DEVICE FOR SEPARATING RF SIGNALS

Abstract: The device for separating RF signals in at least a first and a second frequency range comprises a first electrode (10) for supplying the RF signals and a second electrode (16) for supplying the RF signals in the first frequency range. The first electrode (10) is coupled to the second electrode (16) via a first impedance element (12). The second electrode (16) is coupled to a reference point (18) via a second impedance element (14). A first impedance value of the first impedance element (12) is high for the RF signals in the second frequency range, whereas this first impedance value is comparatively low for the RF signals in the first frequency range. A second impedance value of the second impedance element (14) is substantially equal to zero for the RF signals in the second frequency range, and this second impedance value is unequal to zero for the RF signals in the first frequency range.

Description: Device for separating RF signals. The invention relates to a device for separating RF signals in at least a first and a second frequency range, which device comprises a first electrode for supplying the RF signals and also a second electrode for supplying the RF signals in the first frequency range. The invention also relates to an assembly of an antenna and such a device. A device as described in the opening paragraph is currently often used in motorcars which are equipped with an antenna suitable for receiving both radio and telephony signals and is also suitable for transmitting telephony signals. Examples of such radio signals are AM and FM signals. GSM signals are examples of telephony signals. The radio signals received by such an antenna can be processed and reproduced by a radio receiver. For satisfactorily processing these radio signals, it is necessary that the telephony signals in a device as described in the opening paragraph are separated from the radio signals. However, the use of the known devices leads to a considerable decrease of the sensitivity of the radio reception because the radio signals are also attenuated by these devices. This is a great problem, notably for the reception of AM signals. It is an object of the invention to provide a device for separating RF signals of the type described in the opening paragraph, leaving the amplitude of the RF signals in the first frequency range substantially unchanged. To this end, the device according to the invention is characterized in that the first electrode is coupled to the second electrode via a first impedance element, the second electrode is coupled to a reference point via a second impedance element, and the first impedance element has a first impedance value which is comparatively high for the RF signals in the second frequency range and comparatively low for the RF signals in the first frequency range, the second impedance element having a second impedance value which is substantially equal to zero for the RF signals in the second frequency range and unequal to zero for the RF signals in the first frequency range. The invention is based on the recognition that, in the known devices, the attenuation of the RF signals in the first frequency range is caused by the fact that the known devices are provided with one or more capacitors coupled between the signal path and ground. The use of such capacitors follows from the application of one of the conventional methods of designing filters. In the device according to the invention, the use of said capacitor is not necessary. For the RF signals in the second frequency range, for example GSM signals at 900 MHz, it holds that the impedance of the first impedance element is high and the impedance of the second impedance element is substantially equal to zero. As a result, the first impedance element ensures a considerable attenuation of the RF signals in the second frequency range, while the second impedance element constitutes a short circuit to the reference point for these RF signals, which reference point may be, for example ground. Both effects jointly ensure a substantial suppression of the RF signals in the second frequency range. For the RF signals in the first frequency range, for example AM signals between 150 kHz and 30 MHz and/or FM signals between 16 MHz and 108 MHz, it holds that the impedance of the first impedance element is comparatively low and the impedance of the second impedance element is unequal to zero. As a result, the RF signals in the first frequency range are hardly attenuated by the first impedance element, while the RF signals are not short-circuited to the reference point by the second impedance element. Both effects jointly ensure a substantially unchanged transfer of the RF signals in the first frequency range to the second electrode. An embodiment of the device according to the invention is characterized in that the second impedance element comprises a series arrangement of a coil and a capacitor. As a result, a series-resonant circuit is obtained having an impedance which is substantially equal to zero at a first resonance frequency f res ,. In this case it holds that, when L, is the inductance of the coil and Cj is the capacitance of the capacitor, f res , is equal to For frequencies which are unequal to the first resonance frequency f res ,, it holds that the impedance of the series arrangement is unequal to zero. By dimensioning the series- resonant circuit in such a way that the first resonance frequency f res , is located in the second frequency range, an elegant implementation of the second impedance element is obtained. A further embodiment of the device according to the invention is characterized in that the first impedance element comprises a parallel arrangement of a second coil and a second capacitor. As a result, a parallel-resonant circuit is obtained having an impedance which is high at a second resonance frequency f res 2 , while the impedance at frequencies unequal to this resonance frequency f res 2 is comparatively low. In this case it holds that, when L 2 is the inductance of the second coil and C 2 is the capacitance of the second capacitor, f res 2 is equal to By dimensioning the parallel-resonant circuit in such a way that the second resonance frequency f res 2 is located in the second frequency range, an elegant implementation of the first impedance element is obtained. It is to be noted that the parasitic capacitance of the second coil cannot be left outside consideration in this case. Dependent on its application, this parasitic capacitance alone may already be large enough to reach the desired second resonance frequency f res 2 . Another embodiment of the device according to the invention is characterized in that the second resonance frequency ranges between a fundamental frequency of the RF signals in the second frequency range and a harmonic of said fundamental frequency. It is thereby achieved that both the fundamental frequency and its harmonic are substantially attenuated by the parallel arrangement. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows an embodiment of a device according to the invention. Fig. 2 is an electric circuit diagram of an antenna circuit incorporating a device according to the invention. In the Figures, identical components are denoted by the same reference numerals. Fig. 1 shows an embodiment of a device according to the invention. This device may be used for separating RF signals in different frequency ranges such as, for example radio signals (AM, FM) and telephony signals (GSM). To this end, these RF signals must be applied to a first electrode 10 of the device. This first electrode 10 is coupled to a second electrode 16 via a first impedance element 12. This second electrode 16 is coupled to a reference point 18 via a second impedance element 14, which reference point 18 is grounded. In the device according to the invention, the RF signals applied to the first electrode 10 and located in a first and a second frequency range, respectively, are separated from each other in such a way that substantially only the RF signals in the first frequency range are obtainable from the second electrode 16. As is shown in Fig. 1 , the first impedance element 12 may be constituted by a parallel arrangement of a coil 20 and a capacitor 22. It is known that the impedance of such a parallel arrangement is frequency-dependent, with this impedance showing a peak at a first resonance frequency which is equal to l/27rVL,C 1 . Here, Lj represents the inductance of the coil 20 and C γ represents the capacitance of the capacitor 22. The parallel arrangement has a comparatively low impedance for frequencies which are unequal to the first resonance frequency. It is to be noted that, in many cases, the parasitic capacitance of the coil 20 cannot be left outside consideration when computing the first resonance frequency. As the case may be, this parasitic capacitance alone may already be large enough to reach the desired first resonance frequency. The capacitor 22 can then of course be dispensed with. It is also apparent from Fig. 1 that the second impedance element 14 may be constituted by a series arrangement of a coil 24 and a capacitor 26. Such a series arrangement is also known to have an impedance which is frequency-dependent and is substantially equal to zero at a second resonance frequency which is equal to l/2 τ L 2 C 2 . Here, L 2 represents the inductance of the coil 24 and C 2 represents the capacitance of the capacitor 26. The impedance of the series arrangement is unequal to zero for frequencies which are unequal to the second resonance frequency. By dimensioning said parallel and series arrangements in such a way that the first and the second resonance frequencies are both located in the second frequency range, it is achieved that the RF signals in the second frequency range, for example GSM signals at 900 MHz, are considerably attenuated by the high impedance of the parallel arrangement, while the series arrangement short-circuits these attenuated RF signals to a reference point 18. This reference point may be, for example ground. Both effects jointly ensure a substantial suppression of the RF signals in the second frequency range. It is further achieved by said dimensioning that the RF signals in the first frequency range, for example AM signals between 150 kHz and 30 MHz, are hardly attenuated by the low impedance of the parallel arrangement, while these RF signals are not short-circuited to ground 18 by the series arrangement. Both effects jointly ensure a substantially unchanged transfer of the RF signals in the first frequency range to the second electrode 16. The device according to the invention is preferably used in combination with an antenna which is suitable for receiving radio and telephony signals and is also suitable for transmitting telephony signals. Such antennas are currently often used in motorcars. By means of the device according to the invention, the telephony signals can be prevented from having a disturbing influence on the radio reception. Such an antenna can be coupled to the device according to the invention via the first electrode 10. Fig. 2 is an electric circuit diagram of an antenna circuit incorporating a device according to the invention. This antenna circuit may be coupled to an antenna as described above via a first electrode 10. A telephone may be connected to an electrode 32. On the one hand, telephony signals received by the antenna can be passed on to the telephone via a capacitor 30. On the other hand, the telephony signals generated by the telephone can be applied for transmission to the antenna via the capacitor 30. The device according to the invention is arranged between the first electrode 10 and the second electrode 16. The first impedance element 12 is constituted by the coil 20 which, together with its own parasitic capacitance, constitutes a parallel-resonant circuit. The second impedance element 14 is constituted by the series arrangement of the coil 24 and the capacitor 26. The telephony signals are suppressed to a considerable extent by the device according to the invention as described hereinbefore, so that substantially only the radio signals (AM and FM signals) received via the antenna are present at the second electrode 16. These AM and FM signals are subsequently separated by the antenna circuit whereafter they are amplified. The AM signals are suppressed to a considerable extent by the series arrangement of the capacitor 40 and the coil 42 which, together with the antenna, has a resonance frequency of approximately 100 MHz. Consequently, substantially only the FM signals are present at the base of the transistor 46. Together with the resistors 44 and 50 and the coil 48, this transistor 46 constitutes a negative feedback amplifier which amplifies the FM signals. This FM amplifier has a high input impedance so that the very frequency-dependent source resistance constituted by the capacitor 40 and the coil 42 does not substantially influence the transmission of the FM signals. The emitter of the transistor 46 is connected to ground via the coil 48. This provides a negative feedback for the FM signals. A resistor instead of the coil 48 is often used for this purpose. However, the use of the coil 48 has the advantage that it does not introduce noise and that no losses occur. Together with its own parasitic capacitance, the coil 34 constitutes a parallel-resonant circuit. Since this resonant circuit resonates at approximately 100 MHz, substantially only the AM signals are passed on to the capacitor 52. Unwanted low-frequency components are filtered from these AM signals by this capacitor 52. The AM signals filtered in this way are subsequently amplified in an amplifier comprising two MOS transistors 60 and 62 and the resistors 54, 56, 58 and 64. This amplifier is linear through a comparatively large range because the MOS transistors 60 and 62 are arranged in parallel. Due to this measure, a noise adaptation to the capacitive source is also obtained. The resistors 54 and 56 ensure a further suppression of the telephony signals. The combination of the coil 34 and the input capacitances of the MOS transistors constitutes a resonant circuit. The unwanted frequency dependence of this resonant circuit is limited by the attenuation resistor 36. The resistors 66, 68 and 74, together with the coil 70 and the capacitor 78, determine a desired output impedance and also provide the power supply for the FM-specific part of the antenna circuit. The coil 76 ensures that FM signals which are applied to the electrode 84 via the coil 82 and the capacitor 80 cannot reach the AM-specific part. Similarly, the series arrangement of the coil 82 and the capacitor 80 ensures that AM signals cannot reach the FM-specific part. In the electrode 84, the amplified AM and FM signals are joined again for their supply to a radio receiver.

Claims: CLAIMS: 1. A device for separating RF signals in at least a first and a second frequency range, which device comprises a first electrode (10) for supplying the RF signals and also a second electrode (16) for supplying the RF signals in the first frequency range, wherein the first electrode (10) is coupled to the second electrode (16) via a first impedance element (12), the second electrode (16) is coupled to a reference point (18) via a second impedance element (14), and the first impedance element (12) has a first impedance value which is comparatively high for the RF signals in the second frequency range and comparatively low for the RF signals in the first frequency range, the second impedance element (14) having a second impedance value which is substantially equal to zero for the RF signals in the second frequency range and unequal to zero for the RF signals in the first frequency range. 2. A device as claimed in claim 1, characterized in that the second impedance element (14) comprises a series arrangement of a coil (24) and a capacitor (26). 3. A device as claimed in claim 2, characterized in that the series arrangement has a first resonance frequency which is located in the second frequency range. 4. A device as claimed in claim 3, characterized in that the first resonance frequency is approximately 900 MHz. 5. A device as claimed in any one of the preceding claims, characterized in that the first impedance element (12) comprises a parallel arrangement of a second coil (20) and a second capacitor (22). 6. A device as claimed in claim 5, characterized in that the second capacitor (22) is constituted by a parasitic capacitance of the second coil (20). 7. A device as claimed in claim 5 or 6, characterized in that the parallel arrangement has a second resonance frequency which is located in the second frequency range. 8. A device as claimed in claim 7, characterized in that the second resonance frequency is approximately 900 MHz. 9. A device as claimed in claim 7, characterized in that the second resonance frequency ranges between a fundamental frequency of the RF signals in the second frequency range and a harmonic of said fundamental frequency. 10. An assembly of an antenna and a device as claimed in any one of the preceding claims, the antenna being coupled to the first electrode (10).

Inventors: SESSINK FRANCISCUS JOSEPHUS AL, SESSINK, FRANCISCUS, JOSEPHUS, ALFONSUS, MARIA
Applicants: MANNESMANN VDO AG, SESSINK FRANCISCUS JOSEPHUS AL, SESSINK, FRANCISCUS, JOSEPHUS, ALFONSUS, MARIA
Application year: 1998
Main classification: H03H7
IPC codes: H03H7