Abstract:
A multiband receiver and associated method are provided for simultaneous reception of a number of RF signals with partially different carrier frequencies. An RF/IF converter is provided with a number of local oscillators corresponding to a number of RF signals in order to produce L0 in phase components corresponding to the carrier frequencies and quadrature components; in addition to an in-phase mixer and a quadrature mixer for mixing the respective L0 in-phase and quadrature components with the RF signals in order to produce intermediate frequency signals with IF in-phse components and IF quadrature components. A complex fiber carries out complex filtering on the basis of the intermediate frequency signals in order to suppress reflected intermediate frequency signals.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates, generally, to a multiband receiver as well as to an associated method and, more particularly to a multiband receiver for simultaneous reception of multiband RF signals, some of which are at different carrier frequencies.  
         [0002]     The use of telecommunications networks using, for example, mobile radio systems has developed increasingly in recent years, leading to situations in which, in addition to earlier telecommunications radio networks, new networks, such as cellular networks, are often being added.  
         [0003]     As a consequence, a large number of different transmission standards are increasingly coexisting alongside one another; for example, digital and analog mobile telecommunications networks. By way of example, the GSM Standard (Global System for Mobile Communication), the DECT Standard (Digital European Cordless Telecommunication) and the UMTS Standard (Universal Mobile Telecommunication System) may be mentioned as digital telecommunications networks in Europe. Worldwide, the PCS1900 (Personal Communication Services), DCS1800 (Digital Cellular System) or JDC (Japanese Digital Cellular) Standards furthermore may be mentioned by way of example. This large number of different communications networks and transmission standards are based on either substantially the same or quite often different carrier frequencies.  
         [0004]     By way of example, there is a need to connect a notebook via a “Bluetooth” interface to a mobile telephone, and to connect this mobile telephone via a GSM interface to the Internet, in the process of which two different RF signals have to be received at the same time via different interfaces in the mobile telephone.  
         [0005]     There is, therefore, an increasing requirement to provide a receiver which is at the same time compatible with at least two or more transmission standards and frequency bands. Telecommunications terminals with receivers such as these are normally referred to as dual-band or triband terminals and allow reception of RF signals (radio frequency signals) to two or three transmission standards.  
         [0006]     Two or more radio receivers (radios) that are separate from one another normally have been combined with one another in order to produce such multiband receivers. However, this results in extraordinarily high costs for implementation.  
         [0007]     Furthermore, the document EP 0 945 990 A1 discloses a multiband receiver for reception of multiband RF signals, some of which are at different carrier frequencies, with an RF/IF converter converting the RF signals (radio frequency) to so-called intermediate frequency signals. To be more precise, a local oscillator is, in this case, used to produce two intermediate frequencies for, for example, the different standards GSM1800 and GSM1900, thus producing a dual-band receiver. However, this has the disadvantage that it is restricted to a relatively small number of carrier frequencies, and to carrier frequencies in a similar frequency band, as well as the disadvantage of the use of expensive intermediate frequency paths.  
         [0008]     In contrast, the present invention is directed toward a multiband receiver and an associated method via which multiband RF signals, some of which are at different carrier frequencies, can be received at the same time in a cost-effective and flexibly adaptable manner.  
       SUMMARY OF THE INVENTION  
       [0009]     According to the present invention, the use of an RF/IF converter for conversion of the RF signals to a large number of intermediate frequency signals, with a large number (which corresponds to the large number of RF signals) of local oscillators being used to produce in-phase and quadrature components associated with the carrier frequencies, as well as the use of an in-phase and quadrature mixer for mixing the respective in-phase and quadrature components with the RF signals in order to produce IF in-phase and IF quadrature components in an intermediate frequency band, allow the intermediate frequency signals which are desired for further processing to be filtered out in a simple and very accurate manner using an additional complex filter, on the basis of the intermediate frequency signals. Thus, not only is it possible to use a single radio receiver for a large number of multiband radio frequencies or RF signals at the same time, but this also results in an extremely flexible configuration for implementation of transmission standards which have not yet been agreed or have so far been agreed only with a very great degree of difficulty.  
         [0010]     The intermediate frequency signals preferably have unreflected and mirror-image signals in a low intermediate frequency band (low IF), with the complex filter producing a phase shift between the associated in-phase and quadrature components of these signals such that the undesirable mirror-image signal components are at least partially canceled out. The use of signals in a low intermediate frequency band allows the provision of particularly low-cost multiband receivers. The complex filtering may, in this case, be carried out on an analog or digital basis, using analog or digital intermediate frequency signals. If the mirror-image or unreflected signals all lie in the same positive or negative frequency band, the complex filter can be implemented particularly easily.  
         [0011]     The local oscillators preferably produce LO signals which do not interfere at all with one another, so that the signal quality can be improved considerably. By way of example, a polyphase filter is used as the complex filter.  
         [0012]     In order to further improve the reception quality, the multiband receiver may have a preamplifier in its RF receiving part, which has a frequency splitter for splitting the RF signals into their associated carrier frequency bands, separate amplifier paths for amplification of the RF signals in the respective carrier frequency bands, and a signal combiner for combination of the split and amplified RF signals. So-called low noise preamplification can be carried out for each carrier frequency band, particularly if the amplifier paths each have an RF bandpass filter and a variable RF amplifier.  
         [0013]     The reception quality also can be improved by using an amplifier path with a variable IF amplifier and an IF low-pass filter in an intermediate frequency receiving part.  
         [0014]     Digital signal processing which is arranged on the output side preferably has a large number of local oscillators for production of signal components which are associated with the determined intermediate frequency signals, and a large number of digital low-pass filters for filtering the digital signal components. This results in high-quality further processing of the intermediate frequency signals which are in the low intermediate frequency band.  
         [0015]     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0016]      FIG. 1  shows a simplified block diagram of a multiband receiver according to a first exemplary embodiment of the present invention.  
         [0017]      FIG. 2  shows a simplified block diagram of a multiband receiver according to a second exemplary embodiment of the present invention  FIG. 3  shows a simplified spectral illustration of the signals which are used according to the first exemplary embodiment.  
         [0018]      FIG. 4  shows a simplified spectral illustration of the signals which are used according to the second exemplary embodiment.  
         [0019]      FIG. 5  shows a simplified spectral illustration of signals according to a further exemplary embodiment of the present invention, which is not illustrated. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The present invention will be described in the following text with reference, by way of example, to a dual-band receiver, which receives RF signals at carrier frequencies in an 1800 MHz band and in a 900 MHz band. The stated frequencies will be used in this case only to illustrate the basic principle.  
         [0021]      FIG. 1  shows a simplified block diagram of a multiband receiver for reception of two RF signals which are transmitted in a carrier frequency band at 1800 MHz and 900 MHz. An RF receiving part RF is used for reception of the RF signals via, for example, an antenna  1  and preprocessing via a preamplifier which includes the blocks  2  to  7 . However, the antenna  1  may be replaced in the same way by a coaxial cable connection or some other connection for cable-based reception of RF signals. The preamplifier is used for so-called low noise amplification, and may, in principle also be omitted.  
         [0022]     As is shown in  FIG. 1 , the input signal, which has at least two RF signals and is received at the antenna  1 , is split in a frequency splitter  2  into RF signals with their associated carrier frequency bands. To be more precise, an RF signal RF 1  which is transmitted, by way of example, at the carrier frequency of 1800 MHz is in this case passed to a first amplifier path, which includes the blocks  3  and  5 , and an RF signal RF 2  which is transmitted at the carrier frequency of 900 MHz is passed to the amplifier path which includes the blocks  4  and  6 . In this case, RF bandpass filtering is carried out in the blocks  3  and  4  in order to improve the respective RF signals RF 1  and RF 2 . They are then amplified separately in the RF amplifiers  5  and  6 , with the split, filtered and amplified RF signals RF 1  and RF 2  then being combined again in a signal combiner  7 .  
         [0023]     The RF signals RF 1 , RF 2  which have been processed in this way are now, for example, split equally on the basis of power in a signal splitter  8  and are supplied to a quadrature mixer  9  and to an in-phase mixer  10 , respectively, as input signals. The quadrature mixer  9  and the in-phase mixer  10  are also respectively supplied with a quadrature component Q LO  and an in-phase component I LO  which are produced from a large number of local oscillators LO 1 , LO 2 , . . . corresponding to the large number of RF signals. Since, as is shown in  FIG. 1 , only two RF signals RF 1  and RF 2  at carrier frequencies of 1800 MHz and 900 MHz are used, two and only two local oscillators LO 1  and LO 2  are used according to the present invention, which oscillate at a frequency which is slightly below or slightly above the respective carrier frequency, and which respectively produce in-phase and quadrature components. By way of example, the local oscillator LO 1  oscillates at a frequency of 1798 MHz, and is thus 2 MHz below the carrier frequency of the RF signal RF 1 . On the other hand, the second local oscillator LO 2  oscillates at a frequency of 899 MHz, and is thus 1 MHz below the carrier frequency of the second RF signal RF 2 . The chosen frequency differences (2 MHz, 1 MHz) preferably lie in a similar frequency band. In principle, the carrier frequencies also may, of course, lie in a similar (or identical) frequency band (for example, GSM 1800, GSM 1900, DECT), which results in relatively minor problems in the implementation of the multiband receiver.  
         [0024]      FIG. 3  shows a simplified spectral illustration of the signals which are used in  FIG. 1 . In this highly simplified illustration, the RF signal RF 1  includes a spectral line at 1800 MHz and the RF signal RF 2  includes a spectral line at 900 MHz. The local frequency signals or LO signals lo 1  and lo 2  which are produced by the local oscillators LO 1  and LO 2  in consequence occur as spectral lines at frequencies of 1798 MHz and 899 MHz, which are shifted through different frequency differences of 2 MHz and 1 MHz. In this case,  FIG. 3  identically illustrates both the in-phase components I LO  and the quadrature components Q LO , which have been shifted through 90°, although they are supplied to the different mixers, or to the quadrature mixer  9  and to the in-phase mixer  10 .  
         [0025]     The combination of the mixers  9  and  10  as well as the associated local oscillators LO 1 , LO 2 , . . . results in a so-called RF/IF converter for conversion of the large number of RF signals RF 1 , RF 2 , . . . to a large number of analog intermediate frequency signals IF 1 , IF 2 , as well as the mirror-image signals IF 1 ′ and IF 2 ′ which are associated with them. To be more precise, as shown in  FIG. 1 , the quadrature mixer  9  outputs the respective quadrature components of the unreflected intermediate frequency signals IF 1  and IF 2 , as well as the mirror-image intermediate frequency signals IF 1 ′ and IF 2 ′. In the same way, the in-phase mixer  10  outputs the in-phase components of the unreflected and mirror-image intermediate frequency signals IF 1 , IF 2  as well as IF 1 ′ and IF 2 ′.  
         [0026]     The spectral position of these unreflected and mirror-image intermediate frequency signals IF 1 , IF 2 , IF 1 ′ and IF 2 ′ is illustrated in  FIG. 3 , in which case the convolution of the local oscillator signals lo 1  and lo 2  results in the intermediate frequency signals IF 1  and IF 2  which correspond to the RF signals RF 1  and RF 2  being located in a low intermediate frequency band at the frequencies of 2 MHz and 1 MHz. In this case, the expression “signals in the low intermediate frequency band” is used when their frequency corresponds to the bandwidth of the associated RF signals. If an RF/IF converter is produced for such a low intermediate frequency band, this results in particularly low-cost implementations for the intermediate frequency paths.  
         [0027]     The mirror-image intermediate frequency signals IF 1 ′ and IF 2 ′ now can be canceled out by the use of a downstream analog complex filter AKF as is indicated by the bandpass filtering from about 0 to 3 MHz in  FIG. 3 . By way of example, the analog complex filtering can be carried out by suitable phase shifting of the in-phase and quadrature components I IF , I IF′ , Q IF , Q IF′  that are produced by the RF/IF converter in the intermediate frequency band, thus resulting in the mirror-image intermediate frequency signals IF 1 ′ and IF 2 ′ being at least partially canceled out.  
         [0028]     As is shown in  FIG. 3 , the frequencies of the local oscillators LO 1  and LO 2  and of the associated signals lo 1  and lo 2  may be chosen in such a way that no interference occurs, particularly in the event of convolution in the frequency domain. As such, even RF signals whose carrier frequencies are well separated from one another can be changed to a low-frequency intermediate frequency band (LOW IF) which allow simple and low-cost signal processing. In the same way,  FIG. 3  shows that the signals lo 1  and lo 2  which are produced by the local oscillators LO 1  and LO 2  preferably should be below or above their associated RF signals, so that the associated intermediate frequency signals IF 1  and IF 2  also are respectively located in the same positive or negative frequency band. This allows the undesirable mirror-image intermediate frequency signals IF 1 ′ and IF 2 ′ to be reliably suppressed. A so-called polyphase filter preferably is used for the analog complex filter AKF.  
         [0029]     The RF/IF converter described above thus allows a large number of RF signals with a frequency separation of Δf which is considerably greater than an associated intermediate frequency bandwidth to be converted to virtually the same intermediate frequency band, in which case, particularly in a low-frequency intermediate frequency band, a situation occurs in which positive and negative frequency bands are located around the zero point. A signal which can be further and easily processed then can be produced using the in-phase and quadrature components of these intermediate frequency signals via a complex analog “image rejection” filter. Low-frequency intermediate frequency signals such as these also allow the filter to be integrated in an RF chip, thus further reducing the costs and the space required. Furthermore, the use of the analog complex filter AKF makes it possible to overcome interfering DC voltage components (DC).  
         [0030]     A data signal in the low intermediate frequency band is now produced on the output side of the analog complex filter AKF and optionally can be supplied to a variable IF amplifier  11  and to an IF low-pass filter  12 . The intermediate frequency signal, which is still in analog form up to this point, is then converted to a digital intermediate frequency signal via an AID converter  13  and is further processed by a digital signal processing part DSP in order to produce recovered data signals.  
         [0031]     In this digital further-processing, which is known per se, it is possible, for example, for each intermediate frequency which is produced in an intermediate frequency or IF receiving part IF to be used by a local oscillator to produce signal components which are associated with the intermediate frequency signals IF 1 , IF 2 . By way of example, as is shown in  FIG. 1 , a local oscillator ΔO 1  may oscillate at the frequency of 2 MHz relating to the intermediate frequency signal IF 1 , and in each case can be mixed, via a respective mixer  15  or  17 , with its in-phase and quadrature component to form the digital input signal. The signals that have been mixed in this way then can be further-processed in digital low-pass filters  16  and  18 , thus allowing the signal quality to be improved further. In the same way, this digital signal processing is applied via second local oscillator ΔO 2  to the second intermediate frequency signal IF 2  at a frequency of about 1 MHz.  
         [0032]      FIG. 2  shows a simplified block diagram of a multiband receiver according to a second exemplary embodiment, with the same reference symbols denoting the same or corresponding elements, which will not be described again in the following text.  
         [0033]     As is shown in  FIG. 2 , and in contrast to the exemplary embodiment shown in  FIG. 1 , a digital complex filter DKF now is used to reduce the mirror-image intermediate frequency signals IF 1 ′ and IF 2 . In this case,  FIG. 4  shows the signals which are associated with the second exemplary embodiment shown in  FIG. 2  in a simplified spectral illustration in which there are no differences with respect to the simplified illustration shown in  FIG. 3 .  FIG. 4 , therefore, will not be described again in the following text, and reference should be made to the description in  FIG. 3 .  
         [0034]     The major difference between the first exemplary embodiment shown in  FIG. 1  and the second exemplary embodiment shown in  FIG. 2  is only that the complex filter is now in the form of a digital complex filter DKF and is located in the digital part of the multiband receiver. As a consequence, and as shown in  FIG. 2 , the quadrature and in-phase components I IF , I IF′ , Q IF , Q IF′  which are output from the quadrature mixer  9  and from the in-phase mixer  10  of the intermediate frequency signals IF 1 , IF 2 , . . . IF 1 ′, IF 2 ′, . . . are then supplied, for example, to an IF low-pass filter  14   a  or  14   b  respectively, and are then passed to an IF amplifier path as is known from  FIG. 1 .  
         [0035]     However, since the complex filter is not provided until the digital part is reached, the IF amplifier path must be provided both for the quadrature components and for the in-phase components, that is to say it must be duplicated, and once again includes, for example, a variable IF amplifier  11   a ,  11   b  and an IF low-pass filter  12   a  and  12   b . An A/D converter  13   a  and  13   b , likewise, now must be provided in each case for the quadrature components and for the in-phase components in order subsequently to supply the digitized intermediate frequency signal and its components to the digital complex filter in a known manner for filtering of the mirror-image intermediate frequency signals IF 1 ′ and IF 2 ′. The filtering in this case once again basically corresponds to the filtering in the analog complex filter, for which reason, reference at this point should be made to the description relating to  FIG. 1 .  
         [0036]      FIG. 5  shows a simplified spectral illustration of the signals according to a further exemplary embodiment, which is not illustrated, with the same reference symbols denoting identical or corresponding signal lines, which will not be described again in the following text.  
         [0037]     As is shown in  FIG. 5  and in contrast to the spectral lines in  FIGS. 3 and 4 , the signals lo 1  and lo 2  which are produced by the local oscillators are now above the associated RF carrier frequencies RF 1  and RF 2 . To be more precise, the signal lo 1  which is produced by the local oscillator LO 1  is at 1802 MHz and the signal lo 2  is at 901 MHz. As a consequence, the intermediate frequency signals IF 1  and IF 2  which are produced during the convolution process are now in the negative frequency band at −2 MHz and −1 MHz, while the associated mirror-image intermediate frequency signals IF 1 ′ and IF 2 ′ are at the frequencies +2 MHz and +1 MHz. In an exemplary embodiment such as this, the analog or digital complex filter must be set such that the negative frequency band is now recovered and the mirror-image intermediate frequency signals which lie in the positive band are at least partially canceled out. The rest of the signal processing is once again comparable to that in the exemplary embodiments described above.  
         [0038]     The present invention has been described above with reference to a dual-band telecommunications terminal for carrier frequencies of 1800 MHz and 900 MHz. However, it is not restricted to this and, in fact, relates to all other multiband receivers via which RF signals, some of which are at different frequencies, are received without the use of wires or by cable. In particular, multiband receivers such as these may be used in mobile telecommunications terminals, so-called “software defined radios” and TD-SCDMA products.  
         [0039]     Indeed, although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the present invention s set forth in the hereafter appended claims.