Abstract:
A multilayer substrate ensuring transmission loss as low as when one double-sided substrate is used, while reducing noise, and an LNB converter are provided. The multilayer substrate includes a four-layer substrate having pattern layers sandwiching dielectric layers therebetween. The surface pattern layer provided with a signal line of the microstrip line has a projecting portion. The pattern layer provided with a ground pattern corresponding to the signal line has a portion at least overlapping a root portion of the projecting portion as seen from the top and constituting a surface layer on the other side opposite to the root portion, with no other pattern layer interposed between the relevant portions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a multilayer substrate for use in a satellite broadcast reception apparatus, or a low noise block-down (LNB) converter, employed in satellite broadcasting and satellite communication, and a satellite broadcast reception apparatus (hereinafter, referred to as the “LNB converter”) using the relevant multilayer substrate.  
           [0003]    2. Description of the Background Art  
           [0004]    As a substrate for use in the LNB converter, a so-called double-layer substrate (double-sided substrate) has been employed conventionally. As shown in FIG. 10, the double-layer substrate has a substrate dielectric layer  105  formed of a base material of teflon (R) having its both sides plated with copper to form microstrip lines  101 ,  102 . In the microstrip lines, as shown in FIG. 11, a circuit pattern to be a signal line is formed on the front layer of the substrate, and the entire rear layer is grounded, to obtain low-loss and stable transmission characteristic. That is, as shown in FIG. 12, a radiowave signal having propagated through a waveguide  113  is introduced into a space surrounded by a frame  112  and a chassis  111 , and is transmitted via a probe  120  into the microstrip line formed in the front layer  101  of double-layer substrate  110 . FIG. 13 shows the structure of a portion of the double-layer substrate in FIG. 12 in the vicinity of the transmitting portion. A projecting portion  101   a  constituting probe  120  projecting into a through hole  121  is provided on a projecting portion of substrate dielectric layer  105  continued from the signal line of the microstrip line. In the case where a circuit of the LNB converter is formed on the substrate, it is still advantageous to employ the double-layer substrate in that the entire rear side of the substrate is grounded to keep the overall earth state constant.  
           [0005]    With recent advancement of multi-channel satellite broadcasting and satellite communication as well as reception from multiple satellites, there is a trend to replace a reception system using a plurality of LNB converters with a reception system using a signal LNB converter. Such an LNB converter in the future will be required to accommodate conventional LNB circuits within a single LNB converter without any problems. Further, the future LNB converter will have to switch and distribute signals for output. Thus, good isolation of the signals preventing interference therebetween will be highly required.  
           [0006]    With the conventional double-layer substrate used in the LNB converter, signal lines inevitably cross with each other. Thus, a method to forcibly separate signals using a semi-rigid cable or the like has conventionally been employed. In the LNB converter adapted to the recent multi-satellite system, the signal crossing portion has become further complicated, of which assembly is physically difficult.  
           [0007]    A possible solution therefor is to use a multilayer substrate as shown in FIG. 14. This multilayer substrate has pattern layers of first layer  101 , second layer  102 , third layer  103  and fourth layer  104 , with two substrate dielectric layers  105 ,  107  and a bonding insulating layer  106  arranged therebetween. The multilayer substrate, as shown in FIG. 15, has two double-layer substrates as its base, which are bonded together by the bonding insulating layer  106 . Thus, this multilayer substrate has a four-layer configuration, with a front pattern and a rear pattern included in the respective double-layer substrate. It is also possible to provide multilayer configurations of four layers, six layers and eight layers, by laminating the corresponding numbers of double-layer substrates.  
           [0008]    In the multilayer substrate for use in the LNB converter, parts can be mounted on the outermost layers or the surface layers, e.g., the first and fourth layers in the case of the four-layer substrate shown in FIG. 16. The microstrip line pattern can also be formed in the surface layers. At this time, a pattern corresponding to a ground layer with respect to the microstrip line is formed in an inner layer that is unseen from the surface of the substrate, e.g., in the second and third layers in the case of the four-layer substrate.  
           [0009]    With the multilayer substrate formed by laminating double-layer substrates as described above, however, the ground layer arranged in the inner layer is electrically isolated from an enclosure to which the substrate is secured. Thus, it is likely to suffer transmission loss especially with a high frequency, which becomes the stumbling block preventing the use of the multilayer substrate compared to the double-layer substrate.  
           [0010]    As described above, the connection between the probe and the substrate circuit in the double-layer substrate as in FIGS. 12 and 13 permits low-loss power supply. This is because the portion in the vicinity of the probe is surrounded by the metal chassis and frame to prevent radiowave leakage and thus to minimize the transmission loss. With the multilayer substrate, e.g., the four-layer substrate, if the microstrip line pattern is provided in the first layer and the ground pattern is provided in the second, inner layer, the configuration as in FIGS. 12 and 13 cannot be provided. In this case, intervention of the third and fourth pattern layers results in considerable degradation of transmission characteristic. That is, in the case of the double-layer substrate, the double-layer substrate is sandwiched between the chassis and the frame to prevent the radiowave leakage, and, at the same time, the ground for the probe and the ground surface of the circuit connecting portion are commonly provided to sufficiently stabilize the earth. By comparison, in the case of the four-layer substrate, it is difficult to ensure good contact between the ground for the circuit provided in the inner pattern layer and the ground for the chassis. That is, an additional double-layer substrate is inserted between the ground pattern on the rear surface of the double-layer substrate and the chassis, considerably degrading the transmission characteristic. Currently, this obstructs the use of the multilayer substrate in the LNB converter.  
         SUMMARY OF THE INVENTION  
         [0011]    An object of the present invention is to provide a multilayer substrate which prevents noise and crosstalk that are likely to occur in multi-channel transmission/reception and which ensures transmission loss as low as when one double-layer substrate is employed, and an LNB converter using such a multilayer substrate.  
           [0012]    The LNB converter according to the present invention is provided with a multilayer substrate having a microstrip line and a probe, and causes a radiowave signal from an antenna to propagate through a waveguide to transmit through the probe to the microstrip line. The multilayer substrate is provided with a wave-guiding through hole, and the probe is provided to project from the multilayer substrate into the through hole. Of the multilayer substrate, a pattern layer constituting a surface layer on one side is provided with a signal line of the microstrip line and a projecting portion constituting a portion of the probe. Another pattern layer provided with a ground pattern corresponding to the signal line has a portion at least overlapping a root portion of the projecting portion as seen from the top and constituting a surface layer on the other side opposite to the root portion. There is no other pattern layer interposed between the root portion and the portion constituting the surface layer on the other side.  
           [0013]    The portion constituting the surface layer on the other side opposite to the root portion may be a portion of a substrate surface layer (the undermost layer) on the other side with respect to the substrate surface layer (the topmost layer) of the multilayer substrate in which the projecting portion of the probe is provided. Alternatively, it may be a portion of an inner pattern layer that is exposed by removing a corresponding portion of the undermost layer to provide a surface layer.  
           [0014]    In other words, the present invention provides any of the following structures (1) through (3) in the LNB converter configured with the multilayer substrate, to reduce transmission loss of a high-frequency signal. (1) The structure in which a ground layer is formed of a substrate surface pattern that can contact the earth of the enclosure such as a chassis, rather than an inner pattern layer. (2) The structure in which a ground layer corresponding to the signal line is formed by processing and exposing the inner pattern layer, which is made to contact the enclosure such as a chassis. (3) The structure in which the multilayer substrate as in the structure (1) is employed to form the ground portion with the substrate surface layer pattern, to reduce transmission loss and to facilitate assembly of the LNB converter.  
           [0015]    With the above-described structures (1) through (3), the advantageous features of the multilayer substrate can be enjoyed to the full extent, while ensuring low-loss transmission characteristic. That is, the LNB converter employing the multilayer substrate has the advantages including downsizing by virtue of the multi-layered structure, simplification of complicated wiring, facilitation of assembly, and improvement of reliability. This can generally lead to reduction of manufacturing cost, although the substrate cost is currently still expensive as a single item, with only a small number of such substrates available.  
           [0016]    The present invention provides the following effects. In an LNB converter receiving signals from multiple satellites where the signals inevitably cross with each other, the external cable conventionally employed becomes unnecessary. Instead, the multilayer substrate, simple in assembly, can be employed to realize an LNB converter ensuring high-level isolation and high performance with less transmission loss. As an increasing number of multilayer substrates are adapted to the LNB converters, the substrate unit price will decrease, which will further promote downsizing of the existing LNB converters.  
           [0017]    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  
       [0018]    [0018]FIG. 1 is a cross sectional view of a multilayer substrate used in an LNB converter according to a first embodiment of the present invention.  
         [0019]    [0019]FIG. 2 is a top plan view of the first pattern layer of the multilayer substrate shown in FIG. 1 in the vicinity of a through hole.  
         [0020]    [0020]FIG. 3 is a top plan view of the fourth pattern layer of the multilayer substrate shown in FIG. 1 in the vicinity of the through hole.  
         [0021]    [0021]FIG. 4 is a top plan view of the second and third pattern layers of the multilayer substrate shown in FIG. 1 in the vicinity of the through hole.  
         [0022]    [0022]FIG. 5 is an exploded perspective view of the LNB converter employing the multilayer substrate shown in FIG. 1.  
         [0023]    [0023]FIG. 6 is a cross sectional view of a multilayer substrate used in an LNB converter according to a second embodiment of the present invention.  
         [0024]    [0024]FIG. 7 shows the multilayer substrate of FIG. 6 with a chassis and a frame.  
         [0025]    [0025]FIG. 8 is a cross sectional view of a multilayer substrate used in an LNB converter according to a third embodiment of the present invention.  
         [0026]    [0026]FIG. 9 is a top plan view of the multilayer substrate of FIG. 8 as seen from the chassis side.  
         [0027]    [0027]FIG. 10 is a cross sectional view of a conventional double-sided substrate.  
         [0028]    [0028]FIG. 11 is a cross sectional view of a conventional microstrip line.  
         [0029]    [0029]FIG. 12 is a perspective view of a signal transmitting portion of a conventional circuit substrate.  
         [0030]    [0030]FIG. 13 is a cross sectional view of the signal transmitting portion of the conventional double-sided substrate.  
         [0031]    [0031]FIG. 14 is a cross sectional view of a conventional multilayer substrate.  
         [0032]    [0032]FIG. 15 is a cross sectional view illustrating how the four-layer substrate is formed with double-sided substrates.  
         [0033]    [0033]FIG. 16 is a cross sectional view of a signal transmitting portion of a conventional four-layer substrate. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Hereinafter, embodiments of the present invention will be described with reference to the drawings.  
         [0035]    First Embodiment  
         [0036]    [0036]FIG. 1 is a cross sectional view of the multilayer substrate used in an LNB converter according to the first embodiment of the present invention. This multilayer substrate  10  has two double-sided substrates bonded together by a bonding insulating layer  6 . Multilayer substrate  10  is provided with a through hole  21 , and a probe  20  is provided to project into through hole  21 . Probe  20  is formed of a projecting portion  1   a  which projects from a first pattern layer  1  provided with a microstrip line, and projecting portions  5   a ,  7   a ,  6   a  which project from two substrate insulating layers  5 ,  7  and insulating layer  6 .  
         [0037]    Another surface pattern layer  4  other than the first pattern layer  1  has a portion  4   a  arranged opposite to a root portion  1   b  of the projecting portion  1   a  of the first layer. There is no other pattern layer interposed between the root portion  1   b  and the portion  4   a  opposite to the root portion  1   b . A ground pattern is provided in the fourth pattern layer  4 . This ground pattern contacts chassis  11  in the peripheral portion.  
         [0038]    [0038]FIG. 2 is a top plan view showing a portion of the first pattern layer of multilayer substrate  10  shown in FIG. 1 surrounding the through hole. Projecting portion  1   a  projects into through hole  21 , forming a portion of the probe. FIG. 3 is a top plan view of a portion of the fourth pattern layer surrounding the through hole. As seen from the top, portion  4   a  corresponding to projecting portion  1   a  forms the surface layer portion on the rear side, with no other pattern layer interposed between itself and projecting portion  1   a  of the first layer. FIG. 4 is a top plan view of the second and third pattern layers  2 ,  3 . Second and third pattern layers  2 ,  3  have concave portions  2   a ,  3   a  which are continued from through hole  21  so as not to intervene between root portion  1   b  of the first layer and the portion  4   b  of the fourth layer corresponding to the root portion  1   b.    
         [0039]    The structure of an LNB converter  30  including multilayer substrate  10  is now described. Referring to FIG. 5, multilayer substrate  10  is sandwiched between a metal chassis  11  and a metal frame  12 . In the LNB converter, an electric signal having propagated through a waveguide  13  is input to a circuit portion of the multilayer substrate via the probe  20  protruding from the multilayer substrate. The input of the electric signal may be called power supply or feeding.  
         [0040]    Metal chassis  11  serves to secure multilayer substrate  10  and provide earth commonly for the multilayer substrate and an external terminal, and also functions as a waveguide for transmission of a high-frequency radiowave signal reflected from an antenna. Metal frame  12  cooperates with metal chassis  11  to realize signal transmission to a circuit on the multilayer substrate, wave shield, ground integrated with the chassis, and airtight sealing of the LNB converter.  
         [0041]    On reception, a weak signal from the satellite is transmitted to a circuit of the multilayer substrate as a carrier, high-frequency signal. The signal from the satellite is reflected by the parabola antenna dish, and focuses within the waveguide of the LNB converter. With the probe protruding from the multilayer substrate matching the impedance in the waveguide to that of the circuit, the radiowave propagating through the waveguide is transmitted to the circuit portion of the multilayer substrate, particularly to the microstrip line.  
         [0042]    Using the above-described multilayer substrate ensures good NF (noise figure) performance and reduces transmission loss, while supporting the multi-channel transmission/reception.  
         [0043]    Second Embodiment  
         [0044]    [0044]FIG. 6 is a cross sectional view of the multilayer substrate used in an LNB converter according to the second embodiment of the present invention. In the present embodiment, a projecting portion  1   a  forming a probe is provided in the first pattern layer  1  constituting the microstrip line, as in the first embodiment. This probe includes a portion protruding from the substrate insulating layer  5 ; however, it does not include a portion protruding from any other insulating layer or substrate insulating layer.  
         [0045]    In the second pattern layer  2  and the third pattern layer  3  that cannot contact the chassis, i.e., in the pattern layers corresponding to the inner layers of the multilayer substrate, portions corresponding to the root portion  1   b  of projecting portion  1   a  are removed. By comparison, in the fourth pattern layer  4 , a portion  4   a  corresponding to the root portion  1   b  of projecting portion  1   a  as seen from the top is unremoved. The fourth pattern layer is used as a ground layer.  
         [0046]    In this configuration, the ground layer of the circuit is the second pattern layer  2 . The fourth pattern layer is used for substitute only in a portion necessary to contact chassis  11 , with the second and third pattern layers removed.  
         [0047]    In the LNB converter including the multilayer substrate, the earth condition of the probe portion is particularly important, which considerably affects the NF value. The top plan views of the second and third pattern layers are as in FIG. 4.  
         [0048]    [0048]FIG. 7 is a cross sectional view of the multilayer substrate  10  sandwiched between a chassis  11  and a metal frame  12 . Multilayer substrate  10  has its peripheral portion sandwiched and secured between metal frame  12  and chassis  11 . The functions of metal frame  12  and chassis  11  are as described above. As such, the second and third pattern layers are provided only in the connect portion between the probe and the circuit portion, so that the NF value substantially the same as when using one double-sided substrate can be obtained.  
         [0049]    The optimal conditions for the inner pattern layers, or the second and third layers in this case, need to be determined by fabricating several kinds of multilayer substrates and monitoring their NF values. The best mode will be the one with which the transmission loss becomes small. In the LNB converter having the multilayer substrate, chassis and frame configured by the present invention and assembled as shown in FIG. 5, the NF value becomes substantially the same as in an LNB converter formed of one double-sided substrate, and degradation of noise characteristic can be prevented.  
         [0050]    Third Embodiment  
         [0051]    [0051]FIG. 8 is a cross sectional view of the multilayer substrate used in an LNB converter according to the third embodiment of the present invention. In the multilayer substrate shown in FIG. 8, portions of the third and fourth pattern layers  3  and  4  corresponding to the root portion  1   b  of the projecting portion  1   a  as described above are eliminated. The structure shown in FIGS. 8 and 9 results in less degradation in characteristics and provides ground to the chassis. Accordingly, the present embodiment ensures excellent NF performance and transmission characteristic.  
         [0052]    However, the present embodiment is disadvantageous in that, to produce such a multilayer substrate, the respective substrates need to undergo processing prior to bonding thereof, which increases the number of process steps compared to the case of the first and second embodiments where the substrates are subjected to the processing after bonding thereof.  
         [0053]    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.