Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
   The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2002-296157, filed Oct. 9, 2002, the entire disclosure of which is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a low noise block down converter. More particularly, the present invention relates to a low noise block down converter utilized for a satellite reception system. 
   2. Description of the Background Art 
   In a conventional Low Noise Block down converter (hereinafter referred to as an “LNB”) with a plurality of local oscillators, each local oscillator is completely separated from another local oscillator by a metal wall in order to prevent electromagnetic coupling between a dielectric resonator in each local oscillator and a dielectric resonator in another local oscillator. 
     FIGS. 7A and 7B  are cross-sectional views showing a main portion of the conventional LNB.  FIG. 7A  is a cross-sectional view cut along a line VIIA—VIIA in  FIG. 7B , while  FIG. 7B  is a cross-sectional view cut along a line VIIB—VIIB in  FIG. 7A . 
   In  FIGS. 7A and 7B , two local oscillators  41   a  and  41   b  of the LNB are respectively housed within shielding chambers  40   a  and  40   b  in a metal shielding box  40 , and are electromagnetically shielded by a metal wall  40   c . Local oscillator  41   a  includes a dielectric resonator  42   a , an oscillation device  43   a , a microstrip line  44   a , and a substrate  45   a . Local oscillator  41   a  outputs a signal of a certain frequency (e.g. 9.75 GHz). Local oscillator  41   b  includes a dielectric resonator  42   b , an oscillation device  43   b , a microstrip line  44   b , and a substrate  45   b . Local oscillator  41   b  outputs a signal of another frequency (e.g. 10.6 GHz). In  FIG. 7B , dashed circles show electromagnetic fields radiated from dielectric resonators  42   a  and  42   b.    
   As described above, in the conventional low noise block down converter, metal shielding box  40  is divided by metal wall  40   c  to prevent electromagnetic coupling between dielectric resonators  42   a  and  42   b . Therefore, downsizing of metal shielding box  40  and hence the low noise block down converter has been difficult to achieve. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a compact low noise block down converter. 
   A low noise block down converter according to the present invention includes a plurality of local oscillators each including a dielectric resonator and having an oscillation frequency different from each other, an electromagnetic coupling preventing member preventing electromagnetic coupling between one of the dielectric resonators and another one of the dielectric resonators, and a metal shielding box including one shielding chamber accommodating the plurality of local oscillators and the electromagnetic coupling preventing member. Therefore, the metal shielding box and hence the low noise block down converter can be made small compared to the conventional case in which the plurality of local oscillators are completely separated from each other by a metal wall. 
   Preferably, the electromagnetic coupling preventing member includes a conductive bar having one end extending between any two of the dielectric resonators and receiving a reference potential. In this case, the conductive bar can prevent the electromagnetic coupling between the two dielectric resonators. 
   Preferably, the low noise block down converter includes a substrate having a surface on which the plurality of local oscillators are mounted. The electromagnetic coupling preventing member includes a conductive pattern formed on the surface of the substrate between any two of the dielectric resonators and receiving a reference potential. In this case, the conductive pattern can prevent the electromagnetic coupling between the two dielectric resonators. 
   Preferably, the electromagnetic coupling preventing member further includes a metal plate provided between any two of the dielectric resonators and receiving a reference potential. In this case, the metal plate can prevent the electromagnetic coupling between the two dielectric resonators. 
   The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram showing the overall configuration of a satellite reception system in accordance with an embodiment of the present invention. 
       FIG. 2  is a circuit block diagram showing the configuration of a universal LNB  3  shown in  FIG. 1 . 
       FIGS. 3A and 3B  are cross-sectional views showing the configuration of two local oscillators  13   a  and  13   b  shown in  FIG. 2 . 
       FIGS. 4A and 4B  are cross-sectional views showing a comparative example for the present embodiment. 
       FIGS. 5A and 5B  are cross-sectional views showing a modification of the present embodiment. 
       FIGS. 6A and 6B  are cross-sectional views showing another modification of the present embodiment. 
       FIGS. 7A and 7B  are cross-sectional views showing a main portion of the conventional LNB. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a satellite reception system in accordance with an embodiment of the present invention includes a broadcasting satellite  1 , an antenna  2 , an LNB  3 , an IF (Intermediate Frequency) cable  4 , a DBS Direct Broadcasting Satellite) tuner  5 , and a television  6 . 
   The operation of the satellite reception system shown in  FIG. 1  will now be described. A radio wave in a 12 GHz band (10.70–12.75 GHz) transmitted from broadcasting satellite  1  is received by antenna  2 . The received radio wave is frequency-converted to an IF signal in a 1 GHz band (950–2150 MHz) and low-noise amplified by LNB  3  mounted to antenna  2 . The IF signal output from LNB  3  is introduced indoors via IF cable  4 , demodulated into a video and audio signal by DBS tuner  5 , and then transmitted to television  6 . 
   In  FIG. 2 , universal LNB  3  includes a waveguide  10 , a Low Noise Amplifier (hereinafter referred to as an “LNA”)  11 , a Band Pass Filter (hereinafter referred to as a “BPF”)  12 , local oscillators  13   a  and  13   b , a mixer  14 , an IF amplifier  15 , a power supply unit  16 , condensers  17   a  and  17   b , a coil  18 , and an output terminal  19 . 
   The operation of universal LNB  3  shown in  FIG. 2  will now be described. A vertically polarized wave signal and a horizontally polarized wave signal in the 12 GHz band (10.70–12.75 GHz) transmitted from broadcasting satellite  1  are respectively received at two antenna probes in waveguide  10 . The received signals are low-noise amplified by LNA  11 , and then input to BPF  12 . In BPF  12 , a signal in an image frequency band is removed to produce a signal in a desired frequency band. The signal output from BPF  12  is mixed with a local oscillation signal (9.75 GHz) from local oscillator  13   a  or with a local oscillation signal (10.6 GHz) from local oscillator  13   b  by mixer  14 , and is frequency-converted to the IF signal in the 1 GHz band (950 to 2150 MHz). Two local oscillators  13   a  and  13   b  may be switched therebetween for use. The IF signal output from mixer  14  is amplified to have appropriate noise characteristics and gain characteristics by IF amplifier  15 , condensers  17   a  and  17   b , and coil  18 , and is output from output terminal  19 . It is noted that LNA  11 , local oscillators  13   a  and  13   b , and IF amplifier  15  are powered through power supply unit  16 . 
     FIGS. 3A and 3B  are cross-sectional views showing the configuration of two local oscillators  13   a  and  13   b  shown in  FIG. 2 .  FIG. 3A  is a cross-sectional view cut along a line IIIA—IIIA in  FIG. 3B , while  FIG. 3B  is a cross-sectional view cut along a line IIIB—IIIB in  FIG. 3A . 
   In  FIGS. 3A and 3B , a substrate  24  with two local oscillators  13   a  and  13   b  mounted thereon and a conductive bar  25  are housed within one shielding chamber  20   a  in a metal shielding box  20 . Local oscillator  13   a  includes a dielectric resonator  21   a , an oscillation device  22   a , and a microstrip line  23   a . Local oscillator  13   a  outputs the signal at the frequency of 9.75 GHz. Local oscillator  13   b  includes a dielectric resonator  21   b , an oscillation device  22   b , and a microstrip line  23   b . Local oscillator  13   b  outputs the signal at the frequency of 10.6 GHz. A proximal end of conductive bar  25  is bonded to the middle of a ceiling of metal shielding box  20 . A distal end of conductive bar  25  extends between two dielectric resonators  21   a  and  21   b . Conductive bar  25  and metal shielding box  20  are grounded. Conductive bar  25  prevents coupling of electromagnetic fields (dashed circles in  FIG. 3B ) radiated from two dielectric resonators  21   a  and  21   b.    
     FIGS. 4A and 4B  are cross-sectional views showing a comparative example for the present embodiment.  FIG. 4A  is a cross-sectional view cut along a line IVA—IVA in  FIG. 4B , while  FIG. 4B  is a cross-sectional view cut along a line IVB—IVB in  FIG. 4A . The configuration shown in  FIGS. 4A and 4B  is different from the configuration shown in  FIGS. 3A and 3B  in that conductive bar  25  is not provided between dielectric resonators  21   a  and  21   b . In this case, electromagnetic fields (dashed circles in  FIG. 4B ) radiated from two dielectric resonators  21   a  and  21   b  are coupled to each other. This results in local oscillators  13   a  and  13   b  interfering with each other and failing to produce signals at the desired frequencies (9.75 GHz, 10.6 GHz). 
   In the present embodiment, the electromagnetic coupling between two dielectric resonators  21   a  and  21   b  is prevented by conductive bar  25 . Therefore, metal shielding box  20  and hence the LNB can be smaller compared to the conventional case in which the electromagnetic coupling between two dielectric resonators  21   a  and  21   b  is prevented by metal wall  40   c . In the present embodiment, two local oscillators  13   a  and  13   b  are provided within one shielding chamber  20   a  in metal shielding box  20 . However, it will readily be appreciated that electromagnetic coupling can be prevented even when a plurality of local oscillators are provided in shielding chamber  20   a , as long as conductive bar  25  is provided for each space between adjacent local oscillators. 
     FIGS. 5A and 5B  are cross-sectional views showing a modification of the present embodiment. The configuration shown in  FIGS. 5A and 5B  is different from the configuration shown in  FIGS. 4A and 4B  in that a ground pattern  26  is formed on substrate  24  between dielectric resonators  21   a  and  21   b  and that ground pattern  26  is connected to metal shielding box  20  via a through hole  27 . In this case, ground pattern  26  and through hole  27  prevent coupling between electromagnetic fields (dashed circles in  FIG. 5B ) radiated from two dielectric resonators  21   a  and  21   b.    
     FIGS. 6A and 6B  are cross-sectional views showing another modification of the present embodiment. The configuration shown in  FIGS. 6A and 6B  is different from the configuration shown in  FIGS. 5A and 5B  in that a metal plate  28  is provided on ground pattern  26 . In this case, ground pattern  26 , through hole  27 , and metal plate  28  prevent coupling between electromagnetic fields (dashed circles in  FIG. 6B ) radiated from two dielectric resonators  21   a  and  21   b . Therefore, more ensured prevention of the electromagnetic coupling between two dielectric resonators  21   a  and  21   b  can be achieved. 
   Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Technology Category: 5