JP2014175877A - Automatic gain control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automatic gain control circuit for reducing the attenuation of the voltage of a desired signal while suppressing a change in input impedance.SOLUTION: A variable attenuation circuit 13 attenuates the voltage of a radio signal by a variable degree of attenuation. A detection circuit 14 detects a voltage-attenuated radio signal. A demodulation circuit 15 demodulates the detected radio signal. A control circuit 16 controls the attenuation degree of the variable attenuation circuit 13 on the basis of a first voltage of the demodulated radio signal and a second voltage of the detected radio signal. The variable attenuation circuit 13 is configured so that multiple types of attenuation circuit can be realized. Furthermore, the control circuit 16 selects either of the multiple types of attenuation circuit on the basis of the first and second voltages, exerts control so that other types of attenuation circuit than the selected type of attenuation circuit are turned off.

Description

  The present invention relates to an automatic gain control technique, and more particularly, to an automatic gain control circuit that controls an attenuation level when a received radio signal is attenuated.

  As receiving antennas for in-vehicle radios in recent years, antennas that are relatively inconspicuous, such as a glass antenna provided in a glass window and a small pole antenna provided on a top plate of a vehicle body, are often used. These antennas are small in size compared with rod antennas having a length of about ¼ wavelength at the FM (Frequency Modulation) radio band frequencies that have been used so far. In combination with to compensate for the gain. Since the antenna amplifier is installed directly under the antenna and is electrically between the antenna and the radio receiver, the antenna amplifier is affected by the impedance load fluctuation of the radio receiver.

  An automatic gain control (AGC) circuit of a radio receiver such as a radio receiver has a front end (a circuit configuration of the radio receiver) in order to prevent subsequent circuits (such as a demodulation circuit) from being distorted by an excessive input signal. The circuit block that is electrically closest to the antenna) is used to control the attenuation of the variable attenuation circuit so that radio signals of a certain voltage or higher do not enter the subsequent circuit. After the received radio signal is attenuated, the radio signal is sent to the subsequent circuit. When the variable attenuating circuit operates under the control of an automatic gain control circuit (hereinafter referred to as “AGC circuit”), the input impedance (impedance seen from the antenna and antenna amplifier side) of the radio receiver changes. The impedance load fluctuates and the distortion characteristics of the antenna amplifier deteriorate. In contrast, the output impedance of the variable attenuation circuit used in the AGC circuit is kept constant (see, for example, Patent Document 1), or a variable matching circuit is provided to optimally set the output impedance of the amplifier circuit. (For example, see Patent Document 2).

JP 2002-151993 A JP-A-2005-45440

  However, in the conventional AGC circuit, since the frequency characteristic of the attenuation of the variable attenuation circuit is flat (constant with respect to the frequency), the desired signal (frequency at which the radio receiver is tuned to demodulate voice or the like) When the AGC circuit is operated by an interference signal having a stronger voltage (unnecessary signal having a frequency different from that of the desired signal), the voltage of the desired signal is also attenuated, so that the intelligibility of the output sound is deteriorated. Further, in the conventional amplifier circuit, since the desired signal is assumed to have higher harmonics (frequency of 2 times, 3 times, etc.) as the disturbing signal, the disturbing signal is a signal of another channel in the band of FM radio broadcasting. In this case, since the frequency difference between the desired signal and the interference signal is small, the effect of removing the interference signal and maintaining the output impedance of the desired signal constant cannot be obtained.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an automatic gain control circuit that reduces the attenuation of the voltage of a desired signal while suppressing a change in input impedance.

  In order to solve the above-described problem, an automatic gain control circuit according to an aspect of the present invention includes a variable attenuation unit that attenuates a voltage of a radio signal by a variable attenuation degree, and a radio signal that is attenuated by the variable attenuation unit. Based on the detection unit for detection, the demodulation unit for demodulating the radio signal detected by the detection unit, the first voltage of the radio signal demodulated by the demodulation unit, and the second voltage of the radio signal detected by the detection unit, And a control unit that controls the attenuation of the variable attenuation unit. The variable attenuation unit is configured to be capable of realizing a plurality of types of attenuation circuits, and the control unit selects one of the plurality of types of attenuation circuits based on the first voltage and the second voltage, Control is made to stop the type of attenuation circuit other than the selected type of attenuation circuit.

  It should be noted that any combination of the above-described constituent elements and a conversion of the expression of the present invention between a method, an apparatus, a system, a recording medium, a computer program, etc. are also effective as an aspect of the present invention.

  According to the present invention, it is possible to reduce the attenuation of the voltage of a desired signal while suppressing a change in input impedance.

It is a figure which shows the structure of the automatic gain control circuit which concerns on the Example of this invention. 2A to 2C are diagrams showing an equivalent circuit for the variable attenuation circuit of FIG. 3 is a flowchart showing an operation procedure of the automatic gain control circuit of FIG. 1. It is a figure which shows the spectrum of the radio signal of the 1st example for demonstrating circuit type selection of the variable attenuation circuit of FIG. FIGS. 5A to 5C are diagrams illustrating frequency characteristics of the attenuation of the variable attenuation circuit when the radio signal of the first case illustrated in FIG. 4 is input. 6A to 6C are diagrams illustrating the spectrum of a signal that has passed through the variable attenuation circuit when the radio signal of the first case illustrated in FIG. 4 is input. It is a figure which shows the spectrum of the radio signal of the 2nd example for demonstrating circuit type selection of the variable attenuation circuit of FIG. FIGS. 8A to 8C are diagrams illustrating frequency characteristics of the attenuation of the variable attenuation circuit when the radio signal of the second case illustrated in FIG. 7 is input. 9A to 9C are diagrams illustrating the spectrum of a signal that has passed through the variable attenuation circuit when the radio signal of the second case illustrated in FIG. 7 is input. It is a figure which shows an attenuation degree table when a flat type attenuation circuit is selected as the variable attenuation circuit of FIG. FIGS. 11A to 11C are diagrams showing circuit simulation results of the frequency characteristics of the attenuation for the variable attenuation circuit of FIG. 12A to 12C are diagrams showing circuit simulation results of frequency characteristics of VSWR for the variable attenuation circuit of FIG. It is a figure which shows an attenuation degree table when the LPF type attenuation circuit is selected as the variable attenuation circuit of FIG. It is a figure which shows an attenuation degree table when the HPF type attenuation circuit is selected as the variable attenuation circuit of FIG. It is a figure which shows the table with respect to the variable attenuation circuit of the comparative example from which the HPF type attenuation circuit of this invention and an equivalent attenuation degree are obtained. FIGS. 16A and 16B are diagrams showing a circuit simulation result of the frequency characteristics of the VSWR and an equivalent circuit for the comparative example of this embodiment.

  Before specifically describing the embodiment of the present invention, an outline of the present embodiment will be described. Generally, in a radio receiver, a radio signal received by an antenna includes a desired signal that is a frequency that is demodulated by a demodulation circuit and a disturbing signal that is an unnecessary signal having a frequency other than the desired signal. It is included. The automatic gain control circuit using a variable attenuation circuit provided at the front end of the radio receiver operates in the entire frequency band of the signal received and used by the radio receiver. When the signal voltage is high, automatic gain control is performed on the voltage of the interference signal. For this reason, even if the voltage of the desired signal is low, the voltage of the desired signal may be attenuated because the automatic gain control circuit is operated by the interference signal.

  Furthermore, the distortion characteristics of the antenna amplifier may deteriorate due to fluctuations in the impedance load of the antenna amplifier. For example, when the intermodulation distortion signal generated by the interference signal has the same frequency as the desired signal, the intermodulation distortion characteristic of the antenna amplifier has deteriorated, so the voltage of the intermodulation distortion signal increases and exceeds the voltage of the desired signal. May be input to the radio receiver. When the intermodulation distortion signal included in the output signal of the antenna amplifier is larger in voltage than the desired signal, it is very difficult to separate the intermodulation distortion signal from the desired signal and demodulate only the desired signal on the radio receiver side. There will be interference. Interference is a condition in which the clarity of the broadcast signal of the desired signal has deteriorated due to the sound of the broadcast signal of the desired signal being heard at the same time as the sound of the other station, or the harsh noise sound. The user is uncomfortable.

  For this reason, in this embodiment, the impedance (input impedance) on the antenna side is kept constant, and the attenuation on the low frequency side of the variable attenuation circuit is increased or the high frequency according to the frequency relationship between the desired signal and the interference signal. Switching between increasing the attenuation on the side and making the attenuation constant with respect to the frequency. As a result, deterioration of the distortion characteristics of the antenna amplifier is prevented. Further, the degree of attenuation of the voltage of the desired signal is reduced, and the deterioration of the clarity of the output sound is reduced.

  FIG. 1 shows a configuration of an automatic gain control circuit 10 according to an embodiment of the present invention. In FIG. 1, after a voltage of a radio signal received by an external antenna 11 (hereinafter simply referred to as a signal means a radio signal) is amplified via an antenna amplifier 12, the automatic gain control circuit 10 of the present invention is used. Signal is input.

  A variable attenuation circuit 13 is provided at the front end (most antenna side) of the automatic gain control circuit 10. The signal input to the automatic gain control circuit 10 is first input to the variable attenuation circuit 13. The input signal is a radio signal. The variable attenuation circuit 13 attenuates the voltage of the input signal with a variable attenuation. The variable attenuation circuit 13 outputs the attenuated signal to the detection circuit 14. The detection circuit 14 detects the signal whose voltage has been attenuated by the variable attenuation circuit 13. The detection circuit 14 outputs the detected signal to the demodulation circuit 15. Here, the detection circuit 14 performs wideband detection of a radio signal (including a desired signal and an interference signal) that is an input signal, and outputs a wideband signal voltage Vw that is a detection result to the control circuit 16.

  An external output terminal such as a speaker 17 is connected to the demodulation circuit 15. The demodulation circuit 15 extracts only the desired signal from the input signal, detects the voltage of the desired signal, and outputs the desired signal voltage Vn to the control circuit 16. Further, the demodulation circuit 15 obtains an audio signal by demodulating the desired signal and outputs it to an external output terminal such as the speaker 17. The demodulating circuit 15 includes a low noise amplifier, a mixer, a band limiting filter, and the like, and finally tunes from a radio signal to a desired signal and obtains an audio signal by demodulation processing. When the radio signal is FM radio broadcast, a frequency discrimination circuit or the like can be used as an FM signal demodulation circuit.

  A high-frequency circuit of a radio receiver such as an FM radio receiver is usually designed with a specific input / output impedance (referred to as a reference impedance). For FM radio, the reference impedance of the circuit is 75Ω. Therefore, the output impedance of the antenna amplifier 12 should be 75Ω, and the input impedance of the automatic gain control circuit 10 (that is, the receiver) should be 75Ω. Conventionally, it is well known that the impedance changes when the variable attenuating circuit 13 operates. Therefore, the impedance at a weak electric field input where the variable attenuating circuit 13 does not operate is designed to be the reference impedance. That is, the impedance of the detection circuit 14 (including the demodulation circuit 15) viewed from the variable attenuation circuit 13 is designed with the reference impedance. At this time, under the condition that the variable attenuation circuit 13 does not operate, the input impedance of the automatic gain control circuit 10 becomes equal to the reference impedance, so that impedance matching with the antenna amplifier 12 is achieved.

  The control circuit 16 includes each circuit element constituting the variable attenuating circuit 13 from the wideband signal voltage Vw that is the detection result of the wideband radio signal in the detection circuit 14 and the desired signal voltage Vn that is the detection result of the desired signal in the demodulation circuit 15. The control signal to be output to is determined. This control signal is a signal for controlling the degree of attenuation of the variable attenuation circuit 13. The control circuit 16 outputs a control signal to the variable attenuation circuit 13. As described above, the variable attenuation circuit 13 attenuates the voltage of the radio signal input from the antenna amplifier 12 and outputs the attenuated voltage to the detection circuit 14. The attenuation at that time is set by a control signal from the control circuit 16.

  The variable attenuation circuit 13 is composed of three variable resistance elements and two variable capacitance elements. When viewed from the antenna amplifier 12 side, first, there is a first variable resistance element VR21 connected between the signal line of the radio signal in the variable attenuation circuit 13 and the ground. Next, the second variable resistance element VR22 inserted in series in the signal line of the radio signal between the connection point of the first variable resistance element VR21 and the signal line of the radio signal and the detection circuit 14, and the second variable resistance element VR22 There is a first variable capacitance element VC23 arranged in parallel with the resistance element VR22. Further, a third variable resistance element VR24 inserted between the signal line of the radio signal between the second variable resistance element VR22 and the detection circuit 14 and the ground, and a third variable resistance element VR24 arranged in parallel. There are two variable capacitance elements VC25. Finally, a terminal to which the second variable resistance element VR22 and the third variable resistance element VR24 are connected is an output terminal of the variable attenuation circuit 13, and is connected to the detection circuit 14.

  That is, the variable attenuation circuit 13 is arranged in parallel with the π-type circuit including the first variable resistance element VR21, the second variable resistance element VR22, and the third variable resistance element VR24, and the second variable resistance element VR22. The first variable capacitance element VC23 and the second variable capacitance element VC25 arranged in parallel with the third variable resistance element VR24 connected between the detection circuit side and the ground. The first variable resistance element VR21, the second variable resistance element VR22, and the third variable resistance element VR24 control resistance values by a control signal (or control current, which will be simply referred to as a control signal hereinafter) applied from the outside. It is a circuit element that can. For example, a PIN diode or a transistor can be used. The first variable capacitance element VC23 and the second variable capacitance element VC25 are circuit elements whose capacitance values can be controlled by a control signal applied from the outside. For example, a variable capacitance diode can be used.

  The control circuit 16 determines a control signal to be output to each circuit element constituting the variable attenuation circuit 13 according to the circuit type of the frequency characteristic of the attenuation of the variable attenuation circuit 13 and the required attenuation. The variable attenuation circuit 13 of the automatic gain control circuit 10 of the present embodiment can switch the frequency characteristic of the attenuation degree, as will be described in detail later. That is, the variable attenuation circuit 13 is configured to be able to realize a plurality of types of attenuation circuits. The control circuit 16 has a circuit type (flat type) in which the attenuation is constant with respect to the frequency according to the relationship between the frequency of the desired signal and the interference signal, and a circuit type (LPF (Low Pass) in which the degree of attenuation increases as the frequency increases. Filter (low pass filter) type), circuit type (HPF (High Pass Filter) type) in which the attenuation becomes larger as the frequency becomes lower, and the circuit type with the highest desired signal voltage Vn is selected. The control signal is output to each circuit element of the variable attenuation circuit 13.

  In addition, the control circuit 16 has an “attenuation degree table” for constants set for each circuit element of the variable attenuation circuit so that the variable attenuation circuit 13 realizes a predetermined attenuation for each circuit type of the variable attenuation circuit 13. have. Based on the attenuation table, the control circuit 16 controls the attenuation of the variable attenuation circuit 13 so that the broadband signal voltage Vw output from the detection circuit 14 does not exceed the threshold voltage Vth for starting AGC control. The control signal to be output to each circuit element of the variable attenuation circuit 13 is determined. Here, the configuration of the three types of attenuation circuits in the variable attenuation circuit 13 and the control of the control circuit 16 for them will be specifically described. The variable attenuation circuit 13 switches between three circuit types having different frequency characteristics of attenuation depending on the relationship between the frequency of the desired signal and the interference signal. FIG. 2 shows equivalent circuit diagrams of three circuit types having different frequency characteristics of attenuation of the variable attenuation circuit 13.

  2A to 2C show an equivalent circuit for the variable attenuation circuit 13 of FIG. FIG. 2A is an equivalent circuit diagram of a circuit type (flat type) attenuation circuit in which the degree of attenuation is constant with respect to frequency. When the variable attenuation circuit 13 is a flat attenuation circuit, the resistance values of the second variable resistance element VR22 connected in series to the signal line and the third variable resistance element VR24 connected to the ground on the detection circuit 14 side are set. The voltage of the radio signal input from the antenna amplifier 12 is attenuated and output to the detection circuit 14. At this time, the first variable resistance element VR21, the first variable capacitance element VC23, and the second variable capacitance element VC25, which are not shown in FIG. 2A, are very much in comparison with the impedance viewed from the detection circuit 14 side. Set the impedance so that it can be ignored. As described above, the impedance of the detection circuit 14 is designed to be 75Ω which is the reference impedance in the case of the FM radio receiver. Therefore, for example, the first variable resistance element VR21 is set to 10 kΩ, the first variable capacity element VC23 and the second variable capacity element VC25 are set to 0.1 pF (p is 10 −12 to 15 kΩ at 100 MHz). Equivalent).

  FIG. 2B is an equivalent circuit diagram of a circuit type (LPF type) attenuation circuit in which the degree of attenuation increases as the frequency increases. When the variable attenuation circuit 13 is an LPF type attenuation circuit, the resistance value of the second variable resistance element VR22 connected in series to the signal line and the second variable capacitance element VC25 connected to the ground on the detection circuit 14 side are connected. By adjusting the capacitance value, the voltage of the radio signal input from the antenna amplifier 12 is attenuated and output to the detection circuit 14. At this time, the first variable resistance element VR21, the first variable capacitance element VC23, and the third variable resistance element VR24, which are not shown in FIG. 2B, are very much in comparison with the impedance viewed from the detection circuit 14 side. Set the impedance so that it can be ignored. As described above, the impedance of the detection circuit 14 is designed to be 75Ω which is the reference impedance in the case of the FM radio receiver. Therefore, for example, the first variable resistance element VR21 and the third variable resistance element VR24 may be set to 10 kΩ, and the first variable capacitance element VC23 may be set to 0.1 pF (corresponding to 15 kΩ at 100 MHz). .

  FIG. 2C is an equivalent circuit diagram of a circuit type (HPF type) attenuation circuit in which the degree of attenuation increases as the frequency decreases. When the variable attenuation circuit 13 is an HPF type attenuation circuit, the resistance value of the first variable resistance element VR21 connected to the ground on the antenna amplifier 12 side and the first variable capacitance element VC23 connected in series to the signal line are connected. The capacitance value and the resistance value of the third variable resistance element VR24 connected to the ground on the detection circuit 14 side are adjusted to attenuate the voltage of the radio signal input from the antenna amplifier 12 and output it to the detection circuit 14. . At this time, the second variable resistance element VR22 and the second variable capacitance element VC25, which are not shown in FIG. 2C, have very large impedance compared to the impedance viewed from the detection circuit 14 side, and can be ignored. Set. As described above, the impedance of the detection circuit 14 is designed to be 75Ω which is the reference impedance in the case of the FM radio receiver. Therefore, for example, the second variable resistance element VR22 may be set to 10 kiloΩ, and the second variable capacitance element VC25 may be set to 0.1 pF (corresponding to 15 kiloΩ at 100 MHz).

  In summary, the control circuit 16 selects one of a plurality of types of attenuation circuits based on the wideband signal voltage Vw and the desired signal voltage Vn, and selects a type other than the selected type of attenuation circuit. Control the attenuation circuit to stop. In particular, the control circuit 16 selects one of the plurality of types of attenuation circuits based on the desired signal voltage Vn for each of the plurality of types of attenuation circuits.

  In addition, when the flat-type attenuation circuit is selected, the detection circuit 14 is higher than the impedance of the detection unit with respect to the first variable resistance element VR21, the first variable capacitance element VC23, and the second variable capacitance element VC25. The impedance is set, and the resistance values of the second variable resistance element VR22 and the third variable resistance element VR24 are controlled. When the LPF type attenuation circuit is selected, the detection circuit 14 has an impedance higher than that of the detection unit for the first variable resistance element VR21, the first variable capacitance element VC23, and the third variable resistance element VR24. The resistance value of the second variable resistance element VR22 and the capacitance value of the second variable capacitance element VC25 are controlled by setting. When selecting the HPF type attenuation circuit, the detection circuit 14 sets an impedance higher than that of the detection unit for the second variable resistance element VR22 and the second variable capacitance element VC25, and the first variable resistance element The resistance value of VR21, the capacitance value of first variable capacitance element VC23, and the resistance value of third variable resistance element VR24 are controlled.

  Further, when the detection circuit 14 operates as a flat type attenuation circuit and attenuates a radio signal to a predetermined attenuation or more, the detection circuit 14 sets a value close to the impedance of the detection unit as the resistance value of the second variable resistance element VR22. Set. The detection circuit 14 sets a value close to the impedance of the detection unit as the resistance value of the second variable resistance element VR22 when the radio signal is attenuated to a predetermined attenuation or higher while operating as an LPF type attenuation circuit. . The detection circuit 14 sets a value close to the impedance of the detection unit as the resistance value of the first variable resistance element VR21 when the radio signal is attenuated to a predetermined attenuation level or more while operating as an HPF type attenuation circuit. .

  This configuration can be realized in terms of hardware by a CPU, memory, or other LSI of any computer, and in terms of software, it can be realized by a program loaded in the memory, but here it is realized by their cooperation. Draw functional blocks. Accordingly, those skilled in the art will understand that these functional blocks can be realized in various forms only by hardware, or by a combination of hardware and software.

  The processing operation of the automatic gain control circuit 10 configured as described above will be described below. FIG. 3 is a flowchart showing an operation procedure of the automatic gain control circuit 10. First, when the user turns on the automatic gain control circuit 10 (or a radio receiver such as a radio receiver equipped with the automatic gain control circuit 10), the automatic gain control circuit 10 enters a reception mode (step S41). Next, the detection circuit 14 receives the antenna 11 and amplifies it by the antenna amplifier 12, and then measures the wideband signal voltage Vw of the radio signal input to the automatic gain control circuit 10 (step S42). For example, in the case of FM radio broadcasting, the voltage of the maximum voltage is measured as the broadband signal voltage Vw within the frequency band 76 to 90 MHz of FM radio broadcasting in Japan. The detection circuit 14 sends the broadband signal voltage Vw to the control circuit 16. The control circuit 16 compares the wideband signal voltage Vw measured by the detection circuit 14 with a predetermined threshold voltage Vth (step S43). The threshold voltage Vth is a voltage at which the AGC control of the automatic gain control circuit 10 (that is, the attenuation control of the variable attenuation circuit 13) starts to operate. When the wideband signal voltage Vw is smaller than the threshold voltage Vth (No In the case of determination), the process returns to step S42. When the broadband signal voltage Vw is greater than the threshold voltage Vth (in the case of Yes determination), the process proceeds to step S44.

  The control circuit 16 performs AGC control by sequentially setting the circuit type of the variable attenuation circuit 13 to each of a flat type attenuation circuit, an LPF type attenuation circuit, and an HPF type attenuation circuit in a short time. At this time, every time the circuit type of the variable attenuation circuit 13 is switched, the desired signal voltage Vn detected by the demodulation circuit 15 is recorded by the control circuit 16 (step S44). Specifically, the control circuit 16 first obtains an attenuation degree D (= Vw−Vth) necessary for the variable attenuation circuit 13 from the difference between the wideband signal voltage Vw and the threshold voltage Vth (for example, the wideband signal voltage Vw = 100 dBμV). (Μ is the power of 10 −6), and the threshold voltage Vth = 80 dBμV, the attenuation D = 100−80 = 20 dB. Hereinafter, the numerical values of the voltage and the attenuation are expressed in dB. Next, the control circuit 16 determines the constants of each circuit element of the variable attenuation circuit 13 when the variable attenuation circuit 13 is used as a flat attenuation circuit from the attenuation table (flat attenuation table) of the flat attenuation circuit. Then, a control signal is sent to set the constant of each circuit element of the variable attenuation circuit 13. The control circuit 16 confirms that the broadband signal voltage Vw measured by the detection circuit 14 has dropped to the threshold voltage Vth after the operation of the variable attenuation circuit 13, and then the desired signal measured by the demodulation circuit 15. The voltage Vn is received, and the flat type attenuation circuit is recorded as the desired signal voltage Vn (F) when selected.

  Next, the control circuit 16 determines the constants of the circuit elements of the variable attenuation circuit 13 when the variable attenuation circuit 13 is used as an LPF type attenuation circuit from the attenuation table (LPF type attenuation table) of the LPF type attenuation circuit. Then, a control signal is sent to each circuit element to set the constant of each circuit element of the variable attenuation circuit 13. The control circuit 16 confirms that the broadband signal voltage Vw measured by the detection circuit 14 has dropped to the threshold voltage Vth. When the control circuit 16 determines that the wideband signal voltage Vw measured by the detection circuit 14 is too large or too small compared to the threshold voltage Vth, the circuit of the variable attenuation circuit 13 based on the LPF type attenuation table. Readjust the element constants. After determining that the broadband signal voltage Vw is an appropriate value for the threshold voltage Vth, the control circuit 16 receives the voltage Vn of the desired signal measured by the demodulation circuit 15 and selects the LPF type attenuation circuit. Recorded as desired signal voltage Vn (L).

  Similarly, the control circuit 16 obtains the constant of each element of the variable attenuation circuit 13 when the variable attenuation circuit 13 is used as the HPF type attenuation circuit from the attenuation table (HPF type attenuation table) of the HPF type attenuation circuit. A control signal is sent to set the constant of each circuit element of the variable attenuation circuit 13. The control circuit 16 confirms that the broadband signal voltage Vw measured by the detection circuit 14 has dropped to the threshold voltage Vth. If the control circuit 16 determines that the wideband signal voltage Vw measured by the detection circuit 14 is too large or too small compared to the threshold voltage Vth, the circuit of the variable attenuation circuit 13 based on the HPF type attenuation table. Readjust the element constants. After determining that the broadband signal voltage Vw is an appropriate value for the threshold voltage Vth, the control circuit 16 receives the voltage Vn of the desired signal measured by the demodulation circuit 15 and selects the HPF type attenuation circuit. Recorded as desired signal voltage Vn (H).

  Next, the control circuit 16 measures the desired signal voltage Vn (F) when selecting the flat type attenuation circuit measured at step S44, the desired signal voltage Vn (L) when selecting the LPF type attenuation circuit, and when selecting the HPF type attenuation circuit. The desired signal voltage Vn (H) is compared (steps S45 and S46). The control circuit 16 determines that the desired signal voltage Vn (L) when the LPF type attenuation circuit is selected is larger than the desired signal voltage Vn (F) when the flat type attenuation circuit is selected, and at the same time, the desired signal voltage when the LPF type attenuation circuit is selected. When Vn (L) is larger than the desired signal voltage Vn (H) when the HPF type attenuation circuit is selected (Yes in step S45), the LPF type attenuation circuit is selected as the circuit type of the variable attenuation circuit 13 (step S45). S47). The control circuit 16 sends a control signal for realizing the attenuation amount D required by the variable attenuation circuit 13 to each circuit element of the variable attenuation circuit 13 based on the LPF type attenuation degree table. Hereinafter, the control circuit 16 variably attenuates the broadband signal voltage Vw so as not to exceed the threshold voltage Vth based on the LPF type attenuation table while referring to the broadband signal voltage Vw measured by the detection circuit 14 as needed. The constant of each circuit element of the circuit 13 is controlled.

  The control circuit 16 determines that the desired signal voltage Vn (H) when the HPF type attenuation circuit is selected is larger than the desired signal voltage Vn (F) when the flat type attenuation circuit is selected, and at the same time, the desired signal voltage when the HPF type attenuation circuit is selected. If Vn (H) is greater than the desired signal voltage Vn (L) when the LPF type attenuation circuit is selected (Yes in step S46), the HPF type attenuation circuit is selected as the circuit type of the variable attenuation circuit 13 (step S46). S48). The control circuit 16 sends a control signal for realizing the attenuation amount D required by the variable attenuation circuit 13 to each circuit element of the variable attenuation circuit 13 based on the HPF type attenuation degree table. Hereinafter, the control circuit 16 variably attenuates the broadband signal voltage Vw so as not to exceed the threshold voltage Vth based on the HPF type attenuation table while referring to the broadband signal voltage Vw measured by the detection circuit 14 as needed. The constant of each circuit element of the circuit 13 is controlled.

  The control circuit 16 determines the desired signal voltage Vn (F) when the flat type attenuation circuit is selected, the desired signal voltage Vn (L) when the LPF type attenuation circuit is selected, and the desired signal voltage Vn (H) when the HPF type attenuation circuit is selected. As a result of comparing the magnitudes, when either the desired signal voltage Vn (L) when the LPF type attenuation circuit is selected or the desired signal voltage Vn (H) when the HPF type attenuation circuit is selected does not become the maximum voltage (No in step S46). In the case of determination), a flat attenuation circuit is selected as the circuit type of the variable attenuation circuit 13 (step S49). The control circuit 16 sends a control signal for realizing the attenuation D required by the variable attenuation circuit 13 to each circuit element of the variable attenuation circuit 13 based on the flat attenuation table. Thereafter, the control circuit 16 variably attenuates the broadband signal voltage Vw so as not to exceed the threshold voltage Vth based on the flat attenuation table while referring to the broadband signal voltage Vw measured by the detection circuit 14 as needed. The constant of each element of the circuit 13 is controlled. The control circuit 16 performs AGC control by fixing the circuit type of the variable attenuation circuit 13 to the selected circuit type (step S50). Although not described in FIG. 3, if the wideband signal voltage Vw falls below the threshold voltage Vth during the AGC control in step S50, the AGC control is terminated. The control circuit 16 returns to step S44 after the elapse of a predetermined time, and again determines the optimum circuit type of the variable attenuation circuit 13 (step S51).

  Here, two examples of circuit type selection of the variable attenuation circuit 13 are shown. FIG. 4 shows the spectrum of the radio signal of the first case for explaining the circuit type selection of the variable attenuation circuit 13. That is, this shows the spectrum of the radio signal at the time of input to the automatic gain control circuit 10, that is, before passing through the variable attenuation circuit 13. Here, in the first case, it is indicated that the radio signal input to the automatic gain control circuit 10 includes two signals of a desired signal (frequency fd) and an interference signal (frequency fu). In the first case, the frequency fd of the desired signal is lower than the frequency fu of the disturbing signal, and the voltage Vd of the desired signal is lower than the voltage Vu of the disturbing signal. The voltage Vu of the disturbing signal is higher than the threshold voltage Vth of the AGC control of the automatic gain control circuit 10 by the voltage difference D. The detection circuit 14 of the automatic gain control circuit 10 detects a radio signal and measures the wideband signal voltage Vw. Since the wideband signal voltage Vw is a voltage having the maximum frequency among the signals of the entire frequency band used in the automatic gain control circuit 10, the wideband signal voltage Vw is equal to the voltage Vu of the disturbing signal (Vw). = Vu).

  FIGS. 5A to 5C show the frequency characteristics of the attenuation of the variable attenuation circuit 13 when the radio signal of the first case shown in FIG. 4 is input. More specifically, FIG. 5A is a schematic diagram of the frequency characteristics of the attenuation when a flat attenuation circuit is selected as the variable attenuation circuit 13 when the radio signal of the first case shown in FIG. 4 is input. FIG. 5B is a schematic diagram of the frequency characteristics of the attenuation when the LPF type attenuation circuit is selected as the variable attenuation circuit 13, and FIG. 5C is the case where the HPF type attenuation circuit is selected as the variable attenuation circuit 13. It is a schematic diagram of the frequency characteristic of the degree of attenuation.

  The variable attenuation circuit 13 needs to have an attenuation D at the frequency fu of the interference signal because the wideband signal voltage Vw measured by the detection circuit 14 is higher than the threshold voltage Vth by the voltage difference D. When a flat type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 5A, since the attenuation is constant with respect to the frequency, the attenuation of the frequency fd of the desired signal is also the attenuation D. When the LPF type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 5B, the higher the frequency, the larger the attenuation, so the attenuation of the frequency fd of the desired signal is greater than the attenuation D of the disturbing signal. A small attenuation D (L) is obtained. When the HPF type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 5 (c), the lower the frequency, the larger the attenuation, so the attenuation of the desired signal frequency fd is greater than the attenuation D of the disturbing signal. A large degree of attenuation D (H) is obtained. From the above, regarding the attenuation level of the desired signal, the attenuation level D (L) when the LPF type attenuation circuit is selected <the attenuation level D when selecting the flat type attenuation circuit << the attenuation level D (H when selecting the HPF type attenuation circuit) )

  FIGS. 6A to 6C show the spectrum of a signal that has passed through the variable attenuation circuit 13 when the radio signal of the first case shown in FIG. 4 is input. More specifically, FIG. 6A passes through the variable attenuation circuit 13 when the flat attenuation circuit is selected as the variable attenuation circuit 13 when the radio signal of the first case shown in FIG. 4 is input. 6B shows the spectrum of the signal that has passed through the variable attenuation circuit 13 when the LPF type attenuation circuit is selected as the variable attenuation circuit 13, and FIG. 6C shows the HPF as the variable attenuation circuit 13. This is a spectrum of a signal that has passed through the variable attenuation circuit 13 when the type attenuation circuit is selected.

  When a flat attenuation circuit is selected as the variable attenuation circuit 13, the voltage Vn (F) of the desired signal measured by the demodulation circuit 15 is input to the variable attenuation circuit 13 before being input, as shown in FIG. The desired signal voltage Vn (F) = Vd−D is obtained by subtracting the degree of attenuation D of the variable attenuation circuit 13 from the voltage Vd of the desired signal. When the LPF type attenuation circuit is selected as the variable attenuation circuit 13, the voltage Vn (L) of the desired signal measured by the demodulation circuit 15 is the desired value before the input of the variable attenuation circuit 13 as shown in FIG. The desired signal voltage Vn (L) = Vd−D (L) is obtained by subtracting the attenuation degree D (L) of the variable attenuation circuit 13 from the signal voltage Vd. When the HPF type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 6C, the desired signal voltage Vn (H) measured by the demodulation circuit 15 is the desired value before the input of the variable attenuation circuit 13. The desired signal voltage Vn (H) = Vd−D (H) is obtained by subtracting the attenuation degree D (H) of the variable attenuation circuit 13 from the signal voltage Vd. From the above, for the desired signal voltage detected by the demodulation circuit 15, the desired signal voltage Vn (L) when the LPF type attenuation circuit is selected> the desired signal voltage Vn (F) when the flat type attenuation circuit is selected> This is the desired signal voltage Vn (H) when the HPF type attenuation circuit is selected.

  Therefore, when the LPF type attenuation circuit is selected, the desired signal voltage Vn detected by the demodulation circuit 15 is maximized. Therefore, the control circuit 16 selects the LPF type attenuation circuit as the variable attenuation circuit 13 and performs AGC control. I do. In the conventional technique, since the attenuation of the variable attenuation circuit 13 is constant with respect to the frequency, the attenuation of the desired signal also increases and the voltage of the desired signal decreases, so that the clarity of the sound output from the demodulation circuit is increased. Although deteriorated, the present invention has an effect of improving the intelligibility of the sound output from the demodulation circuit because the attenuation of the desired signal is reduced and the voltage of the desired signal is increased. In the first case, it is shown that the LPF type attenuation circuit is selected as the variable attenuation circuit 13 when the frequency fu of the interference signal is higher than the frequency fd of the desired signal. When fu is lower than the frequency fd of the desired signal, an HPF type attenuation circuit is selected as the variable attenuation circuit 13.

  FIG. 7 shows a spectrum of the radio signal of the second case for explaining the circuit type selection of the variable attenuation circuit 13. In the second case, the radio signal input to the automatic gain control circuit 10 includes three signals of a desired signal (frequency fd), a first disturbance signal (frequency fu1), and a second disturbance signal (frequency fu2). Is included. In the second case, the frequency fu1 of the first jamming signal is the lowest, the frequency fd of the desired signal is the next highest, and the frequency fu2 of the second jamming signal is the highest. That is, the frequency fd of the desired signal is between the frequency fu1 of the first interference signal and the frequency fu2 of the second interference signal. In the second case, the voltages of the first jamming signal and the second jamming signal are both the voltage Vu, which is larger than the voltage Vd of the desired signal. The voltage Vu between the first disturbing signal and the second disturbing signal is higher than the threshold voltage Vth of the AGC control of the automatic gain control circuit 10 by a voltage difference D. Here, the voltages of the first jamming signal and the second jamming signal do not need to be equal, and the following explanation can be applied as long as the voltage is higher than the desired signal voltage Vd. The description will proceed assuming that the voltages of the two disturbing signals are equal. The detection circuit 14 of the automatic gain control circuit 10 detects a radio signal and measures the wideband signal voltage Vw. Since the wideband signal voltage Vw is a voltage having the maximum frequency among the signals of the entire frequency band used in the automatic gain control circuit 10, the wideband signal voltage Vw is the first interference signal and the second interference signal. It becomes equal to the voltage Vu of the signal (Vw = Vu).

  FIGS. 8A to 8C show the frequency characteristics of the attenuation of the variable attenuation circuit 13 when the radio signal of the second case shown in FIG. 7 is input. Specifically, FIG. 8A is a schematic diagram of the frequency characteristics of the attenuation when the flat attenuation circuit is selected as the variable attenuation circuit 13 when the radio signal of the second case shown in FIG. 7 is input. FIG. 8B is a schematic diagram of the frequency characteristics of the attenuation when the LPF type attenuation circuit is selected as the variable attenuation circuit 13, and FIG. 8C is the case where the HPF type attenuation circuit is selected as the variable attenuation circuit 13. It is a schematic diagram of the frequency characteristic of the degree of attenuation. The variable attenuation circuit 13 attenuates at the frequency fu1 of the first disturbance signal and the frequency fu2 of the second disturbance signal because the wideband signal voltage Vw measured by the detection circuit 14 is higher than the threshold voltage Vth by the voltage difference D. The degree needs to be equal to or greater than the voltage difference D.

  When a flat type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 8A, the attenuation is constant with respect to the frequency, and therefore the attenuation of the frequency fd of the desired signal is also the attenuation D. When the LPF type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 8B, the higher the frequency, the larger the attenuation. Therefore, if the attenuation D is obtained at the frequency fu1 of the first disturbance signal, At the frequency fu2 of the 2 interference signals, the attenuation is equal to or greater than D. At this time, the attenuation degree of the frequency fd of the desired signal is an attenuation degree D (L) larger than the attenuation degree D of the first disturbing signal. When the HPF type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 8 (c), the lower the frequency, the larger the attenuation, so if the attenuation D is obtained at the frequency fu2 of the second interference signal, The attenuation degree D is greater than or equal to the frequency fu1 of one interference signal. At this time, the attenuation degree of the frequency fd of the desired signal becomes an attenuation degree D (H) larger than the attenuation degree D of the second disturbing signal. From the above, regarding the attenuation of the desired signal, the attenuation D when the flat attenuation circuit is selected << the attenuation D (L) when the LPF attenuation circuit is selected, and the attenuation D when the flat attenuation circuit is selected. <Attenuation degree D (H) when the HPF type attenuation circuit is selected.

  FIGS. 9A to 9C show the spectrum of the signal that has passed through the variable attenuation circuit 13 when the radio signal of the second case shown in FIG. 7 is input. More specifically, FIG. 9A passes through the variable attenuation circuit 13 when a flat attenuation circuit is selected as the variable attenuation circuit 13 when the radio signal of the second case shown in FIG. 7 is input. 9B shows the spectrum of the signal that has passed through the variable attenuation circuit 13 when the LPF type attenuation circuit is selected as the variable attenuation circuit 13, and FIG. 9C shows the HPF as the variable attenuation circuit 13. This is a spectrum of a signal that has passed through the variable attenuation circuit 13 when the type attenuation circuit is selected.

  When a flat attenuation circuit is selected as the variable attenuation circuit 13, the voltage Vn (F) of the desired signal measured by the demodulation circuit 15 is input to the variable attenuation circuit 13 before being input, as shown in FIG. The desired signal voltage Vn (F) = Vd−D is obtained by subtracting the degree of attenuation D of the variable attenuation circuit 13 from the voltage Vd of the desired signal. When the LPF type attenuation circuit is selected as the variable attenuation circuit 13, as shown in FIG. 9B, the voltage Vn (L) of the desired signal measured by the demodulation circuit 15 is the desired value before the input of the variable attenuation circuit 13. The desired signal voltage Vn (L) = Vd−D (L) is obtained by subtracting the attenuation degree D (L) of the variable attenuation circuit 13 from the signal voltage Vd. When the HPF type attenuation circuit is selected as the variable attenuation circuit 13, the desired signal voltage Vn (H) measured by the demodulation circuit 15 is obtained before the input of the variable attenuation circuit 13 as shown in FIG. The desired signal voltage Vn (H) = Vd−D (H) is obtained by subtracting the attenuation degree D (H) of the variable attenuation circuit 13 from the signal voltage Vd.

  From the above, for the voltage of the desired signal detected by the demodulation circuit 15, the voltage Vn (F) of the desired signal when the flat type attenuation circuit is selected> the voltage Vn (H) of the desired signal when the HPF type attenuation circuit is selected, Further, the voltage Vn (F) of the desired signal when the flat type attenuation circuit is selected> the voltage Vn (L) of the desired signal when the LPF type attenuation circuit is selected. Accordingly, when the flat type attenuation circuit is selected, the desired signal voltage Vn detected by the demodulation circuit 15 becomes the maximum. Therefore, the control circuit 16 selects the flat type attenuation circuit as the variable attenuation circuit 13 and performs AGC control. I do. In the conventional technique, the attenuation of the variable attenuation circuit 13 is always constant with respect to the frequency. However, in the present invention, only when the attenuation of the desired signal is the smallest and the voltage of the desired signal is the highest. The flat type attenuation circuit is selected and operates so that the clarity of the audio signal output from the demodulation circuit 15 is maximized.

  In the following, as an embodiment of the present invention, an attenuation table for each circuit type of the variable attenuation circuit 13 (a list of attenuation and constants of each circuit element, simulation results of attenuation and VSWR (voltage standing wave ratio)) is shown. Show. FIG. 10 shows an attenuation table (flat attenuation table) when a flat attenuation circuit is selected as the variable attenuation circuit 13. The VSWR (Voltage Standing Wave Ratio) is determined by the reflection coefficient of the impedance on the reference side and the load side, and becomes a value larger than 1 as the impedance mismatch is larger. In general radio receivers including in-vehicle FM radio, VSWR is designed to be 2 or less in order to reduce reflection loss due to impedance mismatch. In the case of an in-vehicle FM radio, when VSWR increases, it means that the load impedance of the antenna amplifier 12 deviates from 75Ω, so that not only the reflection loss increases, but also the distortion characteristics of the antenna amplifier 12 deteriorate.

  FIGS. 11A to 11C show circuit simulation results of the frequency characteristics of the attenuation for the variable attenuation circuit 13 of FIG. FIG. 11A shows a circuit simulation result of the attenuation when a flat type attenuation circuit is selected as the variable attenuation circuit 13. Curve A-1, curve A-2, curve A-3, and curve A-4 have an attenuation of 70 MHz at 10 dB (No. 2 in FIG. 10), 20 dB (No. 4 in FIG. 10), and 30 dB, respectively. 10 is a circuit simulation result of attenuation when the constants of the variable attenuation circuit 13 under the conditions of No. 6) and 40 dB (No. 8 in FIG. 10) are applied. From the respective curves, it can be read that the attenuation is constant (flat) from 70 MHz to 110 MHz.

  12A to 12C show circuit simulation results of the frequency characteristics of VSWR for the variable attenuation circuit 13 of FIG. FIG. 12A shows a circuit simulation result of VSWR when a flat type attenuation circuit is selected as the variable attenuation circuit 13. Although the result of the condition (every 10 dB) in which the attenuation of 70 MHz is 10 dB to 40 dB is shown, the resulting curves are overlapped so that they cannot be distinguished visually, and the VSWR is 1.1 or less.

  In FIG. 10, the resistance value of the second variable resistance element VR22 increases as the degree of attenuation increases, and is set to gradually approach 75Ω. By designing in this way, even when the attenuation of the variable attenuation circuit 13 increases, the impedance viewed from the antenna amplifier 12 side can be maintained constant (deterioration of VSWR is prevented). 10, 11 (a), and 12 (a), when a flat type attenuation circuit is selected as the variable attenuation circuit 13, the attenuation is varied from 5 dB to 40 dB, and at the same time, the VSWR of 70 MHz to 110 MHz is 2 or less ( It can be seen that it can be designed to be 1.1 or less at worst. Therefore, by using FIG. 10 which is the attenuation table of the flat type attenuation circuit, the control circuit 16 has an arbitrary attenuation at an arbitrary frequency in the band, and the VSWR viewed from the antenna amplifier side is 2 or less. The constant of the flat attenuation circuit can be set in the variable attenuation circuit 13.

  FIG. 13 shows an attenuation table (LPF type attenuation table) when an LPF type attenuation circuit is selected as the variable attenuation circuit 13 of FIG. FIG. 11B shows a circuit simulation result of the degree of attenuation when an LPF type attenuation circuit is selected as the variable attenuation circuit 13 of FIG. Curve B-1, Curve B-2, Curve B-3, and Curve B-4 have an attenuation of 70 MHz at 10 dB (No. 2 in FIG. 13), 20 dB (No. 4 in FIG. 13), and 30 dB, respectively. 13 is a circuit simulation result of the degree of attenuation when the constant of the variable attenuation circuit 13 under the conditions of No. 6) and 40 dB (No. 8 in FIG. 13) is applied. From the respective curves, it can be seen that from 70 MHz to 110 MHz, it is an LPF type characteristic in which the degree of attenuation increases as the frequency increases.

  FIG. 12B shows a circuit simulation result of VSWR when an LPF type attenuation circuit is selected as the variable attenuation circuit 13. Curve B-1, Curve B-2, and Curve B-3 have attenuations of 70 MHz at 10 dB (No. 2 in FIG. 13), 20 dB (No. 4 in FIG. 13), and 30 dB (No. 4 in FIG. 13), respectively. 6) is a simulation result of the attenuation when the constant of the variable attenuation circuit 13 under the condition of (6) is applied (the simulation result when the attenuation of 70 MHz is 40 dB (No. 8 in FIG. 13) is the attenuation of 70 MHz. Since it overlaps with the curve B-3 which is a simulation result when is 30 dB, only the curve B-3 is shown). In 70 MHz to 110 MHz, VSWR is 2 or less.

  In FIG. 13, the resistance value of the second variable resistance element VR22 increases as the degree of attenuation increases, and is set to gradually approach 75Ω. By designing in this way, even when the attenuation of the variable attenuation circuit 13 is increased, the impedance viewed from the antenna amplifier 12 side can be maintained constant (deterioration of VSWR) (the worst theoretical value). It can be designed with 1.7 or less). 13, 11 (b), and 12 (b), when an LPF type attenuation circuit is selected as the variable attenuation circuit 13, the attenuation is varied from 5 dB to 40 dB, and the VSWR of 70 MHz to 110 MHz is simultaneously reduced to 2 or less. It can be seen that the design can be as follows. Therefore, the control circuit 16 uses FIG. 13 which is an attenuation table of the LPF type attenuation circuit, so that an arbitrary attenuation is obtained at an arbitrary frequency in the band, and the VSWR as viewed from the antenna amplifier side is 2 or less. The constant of the LPF type attenuation circuit can be set in the variable attenuation circuit 13.

  FIG. 14 shows an attenuation table (HPF type attenuation table) when an HPF type attenuation circuit is selected as the variable attenuation circuit 13 of FIG. FIG. 11C shows a circuit simulation result of the attenuation when an HPF type attenuation circuit is selected as the variable attenuation circuit 13. Curves C-1, C-2, C-3, and C-4 have 70 MHz attenuation of 10 dB (No. 2 in FIG. 14), 20 dB (No. 4 in FIG. 14), and 30 dB, respectively. 14 is a circuit simulation result of the attenuation when the constant of the variable attenuation circuit 13 under the conditions of No. 6) and 40 dB (No. 8 in FIG. 14) is applied. From the respective curves, it can be seen that from 70 MHz to 110 MHz, it is an HPF type characteristic in which the attenuation increases as the frequency decreases.

  FIG. 12C shows a circuit simulation result of VSWR when an HPF type attenuation circuit is selected as the variable attenuation circuit 13. The constants of the variable attenuation circuit 13 under the condition that the attenuation of 70 MHz is 10 dB (No. 2 in FIG. 14) and 20 dB (No. 4 in FIG. 13) are applied to the curves C-1 and C-2, respectively. Simulation results when the attenuation at 70 MHz is 30 dB (No. 6 in FIG. 13) and at 40 dB (No. 8 in FIG. 13), the attenuation at 70 MHz is 20 dB. (Because it overlaps with the curve C-2, which is the simulation result of, only the curve C-2 is shown). In 70 MHz to 110 MHz, VSWR is 2 or less.

  In FIG. 14, the resistance value of the first variable resistance element VR <b> 21 decreases as the attenuation increases, and is set to gradually approach 75Ω. By designing in this way, even when the attenuation of the variable attenuation circuit 13 is increased, the impedance viewed from the antenna amplifier 12 side can be maintained constant (deterioration of VSWR) (the worst theoretical value). It can be designed with 1.7 or less). 14, 11 (c), and 12 (c), when the HPF type attenuation circuit is selected as the variable attenuation circuit 13, the attenuation is varied from 5 dB to 40 dB, and at the same time, the VSWR of 70 MHz to 110 MHz is made 2 or less. It can be seen that the design can be as follows. Therefore, the control circuit 16 uses FIG. 14 which is an attenuation table of the HPF type attenuation circuit, so that an arbitrary attenuation is obtained at an arbitrary frequency in the band, and the VSWR viewed from the antenna amplifier side is 2 or less. The constant of the HPF type attenuation circuit can be set in the variable attenuation circuit 13.

  Finally, in order to explain the effect of the embodiment of the present invention, the simulation results of the VSWR of the present invention and the comparative example are compared. FIG. 15 shows a table for an HPF type attenuation circuit of the present invention and a variable attenuation circuit of a comparative example that can obtain the same degree of attenuation. Specifically, it is the circuit constant of the variable attenuation circuit of the comparative example, and the electrical low performance (attenuation degree and VSWR simulation results). FIGS. 16A and 16B show circuit simulation results and equivalent circuits of the frequency characteristics of VSWR for the comparative example of this embodiment. More specifically, FIG. 16A shows a circuit simulation result of the frequency characteristics of the VSWR of the present invention and the comparative example that can obtain the HPF type equivalent attenuation, and FIG. 16B shows an attenuation circuit of the comparative example. FIG.

  In the comparative example, since the first variable resistance element VR21 is not provided, the resistance value of the third variable resistance element VR24 is adjusted. As a result, the attenuation of 70 MHz and the attenuation of 110 MHz were equal between the present invention and the comparative example. However, the VSWR of the comparative example was 193 or more, which was significantly deteriorated as compared with the VSWR (1.2 or less) of the present invention. Therefore, when the variable attenuating circuit 13 is configured as a comparative example, when the AGC control of the automatic gain control circuit 10 is operated, the impedance load as viewed from the antenna amplifier 12 changes greatly, so that the distortion characteristics of the antenna amplifier 12 deteriorate. To do. As a result, the voltage of a distortion signal such as an intermodulation distortion signal generated by the antenna amplifier 12 becomes large and is input to the automatic gain control circuit 10. You will have to hear bad, unpleasant voices. When the variable attenuation circuit 13 has the configuration of the present invention, when the AGC control of the automatic gain control circuit 10 is operated, the impedance load viewed from the antenna amplifier 12 hardly changes, and the distortion characteristics of the antenna amplifier 12 do not change. As a result, since the voltage of the distortion signal such as the intermodulation distortion signal generated by the antenna amplifier 12 does not change, interference does not occur with respect to the voice of the desired signal, and the user can hear a voice with high clarity.

  According to the embodiment of the present invention, it is possible to prevent the occurrence of interference due to the deterioration of the distortion characteristics of the antenna amplifier by maintaining a constant input impedance regardless of the attenuation of the variable attenuation circuit. In addition, regardless of the relationship between the frequency of the desired signal and the disturbing signal, the attenuation of the voltage of the desired signal due to the operation of the variable attenuation circuit can be minimized by switching and using the frequency characteristics of the attenuation of the variable attenuation circuit. . Further, since the attenuation of the voltage of the desired signal due to the operation of the variable attenuation circuit is minimized, it is possible to minimize the deterioration of the intelligibility of the output sound.

  As described above, according to the present embodiment, the variable attenuation circuit 13 of the automatic gain control circuit 10 is a π-type including the first variable resistance element VR21, the second variable resistance element VR22, and the third variable resistance element VR24. The circuit, the first variable capacitance element VC23 arranged in parallel with the second variable resistance element VR22 connected in series to the signal line, and the third variable resistance element VR24 connected between the detection circuit 14 and the ground are arranged in parallel. Since the second variable capacitance element VC25 is configured as described above, it is possible to prevent the distortion characteristics of the antenna amplifier 12 from deteriorating by keeping the impedance (input impedance) on the antenna side constant. Further, depending on the frequency relationship between the desired signal and the disturbing signal, the attenuation on the low frequency side of the variable attenuation circuit 13 is increased, the attenuation on the high frequency side is increased, or the attenuation is constant with respect to the frequency. Therefore, the degree of attenuation of the voltage of the desired signal can be minimized, and the deterioration of the intelligibility of the output voice can be minimized.

  In the above, this invention was demonstrated based on the Example. This embodiment is an exemplification, and it will be understood by those skilled in the art that various modifications can be made to each of those constituent elements or combinations of processing processes, and such modifications are also within the scope of the present invention. .

  The outline of one embodiment of the present invention is as follows. An automatic gain control circuit according to an aspect of the present invention includes a variable attenuation unit that attenuates a voltage of a radio signal with a variable attenuation, a detection unit that detects a radio signal in which the voltage is attenuated in the variable attenuation unit, and a detection unit The degree of attenuation of the variable attenuator is controlled based on the demodulator that demodulates the radio signal detected in step 1, the first voltage of the radio signal demodulated in the demodulator, and the second voltage of the radio signal detected in the detector A control unit. The variable attenuation unit is configured to be capable of realizing a plurality of types of attenuation circuits, and the control unit selects one of the plurality of types of attenuation circuits based on the first voltage and the second voltage, Control is made to stop the type of attenuation circuit other than the selected type of attenuation circuit.

  The control unit may select one of the plurality of types of attenuation circuits based on the first voltage for each of the plurality of types of attenuation circuits.

  The variable attenuating unit includes a first variable resistance element inserted between the signal line of the radio signal in the variable attenuating unit and the ground, and a signal of the radio signal between the connection point of the first variable resistance element and the signal line and the detection unit. A second variable resistance element inserted in series with the line, a third variable resistance element inserted between the signal line of the radio signal between the second variable resistance element and the detector and the ground, and a second variable resistance A first variable capacitance element arranged in parallel to the element and a second variable capacitance element arranged in parallel to the third variable resistance element may be provided. When the control unit selects (1) an attenuation circuit whose attenuation is constant with respect to the frequency, the control unit selects the first variable resistance element, the first variable capacitance element, and the second variable capacitance element. When setting an impedance higher than the impedance of the detection unit, controlling the resistance values of the second variable resistance element and the third variable resistance element, and (2) selecting a type of attenuation circuit in which the degree of attenuation increases as the frequency increases. The first variable resistance element, the first variable capacitance element, and the third variable resistance element are set with impedances higher than the impedance of the detection unit, and the resistance value of the second variable resistance element and the second variable resistance element are set. When the capacitance value of the capacitive element is controlled and (3) the type of attenuation circuit in which the attenuation level increases as the frequency decreases is lower than the impedance of the detection unit for the second variable resistive element and the second variable capacitive element. High impi Set the dance may control the resistance value of the first variable resistance element and the capacitance value of the first variable capacitance element and a resistance value of the third variable resistance element.

  The control unit (1) operates as a type of attenuation circuit in which the attenuation is constant with respect to the frequency, and when the radio signal is attenuated more than a predetermined attenuation, the resistance value of the second variable resistance element When a value close to the impedance of the detector is set and (2) the radio signal is attenuated more than a predetermined attenuation while operating as an attenuation circuit in which the attenuation increases as the frequency increases, the second variable resistor When a value close to the impedance of the detector is set as the resistance value of the element, and (3) when the radio signal is attenuated more than a predetermined attenuation while operating as a type of attenuation circuit in which the attenuation increases as the frequency decreases, A value close to the impedance of the detector may be set as the resistance value of the first variable resistance element.

  DESCRIPTION OF SYMBOLS 10 Automatic gain control circuit, 11 Antenna, 12 Antenna amplifier, 13 Variable attenuation circuit, 14 Detection circuit, 15 Demodulation circuit, 16 Control circuit, 17 Speaker, VR21 1st variable resistance element, VR22 2nd variable resistance element, VC23 A first variable capacitor, VR24, a third variable resistor, VC25, a second variable capacitor.

Claims (4)

A variable attenuation unit for attenuating the voltage of the radio signal with a variable attenuation, and
A detection unit for detecting a radio signal whose voltage has been attenuated in the variable attenuation unit;
A demodulator that demodulates the radio signal detected by the detector;
A controller for controlling the attenuation of the variable attenuator based on the first voltage of the radio signal demodulated by the demodulator and the second voltage of the radio signal detected by the detector;
The variable attenuation unit is configured to be able to realize a plurality of types of attenuation circuits,
The control unit selects one of a plurality of types of attenuation circuits based on the first voltage and the second voltage, and stops a type of attenuation circuit other than the selected type of attenuation circuit. An automatic gain control circuit characterized by controlling.   2. The automatic gain control according to claim 1, wherein the control unit selects one of the plurality of types of attenuation circuits based on a first voltage for each of the plurality of types of attenuation circuits. circuit. The variable attenuation unit is
A first variable resistance element inserted between the signal line of the radio signal and the ground in the variable attenuation section;
A second variable resistance element inserted in series in a signal line of a radio signal between the connection point of the first variable resistance element and the signal line and the detection unit;
A third variable resistance element inserted between a signal line and a ground of a radio signal between the second variable resistance element and the detection unit;
A first variable capacitance element arranged in parallel with the second variable resistance element;
A second variable capacitance element arranged in parallel with the third variable resistance element,
When the control unit selects (1) an attenuation circuit whose attenuation is constant with respect to frequency, the first variable resistance element, the first variable capacitance element, and the second variable capacitance element On the other hand, an impedance higher than the impedance of the detector is set, and the resistance values of the second variable resistance element and the third variable resistance element are controlled, and (2) the attenuation increases as the frequency increases. When the attenuation circuit is selected, an impedance higher than the impedance of the detector is set for the first variable resistance element, the first variable capacitance element, and the third variable resistance element, and the second variable resistance element is set. The resistance value of the variable resistance element and the capacitance value of the second variable capacitance element are controlled, and (3) when selecting an attenuation circuit in which the attenuation level increases as the frequency decreases, the second variable resistance element and Said An impedance higher than that of the detection unit is set for the variable capacitance element, and the resistance value of the first variable resistance element, the capacitance value of the first variable capacitance element, and the resistance of the third variable resistance element 3. The automatic gain control circuit according to claim 1, wherein the value is controlled.   When the control unit attenuates a radio signal to a predetermined attenuation or higher while operating as an attenuation circuit of a type in which the attenuation is constant with respect to the frequency, the resistance of the second variable resistance element is When a value close to the impedance of the detection unit is set as a value, and (2) the radio signal is attenuated to a predetermined attenuation or higher while operating as an attenuation circuit in which the attenuation increases as the frequency increases, the first The resistance value of the variable resistance element 2 is set to a value close to the impedance of the detection unit. (3) While operating as an attenuation circuit in which the attenuation becomes larger as the frequency is lower, the radio signal is more than a predetermined attenuation. 4. The automatic gain control circuit according to claim 3, wherein a value close to the impedance of the detection unit is set as a resistance value of the first variable resistance element in the case of attenuation. .

Images