Abstract:
A multiplexer for a device capable of receiving signals in first and second frequency bands, the multiplexer comprising a first branch for connecting an antenna to a first sub-system configured to process received signals in the first frequency band, a second branch for connecting the antenna to a second sub-system configured to process received signals in the second frequency band, wherein the first branch comprises a filter for attenuating received signals in the second frequency band and the second branch couples the antenna directly to the second sub-system.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of U.K. Patent Application No. GB 0916494.8, filed Sep. 21, 2009, whose disclosure is hereby incorporated by reference in its entirety into the present disclosure. 
     FIELD OF THE INVENTION 
     The present invention relates to a multiplexer, to a device capable of receiving signals in a plurality of frequency bands and to a front-end module for such a device. 
     DESCRIPTION OF RELATED ART 
     Mobile communications devices such as mobile telephones increasingly incorporate multiple functions in a single package. For example, many mobile telephones include a receiver for a positioning system such as the Global Positioning System (GPS) in addition to a radio receiver for receiving radio signals from base stations of a mobile telephone network. 
     To reduce the number of components in such a device, it is commonplace for the positioning system receiver and the radio receiver to use a single common antenna. This is possible because the frequency band used by the radio receiver, which is typically 925-960 MHz for a GSM radio receiver, is different from the frequency used by the positioning system, which is typically around 1576 MHz for a GPS receiver. This sharing of a common antenna is facilitated by a diplexer, which selectively couples the radio receiver and the positioning system to the single antenna according to the frequency of a signal received by the antenna. 
       FIG. 1  is a schematic representation of a known architecture for use in a device such as a mobile telephone having GSM and GPS receivers. The architecture  10  has a single antenna  12  which is coupled to a diplexer  14  having parallel first and second branches  16 ,  18 . The first branch  16  includes a low-pass filter  20  with a cut-off frequency at or just above 960 MHz, such that signals with frequencies higher than 960 MHz are blocked. The second branch  18  includes a high-pass filter  22  which has a cut-off frequency at or just below 1576 MHz, such that signals with frequencies below 1576 MHz are blocked. The low-pass filter  20  has an output which is connected to an input of a GSM surface acoustic wave (SAW) filter  24 , which passes GSM frequency signals to a GSM sub-system  28 , whilst the high-pass filter  22  has an output which is connected to an input of a GPS SAW filter  26 , which passes GPS frequency signals to a GPS sub-system  30 . 
     In use of the architecture  10 , GSM signals received by the antenna  12  are passed by the low-pass filter  20 . allowing them to reach the GSM SAW filter  24  and the GSM sub-system  28 , but are blocked by the high-pass-filter  22 , preventing them from reaching the GPS sub-system  30 . Similarly, GPS signals received by the antenna  12  are passed by the high-pass filter  22  to the GPS SAW filter  26  and on to the GPS sub-system  30 , but are blocked by the low-pass filter  20 , which prevents them from reaching the GSM sub-system  28 . 
     Whilst the architecture illustrated in  FIG. 1  can be effective, it has the disadvantage that the filters  20 ,  22  each give rise to an insertion loss, thus attenuating any signal received by the antenna  12 . Whilst this may not be a particular problem for the GSM sub-system  28 , it can give rise to difficulties for the GPS sub-system  30 , as a received GPS signal may already be weak. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a multiplexer for a device capable of receiving signals in first and second frequency bands, the multiplexer comprising a first branch for connecting an antenna to a first sub-system configured to process received signals in the first frequency band and a second branch for connecting the antenna to a second sub-system configured to process received signals in the second frequency band, wherein the first branch comprises a filter for attenuating received signals in the second frequency band and the second branch couples the antenna directly to the second sub-system. 
     The multiplexer of the present invention permits a single antenna to be used to receive signals in different frequency bands. The multiplexer of the present invention improves the quality of reception of a signal in the second frequency band, as the second branch contains no additional components that could attenuate a received signal, such that the second branch can be said directly to couple the antenna to the second sub-system. The only attenuation of the received signal is that which occurs as a result of the signal passing through the second branch. Additionally, the multiplexer of the present invention includes fewer components than prior art multiplexers and thus has a lower bill of materials cost than prior art systems, whilst permitting greater design flexibility. 
     Preferably the second branch does not include a filter. 
     The filter of the first branch may comprise a diplex filter. 
     The signals in the first frequency band may be transmitted in accordance with a first telecommunications standard and the signals in the second frequency band may be transmitted in accordance with a second telecommunications standard. 
     The first telecommunications standard may be GSM. 
     The first telecommunications standard may alternatively be Bluetooth®. 
     The second telecommunications standard may be GPS. 
     According to a second aspect of the invention there is provided a receiver for a device capable of receiving signals in a plurality of frequency bands, the receiver comprising an antenna coupled to a multiplexer according to the first aspect of the invention. 
     According to a third aspect of the invention there is provided a front-end module for a receiver for a device capable of receiving signals in a plurality of frequency bands, the front-end module comprising a multiplexer according to the first aspect of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, of which: 
         FIG. 1  is a schematic representation of known architecture for use in a device having both GSM and GPS receivers. 
         FIG. 2  is a schematic representation of a receiver architecture including a diplexer according to an embodiment of the present invention. 
         FIG. 3  is a schematic representation of a receiver architecture for use in a device having both Bluetooth® and GPS receivers according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 2 , an architecture for use in a front end module of a receiver is shown generally at  40 . For reasons of clarity and brevity only those components that are relevant to the present invention are illustrated in  FIG. 2 , but it will be appreciated that the receiver  40  will include many other components. 
     The receiver  40  has a single antenna  42  which is able to receive GSM signals in the 869-960 MHz frequency band used by the GSM 850 and GSM 900 standards and GPS signals in the 1574-1576 MHz frequency band. The antenna  42  is connected to a diplexer  44  which has parallel first and second branches  46 ,  48 . It will be appreciated that the diplexer  44  is a form of multiplexer. 
     The first branch  46  couples the antenna  42  to a GSM processing sub-system  60 , and includes a diplex filter  50  with a pass-band having a frequency range of 869 MHz to 960 MHz and a stop-band having a frequency range of 1574 MHz to 1576 MHz, so as to pass signals in the GSM frequency band but block signals at the higher GPS frequencies. In this example the diplex filter  50  is a passive device made up of a capacitor  52  and an inductor  54  connected in series, with a further capacitor  56  being connected in parallel with the series capacitor  52  and inductor  54 . The values of the capacitors  52 ,  56  and the inductor  54  are chosen so that in the illustrated configuration they form a diplex filter with the required pass-band and stop-band. For example, the capacitor  52  may have a value of 3.3 picofarads, the inductor  54  may have a value of 10 nanohenries and the capacitor  56  may have a value of 1.2 picofarads. However, those skilled in the relevant art will appreciate that different component values could be used to create the same effect, and that any filter having the required characteristics could perform the function of the diplex filter  50 . 
     An output of the diplex filter  50  is coupled to an input of the GSM processing sub-system  60 , which includes a surface acoustic wave (SAW) filter  62 , a low-noise amplifier (LNA)  64  and a matching section  66  for matching the output impedance of the SAW filter  62  to the input impedance of the LNA  64 . An output of the LNA  64  is coupled to additional components (not shown) of the receiver  40  for further processing of a received GSM signal. 
     The second branch  48  couples the antenna  42  directly to a GPS sub-system  70 , in the sense that in contrast to the prior art system  10  described above, the second branch  48  does not contain a high-pass filter  16  or any other additional components which would attenuate a received signal passing through the second branch  48 . Thus, any signal received by the antenna  42  is passed directly to the GPS sub-system  70  by the second branch  48  without any substantial attenuation other than that which occurs as a result of the signal passing through the second branch  48 , i.e. without any additional attenuation due to other components in the second branch  48 . 
     The GPS sub-system  70  includes a SAW filter  72  and a GPS low noise amplifier (LNA)  74 . An input of the LNA  74  is coupled to an output of the SAW filter  72  by a matching section  76 , which matches the input impedance of the LNA  74  to the output impedance of the SAW filter  72 . 
     At GPS frequencies the first branch  46  presents an open circuit to the antenna  42  due to the diplex filter  50 . Thus substantially the whole of the received signal is passed by the second branch  48  to the GPS sub-system  70 . In contrast to the prior art system  10  described above, the second branch  48  does not include a high-pass filter  16 , and thus the insertion losses associated with such a high-pass filter are avoided here. Typically the second branch  48  has a total loss of around 0.07 dB, which contrasts with a much greater loss of up to around 0.5 dB incurred by the prior art system  10  described above as a result of the insertion loss of the high-pass filter  16 . 
     As GSM frequencies the diplex filter  50  is effectively a short circuit, so a signal received by the antenna  42  is passed to the GSM sub-system  60  by the first branch  46 . As there is no filter in the second branch  48 , a proportion of the received signal is also passed to the GPS sub-system  70 , which gives rise to an additional loss of around 1.5 dB in the GSM sub-system  60 . 
     The diplexer  44  thus improves the performance of the GPS sub-system  70  by reducing the attenuation of a received GPS signal at the expense of reduced performance of the GSM sub-system  60 . This compromise is generally acceptable, as GSM signals are typically stronger than GPS signals and good performance in relation to reception of GPS signals is seen by many device manufacturers as more desirable than good performance in relation to reception of GSM signals. In hand with this, the diplexer  44  provides a reduced-cost option in comparison to prior art devices as it contains fewer components. This also allows the diplexer  44  to be smaller than prior art devices, and provides a greater degree of design flexibility. 
     Although in the example described above the first and second branches  46 ,  48  are configured for reception of GSM and GPS signals respectively, it will be appreciated that the principles of the present invention can be applied to received signals transmitted in accordance with other telecommunications standards. For example,  FIG. 3  is a schematic illustration of an architecture which can be used in a device having both Bluetooth® and GPS receivers. 
     In the embodiment shown generally at  80  in  FIG. 3 , an antenna  82  is able to receive both GPS signals in the 1574 MHz to 1576 MHz range and Bluetooth® signals in the 2.4 GHz range. 
     The antenna  82  is coupled to a diplexer  84  which has first and second branches  86 ,  88 . The first branch  86  couples the antenna  82  to a Bluetooth® processing sub-system  100 , which includes a surface acoustic wave (SAW) filter  102 , a low-noise amplifier (LNA)  104  and a matching section  106  for matching the output impedance of the SAW filter  102  to the input impedance of the LNA  104 . An output of the LNA  104  is coupled to additional components (not shown) of the receiver  80  for further processing of a received Bluetooth® signal. 
     The first branch  86  includes a diplex filter  90  with a pass-band having a frequency range of 2.4 GHz to 2.4835 GHz and a stop-band having a frequency range of 1574 MHz to 1576 MHz, so as to pass signals in the Bluetooth® frequency band but block signals at the lower GPS frequencies. 
     In this example the diplex filter  90  is a passive device made up of a capacitor  92  and an inductor  94  connected in series, with a further inductor  96  being connected in parallel with the series capacitor  92  and inductor  94 . The values of the capacitor  92  and the inductors  94 ,  96  are chosen so that in the illustrated configuration they form a diplex filter with the required pass-band and stop-band. Appropriate component values will be readily apparent to those skilled in the relevant art. Moreover, it will be understood that any filter having the required characteristics could perform the function of the diplex filter  90 . 
     As in the embodiment shown in  FIG. 2 , the second branch  88  couples the antenna  82  directly to a GPS sub-system  110 , in the sense that in contrast to the prior art system  10  described above, the second branch  88  does not contain a high-pass filter  16  or any other additional components which would attenuate a received signal passing through the second branch  88 . Thus, any signal received by the antenna  82  is passed directly to the GPS sub-system  110  by the second branch  88  without any substantial attenuation other than that which occurs as a result of the signal passing through the second branch  88 , i.e. without any additional attenuation due to other components in the second branch  88 . 
     As in the embodiment illustrated in  FIG. 2 , the GPS sub-system  110  includes a SAW filter  112  and a GPS low noise amplifier (LNA)  114 . An input of the LNA  114  is coupled to an output of the SAW filter  112  by a matching section  116 , which matches the input impedance of the LNA  114  to the output impedance of the SAW filter  112 . 
     At GPS frequencies the first branch  86  presents an open circuit to the antenna  82  due to the diplex filter  90 . Thus substantially the whole of the received signal is passed by the second branch  88  to the GPS sub-system  110 . In contrast to the prior art system  10  described above, the second branch  88  does not include a high-pass filter  16 , and thus the insertion losses associated with such a high-pass filter are avoided here. 
     As Bluetooth® frequencies the diplex filter  90  is effectively a short circuit, so a signal received by the antenna  82  is passed to the Bluetooth® sub-system  100  by the first branch  86 . As there is no filter in the second branch  88 , a proportion of the received signal is also passed to the Bluetooth® sub-system  100 , which gives rise to an additional loss of around 1.5 dB in the Bluetooth® sub-system  100 . As with the embodiment illustrated in  FIG. 2 , the diplexer  84  improves the performance of the GPS sub-system  70  by reducing the attenuation of a received GPS signal at the expense of reduced performance of the Bluetooth® sub-system  100 . This compromise is generally acceptable, as Bluetooth® signals are typically stronger than GPS signals and good performance in relation to reception of GPS signals is seen by many device manufacturers as more desirable than good performance in relation to reception of Bluetooth® signals. 
     It will be appreciated that the present invention is equally suited to other telecommunications standards such as IEEE 802.11 (Wi-Fi™). Moreover, the diplexer  44 ,  84  could include additional branches (meaning that it can no longer be referred to as a diplexer, but only by the more general term multiplexer), with each branch coupling the antenna  42 ,  82  to a sub-system for processing signals transmitted in accordance with a different telecommunications standard. 
     The diplexer  44 ,  84  (or multiplexer) can be implemented in a number of ways. For example, the diplexer  44 ,  84  may be implemented using discrete components surface-mounted on a circuit board of a receiver  40 ,  80  or of a front-end module for use in a receiver  40 ,  80 . 
     While preferred embodiments of the invention have been set forth above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the invention. For example, numerical values are illustrative rather than limiting, as are recitations of specific telecommunication protocols. Therefore, the present invention should be construed as limited only by the appended claims.