200302632 (1) 玖、發明說明 【發明所屬之領域】 本發明是關於一種收訊機；特別是關於一種最適當使 用於具備自動地探察各種廣播電台的頻帶的功能的收訊機 【先前技術】 在很多無線接收機，具備自動地探察各種廣播電台的 頻帶的功能（所謂探察功能）。在探察功能是接收感度 (電場強度）成爲所定位準以上的接收頻帶作爲廣播電台 的頻帶並自動檢測該頻帶。在該探察功能，有探察可選台 的所有廣播電台的DX模態，及僅探察接收電流較強的廣 播電台的局部模態。 DX模態是全部檢測接收電場強度成爲所定位準以上 (例如20dB以上）的頻帶的模態。又，局部模態是接收信 號進入電場強度檢測電路之前某些程度衰減該接收信號的 位準，全部檢測衰減後的信號電場強度成爲所定位準以上 的頻帶的模態。例如，衰減接收信號的位準10dB時，成爲 僅檢測接收電場強度成爲3 0 ( = 20+ 1 0) dB以上的頻帶。 第1圖是表示具備探察功能的習知收訊機的構成的圖 式。在第1圖中，1是接收天線、2是濾波器、3是RF濾波 器、4是混頻器、5是局部振盪電路、6是IF放大器、7是作 爲電場強度檢測電路的比較器、8是衰減電路。 濾波器2是從接收天線1接收的廣播信號抽出目的接收 -5- (2) (2)200302632 頻帶的信號，並輸出至RF放大器3。RF放大器3是對於從 濾波器2所輸出的廣播信號進行高頻放大’輸出放大後的 廣播信號。混頻器4是藉由混合從RF放大器3所輸出的放大 後的廣播信號，及從局部振盪電路5所輸出的所定頻率的振 盪信號，俾輸出變換廣播放大信號的頻率的中頻信號。 IF放大器6是放大從混頻器4所輸出的中頻信號’同時 進行調諧動作。比較器7是比較從IF放大器6所輸出的中頻 信號，及事先所設定的所定位準（例如20dB)的信號，若 中頻信號的位準者較大時，則將須告知有廣送信號的脈衝信 號輸出至該頻帶。 衰減電路8是並聯地連接於濾波器2與RF放大器3之間。 該衰減電路8是具備電容器Cl，C2、二極體Dl，D2、電阻R 、局部開關SW、電源電壓V所構成。被選擇DX模態時，則 局部開關SW是在遮斷狀態，而未進行信號的衰減動作。另 一方面，被選擇局部模態時，則局部開關SW成爲導通狀態 ，進行被輸入於RF放大器3的廣播信號的衰減動作。 亦即，在局部模態，經局部開關SW輸出一定電壓V，以 電阻R進行電流變換並將電流流在二極體Dl，D2。藉由此時 所產生的二極體Dl，D2的導通電阻，經電容器Cl，C2來衰 減被輸入於RF放大器3的廣播信號。 表示於上述第1圖的習知例的情形，在衰減電路8的局 部開關SW成爲導通的局部模態時，流在二極體di，D2的電 流是與接收頻率無關而爲一定之故，因而二極體Dl，D2的 導通電阻是成爲一定値。另一方面，在從天線1至濾波器2 -6 - (3) (3)200302632 之間存在有電容成分，該中間的電阻値是藉由接收頻率而變 動。 以從該天線1至濾波器2之間的電容與二極體Dl，D2的 導通電阻之關係成爲高通濾波器。所以產生在較低接收頻 率，則增大從天線1至濾波器2之間的電阻値，使得信號的 衰減量變多，又在高接收頻率，則減少上述電阻値，而信號 的衰減量變少的現象。因此，在局部模態中，藉由探察功能 所檢測所需的接收感度，在低頻領域者比高頻領域較高的感 度差有較多發生的缺點問題。 本發明是爲了解決此種缺點問題而創作者，其目的是 在於可減少因局部模態時的接收頻率所致的探察感度差。 【發明內容】 本發明的收訊機，屬於具備衰減接收信號，並檢測衰 減後的信號電場強度成爲所定位準以上的廣播電台的頻帶的 功能的收訊機，其特徵爲：具備高頻放大上述接收信號的 高頻放大電路，及衰減被輸入於上述高頻放大電路的信號的 衰減電路；上述衰減電路是利用對應於接收頻率而變動的 調諧電壓，構成能衰減被輸入於上述高頻放大電路的信號。 上述衰減電路所利用的調諧電壓，是例如局部振盪電 路的調諧電壓。該局部振盪電路，是爲了對於進行從高頻 放大電路所輸出的信號的頻率變換的頻率變換電路輸出所定 頻率的振盪信號，原來具備於收訊機者。 上述衰減電路是具備以所定增益放大上述局部振盪電 -7- (4) (4)200302632 路的調諧電壓並輸出的放電路也可以。 依照如上述地構成的本發明，若接收頻率變高，則調 諧電壓變大之故，因而在衰減電路內流較多的電流而導通 電阻變小，與將一定電壓供給於衰減電路的情形相比較， 修正成使信號的衰減量變多。相反地，若接收頻率變低， 則調諧電壓變小之故，因而在衰減電路內流較少電流而導 通電阻變大，與將一定電壓供給於衰減電路的情形相比較 ’修正使信號的衰減量變少。由此，可成爲在所有接收頻帶 不會改變信號的衰減量。 【實施方式】 以下，依據圖式說明本發明的一實施形態。 第2圖是表示依具備探察功能的本實施形態的收訊機的 構成的圖式，例如表示AM無線收訊機。在第2圖中，1是接 收天線、2是濾波器、3是RF放大器（高頻放大電路）、4 是混頻器（頻率變換電路）、5是局部振盪電路、6是IF放 大器、7是作爲電場強度檢測電路的比較器、10是衰減電路 〇 濾波器2是從接收天線1接收的廣播信號抽出目的接收 頻帶的信號，並輸出至RF放大器3。RF放大器3是對於從 濾波器2所輸出的廣播信號進行高頻放大，輸出放大後的 廣播信號。混頻器4是藉由混合從RF放大器3所輸出的放大 後的廣播信號，及從局部振盪電路5所輸出的所定頻率的振 盪信號，俾輸出變換廣播放大信號的頻率的中頻信號。 (5) (5)200302632 IF放大器6是放大從混頻器4所輸出的中頻信號，同時 進行調諧動作。比較器7是比較從IF放大器6所輸出的中頻 信號，及事先所設定的所定位準（例如20dB)的信號，若 中頻信號的位準者較大時，則將須告知有廣送信號的脈衝信 號輸出至該頻帶。 衰減電路10是並聯地連接於濾波器2與RF放大器3之間 。該衰減電路10是具備電容器Cl，C2、二極體Dl，D2、電 阻R、局部開關SW及放大器11所構成。放大器11是以所定增 益放大局部振盪電路5的調諧電壓並加以輸出。 局部振盪電路5所需的調諧電壓是被施加於LC諧振的變 容器（未圖示）。該調諧電壓是欲提高局部振盪電路5的振 盪頻率時需要較高電壓。相反地，欲降低局部振盪電路5的 振盪頻率時，則調諧電壓變低。 被選擇DX模態時，則局部開關SW是在遮斷狀態，而未 進行因衰減電路10所致的信號的衰減動作。另一方面，被 選擇局部模態時，則局部開關SW成爲導通狀態，進入被輸 入於RF放大器3的廣播信號的衰減動作。 亦即，在局部模態，經局部開關SW，從放大器11輸出 以局部振盪電路5的調諧電壓爲依據放大電壓，而以電阻R 進行電流變換並將電流流在二極體D 1，D2。藉由此時所產 生的二極體Dl，D2的導通電阻，經電容器Cl，C2來衰減被 輸入於RF放大器3的廣播信號。 接收頻率變高，則調諧電壓變大而有較多電流流在二 極體Dl，D2之故，因而二極體Dl，D2的導通電阻是變小 (6) (6)200302632 ° S此’修正成與將一定電壓供給於衰減電路時相比較較 多信號的衰減量。相反地，接收頻率變低，則調諧電壓變 小而有較少電流流在二極體D 1，D2之故，因而二極體D 1 ’ D2的導通電阻是變大。由此，修正成與一定電壓供給 於衰減電路時相比較較少信號的衰減量。藉此，在所有接 收頻帶並不會有變更信號的衰減量。 第3圖是表示說明依本實施形態的收訊機的動作所需 的圖式。如表示於第1圖的習知例所示，將衰減電路8所使 用的電壓作爲一定電壓時，如圖A所示地藉由接收頻帶產 生很多探察感度差。亦即，在接收頻率較低領域，信號之 衰減量較多之故，因而除非接收感度相當大，該廣播電台 的頻帶是不被檢測（被要求高接收感度）。另一方面，在 接收頻率較高領域，信號之衰減量較少之故，因而接收感 度較小的廣播電台的頻帶也可檢測（接收感度較小優異） 〇 對此，如本實施形態所示地在衰減電路1 0中使用局部 振盪電路5的調諧電壓時，如圖表B所示，頻率愈低則調諧 電壓是變小，被修正成二極體Dl，D2的導通電阻變大， 使得信號的衰減量變少。又，頻率愈高則調諧電壓是變大 ，被修正成二極體D 1，D2的導通電阻變小，使得信號的 衰減量變多。由此，在局部模態的探察檢測所必需要的接 收感度被修正成如圖表C所示，減少因接收頻率所致的探 察感度差。 又，對應於調諧頻率，將二極體Dl，D2的導通電阻 -10- (7) (7)200302632 可變成某種程度，爲藉由放大器11的增益可加以調整。亦 即，藉由設置放大器1 1，可任意地調整衰減修正量。 又，在此如圖表A所示地，將接收頻率愈高愈降低探 察感度的情形（圖表的傾斜爲負値）例舉爲例子加以說明 ，惟相反地，也有接收頻率愈低愈降低探察感度的情形（ 圖表的傾斜爲正値）。表示感度差的圖表是依存於濾波器2 的特性而決定，例如在電感器成分較多的電路等，有圖表的 傾斜成爲正値的情形。這時候，作爲放大器11使用反轉放大 器，就可有效地減低因接收頻率所致的感度差。 如上所詳述地，依照本實施形態，作爲施加於衰減電 路10的二極體Dl，D2的電壓，利用依存於頻率而變動的調 諧電壓之故，因而可適當地修正信號的衰減量，可減低因局 部模態的接收頻率所致的探察感度相差。 又，作爲調諧電壓，利用一般具備於收訊機的局部振 盪電路5的調諧電壓之故，因而具有不必將特別電路設於調 諧電壓發生用的優點。又，有關於放大器11，在AM / FA收 音機共用的收訊機的情形，藉由將具備於FM收音機的追 踪用者挪用於AM用，並不需要追加零件。由此，不必追 加特別電路而可減低探察感度差。 又，爲了發生調諧電壓，也可設置微電腦或D / A變 換器等。惟此等，必須將此的程式具備在記憶體俾進行所 定控制。因此，該分量使得構成變大之同時，成爲需要微 電腦的負荷。對於此，在本實施形態中，利用局部振盪電 路的調諧電壓來進行類比性修正信號衰減量之故，因而也 -11 - (8) (8)200302632 具有完全不需要特別控制的優點。 又，上述實施形態是僅不過表示實施本發明的具體化 的一例者，藉此並不是本發明的技術性範圍被限定地解決者 。亦即，本發明是在從其精神或其主要特徵不超出之範圍， 可實施各種形態。 本發明是如上述地，具備衰減接收信號，並檢測衰減 後的信號電場強度成爲所定位準以上的頻帶的功能的收訊 機中，利用對應於接收頻率而變動的調諧電壓來衰減上述 接收信號之故，因而可修正成在衰減電路所利用的電壓一 定時所產生的接收頻率所致的衰減量的相差，而可減低因局 部模態的接收頻率所致的感度相差。 (產業上的利用能性） 本發明是在可減少因局部模態時的接收頻率所致的探 察感度相差上有用。 圖式簡單說明 第1圖是表示具備探察功能的習知收訊機的構成的圖 式。 第2圖是表示依具備探察功能的本實施形態的收訊機 的構成的圖式。 第3圖是表示說明依本實施形態的收訊機的動作所需 的圖式。 200302632 Ο) 主要元件對照表 1 接收天線 2 濾波器 3 RF放大器 4 混頻器 5 局部振盪電路 6 IF放大器 7 比較器 8 哀減電路 11 放大器 C 1，C2 電容器 D 1，D2 二極體 R 電阻 S W 局部開關 V 電源電壓200302632 (1) Description of the invention [Field of invention] The present invention relates to a radio receiver; in particular, it relates to a radio receiver most suitable for use with a function of automatically detecting the frequency band of various broadcasting stations [prior art] Many wireless receivers have a function of automatically detecting the frequency band of various broadcasting stations (so-called detection function). In the detection function, the receiving frequency (electric field strength) is equal to or higher than the target reception frequency band as the frequency band of the broadcasting station, and the frequency band is automatically detected. In this detection function, there are DX modes of all the broadcasting stations for optional stations, and only local modes of the broadcasting stations with strong receiving currents. The DX mode is a mode in which all detected reception electric field strengths are in a frequency band above the positioning accuracy (for example, 20 dB or more). In addition, the local mode is a mode in which the received signal attenuates the received signal to some extent before it enters the electric field strength detection circuit, and the detected electric field strength of the attenuated signal is a mode in a frequency band above the positioning standard. For example, when the level of the received signal is attenuated by 10 dB, only the frequency band where the received electric field strength becomes 30 (= 20 + 1 0) dB or more is detected. Fig. 1 is a diagram showing a configuration of a conventional receiver having a detection function. In the first figure, 1 is a receiving antenna, 2 is a filter, 3 is an RF filter, 4 is a mixer, 5 is a local oscillation circuit, 6 is an IF amplifier, 7 is a comparator serving as an electric field strength detection circuit, 8 is an attenuation circuit. The filter 2 extracts a broadcast signal received in the receiving antenna 1 from a destination -5- (2) (2) 200302632 frequency band and outputs it to the RF amplifier 3. The RF amplifier 3 is a high-frequency amplifier for broadcasting signals output from the filter 2 and outputs the amplified broadcasting signals. The mixer 4 mixes the amplified broadcast signal output from the RF amplifier 3 and the oscillation signal of a predetermined frequency output from the local oscillation circuit 5 to output an intermediate frequency signal that converts the frequency of the broadcast amplified signal. The IF amplifier 6 amplifies the intermediate frequency signal 'output from the mixer 4 and performs a tuning operation at the same time. The comparator 7 compares the intermediate frequency signal output from the IF amplifier 6 with the signal of a preset positioning level (for example, 20dB). If the level of the intermediate frequency signal is relatively large, it will be required to inform the broadcaster No. pulse signal is output to this frequency band. The attenuation circuit 8 is connected between the filter 2 and the RF amplifier 3 in parallel. The attenuation circuit 8 includes capacitors C1 and C2, diodes D1 and D2, a resistor R, a local switch SW, and a power supply voltage V. When the DX mode is selected, the local switch SW is in the off state without performing signal attenuation. On the other hand, when the local mode is selected, the local switch SW is turned on, and attenuating the broadcast signal input to the RF amplifier 3 is performed. That is, in a local mode, a certain voltage V is output via the local switch SW, a current is converted with a resistor R, and the current flows through the diodes D1, D2. The on-resistance of the diodes D1 and D2 generated at this time is used to attenuate the broadcast signal input to the RF amplifier 3 via the capacitors Cl and C2. In the case of the conventional example shown in FIG. 1 above, when the local switch SW of the attenuation circuit 8 is turned on in the local mode, the current flowing through the diode di and D2 is constant regardless of the reception frequency. Therefore, the on-resistance of the diodes D1 and D2 becomes constant. On the other hand, there is a capacitive component between the antenna 1 and the filter 2 -6-(3) (3) 200302632, and the intermediate resistance 値 is changed by the receiving frequency. The relationship between the capacitance between the antenna 1 and the filter 2 and the on-resistance of the diodes D1 and D2 becomes a high-pass filter. Therefore, at a lower receiving frequency, the resistance 値 between the antenna 1 and the filter 2 is increased, so that the signal attenuation is increased, and at a high receiving frequency, the resistance 上述 is reduced, and the signal attenuation is reduced. phenomenon. Therefore, in the local modal, the detection sensitivity required by the detection function is lower in the low-frequency domain than in the high-frequency domain, which has many disadvantages. The present invention was created by the present invention to solve such a disadvantage, and its object is to reduce the poor detection sensitivity caused by the reception frequency in the local mode. [Summary of the Invention] The receiver of the present invention belongs to a receiver that has a function of attenuating a received signal and detecting the attenuation of the electric field strength of the signal after the attenuation becomes a frequency band of a broadcasting station above the positioning accuracy. The high-frequency amplifier circuit for receiving a signal and an attenuation circuit for attenuating a signal input to the high-frequency amplifier circuit; the attenuation circuit is configured to attenuate and be input to the high-frequency amplifier by using a tuning voltage that varies according to the reception frequency. Signal of the circuit. The tuning voltage used in the attenuation circuit is, for example, a tuning voltage of a local oscillation circuit. This local oscillation circuit is intended to output an oscillation signal of a predetermined frequency to a frequency conversion circuit that performs frequency conversion of a signal output from a high-frequency amplifier circuit. The attenuation circuit may include an amplifier circuit which amplifies the tuning voltage of the local oscillation circuit with a predetermined gain and outputs the tuning voltage. According to the present invention configured as described above, if the receiving frequency becomes higher, the tuning voltage becomes larger. Therefore, a larger current flows in the attenuation circuit and the on-resistance becomes smaller, which is in contrast to the case where a constant voltage is supplied to the attenuation circuit. For comparison, it is corrected to increase the amount of attenuation of the signal. Conversely, if the receiving frequency becomes lower, the tuning voltage becomes smaller. Therefore, less current flows in the attenuation circuit and the on-resistance becomes larger. Compared with the case where a certain voltage is supplied to the attenuation circuit, the correction causes attenuation of the signal The amount becomes less. As a result, the amount of attenuation of the signal does not change in all the reception frequency bands. [Embodiment] An embodiment of the present invention will be described below with reference to the drawings. Fig. 2 is a diagram showing a configuration of a receiver according to this embodiment having a detection function, and shows, for example, an AM wireless receiver. In the second figure, 1 is a receiving antenna, 2 is a filter, 3 is an RF amplifier (high-frequency amplifier circuit), 4 is a mixer (frequency conversion circuit), 5 is a local oscillation circuit, 6 is an IF amplifier, 7 It is a comparator serving as an electric field strength detection circuit, 10 is an attenuation circuit, and filter 2 is a signal of a destination receiving band extracted from a broadcast signal received by the receiving antenna 1 and output to a RF amplifier 3. The RF amplifier 3 performs high-frequency amplification on a broadcast signal output from the filter 2 and outputs an amplified broadcast signal. The mixer 4 mixes the amplified broadcast signal output from the RF amplifier 3 and the oscillation signal of a predetermined frequency output from the local oscillation circuit 5 to output an intermediate frequency signal that converts the frequency of the broadcast amplified signal. (5) (5) 200302632 The IF amplifier 6 amplifies the intermediate frequency signal output from the mixer 4 and performs a tuning operation at the same time. The comparator 7 compares the intermediate frequency signal output from the IF amplifier 6 with the signal of a preset positioning level (for example, 20dB). If the level of the intermediate frequency signal is relatively large, it will be required to inform the broadcaster No. pulse signal is output to this frequency band. The attenuation circuit 10 is connected between the filter 2 and the RF amplifier 3 in parallel. The attenuation circuit 10 includes capacitors C1, C2, diodes D1, D2, a resistor R, a local switch SW, and an amplifier 11. The amplifier 11 amplifies the tuning voltage of the local oscillation circuit 5 with a predetermined gain and outputs it. The tuning voltage required for the local oscillation circuit 5 is an LC resonance transformer (not shown). This tuning voltage requires a higher voltage in order to increase the oscillation frequency of the local oscillation circuit 5. Conversely, when the oscillation frequency of the local oscillation circuit 5 is to be reduced, the tuning voltage is lowered. When the DX mode is selected, the local switch SW is in an off state, and no signal attenuation operation by the attenuation circuit 10 is performed. On the other hand, when the local mode is selected, the local switch SW is turned on, and the radio signal input to the RF amplifier 3 is attenuated. That is, in the local mode, the amplifier 11 outputs the amplified voltage based on the tuning voltage of the local oscillation circuit 5 via the local switch SW, and performs current conversion with the resistor R and flows the current through the diodes D1, D2. The on-resistance of the diodes D1 and D2 generated at this time is used to attenuate the broadcast signal input to the RF amplifier 3 via the capacitors Cl and C2. As the receiving frequency becomes higher, the tuning voltage becomes larger and more current flows in the diodes D1 and D2. Therefore, the on-resistance of the diodes D1 and D2 becomes smaller (6) (6) 200302632 ° S this' Corrected the amount of signal attenuation more than when a certain voltage is supplied to the attenuation circuit. Conversely, as the receiving frequency becomes lower, the tuning voltage becomes smaller and less current flows through the diodes D1, D2. Therefore, the on-resistance of the diode D1'D2 becomes larger. As a result, the amount of signal attenuation is reduced compared to when a given voltage is supplied to the attenuation circuit. Thereby, there is no change in the amount of attenuation of the signal in all the receiving bands. Fig. 3 is a diagram necessary for explaining the operation of the receiver according to this embodiment. As shown in the conventional example shown in Fig. 1, when the voltage used by the attenuation circuit 8 is set to a constant voltage, as shown in Fig. A, many detection sensitivity differences occur in the reception frequency band. That is, in the area where the reception frequency is low, the signal attenuation is large. Therefore, unless the reception sensitivity is relatively large, the frequency band of the broadcasting station is not detected (high reception sensitivity is required). On the other hand, in areas where the reception frequency is high, the amount of signal attenuation is small, so the frequency band of a broadcast station with a small reception sensitivity can also be detected (smaller reception sensitivity is excellent). Regarding this, as shown in this embodiment When using the tuning voltage of the local oscillation circuit 5 in the attenuation circuit 10, as shown in Figure B, the lower the frequency, the smaller the tuning voltage is. It is corrected to the on-resistance of the diodes D1 and D2, which makes the signal larger. The amount of attenuation decreases. In addition, the higher the frequency, the larger the tuning voltage is. It is corrected so that the on-resistance of the diodes D1 and D2 becomes smaller, which increases the amount of signal attenuation. As a result, the reception sensitivity required for the local mode detection detection is corrected to reduce the detection sensitivity difference due to the reception frequency as shown in Table C. In addition, the on-resistances of the diodes D1 and D2 can be adjusted to a certain degree in accordance with the tuning frequency, so that the gain can be adjusted by the gain of the amplifier 11. That is, the attenuation correction amount can be arbitrarily adjusted by providing the amplifier 11. Here, as shown in Figure A, the case where the detection frequency is lowered as the receiving frequency is higher (the slope of the chart is negative) is taken as an example to explain, but on the contrary, the detection sensitivity is lowered as the receiving frequency is lower. (The slope of the chart is positive). The graph showing the difference in sensitivity is determined depending on the characteristics of the filter 2. For example, in a circuit with a large amount of inductor components, the graph tilt may become positive. At this time, by using an inverting amplifier as the amplifier 11, the sensitivity difference caused by the receiving frequency can be effectively reduced. As described in detail above, according to this embodiment, as the voltages applied to the diodes D1 and D2 of the attenuation circuit 10, a tuning voltage that varies depending on the frequency is used, so that the amount of attenuation of the signal can be appropriately corrected, Reduce the difference in detection sensitivity due to the reception frequency of the local modal. In addition, since the tuning voltage provided in the local oscillator circuit 5 of the receiver is used as the tuning voltage, there is an advantage that it is not necessary to provide a special circuit for the tuning voltage generation. In addition, regarding the amplifier 11, in the case of a receiver shared by the AM / FA receiver, by using a tracking user provided in the FM radio for the AM, no additional parts are required. This eliminates the need for a special circuit and reduces the detection sensitivity. In order to generate a tuning voltage, a microcomputer or a D / A converter may be provided. However, the program must be provided with the desired control in the memory. Therefore, this component makes the structure larger and a load that requires a microcomputer. In view of this, in the present embodiment, the tuning voltage of the local oscillation circuit is used to perform the analog correction of the signal attenuation. Therefore, -11-(8) (8) 200302632 has the advantage that no special control is required at all. It should be noted that the above-mentioned embodiment is merely an example of implementation of the present invention, and therefore, it is not intended to limit the technical scope of the present invention. That is, the present invention can be implemented in various forms without departing from the spirit or main characteristics thereof. According to the present invention, as described above, a receiver having a function of attenuating a received signal and detecting that the electric field strength of the attenuated signal becomes a frequency band above a specified level, attenuates the received signal by using a tuning voltage that fluctuates according to the received frequency. Therefore, it can be corrected to the phase difference of the attenuation amount caused by the reception frequency when the voltage used by the attenuation circuit is constant, and the sensitivity difference caused by the reception frequency of the local mode can be reduced. (Industrial Applicability) The present invention is useful for reducing the difference in detection sensitivity due to the reception frequency in a local mode. Brief Description of Drawings Fig. 1 is a view showing a configuration of a conventional receiver having a detecting function. Fig. 2 is a diagram showing the configuration of a receiver according to this embodiment having a detection function. Fig. 3 is a diagram necessary for explaining the operation of the receiver according to this embodiment. 200302632 〇) Comparison table of main components 1 Receive antenna 2 Filter 3 RF amplifier 4 Mixer 5 Local oscillation circuit 6 IF amplifier 7 Comparator 8 Reduction circuit 11 Amplifier C 1, C2 Capacitor D 1, D2 Diode R Resistance SW Local Switch V Supply Voltage