Abstract:
The invention relates to an electronic device comprising a headset connector adapted to connect a headset to the electronic device and an active amplifier circuit connected to the headset connector. The active amplifier circuit is adapted to amplify radio signals received by an antenna, which is connected to the electronic device via the headset connector. The invention relates equally to a chip comprising such an active amplifier circuit, to a system comprising the electronic device and to a corresponding method for receiving radio signals at such an electronic device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Stage of International Application Number PCT/IB05/003076 filed on Oct. 14, 2005 which was published in English on Apr. 19, 2007 under International Publication Number WO 2007/042855. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to an electronic device, to a system, to a chip and to a method enabling a radio signal reception. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known to enhance mobile communication devices with additional functions that are not directly related to mobile communications. One example for such an enhancement is an analog frequency modulation (FM)-radio receiver implemented in a mobile phone. 
         [0004]    FM radio broadcasting uses frequencies in a range of 88 MHz to 108 MHz. The short wavelength of these frequencies allows using any of the connection lines of a headset connected to a mobile communication device as a passive antenna, from which the FM radio frequency signal can be filtered in the FM-radio receiver. 
         [0005]    The current amplitude modulation (AM) radio broadcasting, in contrast, uses short wave, medium wave and long wave transmissions in an overall frequency range of 150 kHz to 30 MHz. It thus uses wavelengths that are large compared to ultra short waves or FM radio waves, respectively. As a consequence, AM-radio receivers typically require several meters of passive antenna wire for enabling a reception, which is not feasible with mobile phones or other handheld devices. 
         [0006]    Enabling an AM-band reception in handheld devices is also of interest with regard to Digital Radio Mondiale (DRM). DRM is a digital broadcasting system that is defined in the ETSI standards. It provides digital voice, audio, text and image broadcasting and enables fully new services with a global coverage. DRM is designed to be used within the existing AM band. DRM broadcasting has been started in 2003, and in the long term DRM broadcasting will replace the complete analog signal broadcasting within the AM band. Thus, the problem described above for AM-radio reception occurs as well for DRM reception that is to be enabled in a handheld device. 
         [0007]    For illustration, a DRM reception will be described in more detail with reference to  FIGS. 1 to 4 . 
         [0008]      FIG. 1  is a diagram illustrating different spheres above the Earth&#39;s surface  10 . More specifically, the atmosphere  11 , which is closest to the Earth, is followed by the stratosphere  12  and the ionosphere  13 . The ionosphere  13  itself is further composed of a D-layer at approximately 40 km to 90 km from the Earth&#39;s surface  10 , an E-layer at approximately 90 km to 130 km from the Earth&#39;s surface  10  and an F 1 +F 2  layer at approximately 130 km to 250 km from the Earth&#39;s surface  10 . In addition, an AM-band transmitter  21  and an AM-band receiver  22  are depicted. AM-band wave propagation uses reflections at the ionosphere  13  and at the Earth&#39;s surface  10  for propagating globally around the word. 
         [0009]      FIG. 2  is a diagram illustrating short wave propagations in the 3 MHz to 30 MHz band. The ground wave  25 , that is, a direct propagation between a transmitter  21  and receivers  22 , becomes more or less meaningless, as it may be blocked rather quickly by obstacles  23 . Sky waves  26  are reflected to a large extend by the ionosphere  13  and can thus pass very long distances. Sky waves  26  may reach a receiver  22  directly after a reflection at the ionosphere  13 , or after additional reflections, for instance at the Earth&#39;s surface  10  and the ionosphere  13 . A dead zone caused by obstacles  23  might be considered. 
         [0010]      FIG. 3  is a schematic block-diagram of an analog front-end of a homodyne DRM receiver. The depicted components may be integrated, for example, on a single receiver chip. 
         [0011]    The DRM front-end comprises an antenna  300 , which is connected via a preselection filter  301  and a capacitor  302  to the input of a low noise amplifier (LNA) with automatic gain control (AGC)  303 . The output of the LNA  303  is connected on the one hand to an in-phase branch, comprising in this order a first downconversion mixer  310 , a first adjustable amplifier  311 , a first low pass filter  312 , a second adjustable amplifier  313  and a first Delta-Sigma analog-to-digital converter (DS-ADC)  314 . The output of the LNA  303  is connected on the other hand to a quadrature branch, comprising in this order a second downconversion mixer  320 , a third adjustable amplifier  321 , a second low pass filter  322 , a fourth adjustable amplifier  323  and a second DS-ADC  324 . Both the first and the second ADC  314 ,  315  are connected to a digital signal processor  330 . The first and the third adjustable power amplifiers  311 ,  321  are controlled by the digital signal processor  330  via a respective DC-Offset Compensation component  315 ,  325 . 
         [0012]    In addition, an oscillator  304  provides a signal that is fed to a fractional-N phase-locked-loop (PLL)  305 . The output of the PLL  305  is frequency divided by two by a frequency divider  306  and provided as an in-phase local oscillator signal (LO_I) to the first mixer  310  and as a quadrature local oscillator signal (LO_Q) to the second mixer  320 . 
         [0013]    When a signal is received via the antenna  300 , it is band-pass filtered by the preselection filter  301  according to a desired frequency range and amplified by the LNA  303 . The signal is then downconverted by the mixers  310 ,  320  to an analog in-phase baseband signal and an analog quadrature baseband signal using the local oscillator signal LO_I and the local oscillator signal LO_Q, respectively. The analog in-phase and quadrature baseband signals are amplified by amplifier  311 ,  321 , low-pass filtered by low-pass filter  312 ,  322 , further amplified by amplifier  313 ,  323  and converted into a digital baseband signal by ADC  314 ,  324  in the in-phase branch and in the quadrature branch, respectively. The resulting digital baseband signals BB_I and BB_Q are fed to the digital signal processor  330 . The digital signal processor may perform a digital base band processing including a digital source decoding and de-framing, in order to provide digital signals that allow regain the analog audio signal. 
         [0014]    The actual antenna  300  of the DRM front-end of  FIG. 3  can be for example a quarter-wave vertical antenna  300 , as presented in  FIG. 4 . The length of a quarter wave of current  410  and voltage  420  induced into the antenna  300 , which corresponds to the length of the actual antenna  300 , is denoted λ/4, while the length of a half wave of current  410  and voltage  420 , which corresponds to the combined length of the actual antenna  300  and of the mirrored antenna  400  mirrored at the root point  401  of the actual antenna  301 , is denoted λ/2. A is the wavelength of the carrier frequency of received radio signals. It is a disadvantage of this antenna  300  that it has to be very long, namely several meters long, in order to allow receiving DRM short wave signals. 
         [0015]    AM-band reception with a shorter antenna can be realized by employing an active antenna. For use in mobile phones, however, conventional active antennas have significant disadvantages, namely a high weight, high prices or high voltage requirements, respectively. 
       SUMMARY OF THE INVENTION 
       [0016]    It is an object of the invention to enable a feasible reception of lower frequency radio signals, like AM-band signals, in mobile phones and other handheld devices. 
         [0017]    An electronic device is proposed, which comprises a headset connector adapted to connect a headset to the electronic device. In addition, the electronic device comprises an active amplifier circuit connected to the headset connector. The active amplifier circuit is adapted to amplify radio signals received by an antenna, which is connected to the electronic device via the headset connector. 
         [0018]    Moreover, a system is proposed, which comprises such an electronic device and an antenna connected to the electronic device via the headset connector. 
         [0019]    Moreover, a chip for an electronic device is proposed, which comprises an input enabling a connection to a headset connector of the electronic device. The chip further comprises an active amplifier circuit connected to the input. The active amplifier circuit is adapted to amplify radio signals received by an antenna, which is connected to the electronic device via the headset connector. 
         [0020]    Finally, a method for receiving radio signals at an electronic device is proposed. The method comprises receiving radio signals via an antenna, which is connected to the electronic device via a headset connector of the electronic device. The method further comprises amplifying the received radio signals with an active amplifier circuit. 
         [0021]    The invention proceeds from the consideration that an active antenna enables a reduction of its physical dimension compared to a passive antenna. An active antenna comprises a passive part, namely the antenna element, and an active part, namely an active amplifier. 
         [0022]    The effective height h eff  of an active antenna corresponds to the ratio of the output open-circuit voltage of the antenna amplifier U a  to the electrical field strength E: 
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         [0023]    The effective area A eff  of an active antenna corresponds to the ratio of the signal power at the amplifier output P a,out  to the radiation density P n : 
         [0000]    
       
         
           
             
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         [0024]    With an active antenna, the antenna signal can be coupled out high-ohmic, and the matching to the wave resistance, for instance a wave resistance 50Ω in the case of a coaxial cable, can be done at the output of the antenna amplifier. For passive antennas, a 50Ω matching to the wave resistance of coaxial cable has to be done in a passive way, which is a big drawback due to worse antenna properties and more signal attenuation, and since more current will be coupled out of the antenna element. 
         [0025]    As conventional active antennas, which comprise both the passive and the active antenna element, are large and costly, it is proposed that an active amplifier circuit forming the active antenna element is coupled to a headset connector of an electronic device. As a result, a passive antenna element connected to the headset connector, for instance wires of a headset, can be combined with the active amplifier circuit to an active antenna. 
         [0026]    It is an advantage of the invention that the active amplifier circuit enables a lower-frequency radio reception with a rather short antenna, which is thus usable for small electronic devices like mobile phones as well. It is moreover an advantage of the invention that it does not require a dedicated passive antenna element within the electronic device. As a result, the signal reception can be realized with low costs, for example by using a headset cable of a connected headset as an antenna. 
         [0027]    The antenna reception efficiency of the active antenna should be high and the active amplifier circuit should provide a low noise input stage, which is high-omic and low capacitive to reduce the loading of the antenna. In an exemplary embodiment of the invention, the active amplifier circuit comprises one or more Junction-Field-Effect-Transistors (JFET) and/or one or more Metal-Oxide-Semiconductor-Field-Effect-Transistors (MOSFET) as active amplifier(s) for meeting these goals. 
         [0028]    The capability of a semi-conductor based active amplifier circuit depends strongly on the employed semi-conductor technology, as low noise and linear active elements are needed. 
         [0029]    It is an advantage of JFETs that they provide a good compromise between noise behavior and input capacitance. For JFETs, the 1/f noise behavior is negligible for frequencies higher than 1 kHz, whereas for MOSFETs, the 1/f noise behavior is relevant for frequencies up to 100 kHz. Normally, the implementation of JFETs is also a process option in most used semi-conductor technologies. The advantage of MOSFET transistors is their lower input capacitance and their availability in nearly every semi-conductor technology. Thus, in particular if JFETs are not available within the used technology, also MOSFETs can be used, but they are noisier and therefore will increase the overall circuit noise in the signal chain. 
         [0030]    In an exemplary embodiment of the invention, the electronic device and/or the proposed chip further comprise at least one processing component for processing signals amplified by the active amplifier circuit. The at least one processing component may comprise any component of a known radio signal receiver, for instance the components of a conventional DRM receiver presented above with reference to  FIG. 3 , etc. 
         [0031]    The at least one processing component may belong for instance to a radio receiver of the electronic device, which is adapted to process radio signals in a frequency range of 10 kHz to 30 MHz. The antenna for a mobile receiver for this frequency range has to be very small to fulfill the mobility aspect. An active vertical antenna of approximately one meter length would be sufficient to receive the full frequency range from 10 kHz to 30 MHz. 
         [0032]    Such a radio receiver may be for example an AM-radio receiver and/or a DRM radio receiver. In the case of a DRM radio receiver, the relevant frequency range is limited to approximately 150 kHz to 30 MHz. 
         [0033]    In one embodiment of the invention, the headset connector is adapted to connect a wire of at least one headset earspeaker to the active amplifier circuit, when a headset comprising at least one earspeaker is connected to the electronic device via the headset connector. 
         [0034]    This embodiment is particularly suited for a DRM receiver. It is an advantage of DRM that the required signal-to-noise-ratio (SNR) for a stable reception is as low as 15 dB. 
         [0035]    In other embodiment of the invention, the headset connector is adapted to connect a wire of a headset microphone to the active amplifier circuit, when a headset comprising a microphone is connected to the electronic device via the headset connector. In this case, a switch may be provided, which is adapted to disconnect wires of a microphone of a headset from a microphone interface, when a headset comprising a microphone is connected to the electronic device via the headset connector and radio signals are to be received. 
         [0036]    It is to be understood that instead of a switch, another separation component could be used. For example, a low-pass/high-pass filter could separate the audio signal from the antenna signal. However, in this case, the filter should be tuned when moving across the frequency band in order to keep the antenna efficiency high. Using a switch enables a simpler and cheaper implementation. 
         [0037]    This embodiment is particularly suited for an AM-radio receiver. 
         [0038]    In particular for an AM-radio reception, wires of a headset may be used as a kind of a short whip antenna (electrical short antenna) belonging to an active antenna. It has to be noted that in contrast to this approach, a regular whip is normally kept free of obstacles. Using the whip in an active mode for AM-radio reception requires that the headset wires are connected to an active amplifier circuit, which has a high input impedance and a low capacitance input. The wires of the headset earspeakers are less suited to fulfill these requirements, as the audio amplifier and the electro-static discharge (ESD) circuit will load the antenna very hard. It is therefore proposed that a wire of a microphone is used as a short whip antenna for AM-band reception. When using the microphone wire as a high impedance whip antenna, the microphone should be disconnected inside the electronic device before entering the ESD protection circuit. Using the microphone input in combination with some kind of a switch will enable AM-radio reception with the whip concept alternately with the regular use of the microphone. Advantageously, the switch represents a low capacitance load relative to ground, in order to ensure that the capacitive load at the input of the active amplifier circuit is rather low. It has to be noted that the switching advantageously disconnects both balanced wires of the microphone. 
         [0039]    The headset connector may be connected within the electronic device in addition to a frequency-modulation radio receiver in a conventional manner. This means that an analog FM-radio antenna can be reused for AM-band reception by adding an active amplifier circuit behind the FM-radio antenna. 
         [0040]    In practice an FM-band antenna, for instance a headset cable, is not suitable for AM-band reception. But combining for example digital AM-band reception and an active AM-band circuit with a FM antenna leads to good results for digital AM band reception. The invention thus enables for instance the supplementary use of an existing passive antenna in an active antenna, without additional requirements that cannot be fulfilled in a sensible manner by mobile electronic devices from a technical or commercial perspective. 
         [0041]    The invention is of particular advantage for small electronic devices, for instance for handheld devices, but it is to be understood that it may be employed with larger, stationary devices as well. The electronic device can be for instance a wireless communication device, like a mobile phone. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0042]    Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. 
           [0043]      FIG. 1  is a diagram illustrating different spheres above the Earth&#39;s surface; 
           [0044]      FIG. 2  is a diagram illustrating short wave propagations; 
           [0045]      FIG. 3  is a schematic block diagram of a conventional homodyne DRM receiver; 
           [0046]      FIG. 4  is a diagram of a quarter-wave vertical antenna used in the receiver of  FIG. 3 ; 
           [0047]      FIG. 5  is a schematic diagram of a system according to an embodiment of the invention; 
           [0048]      FIG. 6  is a diagram illustrating some exemplary implementation details of the system of  FIG. 5 ; 
           [0049]      FIG. 7  is a diagram illustrating some further exemplary implementation details of the system of  FIG. 5 ; 
           [0050]      FIG. 8  is a flow chart illustrating an operation in the system of  FIG. 5  implemented in accordance with  FIGS. 6 and 7 ; 
           [0051]      FIG. 9  is a diagram illustrating some alternative exemplary implementation details of the system of  FIG. 5 ; and 
           [0052]      FIG. 10  is a flow chart illustrating an operation in the system of  FIG. 5  implemented in accordance with  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0053]      FIG. 5  is a schematic diagram of an exemplary embodiment of a system according to the invention, which enables an AM-band reception without a very long antenna. 
         [0054]    The presented system comprises a mobile phone  50  and a headset  56 . The mobile phone  50 , which constitutes an exemplary electronic device according to the invention, includes a headset connector  51 . Within the mobile phone  50 , the headset connector  51  is not only connected to audio processing components (not shown), but also capacitively coupled to an FM-radio receiver  52  and to an AM-band receiver  53 . The AM-band receiver  53  comprises an active amplifier circuit  54  and further processing components  55 . The headset  56  can be connected by means of a corresponding connector  57  to the headset connector  51  of the mobile phone  50 . 
         [0055]    When a headset  56  is connected to the mobile phone  50 , its cable may be used as an antenna for the FM-radio receiver  52  in a conventional way. Due to the active amplifier circuit  54  of the AM-band receiver  53 , the cable of the headset  56  may also be used as an antenna for the AM-band receiver  53 . 
         [0056]    In one implementation, the AM-band receiver  53  of  FIG. 5  may be a DRM receiver.  FIG. 6  is a diagram presenting exemplary details of the mobile phone of  FIG. 5  comprising such a DRM receiver  53 . More specifically,  FIG. 6  presents how the cable of the headset  56  may be connected via the connectors  51 ,  57  to the FM-radio receiver  52  and to the DRM receiver  53 . 
         [0057]    The headset  56  comprises a left earspeaker  61  and a right earspeaker  62 , which may be physically coupled by a stirrup  67 . A respective ground (Gnd) wire  63 ,  64  of both earspeakers  61 ,  62  is connected via the same parallel connection of an impedance L 1  and a capacitor C 1  to ground. 
         [0058]    In addition, the ground wire  63 ,  64  of both earspeakers  61 ,  62  is connected via the same capacitor C 2  to a common point  68 . Moreover, a left (L) active wire  65  of the left earspeaker  61  and a right (R) active wire  66  of the right earspeaker  62  are connected via a respective capacitor C 3 , C 4  to the common point  68 . 
         [0059]    The headset wires  63  to  66  may have a length of approximately one meter. 
         [0060]    The common point  68  is connected via a series connection of an impedance L 2  and a capacitor C 5  to a first input RF 1  of the FM-radio receiver  52 , and via the series connection of impedance L 2  and capacitor C 5  and a further impedance L 3  to a second input RF 2  of the FM-radio receiver  52 . The first input RF 1  and the second input RF 2  are connected via a respective capacitor C 6 , C 7  to ground, while a ground input RFGND of the FM-radio receiver  52  is connected directly to ground. The FM antenna uses the headset wires  63  to  66  and is implemented as a conventional passive antenna. 
         [0061]    In the implementation of  FIG. 6 , the common point  68  is moreover connected to an input of a DRM receiver  53 . The DRM antenna is implemented as an active antenna. An active antenna consists of a passive part and an active part, as illustrated in  FIG. 7 . 
         [0062]    The passive part  70  of the active antenna is the actual antenna element, which corresponds in the present example to the headset wires  63  to  66 . It can be represented by a voltage source U 1  that is connected in series with an antenna resistor R rad , a loss resistor R loss  and an antenna capacitor C rad . The voltage source U 1  represents the signal level of a signal received via the antenna. 
         [0063]    The active part of the active antenna corresponds to the active amplifier circuit  54  of the DRM receiver  53 . The active amplifier circuit  54  may comprise for example simply an active amplifier  71 , like a JFET or a MOSFET. The output of the JFET or MOSFET  71  is connected to further processing components  55 . The further processing components  55  may comprise for instance an LNA  303  that is connected via an in-phase branch  310 - 315  and a quadrature branch  320 - 325  to a digital signal processor  330 , as described above with reference to  FIG. 3 . Further processing components are provided in the DRM receiver  53  for converting the digital output of the digital signal processor  330  into analog audio signals in a conventional manner. The active part of the antenna  54  and the components  303  to  330 , of which only LNA  303  is depicted in  FIG. 7 , belong to the DRM frontend of the DRM receiver and may be integrated on a single chip  72 . Alternatively, for example, only the analog processing components of the DRM frontend  72  could be implemented on a single chip, while the digital processing components are provided on another chip. 
         [0064]    The passive part of the antenna  70  is connected by AC-coupling with the input stage amplifier circuit realized by the FET  71 , which provides a high-ohmic and low capacitive input-impedance and therefore does not reduce the antenna input signal level. The antenna capacitor C rad  together with the input capacitance is building up a capacitive voltage-divider. The lower the FET input stage capacitance, the more antenna signal voltage is fed into the analog front-end. A low input capacitance gives at the same time a very broadband response. The FET input noise has to be designed as low as possible, but there is a trade-off between input capacitance and noise behavior. 
         [0065]    The active part of the antenna  54  is designed such that it provides a high linearity, even for large signals. As a result, less disturbances by cross-modulation and inter-modulation are caused. Further, the active part of the antenna  54  is designed such that it causes low noise. If the linearity capabilities of the active part of the antenna  54 , of the LNA  303  or of the mixers  310 ,  320  in the analog frontend are not sufficient in the presence of strong interfering signals, in addition a frequency selective filtering may be provided, similarly as in  FIG. 3 . The preselection filter could be arranged at the input of the active part of the antenna  54 . 
         [0066]    These provisions ensure that the inter-modulation robustness is high, meaning the reception of the usually weak wanted signal is stable, even when there are strong interfering signals in the adjacent frequency bands. 
         [0067]    The active antenna reception has to be broadband within the used frequency section, namely short wave (SW), middle wave (MW) or long wave (LW), as the propagation conditions vary over time, and therefore the transmitter frequency of the different channels can change quite often. The broadband reception has to be achieved in a mobile phone  50  with a supply voltage of only approximately 2.5 V. The requirements on the level of the voltage supply can reduced by using low noise input amplifiers, automatic gain control and filtering stages, in order to keep the signal level always in the linear region within the analog signal chain. Nevertheless, such a low supply voltage limits the achievable sensitivity and linearity properties of the active antenna. Still, the required SNR for a stable DRM reception is as low as 15 dB, such that an active vertical antenna of approximately one meter length is sufficient to receive the full frequency range from 10 kHz to 30 MHz. 
         [0068]      FIG. 8  is a flow chart illustrating the operation of a DRM reception by the mobile phone  50  of  FIG. 5  that is implemented according to  FIGS. 6 and 7 . 
         [0069]    A DRM transmitter broadcasts DRM signals, which propagate as described above with reference to  FIGS. 1 and 2 . 
         [0070]    If DRM reception is selected by a user of the mobile phone  50  (step  801 ), DRM signals are received via the earspeakers wires  63 - 66  of a connected headset  56  (step  802 ). The signals are amplified using an active amplifier circuit  54  (step  803 ), more specifically the MOSFET or JFET  71 . The amplified signals are then provided to the LNA  303  etc. for further processing to gain audio signals and/or video signals in a conventional manner (step  804 ). The audio signals may then be output via the earspeakers  61 ,  62  of the headset  56  in a conventional manner (step  805 ). 
         [0071]    The implementation according to  FIGS. 5 and 6  thus provides an antenna proposal for Digital Radio Mondiale for mobile phones, where the FM headset antenna from analog FM radio can be fully reused for AM band reception. 
         [0072]    In another implementation of the mobile phone  50  of  FIG. 5 , the AM-band receiver  53  may be an AM-radio receiver.  FIG. 9  is a diagram presenting exemplary details of the mobile phone of  FIG. 5  comprising such an AM-radio receiver  53 . 
         [0073]    In  FIG. 9 , the earspeakers  91  and the microphone  92  of a headset  56  are depicted. 
         [0074]    The earspeakers  91  are connected in a conventional manner to a respective audio signal source XEARP, XEARN and in addition via an FM interface to FM input ports of a combined FM/AM radio receiver  93 . The wires of the earspeakers  91  are thus used by the FM/AM radio receiver  93  as a passive FM-band antenna. 
         [0075]    The two, balanced wires of the microphone  92  of  FIG. 9  can be connected by a switch  94  to a conventional microphone interface, or be disconnected by the switch  94  from this microphone interface. The switch  94  is controlled by a software output port SWPORT 1  of the FM/AM-radio receiver  93 . 
         [0076]    The microphone  92  is further connected via an AM interface, comprising an active amplification circuit, to an AM input port of the FM/AM-radio receiver  93 . One of the microphone wires is connected more specifically via a capacitor C 1  to a gate of a first transistor T 1  and via capacitor C 1 , a resistor R 1  and a resistor R 2  to ground. The source of the transistor T 1  is connected via a resistor R 3  and resistor R 2  equally to ground. In addition, the source of transistor T 1  is connected to the gate of second transistor T 2 . The source of transistor T 2  is connected via a resistor R 4  to ground. A voltage supply DC is connected between the drain of transistor T 1  and ground and in parallel via an impedance L 1  between the drain of transistor T 2  and ground. The drain of transistor T 2 , finally, is connected via a capacitor C 2  to the AM input of the FM/AM-radio receiver  93 , and within the FM/AM-radio receiver  93  via a variable capacitor C 3  to ground. In this example, the amplification circuit of the active antenna comprising transistors T 1  and T 2 , resistors R 1 -R 4  and impedance L 1  thus realizes a two-stage amplification by means of transistors T 1  and T 2 . It has a high input impedance and a low capacitance input. 
         [0077]      FIG. 10  is a flow chart illustrating the operation of the FM/AM-radio reception by the mobile phone  50  of  FIG. 5  implemented according to  FIG. 9 . 
         [0078]    If a radio reception is selected by a user while a headset  56  is connected to the mobile phone  50  (step  901 ), it is determined whether an AM-radio reception has been selected (step  902 ). Both can be determined e.g. by an appropriate software. 
         [0079]    In case no AM-radio reception has been selected, and thus an FM-radio reception, the FM-band signals are received via the wires of the headset earspeakers  91  (step  903 ). The received signals are provided to the FM/AM-radio receiver via the FM interface and processed in a conventional manner for gaining FM audio signals (step  904 ). The gained audio signals are then output via the headset earspeakers ( 905 ). 
         [0080]    In case AM reception has been selected (step  902 ), in contrast, the FM/AM-radio receiver  93  causes the switch  94  to disconnect both wires of the microphone  92  from the microphone interface (step  906 ). The switch control by the FM/AM-radio receiver  93  can be realized by a software modification in the radio software. The AM-band signals are then received via a wire of the headset microphone  92  and provided to the AM interface (step  907 ). The AM interface applies an active amplification using the active amplification circuit (step  908 ). The amplified signals are provided to the FM/AM-radio receiver for processing to gain AM audio signals (step  909 ). The gained AM audio signals are then output via the headset earspeakers  91  (step  910 ). 
         [0081]    It has to be noted that in general, both the microphone or the earspeaker lines can be used for an AM and/or DRM receiver, but as the AM/DRM receiver may be an additional application in the electronic device then the cheapest solution would be separate wires for separate receivers. The AM/DRM antenna interface requires the high impedance/low capacitance input—which do not fit with FM receiver requirements, nor with the normal noise suppression components found in the audio lines—components which the FM radio antenna interface can accept. By using the microphone lines and a switching system, an AM/DRM receiver can be added as a “module” to an existing electronic device concept, for example an existing mobile phone concept. 
         [0082]    It is to be noted that the described embodiments constitute only some of a variety of possible embodiments of the invention.