrier sense unit of the reception level detecting means when operating said modulating means and said inverse modulation means Using the output of the output and the carrier sense unit of the reception level detecting means when the operation of the modulation means and the inverse modulation means stopping a receiver determines the presence or absence of the carrier. Since the presence / absence of the image frequency signal can be determined only by the reception level detection means and the carrier sense means, the circuit can be simplified.

  Embodiments of the present invention will be described below with reference to the drawings.

Example 1
FIG. 1 is a block diagram illustrating a configuration of a receiver according to a first embodiment of the present invention. FIG. 2 is an explanatory diagram for explaining the frequency relationship of signals processed by the receiver of the present invention. A receiver according to this embodiment will be described with reference to FIGS.

  In FIG. 1, 1 is a first local signal source, 2 is a modulation means, 3 is a second local signal source, 4 is an inverse modulation means, 5 is a first mixer, 6 is a second mixer, and 7 is an arithmetic operation. Means 8, modulation component detection means 9, carrier sense means 10, determination means 11, demodulation means 11, channel selection filter 12, frequency divider 13, frequency divider 14, reception amplifier 15, antenna 15.

  The receiver of FIG. 1 employs an image rejection mixer in order to omit the reception filter directly under the antenna, thereby preventing interference due to input of an image frequency signal. Here, in the image rejection mixer, the received signal and the signal of the first local signal source 1, that is, the local signal, are input to the pair of the first mixer 5 and mixed. However, the received signal is divided into two and input to the pair of first mixers 5 respectively. On the other hand, the first local signal source 1 is frequency-modulated by the modulation means 2. Here, the modulation frequency of the modulation by the modulation means 2 is set to a frequency band different from the frequency band in which the modulation frequency component of the received signal, that is, the baseband frequency exists.

  In this embodiment, the modulation frequency of the reception signal is set to 1.2 kHz or less, whereas the modulation frequency of the modulation means 2 is set to 2.0 kHz. Then, the output signal of the first local signal source 1 is input to the frequency divider 13 to divide the frequency by ½, and two local signals having a relative phase difference of 90 ° are generated. . At this time, the output signal of the frequency divider 13 is subjected to frequency modulation with a modulation frequency of 2.0 kHz. The two local signals are input to the pair of first mixers 5 respectively. Here, the 2 frequency divider 13 is provided in order to accurately obtain a relative phase difference of 90 °. The respective outputs of the first mixer 5 pair are input to the second mixer 6 pair. Here, two local signals having a relative phase difference of 90 ° by dividing the signal of the second local signal source 3 by 2 are input to the pair of the second mixer 6. It is mixed with the output signal of the mixer 5.

  Here, the second local signal source 3 is subjected to characteristic modulation. That is, the second local signal source 3 is frequency-modulated by the inverse modulation means 4. Here, the modulation frequency is the same as the modulation frequency of the first local signal source 1, and the frequency displacement is also set to be the same. However, since the second local signal source 3 has a frequency that is one tenth of that of the first local signal source 1, the ratio of the frequency displacement amount to the signal source frequency is larger in the second local signal source 4. ing. The modulation phase of the frequency modulation is inverted between the first and second local signal sources. In this embodiment, the local signal is set to a lower-local frequency, which is a lower frequency than the received signal. Here, when the Upper-Local frequency is set, the modulation phase is in phase. By performing the modulation by the modulation means 2 and the inverse modulation by the inverse modulation means 4, the modulation can be given only when the image frequency signal is input.

  With respect to this, the frequency relationship of signals processed by the receiver of the present embodiment will be described with reference to FIG.

As shown in FIG. 2, a desired signal ((a) in the figure) and an image signal ((c) in the figure) exist in the reception frequency band. The local signal (first local signal) ((b) in the figure) is an intermediate frequency between the two signals. The desired signal, the image signal, and the first local signal are mixed by a first mixer, and frequency conversion is performed to a first intermediate frequency that is a lower frequency. Here, if the first local signal is frequency-modulated by the modulation means and the frequency changes to a lower side at a certain moment (see the horizontal arrow in FIG. 2), among the first intermediate frequency signals, The desired signal component ((d) in the figure) changes to the higher frequency side. On the other hand, the component of the image signal ((e) in the figure) changes to the lower side. Thus, the behavior is different.

  Next, the first intermediate frequency signal is input to the second mixer and mixed with the second local signal. Here, the second local signal source is subjected to frequency modulation in which the modulation phase is inverted with the same modulation frequency and the same frequency displacement as the modulation of the first local signal source. That is, inverse modulation is performed so as to cancel the frequency modulation by the first mixer. Therefore, the output of the second mixer has no frequency change for the desired signal ((f) in the figure). On the other hand, the frequency displacement of the image signal ((g) in the figure) is doubled, and the frequency change is large. That is, when the image signal is input, the above-described modulation component remains, but when the desired signal is input, the modulation component is canceled and does not remain.

  Next, as shown in FIG. 1, the pair of outputs of the second mixer 6 is input to the computing means 7. Here, the image frequency component is suppressed by performing addition or subtraction arithmetic processing so that the desired signal has the same phase and the image signal has the opposite phase. The output of the calculation means 7 is input to the demodulation means 11 after unnecessary components are removed by the channel selection filter 12. The demodulator 11 includes a circuit that outputs a voltage corresponding to the level of the input signal, that is, a level detection circuit generally called an RSSI circuit. When the output of the level detection circuit exceeds a preset level, that is, a carrier sense level, the carrier sense means 9 generates a signal.

  Here, the carrier sense level is -110 dBm in terms of antenna input level.

  Further, a modulation component detection means 8 is configured. The modulation component detection means 8 detects a modulation component included in the output of the channel selection filter 12, that is, a frequency modulation component having a modulation frequency of 2 kHz. The modulation component detecting means 8 is composed of a circuit for converting a component having a modulation frequency of 2 kHz into a voltage change, a filter for extracting only a frequency component in the vicinity of 2 kHz, and a smoothing circuit. When present, it outputs a DC voltage.

  Then, both the output of the carrier sense unit 9 and the output of the modulation component detection unit 8 are input to the determination unit 10. The determination means 10 is configured as a part of the microcomputer function. The determination means 10 determines the presence or absence of a carrier as follows.

  First, when there is no carrier detection output from the carrier sense unit 9, the determination unit 10 determines that there is no carrier of the desired channel. Next, when there is a carrier detection output from the carrier sense means 9, if the output voltage of the modulation component detection means 8 is greater than a preset value, it is determined that there is no carrier in the desired channel. In this case, it is considered that an image frequency signal is input.

  Further, when there is a carrier detection output from the carrier sense means 9, if the output voltage of the modulation component detection means 8 is smaller than a preset value, it is determined that there is a carrier of the desired channel.

  In this way, in order to determine the presence or absence of the carrier of the desired channel based on the information from the modulation component detection means 8 in addition to the carrier sense means 9, it is erroneously determined that there is a carrier based on the image frequency signal, and the reception operation There is no need to consume unnecessary battery power.

In addition, there is a rule that a specific low power radio device or the like performs a carrier sense operation for confirming whether there is a carrier in a use channel before transmission, and cannot shift to a transmission operation when there is a carrier. Even in this case, in the configuration of the present embodiment, it is possible to prevent the transmission failure due to the erroneous determination that the carrier of the used channel is present based on the image frequency signal.

  The modulation component detection means 8 can be configured to detect by using the demodulation means 11 and filtering the demodulated analog output signal in the vicinity of the modulation frequency.

  Further, the modulation frequency and frequency displacement value by the modulation means 2 may be set to arbitrary values. For example, in this embodiment, it is set to 2 kHz, which is larger than the modulation frequency of the received signal, but it can be selected to be a small value such as 0.5 kHz.

  Further, the modulation component detection means passes the output ((h) (i) in the figure) of the channel selection filter as shown in FIG. 2 through a filter having a gradient in the pass amplitude characteristic with respect to the frequency. It is possible to use a configuration in which frequency modulation is converted into amplitude fluctuation and the amplitude fluctuation signal is detected by envelope detection or the like. In this case, the amplitude of the desired signal is not generated, but the image signal is amplitude-modulated, so that it can be determined.

  In this embodiment, frequency modulation is used for modulation and inverse modulation, but phase modulation may be used. In phase modulation, since modulation can be performed accurately using a quadrature modulator or the like, there is an advantage that modulation and inverse modulation can be performed accurately.

  Further, although the first local signal source 1 and the second local signal source 3 are configured independently, the second signal source 3 can use a signal obtained by dividing the first signal source 1. Alternatively, a signal obtained by multiplying the second signal source by using a PLL circuit or the like as the first local signal may be used.

  In addition, as a configuration for making the modulation shift amount (frequency shift amount) of the first and second local signal sources the same, another signal and mixer that are not modulated based on one modulated signal. The signal of the first or second local signal source can be obtained by mixing and frequency-converting at (1). In particular, since the ratio of the frequency shift amount of the second local signal source to the signal frequency is significantly larger than that of the first local signal source, it is effective to obtain the same frequency shift amount using a mixer.

(Example 2)
FIG. 3 is a block diagram illustrating a configuration of the receiver according to the second embodiment of the present invention. In FIG. 3, 16 is a phase shift means. Also, the same components as those in FIG. The receiver according to the present embodiment will be described with reference to FIG.

  The difference between the present embodiment and the first embodiment is in the position where the inverse modulation means is arranged. In the present embodiment, the signal of the second local signal source 13 input to the second mixer 6 arranged at the subsequent stage of the image rejection mixer composed of the first mixer 5, the phase shift means 16 and the arithmetic means 7. Is modulated by the inverse modulation means 4 so as to cancel the modulation by the modulation means 2. That is, it is not necessary to incorporate a reverse modulation mechanism in the image rejection mixer. Here, the phase shift means 16 is provided to relatively change the phase of the first intermediate frequency signal, which is the output signal of the pair of the first mixer 5, by 90 °, respectively. It uses the property that the phase changes near the shoulder of the pass filter.

  With the configuration as described above, the second mixer 6 can be configured as a single unit, and the circuit scale can be reduced. In addition, there is an advantage that the inverse modulation operation can be performed more reliably than when a pair of mixers are used. .

  This embodiment has a double superheterodyne configuration.

(Example 3)
FIG. 4 is a block diagram showing the configuration of the receiver according to the third embodiment of the present invention. In FIG. 4, 17 is a frequency variable means, 18 is a level fluctuation detecting means, and 19 is a variable filter. In addition, the same components as those in FIGS. 1 and 3 are denoted by the same reference numerals.

  The feature of the present embodiment is that the frequency modulation means 17 for changing the frequency characteristic of the variable filter 19 is used, whereas the inverse modulation means is used in the first and second embodiments. In other words, the level fluctuation detecting means 18 is used in place of the modulation component detecting means.

  The signal from the first local signal source 1 modulated by the modulation means 2 and the received signal are mixed by the first mixer 5 to be modulated. When the modulation by the modulation means 2 is frequency modulation, the frequency of the first intermediate frequency signal that is the output of the first mixer 5 changes. The output of the first mixer 5 is input to the channel selection filter 12 via the phase shift means 15 and the calculation means 7 and then input to the variable filter 19. Here, the frequency variable means 17 changes the frequency characteristic of the variable filter so that the output level of the variable filter is constant with respect to the signal of the desired channel. The variable filter 19 is a characteristic having a gradient in the frequency characteristic of the pass amplitude in the vicinity of the first intermediate frequency, and the frequency characteristic of the variable filter 19 is changed in the frequency axis direction or the pass amplitude axis direction in synchronization with the modulation of the modulation means 2. By shifting, the output level of the variable filter is controlled to be constant with respect to the desired channel signal.

  At this time, when the received signal is an image frequency signal, the frequency change of the variable filter 19 and the frequency change of the first intermediate frequency signal are in opposite directions, so the output level of the variable filter 19 changes. Is detected by the level fluctuation detecting means 18. When the output of the level fluctuation detection means 18 input to the determination means 10 is larger than a preset value, it is determined that there is no carrier. On the other hand, when the received signal is a desired channel signal, since the output of the level fluctuation detecting means 18 is smaller than a preset value, it can be determined that there is a carrier.

  In this embodiment, components such as the inverse modulation means, the second local signal source, or the second mixer for the inverse modulation operation are unnecessary, and there is an advantage that the circuit can be simplified.

  Further, there is an advantage that it can be applied to the configuration of a single superheterodyne system.

  In this embodiment, the variable filter 19 having a gradient in the passing amplitude with respect to the frequency is used, but a channel selection filter can be used. That is, the center frequency of the pass band of the channel selection filter can be changed in synchronization with the modulation. The frequency variable means 17 controls the center frequency of the channel selection filter so that it always matches the center frequency of the desired channel signal. As a result, when the received signal is the desired channel signal, the output level does not change, but when the received signal is an image frequency signal, the frequency of the intermediate frequency signal changes outside the pass band of the channel selection filter, so the pass level is large. Change. By detecting this change in the passing level by the level fluctuation detecting means 18, it is possible to prevent erroneous carrier sense due to the image frequency signal.

  Alternatively, a variable filter 19 having a gradient in the pass amplitude characteristic within the pass band of the channel selection filter may be used. In this case, the image frequency signal can be detected even when the amount of frequency displacement by the modulation means 2 is relatively small.

Example 4
FIG. 5 is a block diagram showing a configuration of a receiver according to the fourth embodiment of the present invention. FIG. 6 is an explanatory diagram for explaining the frequency relationship of signals processed by the receiver of the present invention. The receiver according to the present embodiment will be described with reference to FIGS.

  In FIG. 5, reference numeral 20 denotes a reception level detection means. Also, the same components as those in FIG.

  The difference between the present embodiment and the first embodiment is that the image frequency signal is compared by comparing the reception levels in two states when the modulation operation by the modulation means 1 and the inverse modulation operation by the inverse modulation means 4 are performed and not. Is to prevent erroneous carrier sense.

  In this embodiment, it is possible to switch between two states, a state in which the modulation unit 2 and the inverse modulation unit 3 are operated, and a state in which the modulation unit 2 and the inverse modulation unit 3 are not operated. In addition, reception level detection means 20 for detecting the signal level of the output of the channel selection filter 12 is provided.

  It is assumed that the signal level detected by the reception level detection means 20 is at the first level when the modulation means 2 and the inverse modulation means 4 are not operating (state (A) in FIG. 6).

  Next, when the modulation means 1 and the inverse modulation means 4 are operated, if the received signal is a desired channel signal, the detected level becomes substantially equal to the first level. On the other hand, the level detected in the case of an image frequency signal is smaller than the first level. This is because the image frequency signal undergoes large frequency modulation by the operation of modulation and inverse modulation, and its spectrum spreads wider than the bandwidth of the channel selection filter 12. Since a part or most of the signal component is attenuated by the channel selection filter 12, the detection level at the reception level detection means 20 becomes small (state (B) in FIG. 6).

  Therefore, even when the carrier sense means 9 outputs a signal with a carrier, when the detection level by the reception level detection means 20 when there is modulation is significantly lower than the preset width compared to when there is no modulation. The determination means 10 determines that there is no carrier sense. Thereby, it can be prevented that the presence of the carrier is erroneously determined by the image frequency signal.

  As described above, by providing two states, it is possible to reduce power consumption during standby reception. In other words, the carrier sense operation is first performed without the operation of modulation and inverse modulation, and when there is an output from the carrier sense means 9, the operation of modulation and inverse modulation is performed to check whether the signal is an image frequency signal. It can be performed. As a result, most of the operation time can be performed without modulation and inverse modulation, so that the power consumed by the operation of modulation and inverse modulation can be reduced.

In the configuration of this embodiment, it is possible to improve the sensitivity suppression characteristic of reception due to the input of the image frequency signal in the demodulation operation. In the demodulating operation by the demodulating unit 11, by operating the modulating unit 2 and the inverse modulating unit 4, the image frequency signal can be frequency-modulated to spread the spectrum. Here, the modulation frequency shift amount of the modulation means 2 is set larger than the frequency band of the channel selection filter 12. For this reason, the image frequency signal to which the modulation by the modulation unit 2 and the inverse modulation by the inverse modulation unit 4 have been applied undergoes frequency spreading and has a broadened spectrum. For example, if the modulation frequency shift amount is set to 5 times the pass band of the channel selection filter 12, the spectrum of the image frequency component is spread to 10 times the pass band of the channel selection filter after receiving the inverse modulation operation. . In this case, the level of the image frequency component passing through the channel selection filter 12 is reduced to 1/10. On the other hand, the modulation of the desired channel signal is canceled by inverse modulation, so that all components pass through the channel selection filter 12. Therefore, since the level ratio between the desired channel signal and the image frequency signal can be improved, the sensitivity suppression characteristic is 10 dB.
Improved. That is, it is characterized in that the modulation is set to be larger than the band of the channel selection filter 12 and thereafter, most of the components of the image frequency signal are removed by the channel selection filter 12.

  Furthermore, it is possible to prevent the image frequency signal from being demodulated by mistake. That is, since the image frequency signal is greatly modulated by the modulation means and the inverse modulation means in addition to the original modulation, the demodulation means 11 cannot demodulate the original modulation. Thereby, erroneous demodulation can be avoided.

  As described above, the receiver according to the present invention has the following effects. A first local signal source; a modulating means; a mixer; an inverse modulating means; a channel selection filter; and a demodulating means. The modulating means modulates the first local signal source; The output of the signal source and the received signal are mixed by the mixer, and the inverse modulation means inversely modulates the mixer output so as to cancel the modulation component of the mixer output, and the output of the inverse modulation means is the channel selection. It is configured to demodulate by inputting to the demodulating means through a filter, and when the image frequency signal is received together with the desired signal, the sensitivity suppression characteristics are improved by reducing the image frequency signal component by the modulation and inverse modulation operations. Further, there is an effect that it is possible to prevent the image frequency signal from being demodulated erroneously.

1 is a block diagram of a receiver according to a first embodiment of the present invention. Explanatory drawing of the frequency relationship of the signal processed with the receiver in Example 1 of this invention Block diagram of a receiver in Embodiment 2 of the present invention Block diagram of a receiver in Embodiment 3 of the present invention Block diagram of a receiver in Embodiment 4 of the present invention Explanatory drawing of the frequency relationship of the signal processed with the receiver in Example 4 of this invention Block diagram of a conventional receiver

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st local signal source 2 Modulation means 3 2nd local signal source 4 Inverse modulation means 5 1st mixer 6 2nd mixer 7 Calculation means 8 Modulation component detection means 9 Carrier sense means 10 Determination means 11 Demodulation means 12 Channel selection filter 13 Divider 14 Receive amplifier 15 Antenna 16 Phase shift means 17 Frequency variable means 18 Level fluctuation detection means 19 Variable filter 20 Reception level detection means

Claims (4)

A first local signal source; a modulating means; a mixer; an inverse modulating means; a channel selection filter; and a demodulating means. The modulating means modulates the first local signal source; The output of the signal source and the received signal are mixed by the mixer, and the inverse modulation means inversely modulates the mixer output so as to cancel the modulation component of the mixer output, and the output of the inverse modulation means is the channel selection. A receiver that performs demodulation by inputting to the demodulation means via a filter. The receiver according to claim 1, wherein the modulation means performs frequency modulation. The receiver according to claim 1, wherein the modulation means performs phase modulation. A first local signal source; a modulation unit; a mixer; an inverse modulation unit; a channel selection filter; a carrier sense unit; a reception level detection unit; and a determination unit. The modulation unit includes the first local unit The signal source is modulated, the output of the first local signal source and the received signal are mixed by the mixer, and the inverse modulation means inversely modulates the output of the mixer so as to cancel the modulation component of the output of the mixer , The output of the inverse modulation means is input to the channel selection filter, the reception level detection means detects the output level of the channel selection filter, the signal level of the channel selection filter output is detected by the carrier sense means, the determination means is odd the output of the output and the carrier sense unit of the reception level detecting means when operating said modulating means and said inverse modulation means The receiver determines the presence or absence of the carrier using the output of the output and the carrier sense unit of the reception level detecting means when stopping the operation of the means and the inverse modulation means.