JP2922529B2 - Receiving antenna attenuator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a receiving antenna attenuator for preventing interference in a strong electric field such as a broadcast receiver.

(Related Art) A broadcast receiver is provided with an AGC (automatic gain control) circuit that adjusts the amplification degree and always outputs a signal of a constant level even if the level of a received signal received by an antenna fluctuates. .

FIG. 5 is a block diagram showing an example of the configuration of an FM broadcast receiver provided with such an AGC circuit. In this figure,
An FM broadcast receiver that receives a reception signal received by the antenna 1 includes, for example, a high-frequency amplifier circuit 2, a frequency conversion circuit 3, a local oscillator circuit 4, an intermediate frequency amplifier circuit 5, an output level detection circuit 6 for AGC, and an FM detection circuit. 7, a low-frequency amplifier circuit 8, a power amplifier circuit 9, and a speaker 10.

In this FM broadcast receiver, a signal received from an antenna 1 is amplified by a high-frequency amplifier circuit 2, then converted to an intermediate frequency signal by a frequency conversion circuit 3, and an intermediate frequency signal is amplified by an intermediate frequency amplifier circuit 5. Here, the output level of the intermediate frequency amplification circuit 5 is always detected by the output level detection circuit 6 for AGC, rectified by the output level detection circuit 6, and fed back to the high frequency amplification circuit 2 as a DC control voltage. Control the degree. The control operation of the AGC circuit reduces the amplification degree when the output level of the intermediate frequency amplification circuit 5 is high, and increases the amplification degree when the output level is low. As a result, regardless of the received signal level, Automatic control is performed so that a constant level signal is always output.

As described above, the output signal of the intermediate frequency amplification circuit 5 controlled to a constant level by the AGC circuit is detected by the FM detection circuit 7 and an audio signal is extracted. After being amplified to a predetermined level by the amplifier circuit 9, the signal is output from the speaker 10 as sound.

(Problems to be Solved by the Invention) By the way, in the case where the FM broadcast receiver is used for a vehicle, after selecting the A broadcast station at a certain point, the location is moved and the other B broadcast station, which is the FM broadcast station, is selected. When passing through the area near the transmitting antenna of the station, the broadcasting of the receiving station A cannot be normally received due to the strong electric field interference of the station B. This phenomenon is because the high frequency amplifier circuit 2, which is the foremost stage of the FM broadcast receiver, is saturated by an excessive input signal input level due to a strong electric field and loses the frequency selection function.
The control of the amplification degree of the high-frequency amplifier circuit 2 by the AGC circuit alone cannot be used.

In view of the above, an object of the present invention is to provide a receiving antenna attenuator capable of effectively preventing a reception failure due to a strong electric field with a simple configuration.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a method in which a resonance circuit or a filter including a coil wound around a high-frequency magnetic core is provided between an antenna and a front-end circuit on a receiver input side. And applying a DC magnetic field to the high-frequency core to change the magnetic permeability, thereby changing the resonance frequency of the resonance circuit or the pass-stop band frequency of the filter to a frequency band that attenuates the reception signal received by the antenna. The configuration is such that

(Operation) In the receiving antenna attenuator of the present invention, when the antenna enters a strong electric field region of a specific radio wave, a required DC magnetic field is applied to the high-frequency core of the coil to set the resonance frequency or the resonance frequency of the resonance circuit including the coil. The pass band of the filter can be changed to substantially match the frequency band of the specific radio wave. As a result, an excessive reception signal of the antenna caused by the strong electric field of the specific radio wave is added to the receiver after receiving a required attenuation by the resonance circuit or the filter, and the high-frequency amplification circuit or the like at the front stage of the receiver is used. Can be prevented from being saturated. For this reason, the receiver can recover the original frequency selection characteristic and can receive a desired broadcast wave.

(Example) Hereinafter, an example of a receiving antenna attenuator according to the present invention will be described with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention. In this figure, the portion surrounded by a dotted line X is the FM broadcast receiver shown in FIG.
The frequency selection tuning circuit 11 and the amplifying transistor 12 of the first stage high-frequency amplifier circuit 2 are illustrated. A parallel resonance circuit 20 including a coil 21 and a capacitor 22 wound around a high-frequency magnetic core 23 is inserted between the input side of the FM broadcast receiver, that is, between the antenna 1 and the input terminal of the high-frequency amplifier circuit 2. . Here, as the high-frequency magnetic core 23, a high-frequency ferrite having a property of decreasing the magnetic permeability by applying a DC magnetic field can be used.

Note that the DC magnetic field Hd is applied to the high frequency magnetic core 23 of the coil 21.
c can be applied. The DC magnetic field Hdc is applied by applying a winding to generate a DC magnetic field by passing a DC current around the high-frequency magnetic core 23, moving a permanent magnet for generating a DC magnetic field from a separated position to a close position, or generating a DC magnetic field. Arbitrary means such as using another magnetic circuit can be adopted.

In the configuration of the first embodiment, a case where an FM broadcast is received will be described as an example. In a normal electric field state, no DC magnetic field Hdc is applied to the high-frequency core 23 of the coil 21. Therefore, the magnetic permeability of the high-frequency core 23 is a large value, the inductance value of the coil 21 is large, and the resonance frequency of the parallel resonance circuit 20 is low. For example, FM at about 10 MHz
The received signal of the broadcast is hardly attenuated. As a result, broadcast reception by the FM broadcast receiver is possible in the same manner as in a normal case.

On the other hand, if a reception failure due to interference occurs when entering a strong electric field area of a specific broadcasting station, the coil is operated manually by an operator of the FM broadcasting receiver or by automatic means inside the receiver.
A required DC magnetic field Hdc is applied to the 21 high-frequency core 23. As a result, the magnetic permeability of the high-frequency core 23 decreases, the inductance value of the coil 21 decreases, and the resonance frequency of the parallel resonance circuit 20 increases. For example, an FM broadcast frequency band of about 80 MHz to 90 MHz is used to greatly attenuate the received signal of the FM broadcast. As a result, FM due to excessive received signal input
It is possible to prevent the occurrence of a saturation phenomenon at the forefront stage (for example, the high-frequency amplifier circuit 2) of the broadcast receiver and to restore the original frequency selection performance of the receiver. Therefore, radio waves of a desired broadcasting station can be satisfactorily received by removing interference.

FIG. 2 shows a second embodiment of the present invention. In this figure, the portion surrounded by a dotted line X is the FM broadcast receiver shown in FIG.
The frequency selection tuning circuit 11 and the amplifying transistor 12 of the first stage high-frequency amplifier circuit 2 are illustrated. Then, between the input side of the FM broadcast receiver, that is, between the input end of the high frequency amplifier circuit 2 connected to the antenna 1 and the ground side, a series resonance circuit 30 of a coil 31 and a capacitor 32 wound around a high frequency magnetic core 33. Is inserted. Here, as the high-frequency magnetic core 33, a high-frequency ferrite having a property of decreasing magnetic permeability by applying a DC magnetic field can be used.

The DC magnetic field Hd is applied to the high-frequency core 33 of the coil 31.
c can be applied.

Also in the case of the second embodiment, the reception failure caused by the strong electric field is achieved by setting the resonance frequency of the series resonance circuit 30 low without the DC magnetic field and setting the resonance frequency high by applying the DC magnetic field so as to be in the FM broadcast frequency band. Can be prevented.

FIG. 3 shows a third embodiment of the present invention. In this figure, the portion surrounded by a dotted line X is the FM broadcast receiver shown in FIG.
The frequency selection tuning circuit 11 and the amplifying transistor 12 of the first stage high-frequency amplifier circuit 2 are illustrated. Then, a T-type band rejection filter 40 is inserted between the input side of the FM broadcast receiver, that is, between the antenna 1 and the input end of the high frequency amplifier circuit 2. Here, the elements 41, 4 of the band rejection filter 40
Reference numeral 2 denotes a parallel resonance circuit in which a capacitor is connected in parallel to a coil wound around a high-frequency core, and an element 43 is formed from a series resonance circuit in which a capacitor is connected in series to a coil wound around a high-frequency core. Here, as each of the high-frequency magnetic cores, a high-frequency ferrite having a property of decreasing magnetic permeability by applying a DC magnetic field can be used, and a DC magnetic field Hdc can be applied to each of the high-frequency magnetic cores.

Also in the case of the third embodiment, by setting the pass rejection band frequency of the T-type band rejection filter 40 low without the DC magnetic field and setting the pass rejection band frequency high with the DC magnetic field to be in the FM broadcasting frequency band, Can prevent a reception failure caused by the In this case, a filter combining multiple resonance circuits
Since 40 is adopted, the amount of attenuation for an excessive antenna reception signal can be increased.

FIG. 4 is an application example of the present invention, and shows a configuration in which an interference state caused by a strong electric field is automatically detected and a DC magnetic field is applied to a high frequency magnetic core of a coil in a resonance circuit or a filter. In this figure, the DC control voltage of the output level detection circuit 6 for the AGC of the FM broadcast receiver shown in FIG. When the DC control voltage, which is substantially proportional to the intensity of the antenna reception signal, exceeds a certain value, the DC magnetic field generator 50 determines that the input is excessive reception input, and determines the first to third embodiments. Resonant circuits 20, 3 shown in
A DC magnetic field is applied to 0 or the high-frequency core of the filter 40 to attenuate an excessive reception input. In FIG. 4, other circuit portions are the same as those in FIG.

In the above embodiments, the case of FM broadcast reception has been described as an example. However, the present invention can be applied to a receiver such as an in-vehicle mobile radio, and the frequency region to be attenuated can be set arbitrarily.

On the other hand, instead of using an LC resonant circuit or filter,
Although it is conceivable to prevent reception failure due to a strong electric field by switching with a semiconductor (eg, a pin diode), noise generated by the used semiconductor becomes a problem in order to increase the amplification of the receiver.

(Effects of the Invention) As described above, according to the receiving antenna attenuator of the present invention, it is possible to effectively prevent a reception failure due to a strong electric field with a simple configuration. Further, since the LC resonance circuit or the LC filter is used, there is no possibility of noise induction and the S / N ratio is not deteriorated.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of a receiving antenna attenuator according to the present invention, FIG. 2 is a circuit diagram showing a second embodiment of the present invention, and FIG. 3 is a third embodiment of the present invention. FIG. 4 is a circuit diagram showing an application example of the present invention, and FIG. 5 is a block diagram showing a conventional general FM broadcast receiver. DESCRIPTION OF SYMBOLS 1 ... Antenna, 2 ... High frequency amplifier circuit, 3 ... Frequency conversion circuit, 4 ... Local oscillation circuit, 5 ... Intermediate frequency amplifier circuit, 6 ... Output level detection circuit, 7 ... FM detection circuit, 8
...... Low frequency amplifier circuit, 9 ... Power amplifier circuit, 10 ... Speaker, 11 ... Selection tuning circuit, 12 ... Transistor, 20 ...
... Parallel resonance circuit, 21,31 ... Coil, 22,32 ... Capacitor, 23,33 ... High frequency magnetic core, 30 ... Series resonance circuit, 40 ...
... Band rejection filter.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H04B 1/10-1/26 H03H 5/00-7/01 H03G 3/30

Claims (3)

(57) [Claims] A resonance circuit or a filter including a coil wound around a high-frequency core is provided between an antenna and a front-end circuit on the input side of a receiver, and a DC magnetic field is applied to the high-frequency core to reduce magnetic permeability. A receiving antenna attenuator characterized by changing a resonance frequency of the resonance circuit or a pass-stop band frequency of a filter to a frequency band that attenuates a reception signal received by the antenna by changing the frequency. 2. When the level of a received signal received by the antenna is equal to or higher than a predetermined value, the DC magnetic field is applied to change the resonance frequency of the resonance circuit or the pass-stop band frequency of the filter. 2. The receiving antenna attenuator according to claim 1, wherein a receiving signal from the antenna applied to the front-end circuit is attenuated at. 3. The receiving antenna attenuator according to claim 1, wherein said high-frequency magnetic core is a high-frequency ferrite.