JP2015100023A - Direct conversion receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a direct conversion receiver capable of obtaining more excellent reception characteristics by suppressing attenuation of the receiving signals to necessary minimum when suppressing unwanted waves.SOLUTION: In an unwanted wave analysis circuit 71, by using two outputted digital signals, presence or absence of an unwanted wave included in the received high frequency signal and the frequency of the unwanted wave are analyzed. When the unwanted wave is present within a desired band of receiving signals, the frequency of output waves of a variable local oscillator 25 is changed to the same frequency as that of the unwanted wave or the frequency near that of the unwanted wave. As a result, the unwanted wave after demodulation becomes zero in the DC component and is suppressed by suppression band of a first capacitor 33 and a second capacitor 34 for DC cut.

Description

  The present invention relates to a direct conversion receiver used in a wireless communication device or the like.

  Conventionally, a direct conversion receiver is used in a wireless communication device or the like. Patent Document 1 discloses a direct conversion receiver that suppresses unnecessary waves existing in a desired band of a received signal.

  In the direct conversion receiver of Patent Document 1, a high-pass filter (hereinafter referred to as “HPF”) is connected to the output ends of two mixers that synthesize a high-frequency signal received by an antenna and an output wave of a local oscillator. ing. Here, the frequency of the output wave of the local oscillator is set to the same frequency as the center frequency of the received signal.

The operation of the direct conversion receiver disclosed in Patent Document 1 will be described with reference to FIGS.
FIG. 11 is a characteristic diagram showing signal strength-frequency characteristics of the received signal 81, the unnecessary wave 82, and the output wave 83 of the local oscillator before demodulating the frequency. In the figure, the vertical axis represents signal intensity (Level) [dB], and the horizontal axis represents frequency f [Hz]. As shown in FIG. 11, an unnecessary wave 82 exists in a desired band of the received signal 81 received by the antenna. The mixer demodulates the frequency by synthesizing the received signal 81 and the unnecessary wave 82 with the output wave 83 of the local oscillator, and converts the high-frequency signal into a baseband signal.

  FIG. 12 is a characteristic diagram showing signal strength-frequency characteristics of the reception signal 91, the unnecessary wave 92, and the suppression band 93 of the HPF after the frequency is demodulated. As shown in FIG. 12, the unwanted wave 92 exists within the desired band of the received signal 91 even after demodulation. At this time, by changing the cutoff frequency of the HPF, the suppression band 93 of the HPF is changed to suppress the unnecessary wave 92 of the baseband signal.

JP-A-5-235463

  The direct conversion receiver of Patent Document 1 suppresses unnecessary waves by changing the cutoff frequency of the HPF after converting from a high frequency signal to a baseband signal with an output wave of a local oscillator having the same frequency as the center frequency of the received signal. ing. Therefore, particularly when an unnecessary wave is present in a desired band of the received signal, there is a problem that when the unnecessary wave is suppressed, the received signal is greatly attenuated to deteriorate the reception characteristics.

  The present invention has been made in order to solve the above-described problems, and is a direct conversion capable of obtaining a better reception characteristic by minimizing reception signal attenuation when suppressing unnecessary waves. The purpose is to provide a receiver.

  The direct conversion receiver of the present invention has an antenna that receives a modulated high-frequency signal, a variable local oscillator that outputs a local oscillation wave, and a high-frequency signal and a local oscillation wave that have a phase difference of 90 degrees from each other. A first mixer and a second mixer for converting into a two-wave baseband signal; a first capacitor connected to the output terminal of the first mixer; a second capacitor connected to the output terminal of the second mixer; and a high-frequency signal And an unnecessary wave detection circuit unit that detects the frequency of the unnecessary wave included in the signal and changes the frequency of the local oscillation wave to the same frequency as the frequency of the unnecessary wave.

  According to the direct conversion receiver of the present invention, it is possible to suppress the attenuation of the reception signal when suppressing unnecessary waves to the minimum necessary, and to obtain better reception characteristics.

It is a block diagram of the direct conversion receiver of Embodiment 1 of this invention. It is a characteristic view which shows the signal strength-frequency characteristic before the demodulation of the direct conversion receiver of Embodiment 1 of this invention. It is a characteristic view which shows the signal strength-frequency characteristic after the demodulation of the direct conversion receiver of Embodiment 1 of this invention. It is a block diagram of the direct conversion receiver of Embodiment 2 of this invention. It is a block diagram of the direct conversion receiver of Embodiment 3 of this invention. It is a characteristic view which shows the signal strength-frequency characteristic before the demodulation of the direct conversion receiver of Embodiment 3 of this invention. It is a characteristic view which shows the signal strength-frequency characteristic after the demodulation of the direct conversion receiver of Embodiment 3 of this invention. It is a block diagram of the direct conversion receiver of Embodiment 4 of this invention. It is a block diagram of the direct conversion receiver of Embodiment 5 of this invention. It is a block diagram of the direct conversion receiver of Embodiment 6 of this invention. It is a characteristic view which shows the signal strength-frequency characteristic before the demodulation of the conventional direct conversion receiver. It is a characteristic view which shows the signal strength-frequency characteristic after the demodulation of the conventional direct conversion receiver.

Embodiment 1 FIG.
A direct conversion receiver according to the first embodiment will be described with reference to FIG.
In the figure, 11 is an antenna. A band pass filter (hereinafter referred to as “BPF”) 12 and a low noise amplifier 13 are connected in series between the antenna 11 and the first mixer 21 and the second mixer 22. A π / 2 phase shifter (90-degree phase shifter) 24 is connected between the first mixer 21 and the second mixer 22 in parallel with the BPF 12 and the low noise amplifier 13. The variable local oscillator 25 outputs two output waves (local oscillation waves) whose phases are shifted by π / 2 (90 degrees) from each other by the π / 2 phase shifter 24 between the first mixer 21 and the second mixer 22. Is connected.

  A first capacitor 33 for cutting direct current (hereinafter referred to as “DC”) is connected to the output terminal of the first mixer 21, and a second capacitor 34 for cutting DC is connected to the output terminal of the second mixer 22. A first analog / digital (hereinafter referred to as “A / D”) converter 61 is connected to the output terminal of the first capacitor 33 via a first low-pass filter (hereinafter referred to as “LPF”) 41 and a first variable amplifier 53. They are sequentially connected in series. Similarly, the second A / D converter 62 is sequentially connected in series to the output terminal of the second capacitor 34 via the second LPF 42 and the second variable amplifier 54.

  An unnecessary wave analysis circuit (unnecessary wave detection circuit unit) 71 is connected between the output terminals of the first A / D converter 61 and the second A / D converter 62 and the variable local oscillator 25.

The operation of the direct conversion receiver 100 configured as described above will be described.
The frequency of the output wave of the variable local oscillator 25 is set in advance to the same frequency as the received signal.
First, the antenna 11 receives a modulated high frequency signal. The high frequency signal received by the antenna 11 passes through the BPF 15 and is then amplified by the low noise amplifier 13 and divided into two high frequency signals. Next, the two high-frequency signals are mixed with the local oscillation wave output from the variable local oscillator 25 by the first mixer 21 and the second mixer 22, respectively, and converted into two baseband signals. At this time, the local oscillation waves output from the variable local oscillator 25 to the second mixer 22 cause a phase difference of π / 2 by the π / 2 phase shifter 24. Therefore, the phase difference between the two baseband signals is 90 degrees.

  The DC components generated in the two baseband signals by the first mixer 21 and the second mixer 22, respectively, pass through the first capacitor 33 and the second capacitor 34 and are removed. Next, unnecessary signals that pass through the first LPF 41 and the second LPF 42 and are outside the band of the baseband signal are suppressed. Next, the signals are amplified by the first variable amplifier 53 and the second variable amplifier 54, respectively, converted into two digital signals by the first A / D converter 61 and the second A / D converter 62, and output.

At this time, the unnecessary wave analysis circuit 71 suppresses unnecessary waves in the desired band of the received signal as follows.
That is, the unnecessary wave analysis circuit 71 analyzes the presence / absence of an unnecessary wave included in the received high-frequency signal and the frequency of the unnecessary wave, using the two output digital signals. When an unnecessary wave exists in the desired band of the received signal, the frequency of the output wave of the variable local oscillator 25 is changed to the same frequency as the frequency of the unnecessary wave or a frequency close to the frequency of the unnecessary wave.
Note that when there is no unnecessary wave, the frequency of the output wave of the variable local oscillator 25 is not changed.

  FIG. 2 is a characteristic diagram showing signal strength-frequency characteristics of the received signal 81, the unnecessary wave 82, and the output wave 84 of the local oscillator before demodulating the frequency. In the figure, the vertical axis represents signal intensity (Level) [dB], and the horizontal axis represents frequency f [Hz]. As shown in FIG. 2, an unnecessary wave 82 exists in a desired band of the received signal 81 received by the antenna 11. The first mixer 21 and the second mixer 22 synthesize the output wave 84 of the variable local oscillator 25 with the received signal 81 and the unnecessary wave 82 to demodulate the frequency, and convert the high frequency signal into a baseband signal.

FIG. 3 is a characteristic diagram showing the signal strength-frequency characteristics of the reception signal 94, the unnecessary wave 95, and the suppression band 96 of the first capacitor 33 and the second capacitor 34 after the frequency is demodulated. As shown in FIG. 3, the unwanted wave 95 exists within the desired band of the received signal 94 even after demodulation.
Here, since the unnecessary wave analysis circuit 71 sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as the frequency of the unnecessary wave 82 or a frequency close to the frequency of the unnecessary wave 82, the unnecessary wave 95 after demodulation is The DC component has a frequency of 0. Therefore, the unnecessary wave 95 is suppressed by the suppression band 96 of the first capacitor 33 and the second capacitor 34 for DC cut similarly to the DC component generated in the first mixer 21 and the second mixer 22.

As described above, the direct conversion receiver 100 sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as the frequency of the unnecessary wave 82 or a frequency close to the frequency of the unnecessary wave 82. Therefore, it is possible to suppress the unnecessary wave 95 in the desired band of the demodulated reception signal 94 without expanding the suppression band by the first capacitor 33 and the second capacitor 34.
As a result, it is possible to suppress the attenuation of the reception signal when suppressing unnecessary waves to the minimum necessary and to obtain good reception characteristics.

  If the frequency of the unnecessary wave is known, the frequency of the output wave of the variable local oscillator 25 may be set in advance to the same frequency as the frequency of the unnecessary wave or a frequency close to the frequency of the unnecessary wave. With this configuration, even in the initial reception signal that has started reception, attenuation when suppressing unnecessary waves can be suppressed to the minimum necessary, and good reception characteristics can be obtained.

Embodiment 2. FIG.
With reference to FIG. 4, a direct conversion receiver provided with a frequency measurement circuit instead of the unwanted wave analysis circuit will be described.
In the figure, reference numeral 72 denotes a frequency measurement circuit (unwanted wave detection circuit unit). The direct conversion receiver according to the first embodiment except that the frequency measurement circuit 72 is interposed between the output terminal of the low noise amplifier 13 and the variable local oscillator 25 instead of the unnecessary wave analysis circuit 71 shown in FIG. The same constituent members as those in FIG.

  The operation of the direct conversion receiver 101 configured as described above will be described. After the antenna 11 receives the high frequency signal, the first mixer 21 and the second mixer 22 convert the baseband signal, and then the first A / D converter 61 and the second A / D converter 62 convert the digital signal. About the operation | movement until converting and outputting, it operate | moves similarly to the direct conversion receiver 100 of Embodiment 1. FIG.

At this time, the frequency measurement circuit 72 suppresses unnecessary waves in the desired band of the received signal as follows.
That is, the frequency measurement circuit 72 uses the high-frequency signal before demodulation received by the antenna 11 to measure the frequency of the unwanted wave that exists in the desired band of the received signal. Next, the frequency of the output wave of the variable local oscillator 25 is set to the same frequency as the frequency of the unnecessary wave or a frequency close to the frequency of the unnecessary wave by using the measured information regarding the frequency of the unnecessary wave.
As a result, as in the first embodiment, the unnecessary wave after demodulation becomes a DC component having a frequency of 0. Therefore, the unnecessary wave is suppressed by the suppression band of the first capacitor 33 and the second capacitor 34 for DC cut similarly to the DC component generated in the first mixer 21 and the second mixer 22.

As described above, the direct conversion receiver 101 sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as the frequency of the unnecessary wave or a frequency close to the frequency of the unnecessary wave. Therefore, unnecessary waves in the desired band of the demodulated received signal can be suppressed without expanding the suppression band by the first capacitor 33 and the second capacitor 34.
As a result, it is possible to suppress the attenuation of the reception signal when suppressing unnecessary waves to the minimum necessary and to obtain good reception characteristics.

Embodiment 3 FIG.
With reference to FIG. 5, a direct conversion receiver having a variable capacitor instead of a capacitor will be described.
In the figure, 35 is a first variable capacitor, and 36 is a second variable capacitor. In addition, instead of the first capacitor 33 and the second capacitor 34 shown in FIG. 1, a first variable capacitor 35 and a second variable capacitor 36 are provided, and unnecessary waves are provided in the first variable capacitor 35 and the second variable capacitor 36. Except for the configuration in which the analysis circuit 71 is connected, the configuration is the same as that of the first embodiment, and the same components are denoted by the same reference numerals and the description thereof is omitted.

  The operation of the direct conversion receiver 102 configured as described above will be described. In addition, after the antenna 11 receives the high frequency signal, the first mixer 21 and the second mixer 22 perform frequency conversion into a baseband signal, and then the first A / D converter 61 and the second A / D converter 62 are digital signals. About the operation | movement until converting and outputting, it operate | moves similarly to the direct conversion receiver 100 of Embodiment 1. FIG.

At this time, the unnecessary wave analysis circuit 71 suppresses unnecessary waves having a predetermined bandwidth existing within a desired band of the received signal as follows.
That is, the unnecessary wave analysis circuit 71 analyzes the presence / absence of an unnecessary wave included in the received high-frequency signal and the frequency of the unnecessary wave, using the two output digital signals. When an unnecessary wave exists in the desired band, the frequency of the output wave of the variable local oscillator 25 is changed to the same frequency as the center frequency of the unnecessary wave or a frequency close to the center frequency of the unnecessary wave. Further, the capacitance values of the first variable capacitor 35 and the second variable capacitor 36 are varied according to the bandwidth of the unwanted wave.
On the other hand, when there is no unnecessary wave, the frequency of the output wave of the variable local oscillator 25 is not changed, and the capacitance values of the first variable capacitor 35 and the second variable capacitor 36 are not changed.

  FIG. 6 is a characteristic diagram showing signal strength-frequency characteristics of the received signal 81, the unnecessary wave 85, and the output wave 84 of the variable local oscillator 25 before demodulating the frequency. In the figure, the vertical axis represents signal intensity (Level) [dB], and the horizontal axis represents frequency f [Hz]. As shown in FIG. 6, an unnecessary wave 85 having a predetermined bandwidth exists within a desired band of the received signal 81 received by the antenna 11. In this case, the unnecessary wave analysis circuit 71 analyzes the frequency of the unnecessary wave 85 and sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as the center frequency of the unnecessary wave 85 or a frequency close to the center frequency of the unnecessary wave 85. To do. Further, the capacitance values of the first variable capacitor 35 and the second variable capacitor 36 are varied according to the bandwidth of the unnecessary wave 85.

FIG. 7 is a characteristic diagram showing the signal strength-frequency characteristics of the reception signal 94, the unnecessary wave 97, and the suppression band 98 of the first variable capacitor 35 and the second variable capacitor 36 after demodulating the frequency. As shown in FIG. 7, even after demodulation, an unnecessary wave 97 having a predetermined bandwidth exists in the desired band of the received signal 94.
Here, since the frequency of the output wave of the variable local oscillator 25 is set to the same frequency as the center frequency of the unnecessary wave 85 or a frequency close to the center frequency of the unnecessary wave 85, the unnecessary wave 97 after demodulation has a center frequency of 0. DC component. Therefore, the unnecessary wave 97 is suppressed by the first variable capacitor 35 and the second variable capacitor 36 for DC cut, similarly to the DC component generated in the first mixer 21 and the second mixer 22.
Further, since the capacitance values of the first variable capacitor 35 and the second variable capacitor 36 are varied in accordance with the bandwidth of the unnecessary wave 85, the suppression band 98 is the minimum necessary for suppressing the unnecessary wave 97. It is bandwidth.

As described above, the direct conversion receiver 102 sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as the center frequency of the unnecessary wave 85 or a frequency close to the center frequency of the unnecessary wave 85, and the unnecessary wave. The capacitance values of the first variable capacitor 35 and the second variable capacitor 36 are varied according to the bandwidth of 85. Therefore, it is possible to suppress unnecessary waves having a predetermined bandwidth existing within a desired band of the received signal while minimizing the suppression band by the first variable capacitor 35 and the second variable capacitor 36.
As a result, it is possible to suppress the attenuation of a desired signal when suppressing unnecessary waves and to obtain a good reception characteristic.

Embodiment 4 FIG.
With reference to FIG. 8, a direct conversion receiver provided with a frequency measurement circuit instead of the unwanted wave analysis circuit will be described.
In the figure, reference numeral 72 denotes a frequency measurement circuit (unnecessary wave detection circuit unit). Instead of the unnecessary wave analysis circuit 71 shown in FIG. 5, a frequency measurement circuit 72 is interposed between the output terminal of the low noise amplifier 13 and the variable local oscillator 25, so that the first variable capacitor 35 and the second variable capacitor The configuration is the same as that of the direct conversion receiver 102 of the third embodiment except for the configuration in which the frequency measurement circuit 72 is connected to 36, and the same components are denoted by the same reference numerals and the description thereof is omitted.

  In the direct conversion receiver 103 configured as described above, the frequency measurement circuit 72 sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as the center frequency of the unnecessary wave, similarly to the direct conversion receiver 102 of the third embodiment. Alternatively, the frequency is set to a frequency close to the center frequency of the unwanted wave, and the capacitance values of the first variable capacitor 35 and the second variable capacitor 36 are varied according to the bandwidth of the unwanted wave.

Therefore, unnecessary waves existing in the desired band of the received signal can be suppressed while the suppression band by the first variable capacitor 35 and the second variable capacitor 36 is minimized.
As a result, it is possible to suppress the attenuation of a desired signal when suppressing unnecessary waves and to obtain a good reception characteristic.

Embodiment 5 FIG.
A direct conversion receiver provided with a variable LPF instead of the LPF will be described with reference to FIG.
In the figure, 43 is a first variable LPF, and 44 is a second variable LPF. A first variable LPF 43 and a second variable LPF 44 are provided in place of the first LPF 41 and the second LPF 42 shown in FIG. 1, and the unnecessary wave analysis circuit 71 is connected to the first variable LPF 43 and the second variable LPF 44. Similar components are provided with the same reference numerals, and description thereof is omitted.

  The operation of the direct conversion receiver 104 configured as described above will be described. After the antenna 11 receives the high frequency signal, the first mixer 21 and the second mixer 22 convert the baseband signal, and then the first A / D converter 61 and the second A / D converter 62 convert the digital signal. About the operation | movement until converting and outputting, it operate | moves similarly to the direct conversion receiver 100 of Embodiment 1. FIG.

  Similarly to the direct conversion receiver 100 of the first embodiment, the unnecessary wave analysis circuit 71 analyzes the frequency of the unnecessary wave when the unnecessary wave exists in the desired band of the received signal, and the variable local oscillator 25 The frequency of the output wave is set to the same frequency as the frequency of the unnecessary wave or a frequency close to the frequency of the unnecessary wave.

  At this time, the bandwidth of the converted baseband signal changes according to the frequency of the unwanted wave. On the other hand, the unnecessary wave analyzing circuit 71 varies the cutoff frequency band of the first variable LPF 43 and the second variable LPF 44 according to the frequency of the unnecessary wave. As a result, unnecessary signals outside the band of the baseband signal can be suppressed, and noise captured during A / D conversion can be minimized.

  As described above, the direct conversion receiver 104 sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as the frequency of the unnecessary wave or a frequency close to the frequency of the unnecessary wave, and also according to the frequency of the unnecessary wave. Thus, the cutoff frequency band of the first variable LPF 43 and the second variable LPF 44 is varied.

Therefore, it is possible to suppress unnecessary waves in the desired band of the received signal while minimizing the suppression band due to the first capacitor 33 and the second capacitor 34. Further, it is possible to suppress the noise band to be taken in at the time of A / D conversion to the minimum necessary.
As a result, it is possible to suppress the attenuation of a desired signal when suppressing unnecessary waves and to obtain a good reception characteristic.

Embodiment 6 FIG.
With reference to FIG. 10, a direct conversion receiver provided with a frequency measurement circuit instead of the unwanted wave analysis circuit will be described.
In the figure, reference numeral 72 denotes a frequency measurement circuit (unwanted wave detection circuit unit). In place of the unnecessary wave analysis circuit 71 shown in FIG. 9, a frequency measurement circuit 72 is interposed between the output terminal of the low noise amplifier 13 and the variable local oscillator 25 to measure the frequency in the first variable LPF 43 and the second variable LPF 44. Except for the configuration in which the circuit 72 is connected, the configuration is the same as that of the direct conversion receiver 104 of the fifth embodiment, and the same components are denoted by the same reference numerals and description thereof is omitted.

  In the direct conversion receiver 105 configured as described above, the frequency measurement circuit 72 sets the frequency of the output wave of the variable local oscillator 25 to the same frequency as that of the unnecessary wave, similarly to the direct conversion receiver 104 of the fifth embodiment. Alternatively, a frequency close to the frequency of the unnecessary wave is set, and the cutoff frequency band of the first variable LPF 43 and the second variable LPF 44 is varied according to the frequency of the unnecessary wave.

Therefore, it is possible to suppress unnecessary waves in the desired band of the received signal while minimizing the suppression band due to the first capacitor 33 and the second capacitor 34. Further, it is possible to suppress the noise band to be taken in at the time of A / D conversion to the minimum necessary.
As a result, it is possible to suppress the attenuation of a desired signal when suppressing unnecessary waves and to obtain a good reception characteristic.

  In the present invention, within the scope of the invention, any combination of the embodiments, any modification of any component of each embodiment, or omission of any component in each embodiment is possible. .

  11 antenna, 12 band pass filter, 13 low noise amplifier, 21 first mixer, 22 second mixer, 24 π / 2 phase shifter (90 degree phase shifter), 25 variable local oscillator, 33 first capacitor, 34 first 2 capacitor, 35 first variable capacitor, 36 second variable capacitor, 41 first low pass filter, 42 second low pass filter, 43 first variable low pass filter, 44 second variable low pass filter, 53 first variable amplifier, 54 Second variable amplifier, 61 first analog / digital converter, 62 second analog / digital converter, 71 unnecessary wave analysis circuit (unwanted wave detection circuit section), 72 frequency measurement circuit (unwanted wave detection circuit section), 100, 101, 102, 103, 104, 105 Direct conversion receiver.

Claims (5)

An antenna for receiving the modulated high frequency signal;
A variable local oscillator that outputs a local oscillation wave;
A first mixer and a second mixer that synthesize the high-frequency signal and the local oscillation wave to convert them into two baseband signals each having a phase difference of 90 degrees;
A first capacitor connected to an output terminal of the first mixer;
A second capacitor connected to an output terminal of the second mixer;
An unnecessary wave detection circuit unit that detects the frequency of the unnecessary wave included in the high-frequency signal and sets the frequency of the local oscillation wave to the same frequency as the frequency of the unnecessary wave;
A direct conversion receiver comprising: A band-pass filter and a low noise amplifier connected in series between the antenna and the first mixer and the second mixer;
A first analog / digital converter connected to the output terminal of the first capacitor via a first low-pass filter and converting the baseband signal into a digital signal;
A second analog / digital converter connected to the output terminal of the second capacitor via a second low-pass filter and converting the baseband signal into the digital signal;
A 90 degree phase shifter interposed between the variable local oscillator and the second mixer;
The unnecessary wave detection circuit unit is connected between the output terminals of the first analog / digital converter and the second analog / digital converter and the variable local oscillator, and uses the digital signal to set the frequency of the unnecessary wave. The direct conversion receiver according to claim 1, comprising an unnecessary wave analysis circuit for analysis. A band-pass filter and a low noise amplifier connected in series between the antenna and the first mixer and the second mixer;
A first analog / digital converter connected to the output terminal of the first capacitor via a first low-pass filter and converting the baseband signal into a digital signal;
A second analog / digital converter connected to the output terminal of the second capacitor via a second low-pass filter and converting the baseband signal into the digital signal;
A 90 degree phase shifter interposed between the variable local oscillator and the second mixer;
The unnecessary wave detection circuit unit is configured by a frequency measurement circuit that is connected between the output terminal of the low noise amplifier and the variable local oscillator and measures the frequency of the unnecessary wave using the high frequency signal. The direct conversion receiver according to claim 1, wherein: The first capacitor and the second capacitor are composed of variable capacitors,
The direct conversion receiver according to claim 2, wherein the unnecessary wave detection circuit unit varies a capacitance value of the variable capacitor according to a frequency of the unnecessary wave. The first low-pass filter and the second low-pass filter are composed of variable low-pass filters,
The direct conversion receiver according to claim 2, wherein the unnecessary wave detection circuit unit varies a cutoff frequency of the variable low-pass filter in accordance with a frequency of the unnecessary wave.

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