JP2012249136A - Reception circuit, transmission circuit, radio transmission/reception circuit and frequency conversion method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide reception circuit, transmission circuit, radio transmission/reception circuit and frequency conversion method, capable of varying a shift frequency without increasing a processing load.SOLUTION: A reception circuit 100 comprises: a mixer 60 for converting the radio frequency of an input signal input by radio communication into an intermediate frequency; and a variable filter 70 for passing a signal of a predetermined frequency band in the input signal converted into the intermediate frequency. The mixer 60 varies the intermediate frequency of the input signal to be output to the variable filter 70, according to the variation of the radio frequency of the input signal. The variable filter 70 varies the pass band of the input signal to be passed, according to the intermediate frequency of the input signal output from the mixer 60.

Description

  The present invention relates to a receiving circuit that receives a radio signal whose shift frequency changes, converts the frequency of the radio signal into an intermediate frequency, and performs signal processing.

  The transmission frequency, reception frequency, and shift frequency that can be used for wireless communication are determined in advance. Here, the shift frequency indicates a difference between the transmission frequency and the reception frequency. However, different values for the transmission frequency, the reception frequency, and the shift frequency are determined for each country or region. Therefore, a wireless transmission / reception apparatus used for wireless communication is required to transmit / receive signals having different transmission frequencies or reception frequencies for each country or region.

  Patent Document 1 discloses a transmission / reception frequency converter that can transmit and receive signals having different transmission frequencies or reception frequencies by changing the shift frequency. The transmission / reception frequency converter of Patent Document 1 uses two oscillators in an RF (Radio Frequency) band. Thereby, the transmission local oscillation frequency and the reception local oscillation frequency can be individually controlled, and the shift frequency can be changed.

  In Patent Documents 2 and 3, there is a wireless communication apparatus that uses one RF band oscillator and that can remove an interference wave using a filter that controls the pass band width of a radio frequency signal in the RF band. It is disclosed.

Japanese Patent Laid-Open No. 06-61744 Japanese Patent Laid-Open No. 2006-186556 JP 2006-262434 A

  However, since the transmission / reception frequency converter of Patent Document 1 uses two RF band oscillators, there is a problem that the apparatus becomes expensive. In addition, since the wireless communication devices of Patent Documents 2 and 3 have one RF band oscillator, the device is less expensive than the transmission / reception frequency converter of Patent Document 1. However, the wireless communication devices of Patent Documents 2 and 3 are provided with a filter that removes interference waves of radio frequency signals in the RF band. For this reason, the filter requires processing with a high-frequency signal used in the RF band, and there is a problem that the processing load increases.

  In order to solve such a problem, an object of the present invention is to provide a reception circuit, a transmission circuit, a wireless transmission / reception circuit, and a frequency conversion method that can change the shift frequency without increasing the processing load.

  A receiving circuit according to a first aspect of the present invention includes a first mixer that converts a radio frequency of an input signal input by radio communication into an intermediate frequency, and a predetermined frequency in the input signal that has been converted to the intermediate frequency. A first variable filter that passes a signal in a band, and the first mixer outputs an intermediate frequency of the input signal to be output to the first variable filter in accordance with a change in radio frequency of the input signal. The first variable filter is configured to change a pass band of the input signal to be passed according to an intermediate frequency of the input signal output from the first mixer.

  A transmission circuit according to a second aspect of the present invention includes a third mixer that converts an intermediate frequency of an output signal output by radio communication into a radio frequency, and an output signal having a predetermined fixed frequency to the intermediate frequency. A fourth mixer for converting, and a second variable filter for passing a signal in a predetermined frequency band of the output signal converted to the intermediate frequency, wherein the fourth mixer is a radio of the output signal. The intermediate frequency of the output signal output to the second variable filter is changed in accordance with a change in frequency, and the second variable filter is in accordance with the intermediate frequency of the output signal output from the fourth mixer. Thus, the pass band of the output signal to be passed is changed.

  A radio transmission / reception circuit according to a third aspect of the present invention includes a transmission circuit that transmits an output signal of a first radio frequency that is fixedly determined, and an input signal of a second radio frequency that varies depending on the opposing device. Receiving circuit, and the receiving circuit converts the second radio frequency of the input signal input by wireless communication into an intermediate frequency, and converts the input signal into the intermediate frequency. A first variable filter that passes a signal in a predetermined frequency band of the input signal, and the first mixer outputs to the first variable filter in accordance with a change in radio frequency of the input signal. The first variable filter changes the pass band of the input signal to be passed according to the intermediate frequency of the input signal output from the mixer. That.

  In the frequency conversion method according to the fourth aspect of the present invention, the intermediate frequency of the input signal to be output is changed according to the change of the radio frequency of the input signal input by wireless communication, and the change of the intermediate frequency of the input signal Accordingly, the pass band of the variable filter is changed, and the input signal is passed based on the pass band of the variable filter.

  According to the present invention, it is possible to provide a reception circuit, a transmission circuit, a wireless transmission / reception circuit, and a frequency conversion method that can change the shift frequency without increasing the processing load.

1 is a configuration diagram of a receiving circuit according to a first exemplary embodiment; 1 is a configuration diagram of a radio transmission / reception device according to a first exemplary embodiment; It is a figure which shows the frequency conversion in the mixer concerning Embodiment 1. FIG. 1 is a configuration diagram of a circuit that controls a VCO and a variable filter according to a first embodiment; FIG. 3 is a flowchart of signal processing of the receiving circuit according to the first exemplary embodiment; FIG. 3 is a configuration diagram of a radio transmission / reception device according to a second exemplary embodiment; It is a block diagram of the variable filter concerning other embodiment.

(Embodiment 1)
Embodiments of the present invention will be described below with reference to the drawings. A configuration example of the receiving circuit 100 according to the first exemplary embodiment of the present invention will be described with reference to FIG. The receiving circuit 100 includes a mixer 60 and a variable filter 70.

  The receiving circuit 100 may be provided in, for example, a mobile communication device or a base station device. Alternatively, the receiving circuit 100 may be provided in a relay device that relays data between base station devices. Alternatively, the receiving circuit 100 may be provided in a relay device that configures a LAN line.

  The mixer 60 converts the radio frequency of the input signal input by radio communication into an intermediate frequency. The wireless communication is, for example, communication performed based on a predetermined wireless standard between the mobile communication device and the base station device. Alternatively, the wireless communication may be communication executed between mobile communication devices based on a predetermined wireless standard. The radio frequency is a frequency defined in a radio standard, and is a frequency used when transmitting / receiving a radio signal. The intermediate frequency is a frequency defined as a frequency used when processing a signal in the apparatus or in a circuit. The input signal is a signal transmitted from the opposite device, and may be a data signal in which user data is set, a control signal, or the like.

  The mixer 60 changes the intermediate frequency of the input signal output to the variable filter 70 in accordance with the change in the radio frequency of the input signal. The receiving circuit 100 receives an input signal in which various radio frequencies are set in accordance with a region where the apparatus is used and a radio standard used. Therefore, when the radio frequency of the input signal input to the mixer 60 changes, the intermediate frequency of the signal output from the mixer 60 also has a different value. For example, the mixer 60 converts a radio frequency of several GHz set in the input signal into an intermediate frequency of several MHz.

  The variable filter 70 passes a signal in a predetermined frequency band in the input signal converted to the intermediate frequency. The variable filter 70 changes at least one of the upper limit value and the lower limit value of the passband. The variable filter 70 outputs from the mixer 60 in order to extract a signal in a predetermined frequency band according to the intermediate frequency of the signal output from the mixer 60 or to exclude a signal outside the predetermined frequency band. The pass band of the signal to be changed is changed.

  As described above, by using the receiving circuit 100 in FIG. 1, the mixer 60 can output a signal in which the intermediate frequency is changed according to the radio frequency of the input signal to the variable filter 70. Furthermore, the variable filter 70 can change the passband of the intermediate frequency signal according to the frequency of the intermediate frequency signal output from the mixer 60. For this reason, the receiving circuit 100 can extract a signal in a predetermined frequency band even when the frequency of the input signal changes. Moreover, since the variable filter 70 performs signal processing of the intermediate frequency signal, the processing load is reduced as compared to performing signal processing of the radio frequency signal.

  Next, a detailed configuration example of the wireless transmission / reception device 50 according to the first embodiment of the present invention will be described with reference to FIG. The wireless transmission / reception device 50 includes the reception circuit 100 described with reference to FIG. The wireless transmission / reception device 50 includes an output terminal (IF) 1, amplifiers 2, 5, 7, 8, 11, 13, 16, 19, 22, and 23, mixers 3, 6, 12, and 15, a filter 4, An output end (RF) 9, an input end (IF) 10, a variable filter 14, an input end (RF) 17, and VCOs 18, 20 and 21 are provided. The output terminal (IF) 1 and the amplifier 2 constitute a TX1stIF region. The filter 4 and the amplifier 5 constitute a TX2ndIF region. The amplifiers 7 and 8 and the output end (RF) 9 constitute a TXRF region. The input terminal (IF) 10 and the amplifier 11 constitute an RX2ndIF region. The amplifier 13 and the variable filter 14 constitute an RX1stIF region. The amplifier 16 and the input terminal (RF) 17 constitute an RXRF region.

  The TX1stIF region is a region for processing a signal having a fixed intermediate frequency (hereinafter referred to as TX1stIF) used in the wireless transmission / reception device 50. The TX2ndIF region is a region for processing a signal having a variable intermediate frequency (hereinafter referred to as TX2ndIF). The TXRF region is a region for processing a signal having a radio frequency (hereinafter referred to as TXRF) transmitted from the wireless transmission / reception device 50 to the opposite device. The RX2ndIF region is a region for processing a signal having a fixed intermediate frequency (hereinafter referred to as RX2ndIF) used in the wireless transmission / reception device 50. The RX1stIF region is a region for processing a signal having an intermediate frequency that is a variable value. The RXRF region is a region for processing a signal transmitted from the opposite device and set with a radio frequency (hereinafter referred to as RXRF).

  Here, TX1stIF and RX2ndIF are set as fixed values and are not changed. TXRF and RXRF change according to the value of the radio frequency used between the opposite devices. Alternatively, one of TXRF and RXRF may be a fixed value. The difference between TXRF and RXRF is the shift frequency. TX2ndIF and RX1stIF change according to the values of TXRF and RXRF.

  First, a configuration related to transmission processing will be described. The output terminal (IF) 1 is subjected to signal processing in the wireless transmission / reception device 50 and outputs a signal having a frequency of TX1stIF to the amplifier 2. The amplifier 2 is an amplifier corresponding to TX1stIF, and amplifies the signal output from the output terminal (IF) 1 to an arbitrary level.

  The VCO 20 generates a reference signal that is used to convert TX1stIF into TX2ndIF. The VCO 20 changes the frequency set in the reference signal according to the value of TX2ndIF. The VCO 20 outputs the generated reference signal to the amplifier 22. The amplifier 22 amplifies the output reference signal and outputs it to the mixer 3.

  The mixer 3 uses the reference signal output from the amplifier 22 to convert the frequency of the signal output from the amplifier 2 from TX1stIF to TX2ndIF. The mixer 3 outputs the frequency converted signal to the filter 4. When the frequency of the reference signal is Fref1, TX2ndIF is expressed as follows, for example.

  TX2ndIF = Fref1-TX1stIF (Equation 1)

  FIG. 3 shows the frequency conversion in the mixer 3. The desired wave is TX2ndIF, and the disturbing wave is a signal existing around TX2ndIF. By using the mixer 3, not only the desired wave but also the interference wave is frequency-converted. The interference wave is also referred to as an interference wave.

  The filter 4 is, for example, a bandpass filter (BPF), receives a signal output from the mixer 3 and passes the frequency band of TX2ndIF. As a result, in the signal output from the mixer 3, the nearby interference wave such as the adjacent channel of TX2ndIF can be removed. The amplifier 5 amplifies the signal that has passed through the filter 4. At this time, since the interference wave is removed from the signal that has passed through the filter 4, in the amplifier 5, the TX2ndIF signal is mainly amplified, and the interference wave is not greatly amplified.

  The VCO 18 generates a reference signal that is used to convert TX2ndIF into TXRF. The VCO 18 outputs the generated reference signal to the amplifier 19. The amplifier 19 amplifies the output reference signal and outputs it to the mixer 6.

  The mixer 6 uses the reference signal output from the amplifier 19 to convert the frequency of the signal output from the amplifier 5 from TX2ndIF to TXRF. The mixer 6 outputs the frequency converted signal to the amplifier 7. If the frequency of the reference signal is Fref2, TXRF is expressed as follows, for example.

  TXRF = TX2ndIF + Fref2 (Expression 2)

  The amplifiers 7 and 8 amplify the signal output from the mixer 6. The output terminal (RF) 9 transmits the signal amplified by the amplifier 8 to the opposite apparatus using wireless communication.

  Next, a configuration related to reception processing will be described. The input terminal (RF) 17 receives a signal transmitted from the opposite device. The signal transmitted from the opposite device has a radio frequency set, and the set radio frequency is RXRF. The radio frequency of the signal transmitted from the opposite device varies depending on the opposite device. In FIG. 2, the radio frequency of the signal output from the output terminal (RF) 9 is constant. Thus, the shift frequency indicating the frequency difference between the signal output from the output terminal (RF) 9 and the signal input to the input terminal (RF) 17 is the same as that in the signal input to the input terminal (RF) 17. It changes according to the change of frequency. The input terminal (RF) 17 outputs the received signal to the amplifier 16. The amplifier 16 amplifies the signal output from the input terminal (RF) 17 and outputs the amplified signal to the mixer 15.

  The VCO 18 generates a reference signal having a specific frequency and performs output to the amplifier 19 in order to perform frequency conversion in the mixer 6 and the mixer 15. The amplifier 19 amplifies the reference signal output from the VCO 18 and outputs it to the mixer 15. Here, since the radio frequency of the signal output from the output terminal (RF) 9 is constant, the frequency of the reference signal output from the VCO 18 is also specified or set to a constant value.

  The mixer 15 uses the reference signal output from the amplifier 19 to convert the frequency of the signal output from the amplifier 16 from RXRF to RX1stIF. As described above, since the shift frequency is changed and the frequency of the reference signal output from the VCO 18 is fixed, the RX1stIF is in accordance with the radio frequency of the input signal input to the input terminal (RF) 17. Change. The mixer 15 outputs the frequency converted signal to the variable filter 14. If the frequency of the reference signal is Fref2 as in Equation 2, RX1stIF is expressed as follows, for example.

  RX1stIF = RXRF−Fref2 (Formula 3)

  The variable filter 14 is, for example, a bandpass filter (BPF) capable of variably controlling the passband, and receives a signal from the mixer 15 and passes the frequency band of RX1stIF. Thereby, in the signal output from the mixer 15, interference in the vicinity of the adjacent channel of the RX1stIF can be removed. The amplifier 13 amplifies the signal that has passed through the variable filter 14. The amplifier 13 outputs the amplified signal to the mixer 12.

  The VCO 21 generates a reference signal for frequency conversion in the mixer 12. The reference signal is used to convert RX1stIF of the signal output from the amplifier 13 to the mixer 12 into RX2ndIF. RX2ndIF is a predetermined frequency in the apparatus. That is, RX2ndIF is a fixed value. Therefore, the VCO 21 changes the frequency set in the reference signal so as to convert the variable value RX1stIF into the fixed value RX2ndIF. The variable filter 14 performs passband change control in synchronization with the reference signal frequency change control in the VCO 21. A specific example in the passband change control will be described in detail later. The VCO 21 outputs the generated reference signal to the amplifier 23. The amplifier 23 amplifies the output reference signal and outputs it to the mixer 12.

  The mixer 12 uses the reference signal output from the amplifier 23 to convert the frequency of the signal output from the amplifier 13 from RX1stIF to RX2ndIF. The mixer 12 outputs the frequency converted signal to the amplifier 11. If the frequency of the reference signal output from the VCO 21 is Fref3, RX2ndIF is expressed as follows, for example.

  RX2ndIF = RX1stIF−Fref3 (Expression 4)

  The amplifier 11 amplifies the signal output from the mixer 12 and outputs the amplified signal to the input terminal (IF) 10.

  Next, a circuit configuration example for controlling the VCO 21 and the variable filter 14 will be described with reference to FIG. The output frequency of the VCO 21 is controlled using a PLL circuit including a phase comparator 31, a loop filter 32, and a frequency divider 33. The phase comparator 31 outputs a control voltage corresponding to the difference between the frequency of the input signal and the frequency of the reference signal. The loop filter 32 averages the waveform related to the control voltage output from the phase comparator 31 and outputs the averaged waveform to the VCO 21. The VCO 21 outputs a signal having a frequency determined according to the value of the received control voltage to the amplifier 23. The signal output from the VCO 21 is fed back to the phase comparator 31 via the frequency divider 33. Here, control is performed so that the frequency of the signal input to the phase comparator 31 is changed in accordance with the change in RXRF, that is, the change in shift frequency. Or you may control so that the frequency of the signal input into the phase comparator 31 may be changed according to the change of an intermediate frequency. For example, a table or the like in which the shift frequency value is associated with the frequency of the signal input to the phase comparator 31 may be stored in the memory. A control circuit or the like (not shown) may detect a change in shift frequency and determine a frequency of a signal output to the phase comparator 31 by referring to a table held in the memory.

  Here, the phase comparator 31 changes the output control voltage according to the change of the shift frequency. The loop filter 32 outputs the control voltage to the VCO 21 and also to the filter control circuit 34. The filter control circuit 34 changes the band passing through the variable filter 14 according to the control voltage. For example, the filter control circuit 34 may manage the control voltage and the pass band in the variable filter 14 in association with each other.

  In this way, by outputting the control voltage from the loop filter 32 to the VCO 21 and the filter control circuit 34, the reference signal output control in the VCO 21 and the passband change control in the variable filter 14 can be synchronized. .

  Next, an operation example in the wireless transmission / reception device 50 of FIG. 2 will be described. If RXRF is 1 GHz and the shift frequency is 100 MHz, TXRF is 0.9 GHz. If Fref2 is 500 MHz, RX1stIF is 500 MHz, and TX2ndIF is 400 MHz. Here, if the TXRF is not changed and the shift frequency is 200 MHz, RXRF becomes 1.1 GHz. Since TXRF does not change, Fref2 does not change. As a result, RX1stIF is shifted by 100 MHz to 600 MHz. RX2ndIF is predetermined in the apparatus. Therefore, Fref3 changes by 100 MHz, and the mixer 12 changes RX1stIF to RX2ndIF.

  Fref3 is controlled using a PLL circuit and is determined according to the voltage applied to the VCO 21. Similarly to the VCO 21, the variable filter 14 has a pass band determined according to the voltage applied to the filter control circuit 34 and the variable filter 14. The voltage applied to the VCO 21 and the filter control circuit 34 is determined according to the shift frequency or the value of the intermediate frequency of the signal output from the mixer 15.

  Subsequently, a flow of signal processing of the receiving circuit according to the first exemplary embodiment of the present invention will be described with reference to FIG. First, the mixer 15 converts RXRF of the input signal received via the input terminal (RF) 17 into RX1stIF (S11). Next, the control circuit in the receiving circuit 100 determines whether or not the shift frequency has changed (S12). When it is determined that the shift frequency has not changed, the control circuit executes the process of step S15. When the control circuit determines that the shift frequency has changed, the control voltage value is changed by changing the frequency of the signal output to the phase comparator 31, and the filter control circuit 34 passes through the variable filter 14. The bandwidth is changed (S13).

  Next, the VCO 21 changes the value of the reference signal Fref3 output to the mixer 12 via the amplifier 23 (S14). Fref3 is changed so that RX1stIF is converted to a fixed value of RX2ndIF. Next, the mixer 12 converts the RX1stIF of the input signal into RX2ndIF using the reference signal output from the amplifier 23 (S15).

  As described above, by using the receiving circuit according to the first embodiment of the present invention, the control for changing the reference signal Fref3 output from the VCO 21 and the control for changing the passband in the variable filter 14 are synchronized. Can do. As a result, even when the shift frequency changes based on the change in RXRF, the variable filter 14 can pass the predetermined RX1stIF, and the mixer 12 can change RX1stIF to RX2ndIF which is a fixed value. it can.

(Embodiment 2)
Next, a configuration example of the wireless transmission / reception device 110 according to the second exemplary embodiment of the present invention will be described with reference to FIG. 6 differs from the wireless transmission / reception device 50 in FIG. 2 in that the variable filter 114 is disposed in the TX2ndIF region and the filter 124 is disposed in the RX1stIF region. Another difference is that the mixer 113 and the variable filter 114 are controlled in synchronization with the signal output from the VCO 130. Furthermore, the VCO 130 and the variable filter 114 have the same configuration as that in FIG. An output terminal (IF) 111, an output terminal (RF) 119, an input terminal (IF) 120, an input terminal (RF) 127, and amplifiers 112, 115, 117, and 118, which are other configurations of the wireless transmission / reception device 110 in FIG. 121, 123, 126, 128, 132 and 133, the mixers 116, 122 and 125, and the VCOs 128 and 131 are the same as those of the wireless transmission / reception apparatus 50 in FIG.

  The VCO 130 generates a reference signal for frequency conversion in the mixer 113. The reference signal is used to convert TX1stIF of the signal output from the amplifier 112 to the mixer 113 into TX2ndIF. TX1stIF is a predetermined frequency in the apparatus. That is, TX1stIF is a fixed value. In the wireless transmission / reception apparatus of FIG. 6, the radio frequency RXRF of the signal input to the input terminal (RF) 127 is set to a fixed value. Accordingly, the frequency of the reference signal output from the VCO 128 also becomes a specific value. If the radio frequency TXRF of the signal output from the output terminal (RF) 119 is variable, TX2ndIF becomes a value that changes according to the value of TXRF. TX2ndIF is calculated using Equation 1 described above.

  Next, an operation example in the wireless transmission / reception apparatus 110 in FIG. 6 will be described. If RXRF is 1 GHz and the shift frequency is 100 MHz, TXRF is 0.9 GHz. If Fref2 is 500 MHz, RX1stIF is 500 MHz, and TX2ndIF is 400 MHz. Here, if RXRF is not changed and the shift frequency is 200 MHz, TXRF becomes 0.8 GHz. Since RXRF does not change, Fref2 does not change. Thereby, TX2ndIF is shifted by 100 MHz to 300 MHz. TX1stIF is predetermined in the apparatus. Therefore, the frequency Fref1 of the reference signal output from the VCO 130 changes by 100 MHz, and the mixer 113 changes Tx1stIF to TX2ndIF.

  Similar to the VCO 21 in FIG. 2, Fref1 is controlled using a PLL circuit and is determined according to the voltage applied to the VCO 130. Similarly to the VCO 130, the pass band of the variable filter 114 is determined according to the voltage applied to the variable filter 114.

  As described above, by using the radio transmission / reception device 110 according to the second exemplary embodiment of the present invention, the change control of the reference signal Fref1 output from the VCO 130 and the change control of the passband in the variable filter 114 are synchronized. can do. Thereby, even when the shift frequency changes based on the change in TXRF, the variable TX 114 can pass a predetermined TX2ndIF.

  Note that the present invention is not limited to the above-described embodiment, and can be changed as appropriate without departing from the spirit of the present invention. For example, the variable filters 14 and 114 may be configured using a plurality of BPFs as shown in FIG. A plurality of BPFs having different passbands may be arranged in parallel, and each BPF may be switched using the switches 151 and 153 to select an arbitrary BPF.

1, 111 Output terminal (IF)
2, 5, 7, 8, 11, 13, 16, 19, 22, 23, 112, 115, 117, 118, 121, 123, 126 Amplifier 3, 6, 12, 15, 113, 116, 122, 125 Mixer 4,124 Filter 9,119 Output terminal (RF)
10, 120 Input terminal (IF)
14, 114 Variable filter 17, 127 Input terminal (RF)
18, 20, 21, 128, 130, 131 VCO
31 phase comparator 32 loop filter 33 frequency divider 34 filter control circuit 50 wireless transmission / reception device 60 mixer 70 variable filter 100 reception circuit 151, 153 switch 152 BPF

Claims (9)

A first mixer that converts a radio frequency of an input signal input by radio communication into an intermediate frequency;
A first variable filter that passes a signal of a predetermined frequency band in the input signal converted to the intermediate frequency,
The first mixer is:
Changing an intermediate frequency of the input signal to be output to the first variable filter in accordance with a change in radio frequency of the input signal;
The first variable filter is:
A receiving circuit that changes a pass band of the input signal to be passed according to an intermediate frequency of the input signal output from the first mixer. A second mixer for converting an intermediate frequency of the input signal that has passed through the first variable filter into a predetermined fixed intermediate frequency;
An oscillator that determines a frequency of a reference signal output to the second mixer according to an intermediate frequency of the input signal output from the first mixer;
The first variable filter is:
The receiving circuit according to claim 1, wherein a pass band of the input signal to be passed is changed in accordance with a change in frequency of a reference signal output from the oscillator. The oscillator is
Determining a frequency of the reference signal by applying a voltage determined based on an intermediate frequency of the input signal;
The first variable filter is:
The receiving circuit according to claim 2, wherein a pass band of the input signal to be passed is changed based on a voltage applied to the oscillator. A third mixer for converting an intermediate frequency of an output signal output by wireless communication into a radio frequency;
A fourth mixer for converting an output signal having a predetermined fixed frequency into the intermediate frequency;
A second variable filter that passes a signal of a predetermined frequency band of the output signal converted to the intermediate frequency, and
The fourth mixer is
Changing the intermediate frequency of the output signal to be output to the second variable filter in accordance with a change in radio frequency of the output signal;
The second variable filter is:
A transmission circuit that changes a pass band of the output signal to be passed according to an intermediate frequency of the output signal output from the fourth mixer. An oscillator that determines a frequency of a reference signal to be output to the fourth mixer in accordance with a change in radio frequency of the output signal output from the third mixer;
The second variable filter is:
The transmission circuit according to claim 4, wherein a pass band of the output signal to be passed is changed in accordance with a change in a frequency of a reference signal output from the oscillator. The oscillator is
Determining a frequency of the reference signal by applying a voltage determined based on a radio frequency of the output signal;
The second variable filter is:
The receiving circuit according to claim 5, wherein a pass band of the output signal to be passed is determined based on a voltage applied to the oscillator. A transmission circuit for transmitting an output signal of a first radio frequency fixedly defined;
A receiving circuit that receives an input signal of a second radio frequency that changes according to the opposing device,
The receiving circuit is
A first mixer that converts the second radio frequency of the input signal input by wireless communication into an intermediate frequency;
A first variable filter that passes a signal of a predetermined frequency band of the input signal converted to the intermediate frequency,
The first mixer is:
Changing an intermediate frequency of the input signal to be output to the first variable filter in accordance with a change in radio frequency of the input signal;
The first variable filter is:
A radio transmission / reception circuit that changes a pass band of the input signal to be passed according to an intermediate frequency of the input signal output from the mixer. A receiving circuit for receiving an input signal having a fixed third radio frequency;
A transmission circuit that transmits an output signal of a fourth radio frequency that changes according to the opposing device,
The transmission circuit includes:
A third mixer for converting an intermediate frequency of an output signal output by wireless communication into the fourth wireless frequency;
A fourth mixer for converting an output signal having a predetermined fixed frequency into the intermediate frequency;
A second variable filter that passes a signal of a predetermined frequency band in the output signal converted to the intermediate frequency,
The fourth mixer is
Changing the intermediate frequency of the output signal to be output to the second variable filter in accordance with a change in radio frequency of the output signal;
The second variable filter is:
A radio transmission / reception circuit that changes a pass band of the output signal to be passed according to an intermediate frequency of the output signal output from the fourth mixer. Changing the intermediate frequency of the input signal to be output according to the change of the radio frequency of the input signal input by wireless communication;
In response to the change in the intermediate frequency of the input signal, the pass band of the variable filter is changed,
A frequency conversion method for passing the input signal based on a pass band of the variable filter.

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