JP2009200571A - Reception device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reception device which can be switched to suitable filter characteristics by considering the degree of influence of an adjacent channel signal on a desired signal. <P>SOLUTION: A power measuring circuit 9 measures electric power of a desired signal when measuring electric power of an output signal from a first band-pass filter 4, and measures electric power of the desired signal and the adjacent channel signal when measuring electric power of an output signal from a second band-pass filter 5. As the degree of influence of the adjacent channel signal on the desired signal, the SN ratio of both the signals is calculated from the measured values. On the basis of the SN ratio and a frequency error amount by automatic frequency control, a CPU 7 outputs an AFC signal for following a reception signal to a local oscillator 2, and also outputs a signal (c) to vary characteristics of the first band-pass filter 4 thereto. The first band-pass filter 4 removes the adjacent channel signal and also passes the desired signal therethrough. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a receiving apparatus having automatic frequency control means, and more particularly to a receiving apparatus that improves interference characteristics due to adjacent channel signals.

  Conventionally, automatic frequency control (hereinafter referred to as “AFC”) is known as one of means for following a received signal. By using AFC, it is possible to follow the received signal even when the frequency of the received signal deviates from the target frequency. If the follow-up to the received signal is possible, the signal can be demodulated. In particular, in a synchronous detection type receiver, it is necessary to create a local oscillation signal whose frequency and phase are synchronized with the received signal, which requires AFC. However, by changing the local oscillation frequency of the receiving device with AFC, the frequency difference with the adjacent channel signal is reduced, the removal amount in the intermediate frequency filter, the baseband filter, etc. is reduced, and the adjacent channel interference characteristic is reduced. There was a problem.

  As a method for solving the above problem, a direct conversion receiver described below has been proposed (see Patent Document 1). This direct conversion receiver will be briefly described with reference to FIG. The direct conversion receiver shown in FIG. 6 includes a reception antenna 101 that receives a radio signal, a reception signal amplifier 102 that amplifies the reception signal from the reception antenna 101 and outputs a modulated signal a, and a carrier frequency of the received transmission wave. A first local oscillator 103 that generates a first local signal b1 having a frequency substantially equal to the first local signal b1, a 90-degree phase shifter 104 that shifts the first local signal a1 by 90 degrees, a modulation signal a1, and a first A first signal mixer 105 that mixes the local signal b1 and a second signal mixer 106 that mixes the modulated signal a1 and the first local signal b1 shifted by 90 degrees are provided.

  The direct conversion receiver also includes an I low pass filter 107 and a Q low pass filter 108. The I low-pass filter 107 includes cutoff frequency switching means for controlling the cutoff frequency according to the automatic frequency control by the baseband filter control signal g1, and the high frequency according to the cutoff frequency from the output signal of the first signal mixer 105. The component is removed and the I baseband signal c1 is extracted. Further, the Q low-pass filter 108 includes cutoff frequency switching means for controlling the cutoff frequency according to the automatic frequency control by the baseband filter control signal g1, and according to the cutoff frequency from the output signal of the second signal mixer 106. The high frequency component is removed to extract the Q baseband signal d1.

The direct conversion receiver further processes the I baseband signal c1 and the Q baseband signal d1 and outputs a demodulated signal e1, and receives the received frequency signal and the first frequency from the demodulated signal e1 from the demodulator 109. A frequency shift detector 111 for detecting the frequency error of the local oscillator 103 and an automatic output of an automatic frequency control signal f1 for controlling the frequency error of the first local oscillator 103 by the output signal of the frequency shift detector 111. A frequency control unit 110 and a baseband filter control unit 112 for controlling the cutoff frequency of the I and Q low-pass filters 7 and 8 by the output signal of the frequency shift detection unit 111 are provided. Since this direct conversion receiver has the above-described configuration, it is possible to control adjacent frequencies even if the AFC is operated by controlling the cutoff frequencies of the I and Q low-pass filters 7 and 8 in conjunction with automatic frequency control. The jamming characteristics will be improved.
Japanese Patent Laid-Open No. 10-313344

  However, the above conventional technique shows an operation in which the cutoff frequency of the filter fluctuates according to the operation of the AFC regardless of whether there is an interference signal of the adjacent channel. In normal wireless communication, radio waves are not continuously output to a specific channel, and there is always a time during which no radio waves are output. In some cases, data communication or the like is performed for several minutes at a predetermined time once a day. As described above, in the conventional technique, there is a case where the cut-off frequency is lowered even though there is no interference signal of the adjacent channel, and there is a problem that an unnecessary operation is performed.

  In addition, as in the prior art, if an attempt is made to simply lower the cut-off frequency of the low-pass filter and remove the adjacent channel signal, there is a problem that the reception sensitivity is deteriorated by attenuation to the pass band of the desired signal. For example, in the specific low power radio ARIB STD-T67, the occupied bandwidth of the desired signal is 8.5 kHz or 16 kHz, and the channel interval is 12.5 kHz or 25 kHz. Is limited, and it is difficult to sufficiently remove adjacent channel signals.

  As described above, in the conventional technique, there is a problem that the desired signal is attenuated if the adjacent channel signal is sufficiently removed. Further, if the cut-off frequency is suppressed so as not to attenuate the desired signal, there is a problem that the adjacent channel signal cannot be sufficiently removed, the SN ratio is deteriorated and the reception sensitivity is lowered.

  Also, the conventional technique cannot know the adjacent channel signal level. Therefore, the desired signal level is sufficient, and the filter cutoff frequency is changed according to the AFC operation, even if the cutoff frequency of the filter is not changed or the communication performance is not affected from the SN ratio with the adjacent channel signal level. There is a problem of performing unnecessary operations.

  Therefore, there is a problem to be solved in that the frequency characteristic of the filter for removing the adjacent channel signal is changed based on the S / N ratio between the desired signal level and the adjacent channel signal level by knowing the adjacent channel signal level. .

  An object of the present invention is to provide a receiving apparatus that solves the above-described problem and can switch to an optimum filter characteristic in consideration of the influence of an adjacent channel signal on a desired signal.

  To achieve the above object, according to the present invention, in a receiving apparatus having automatic frequency control, a first band-pass filter that passes a desired signal among received signals, and a second that passes the desired signal and an adjacent channel signal. And a power measurement circuit that measures the power of the output signal from the first bandpass filter and the output signal from the second bandpass filter, and a value measured by the power measurement circuit The characteristic of the first band pass filter is varied based on the S / N ratio between the desired signal and the adjacent channel signal calculated in addition and the frequency error amount by the automatic frequency control.

  According to this receiving apparatus, when the power measurement circuit measures the power of the output signal from the first band pass filter, the power of the desired signal is measured, and the power of the output signal from the second band pass filter is measured. When doing so, the power of the desired signal and the adjacent channel signal is measured. From these measured values, the respective powers of the desired signal and the adjacent channel signal can be calculated, and the SN ratio of both signals can be determined as the degree of influence of the adjacent channel signal on the desired signal. Since the characteristics of the first bandpass filter are variable based on the S / N ratio and the frequency error amount by automatic frequency control, the adjacent channel signal is removed and the desired signal is allowed to pass through the first bandpass filter. be able to.

  In the receiving apparatus, when the reception power of the adjacent channel signal on the upper side and the reception power of the adjacent channel signal on the lower side are measured, and the signal level on the adjacent channel side whose frequency approaches by the automatic frequency control is low, The first band pass filter can be prevented from being varied. Thereby, useless variable operation of the first bandpass filter can be avoided.

  In the receiving apparatus, the first band pass filter may be used when the SN ratio is greater than or equal to the SN ratio threshold set based on a predetermined relationship with respect to the frequency error amount in the automatic frequency control. If the S / N ratio is not more than the threshold value without being varied, the first band pass filter can be varied. Whether or not the first band pass filter is varied can be determined based on the magnitude of the SN ratio. That is, when it is judged that the SN ratio is larger than the threshold value and the influence of the adjacent channel signal is small, the filter bandwidth of the first bandpass filter is not changed, and the influence of the adjacent channel signal is reduced when the SN ratio is less than the threshold value. If it is determined that the value is large, the characteristic can be made variable so as to narrow the filter bandwidth of the first bandpass filter according to the difference from the threshold.

  In the above receiver, the amplifier for amplifying the received signal, the local oscillator for outputting the local oscillation signal, the mixer for mixing the output signal amplified from the amplifier and the local oscillation signal of the local oscillator, and the mixing The first band pass filter for band limiting the intermediate frequency signal output from the mixer, the second band pass filter for band limiting the intermediate frequency signal output from the mixer, and the first band pass. A demodulation circuit that demodulates the intermediate frequency signal output from the filter, a CPU that processes the demodulated signal output from the demodulation circuit, an output from the first band-pass filter, or from the second band-pass filter A first changeover switch for switching output, the power measurement circuit for measuring output power from the first changeover switch, and the power measurement circuit. Characterized in a memory for storing and holding power values through the CPU, and it is composed of. The power measurement circuit measures the power of the desired signal when the first changeover switch is operated to measure the power of the output signal from the first bandpass filter, and the first changeover switch is operated to switch the first changeover switch. When measuring the power of the output signal from the second band pass filter, the power of the desired signal and the adjacent channel signal is measured, and the measured value is stored in the memory. The signal-to-noise ratio of both signals can be obtained as the degree of influence of the adjacent channel signal on the desired signal.

  In the receiving apparatus, a third bandpass filter is provided in parallel with the second bandpass filter, and the first bandpass filter output and the second band are used instead of the first changeover switch. In the configuration in which the second changeover switch for switching between the pass filter output and the third band pass filter output is provided, the second band pass filter has a characteristic that only the desired signal and the upper adjacent channel signal pass, The band-pass filter can have a characteristic that only the desired signal and the lower adjacent channel signal pass. Thereby, both the upper side and the lower side can be handled as the adjacent channel signal.

  Further, in the receiving apparatus, the first bandpass filter is configured such that a bandpass filter unit provided with a switch in parallel is connected in at least two or more stages, and each of the bandpass filter units has a passband width. It can have different characteristics of attenuation characteristics. As a result, it is possible to select one of two or more stages of bandpass filter units having different characteristics of the passband width and the attenuation characteristic by operating the switches provided in parallel.

As described above, according to the present invention, it is possible to solve the problem of performing an unnecessary operation of changing the characteristics of a filter even though there is no adjacent channel signal that causes interference as in the prior art. become. This eliminates unnecessary power consumption.
In addition, according to the present invention, it is possible to solve the problem of attenuating the desired signal when the adjacent channel signal is sufficiently removed as in the prior art, and to sufficiently remove the adjacent channel signal. . As a result, both reception sensitivity and interference characteristics can be ensured, and communication quality is improved.

In addition, according to the present invention, since the second passband filter 5 or the third bandpass filter 11 also passes the adjacent channel frequency, a filter with a wide bandwidth may be used, so that there is a merit that is possible with a simple configuration. is there.
Further, according to the present invention, the problem that the adjacent channel signal level cannot be known as in the prior art can be solved, and furthermore, by measuring the SN ratio between the desired signal level and the adjacent channel signal level, It becomes possible to change to optimum filter characteristics after AFC execution, and communication quality is greatly improved.

  Hereinafter, embodiments of a receiving apparatus according to the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a configuration diagram of a receiving apparatus according to a first embodiment of a receiving apparatus according to the present invention. 1 includes an amplifier 1 that amplifies a received signal, a local oscillator 2 that outputs a local oscillation signal, and a mixer 3 that mixes the output signal amplified from the amplifier 1 and the local oscillation signal of the local oscillator 2. A first bandpass filter 4 for band-limiting the intermediate frequency signal output from the mixer 3, a second bandpass filter 5 for band-limiting the intermediate frequency signal output from the mixer 3, and a first A demodulating circuit 6 for demodulating the intermediate frequency signal output from the band-pass filter 4, a CPU 7 for processing the demodulated signal output from the demodulating circuit 6, and an output from the first band-pass filter 4 or the second band A first changeover switch 8 that switches the output from the pass filter 5, a power measurement circuit 9 that measures the output power from the first changeover switch 8, and the power value of the power measurement circuit 9 is set to the CPU 7. A memory 10 for storing and holding through, and a. Here, the local oscillator 2 is, for example, a voltage controlled oscillator.

  The measurement procedure of the reception operation will be described below. To simplify the explanation, consider a case where the frequency of the local oscillation signal is not shifted, the frequency of the received signal is shifted, and only the upper adjacent channel signal exists. As shown in FIG. 1, a received signal including a desired signal and an interference signal is amplified by an amplifier 1. The amplified received signal and the local oscillation signal from the local oscillator 2 are mixed by the mixer 3 to output an intermediate frequency signal. The intermediate frequency signal is band-limited by the first band pass filter 4 and sent to the demodulation circuit 6. The demodulated signal demodulated by the demodulating circuit 6 is sent to the CPU 7 where the CPU 7 performs signal processing.

  Here, since the first band-pass filter 4 is before the AFC operation, only the desired signal passes (FIG. 2a). The power of the desired signal (hereinafter referred to as “S”) is obtained by measuring the first changeover switch 8 to the output of the first band-pass filter 4 by the switch power measurement circuit 9 by the switch change signal e from the CPU 7. can get. This value is stored and held in the memory 10 via the CPU 7.

  When the reception frequency shift is large and the intermediate frequency signal is largely shifted, the intermediate frequency signal moves out of the band of the first bandpass filter 4 and the power cannot be measured accurately. However, since the allowable value of the frequency deviation is determined according to the application by the radio wave method, it is designed to be suppressed to fluctuations in the pass band of the normal filter. For example, in the specific low power radio ARIB STD-T67 (429 MHz band), the transmission frequency deviation is within 4 ppm. In the case of a band limiting filter with an intermediate frequency of 450 kHz, since the 3 dB bandwidth is normally set to about 8 to 12 kHz, the maximum deviation between the communication devices 8 ppm (= 4 ppm − (− 4 ppm)) is changed to 3.4 kHz (8 ppm). And enters the passband.

  Next, the desired signal and the adjacent channel signal are input to the second bandpass filter 5 (FIG. 2b). The first changeover switch 8 is switched to the output of the second bandpass filter 5 by the switch changeover signal e from the CPU 7, and the power measurement circuit 9 measures the power of the desired signal and the adjacent channel signal (hereinafter referred to as “S '") Is obtained. This value is stored and held in the memory 10 via the CPU 7.

Here, when the signal level of the adjacent channel is N, the power N of the adjacent channel signal is obtained by the following equation (1).
N = S′−S (1)
This value is stored and held in the memory 10 via the CPU 7. Here, the S / N ratio is calculated by the CPU 7 from the power S of the desired signal obtained previously and the power N of the adjacent channel signal obtained from the equation (1), and stored in the memory 10.

  Next, the AFC operation is started. From the frequency error signal output terminal a of the demodulation circuit 6, a frequency error as a frequency deviation from a desired intermediate frequency is sent to the CPU 7 as a frequency error signal b. The CPU 7 outputs and controls the AFC control signal d to the local oscillator 2 so that the frequency error input as the frequency error signal b becomes zero, and sets the output frequency of the local oscillator 2 to a desired local oscillation frequency. To follow the received signal. Here, since the local oscillation frequency has changed after the end of the AFC operation, the frequency difference with the adjacent channel signal changes (FIG. 3a).

  As shown in FIG. 3a, when the local oscillation frequency is increased by the AFC operation, the frequency is close to the adjacent channel on the upper side, so that the adjacent channel signal comes close to the pass band of the first band pass filter 4, so that The amount of removal by the first band pass filter 4 is reduced, and the reception characteristics are deteriorated due to interference. At this time, as shown in FIG. 3b, it is possible to remove the adjacent channel signal by narrowing the bandwidth of the band pass filter 4 and improving the attenuation characteristic. The received signal until the AFC operation is completed is a signal dedicated to frequency adjustment such as a preamble and does not include actual information. Therefore, an error occurs in the demodulated signal by switching the first bandpass filter 4. Problems such as occurrence do not occur.

  FIG. 4 a shows an example of the configuration example of the first bandpass filter 4. In this description, a two-stage dependent connection filter configuration will be described, but it is obvious that it is possible to have any number of stages. As shown in FIG. 4a, the first band-pass filter unit 20 and the second band-pass filter unit 21 are cascade-connected, and a switch 22 is provided in parallel with the first band-pass filter unit 20 to provide a second band-pass filter. A switch 23 is provided in parallel with the filter unit 21. Here, the first band-pass filter unit 20 has standard characteristics in terms of bandwidth and attenuation characteristics, and the second band-pass filter unit 21 is a filter having a narrow bandwidth and good attenuation characteristics.

  When the switch 22 is turned off and the switch 23 is turned on, the bandwidth and attenuation characteristics are switched to the first bandpass filter unit 20 having standard characteristics. When the switch 22 is turned on and the switch 23 is turned off, the band It is possible to switch to the second bandpass filter unit 21 having a narrow band and good attenuation characteristics. The characteristics of the first band pass filter 4 can be switched by controlling the switch 22 and the switch 23 by the filter switching signal c from the CPU 7.

The characteristic switching determination method of the first bandpass filter 4 will be described with reference to FIG.
The horizontal axis is the frequency error amount, the vertical axis is the SN ratio, and the SN ratio of the received signal is A. When the amount of frequency error is small (point P), the influence of the adjacent channel signal is small, so the first band pass filter 4 is switched to a filter with standard characteristics. When the frequency error amount increases (Q point), the influence of the adjacent channel signal increases, so the first band-pass filter 4 is switched to a narrow-band characteristic filter.

  Thus, there is a characteristic switching threshold value (R point) of the first bandpass filter 4 between the SN ratio and the frequency error amount, and when the threshold values at other SN ratios are plotted, FIG. Such a switching determination line (straight line or curve) can be represented. If the characteristics of FIG. 4 b are stored in the memory 10, the first bandpass filter 4 can be switched to the optimum filter characteristics based on the SN ratio and frequency error amount measured at the time of reception.

Although a detailed description is omitted, if characteristics of a plurality of filters are prepared as shown in FIG. 4c, it is possible to switch to more optimal filter characteristics. For example, the first band-pass filter 4 can be set as a standard characteristic filter by default, and the filter can be switched to a filter having an appropriate filter characteristic based on the S / N ratio and the frequency error amount by the filter switching signal c. When switching three types of filters as shown in FIG. 4c, the filter unit shown in FIG. 4a has another cascade connection structure, and one of the switches is turned off depending on the filter characteristics to be selected. The switch is switched by the filter switching signal c so that the switch is turned on.
In this way, when the signal is received, the first bandpass filter 4 is set to a standard characteristic filter, and it is only necessary to perform the switching operation when the characteristic needs to be switched.

  Next, a second embodiment of the receiving apparatus according to the present invention will be described. Here, in order to simplify the description, consider a case where only the lower adjacent channel signal exists. FIG. 5a shows the configuration of the receiving apparatus according to the second embodiment. As shown in FIG. 5a, the difference from the first embodiment of the present invention is that a third bandpass filter 11 is provided in parallel with the second bandpass filter 5, and the first changeover switch 8 is changed. The second switching switch 12 is provided to switch the output of the first bandpass filter 4, the output of the second bandpass filter 5, and the output of the third bandpass filter 11.

The characteristics of the second bandpass filter 5 are shown in FIG. The center frequency of the filter is between the center frequency of the intermediate frequency and the upper adjacent channel frequency, and the desired signal and the upper adjacent channel signal pass.
The characteristics of the third band pass filter 11 are shown in FIG. The center frequency of the filter is between the center frequency of the intermediate frequency and the lower adjacent channel frequency, and the desired signal and the lower adjacent channel signal pass through.

  The power S of the desired signal can be obtained by measuring the second changeover switch 12 to the output of the first bandpass filter 4 by the switching power measurement circuit 9. This value is stored and held in the memory 10 via the CPU 7. Next, the power S 'of the desired signal and the upper adjacent channel signal is obtained by measuring the second changeover switch 12 to the output of the second bandpass filter 5 and measuring it with the power measurement circuit 9. This value is stored and held in the memory 10 via the CPU 7. Next, the changeover switch 12 is switched to the output of the third bandpass filter 11 and measured by the power measurement circuit 9 to obtain the desired signal and the power S ″ of the lower adjacent channel. This value is obtained via the CPU 7. Stored in the memory 10.

Here, as can be seen from FIGS. 5b and 5c, there is no upper adjacent channel signal, so S = S ′ (2)
And from equation (1)
N = 0 (3)
It becomes. Although the S / N ratio appears to be infinite from the equations (2) and (3), the desired signal is actually a signal having a certain S / N ratio due to thermal noise or noise generated in the receiver. . Equation (3) means that there is no signal that can be detected by the power measurement circuit 9 in the adjacent channel, and noise such as thermal noise also exists in the adjacent channel.
Also, since there is a signal in the lower adjacent channel, if the power of the lower adjacent channel signal is N ′,
N ′ = S ″ −S (4)
It becomes.

  Next, when the AFC operation is executed, the output of the first bandpass filter 4 becomes as shown in FIG. 5d. By shifting the local oscillation signal as shown in FIG. 5d, the upper adjacent channel frequency approaches the filter passband, but there is no signal in the upper adjacent channel. This can be understood from the values stored in the memory 10 from the expressions (2) and (3), so that the first band-pass filter 4 can be secured with a normal characteristic filter to ensure communication performance. The filter is not switched.

  When the local oscillation signal shifts downward after the AFC operation, the signal approaches the lower adjacent channel, so that the desired signal power S and the SN ratio and frequency error signal by the lower adjacent channel signal power N ′ according to the equation (4) are used. To determine whether to switch the first bandpass filter 4.

The block diagram which shows the structure of the receiver which concerns on 1st Example of this invention Schematic diagram of frequency spectrum of first band pass filter output of the present invention Schematic diagram of frequency spectrum of second bandpass filter output of the present invention Schematic diagram of frequency spectrum after AFC execution of first band pass filter output of the present invention The schematic diagram of the frequency spectrum after switching the characteristic of the 1st band pass filter after AFC execution of the 1st band pass filter output of this invention The block diagram which shows the specific structure of the 1st band pass filter of this invention. The schematic diagram which shows the relationship between the frequency error amount and S / N ratio which determines switching of the 1st bandpass filter of this invention Another schematic diagram showing the relationship between the frequency error amount and the SN ratio for determining the switching of the first bandpass filter of the present invention. The block diagram which shows the structure of the receiver which concerns on 2nd Example of this invention. Schematic diagram of frequency spectrum of second bandpass filter output of the present invention Schematic diagram of frequency spectrum of third band-pass filter output of the present invention Schematic diagram of frequency spectrum after AFC execution of first band pass filter output of the present invention The block diagram which shows the structure of the receiver of a prior art

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Amplifier 2 Local oscillator 3 Mixer 4 1st band pass filter 5 2nd band pass filter 6 Demodulation circuit 7 CPU 8 1st changeover switch 9 Power measurement circuit 10 Memory 11 3rd band pass filter 12 2nd Selector switch 20 first bandpass filter unit 21 second bandpass filter unit 22 first switch 23 second switch a frequency error output terminal b frequency error signal c filter switching signal d AFC control signal e switch switching signal 101 Antenna 102 reception signal amplifier 103 first local oscillator 104 90-degree phase shifter 105 first signal mixer 106 second signal mixer 107 I low-pass filter 108 Q low-pass filter 109 demodulating means 110 automatic frequency control Means 111 Frequency deviation detection means 112 Base Band filter control means 113 Signal processing means a1 Modulated signal b1 First local signal c1 I Baseband signal d1 Q Baseband signal e1 Demodulated signal f1 Automatic frequency control signal g1 Demodulated signal

Claims (7)

In a receiving apparatus having automatic frequency control, a first bandpass filter that passes a desired signal among received signals;
A second bandpass filter that passes the desired signal and an adjacent channel signal;
A power measurement circuit for measuring the power of the output signal from the first band pass filter and the output signal from the second band pass filter;
Varying the characteristics of the first bandpass filter based on the S / N ratio between the desired signal and the adjacent channel signal calculated from the value measured by the power measurement circuit and the frequency error amount by the automatic frequency control. A receiving device.   The received power of the adjacent channel signal on the upper side and the received power of the adjacent channel signal on the lower side are measured, and when the signal level on the adjacent channel side whose frequency approaches by the automatic frequency control is low, the first band pass The receiving apparatus according to claim 1, wherein the filter is not variable.   If the S / N ratio is equal to or greater than the S / N ratio threshold value set based on a predetermined relationship with respect to the frequency error amount in the automatic frequency control, the S / N ratio is not changed. 3. The receiving apparatus according to claim 1, wherein the first bandpass filter is varied when the value is less than or equal to the threshold value. 4.   4. The receiving apparatus according to claim 1, wherein when the SN ratio is equal to or less than the threshold value, a filter bandwidth of the band-pass filter is narrowed according to a difference from the threshold value. An amplifier for amplifying the received signal;
A local oscillator that outputs a local oscillation signal;
A mixer for mixing the output signal amplified from the amplifier and the local oscillation signal of the local oscillator;
The first bandpass filter for band limiting the intermediate frequency signal output from the mixer;
The second bandpass filter for band-limiting the intermediate frequency signal output from the mixer;
A demodulation circuit for demodulating the intermediate frequency signal output from the first bandpass filter;
A CPU for processing the demodulated signal output from the demodulation circuit;
A first changeover switch for switching an output from the first bandpass filter or an output from the second bandpass filter;
The power measuring circuit for measuring output power from the first changeover switch;
A memory for storing and holding the power value of the power measurement circuit via the CPU;
The receiving device according to claim 1, comprising:   A third bandpass filter is provided in parallel with the second bandpass filter, and instead of the first changeover switch, the first bandpass filter output, the second bandpass filter output, and the third In the configuration in which the second changeover switch for switching the output of the bandpass filter is provided, the second bandpass filter has a characteristic that only the desired signal and the upper adjacent channel signal pass, and the third bandpass filter is the desired signal. 6. The receiving apparatus according to claim 5, wherein only the lower adjacent channel signal passes.   The first bandpass filter has a configuration in which bandpass filter units provided with switches in parallel are connected in at least two or more stages, and each of the bandpass filter units has characteristics different in passband width and attenuation characteristics. The receiving apparatus according to claim 1, wherein

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