Abstract:
The invention proposes an upgradable product capable of covering two sub-bands. An outdoor unit comprises, on the uplink, a switchable oscillator coupled to a waveguide featuring a removable cover enabling the said waveguide to be converted into a band rejector filter.

Description:
This application claims the benefit, under 35 U.S.C. §365 of International Application PCT/FR04/050429, filed Sep. 13, 2004, which was published in accordance with PCT Article 21(2) on Mar. 31, 2005 in French and which claims the benefit of European patent application No. 03292300.5, filed Sep. 18, 2003. 
     1. Background of the Invention 
     The invention relates to a bi-directional user terminal with configurable transmission frequencies, particularly a satellite terminal with return channel capable of operating in a frequency band such as the Ku, Ka or other bands. 
     The present invention will be described by referring to a Ka band bi-directional terminal. 
     2. Description of the Prior Art 
     Hence,  FIG. 1  illustrates an example of standard architecture of a Ka band frequency transposition circuit or BUC (Block Up Conversion) placed in an outdoor transmission unit (or ODU for “Outdoor Unit”). The RF signal at an intermediate frequency IF in the 0.95-1.45 GHz band is from the indoor unit (or IDU) and is transposed into the Ka band by implementing a subharmonic mixer (X 2 ) and a local oscillator (hereafter LO) operating at the Ku band. The output of the mixer X 2  is sent to a band-pass filter  1 . Indeed, a highly selective band-pass filtering is required in particular to eliminate the residual Ka band (2*LO) component that is twice the frequency of the local oscillator, which must not be radiated by the terminal. 
     In a known manner, the output of filter  1  is sent to an amplifier  2  whose output is connected to the source  3  of an antenna  4 . 
     For implementation reasons, operators require a Ka band application with a wideband transmission that can be selected from two frequency bands, for example the 28.4-28.6 GHz band and the 29.5-30 GHz band, either of these bands being assigned to the user according to his requirements and/or his geographical location. For such an arrangement, the transmission bands correspond to the frequencies of the local oscillator LO of the BUC, respectively 13.725 GHz and 14.275 GHz. The unwanted components to filter corresponding to 2*LO are then 27.45 and 28.55 GHz. As shown in  FIG. 2  which illustrates the frequency plans corresponding to the two Ka band frequencies emitted (respectively in highband and lowband), the 2*LO components (28.55 GHz and 27.45 GHz) are outside of the plans. One approach typically implemented in this case is to propose two types of separate terminals capable of covering one or other of the frequency bands, this to the detriment of the cost of the terminal. 
     SUMMARY OF THE INVENTION 
     The invention therefore proposes an upgradable product capable of covering several bands or sub-bands, which can be easily configured and installed on site without the intervention of a professional so as to noticeably reduce installation costs. 
     Moreover, the invention proposes only one type of terminal that can cover the different bands, which is of significant economic interest. Hence, the minimisation of the industrialisation costs and the increase of production volumes enable the cost of the terminal to be reduced. Moreover, several operators can use the same product. 
     The invention relates more particularly to an outdoor unit of a reception terminal including a return channel. The return channel (BUC) comprises:
         a local oscillator providing a signal with a frequency that can be selected from at least two frequencies,   a transposition means that transposes a signal to be transmitted by using the signal provided by the local oscillator,   a wideband filtering means that allows through signals whose frequency corresponds to the transposed signal independently from the frequency of the local oscillator local, and   a waveguide element having a cover that depends on the frequency selected for the local oscillator.       

     According to a characteristic of the invention, the waveguide cover transforms the waveguide into a band rejector filter that rejects a bandwidth corresponding to a leak of the transposition frequency in the wideband. 
     According to a first embodiment, the cover is either a flat cover or a cover including slot-coupled resonant cavities. 
     According to another embodiment, the waveguide comprises slot-coupled resonant cavities and the cover is either a flat cover, or a cover comprising elements that electrically plug the slots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood, and other specific features and advantages will emerge from reading the following description, the description making reference to the annexed drawings wherein: 
         FIG. 1  already described shows a BUC architecture according to the prior art, in the case of a terminal operating in the Ka band. 
         FIG. 2  already described shows the transmission frequency plans of a system using two sub-bands, as described in  FIG. 1 . 
         FIG. 3  diagrammatically shows an embodiment of the invention, 
         FIG. 4  shows the perspective configuration of a standard band-stop filter, 
         FIGS. 5   a  and  5   b  diagrammatically show a first embodiment of the present invention, 
         FIGS. 6   a  and  6   b  diagrammatically show a second embodiment of the present invention, and 
         FIG. 7  shows the transmission frequency plans corresponding to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3  shows the radio architecture of a BUC compliant with the present invention in the case of a bi-directional terminal operating in the Ka band. The BUC proposed is capable of covering the two previously cited frequency bands, namely 28.4-28.6 GHz and 29.5-30 GHz. As explained below with reference to  FIG. 7 , the BUC implements a wideband band-pass filtering covering the two frequency bands, namely 28.4-30 GHz, and capable of rejecting the lowest 2*LO frequency (corresponding to the lowband LB). 
     More specifically, the return channel to the BUC of  FIG. 3  thus comprises a subharmonic mixer X 2  receiving respectively as an input the RF signal at the IF intermediate frequency in the bandwidth 0.95-1.45 GHz and the signal from a local oscillator  10  whose oscillation frequency LO can be adjusted to 13.725 GHz or 14.275 GHz according to the high or low band operation selected. 
     The output of the mixer X 2  is sent to a band-pass filter  11  covering the two bands, namely 28.4-30 GHz in the embodiment shown. The output of the band-pass filter  11  is sent to a rejector filter  12 . In accordance with the invention, the rejector filter  12  is a configurable filter and is capable of effectively rejecting the highest 2*LO frequency (corresponding to the highband HB). The rejector filter  12  is, for example, a waveguide rejector filter that can easily be connected to a band-pass filter featuring guide accesses itself. The rejector filter  12  is connected to the feed of the antenna  4 . 
     An example of rejector filter or band-stop filter is shown in  FIG. 4   a . In this case, it is a three-pole filter, namely a rectangular waveguide  20  coupled by slots  21  with three resonant cavities  22  attuned to the frequency to reject. More specifically, the resonant cavities  20  that form the resonant elements LC have a length noticeably equal to λg/2, where λg is the guided wavelength calculated at the rejection frequency. The cavities are coupled to the main guide by inductive slots  21 . The distance between two slots is preferably equal to 3 λg/4 to prevent coupling effects between the slots, although theoretically, it could be λg/4. 
     The terminal thus described can be configured simply by modifying the frequency of the local oscillator  10  and by activating/deactivating the rejector filter  12 . The frequency of the local oscillator  10  is modified for example in a ‘mechanical’ manner by operating a switch accessible to the operator. As a variant, the local oscillator frequency can also be modified by the intermediary of the indoor unit or IDU that then controls an outdoor unit or ODU by a bus of the Disecq type for example. 
     It is considered that the rejector filter is an integral part of the feed of the antenna, so that the extra cost brought by this function remains minimal. To activate or deactivate a filter of the type of the one shown in  FIG. 4 , namely to make this filter configurable, the following two embodiments are possible: 
     The first embodiment, shown in  FIGS. 5   a  and  5   b , consists of a guided structure  30  whose cover  31  is flat if no band-stop filtering is required, as shown in  FIG. 5   a . Otherwise, this cover is replaced by a cover  32  that contains the coupling slots  33  as well as the resonant cavities shown in  FIG. 5   b.    
     The second embodiment, shown in  FIGS. 6   a  and  6   b , consists of a guided structure  40  including the coupling slots  41  and the resonant cavities  42  but open at the upper part. In the case of a non-filtering guide, the cover  43  comprises profiled elements  44  enabling the apertures that are the slots  41  and the open cavities  42  to be plugged as shown in  FIG. 6   a . Otherwise, the guide becomes filtering by simply fixing a flat cover  45  above the guided structure. 
       FIG. 7  shows the two frequency plans on the  FIG. 6   b  (lowband and highband) with switching of the local oscillator at the LO frequency and activation/deactivation of a rejector filter at 28.55 GHz. 
     This upgradable terminal can easily be configured by the user without the intervention of a professional, thanks to a manual switch (or automatic, controlled by the IDU) and by modifying the filtering by changing the cover of a waveguide. This system can noticeably reduce the installation cost. The same concern applies for reducing the terminal installation costs, this technique can naturally be extended to any other multi-band transmission device. 
     The present invention was described by referring to a terminal operating in the Ka band with a rejector filter constituted by a 3-pole rectangular waveguide. It is evident to those in the profession that it can be used in terminals operating at other bands and with different waveguide rejector filters. For example, the present invention can also be implemented in high frequency multi-band user terminals for MMDS (Microwave Multipoint Distribution System) type applications operating in the 40 GHz bands.