Patent Publication Number: US-2023163438-A1

Title: Signal transmission system and dielectric waveguide

Description:
RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application Serial No. 2021-188763 filed on Nov. 19, 2021, which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to a signal transmission system that transmits signals via a dielectric waveguide. 
     BACKGROUND 
     Patent Document 1 noted below discloses a signal transmission device having two circuit boards where millimeter wave signals are transmitted and received between the two circuit boards via a dielectric waveguide. The dielectric waveguide is electromagnetically coupled to a semiconductor chip via an antenna that is a transmission path coupling part.
     Prior Art Documents: Patent Documents: Patent Document 1: WO 2012/111484.   

     SUMMARY 
     For the device disclosed in Patent Document 1, reduction in energy losses during signal transmission via the dielectric waveguide is desirable. 
     (1) An example of a signal transmission system proposed in the present disclosure includes a circuit board, a semiconductor package including an RF circuit mounted on the circuit board, and a dielectric waveguide. The semiconductor package includes a package surface and an antenna formed on the package surface. The dielectric waveguide includes a waveguide end surface facing the antenna. An air gap is ensured between the waveguide end surface and the antenna. With this system, the air gap can reduce insertion losses. 
     (2) The signal transmission system of (1) may have at least one support part that supports the dielectric waveguide on the circuit board and ensures the air gap. Thus, the relative position between the antenna and the dielectric waveguide can be optimized. 
     (3) In the signal transmission system of (2), the at least one support part may be formed on the dielectric waveguide. Herein, the number of parts can be reduced. 
     (4) In the signal transmission system in (2), where the at least one support part includes two support parts positioned on mutually opposite sides of the semiconductor package. Thus, the support stability of the dielectric waveguide can be ensured. 
     (5) In the signal transmission system of (1), the air gap may be 0.025 mm or more and 0.5 mm or less. Thus, insertion losses can be reliably reduced by the air gap. 
     (6) An example of a dielectric waveguide according to the present disclosure includes: 
     a waveguide main body;
 
a waveguide end surface for facing the antenna formed on the surface of a semiconductor package, which is an end surface of the waveguide main body in the extending direction of the waveguide main body; and
 
at least one support part extending beyond the waveguide end surface that ensures an air gap between the antenna supported on a circuit board on which the semiconductor package is mounted and the waveguide end surface.
 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram illustrating an example of a signal transmission system proposed in the present disclosure. 
         FIG.  2 A  is a front view illustrating a first example of a signal transmission system. 
         FIG.  2 B  is a perspective view illustrating a connecting part provided on the signal transmission system illustrated in  FIG.  2 A . 
         FIG.  2 C  is a perspective view of the signal transmission system illustrated in  FIG.  2 A . 
         FIG.  2 D  is a cross-sectional view of a signal transmission system obtained along the IId-IId line illustrated in  FIG.  2 C . 
         FIG.  3    is a graph illustrating the relationship between an air gap and insertion losses. 
         FIG.  4 A  is a perspective view illustrating a second example of a signal transmission system. 
         FIG.  4 B  is a cross-sectional view of a signal transmission system obtained along the IVa-IVa line in  FIG.  4 A . 
         FIG.  5    is a cross-sectional view illustrating a third example of a signal transmission system. 
         FIG.  6    is a cross-sectional view illustrating a fourth example of a signal transmission system. 
         FIG.  7 A  is a perspective view illustrating a fifth example of a signal transmission system. 
         FIG.  7 B  is a cross-sectional view of the signal transmission system obtained along the VIIb-VIIb line illustrated in  FIG.  7 A . 
         FIG.  8    is a cross-sectional view illustrating a sixth example of a signal transmission system. 
         FIG.  9 A  is a side view illustrating a seventh example of a signal transmission system. 
         FIG.  9 B  is a perspective view of the signal transmission system illustrated in  FIG.  9 A . 
         FIG.  9 C  is a perspective view illustrating the arrangement of relay fittings in the signal transmission system illustrated in  FIG.  9 A . 
         FIG.  9 D  is a cross-sectional view of a signal transmission system obtained along the IXd-IXd line illustrated in  FIG.  9 A . 
         FIG.  10 A  is a side view illustrating an eighth example of a signal transmission system. 
         FIG.  10 B  is a perspective view illustrating the arrangement of relay fittings in the signal transmission system illustrated in  FIG.  10 A . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description is provided for a transmission system proposed by the present disclosure.  FIG.  1    is a diagram illustrating a signal transmission system  1  as an example of a signal transmission system proposed by the present disclosure. The signal transmission system  1  includes portable terminals (for example a smartphone), personal computers, a server device, a game device, and the like but is not necessarily limited thereto. 
     The signal transmission system  1  has a first circuit board  10 A and a second circuit board  10 B. The circuit boards  10 A and  10 B are so-called rigid circuit boards such as a glass epoxy board, a composite board with paper epoxy and glass epoxy as a base material, an alumina board, or the like. The circuit boards  10 A and  10 B may be a Flexible Printed Circuit (FPC) composed of resin such as polyimide, polyester, or the like. 
     The signal transmission system  1  has a dielectric waveguide  21 . High frequency signals are transmitted and received between the first circuit board  10 A and the second circuit board  10 B via the dielectric waveguide  21 . In the present specification, “high frequency” means millimeter waves (28 GHz to 300 GHz) and sub-millimeter waves (300 GHz or higher). 
     The first circuit board  10 A is provided with a semiconductor package  12 A and an antenna  12   e . The second circuit board  10 B is provided with a semiconductor package  12 B and antenna  12   e . In addition, a SerDes part  11 A may be provided on the first circuit board  10 A and a SerDes part  11 B may be provided on the second circuit board  10 B. 
     The SerDes part  11 A of the first circuit board  10 A may have a serializer  11   a . The SerDes part  11 B of the second circuit board  10 B may have a deserializer  11   b . Digital signals are input into the serializer  11   a  through one or a plurality of electronic components built-in to the signal transmission system  1 . 
     For example, as illustrated in  FIG.  1   , a plurality of electronic component output signals (digital signals) are input to the serializer  11   a . The electronic component may be, for example, a sensor. Specifically, the electronic component may be an acceleration sensor built-in to the signal transmission system  1  or a temperature sensor to detect the temperature of a battery (not shown) built-in to the signal transmission system  1 . The electronic component may be a Wi-Fi (registered trademark) wireless communication module, a communication module for a mobile communication system (for example, 5th generation mobile communication system), or a Global Navigation Satellite System (GNSS) receiver. An output signal of an electronic component may be input to the serializer  11   a  via an A/D converter (not shown). 
     The serializer  11   a , for example, collects and serializes the output signals of the plurality of electronic components. In other words, the serializer  11   a  generates a series of serial signals containing the output signals of a plurality of electronic components. The deserializer  11   b  of the SerDes part  11 B receives the serialized output signals of electronic components via the dielectric waveguide  21 , separates the plurality of output signals that were serialized, and outputs the signals. 
     A parallel signal may be input from one electronic component to the serializer  11   a . The serializer  11   a  may then convert this parallel signal into a serial signal. For example, the electronic component may be an image sensor (for example, a CMOS image sensor). A parallel signal may be input from various sensors to the serializer  11   a , and the serializer  11   a  may convert these parallel signals into a serial signal. In this case, the deserializer  11   b  may convert the serial signals received via the dielectric waveguide  21  to the original parallel signals and output the signals. 
     The output of the deserializer  11   b  is input to another electronic component built-in to the signal transmission system  1 . Electronic components that acquire signals from the deserializer  11   b  may be, for example, a control IC including a CPU (Central Processing Unit) or memory. 
     Differing from the example shown in  FIG.  1   , the SerDes part  11 A of the first circuit board  10 A may have a deserializer in addition to the serializer  11   a . In this case, the SerDes part  11 B of the second circuit board  10 B may have a serializer in addition to the deserializer  11   b.    
     As illustrated in  FIG.  1   , the SerDes part  11 A (serializer  11   a ) is connected to the semiconductor package  12 A on the first circuit board  10 A via a differential transmission line  15 A formed on the first circuit board  10 A. In a similar manner, the SerDes part  11 B (deserializer  11   b ) is connected to the semiconductor package  12 B on the second circuit board  10 B via a differential transmission line  15 B formed on the second circuit board  10 B. The differential transmission lines  15 A and  15 B may be microstrip lines or strip lines. 
     As illustrated in  FIG.  1   , the semiconductor package  12 A may have a modulating part  12   a  and a transmitting part  12   b . In addition, the semiconductor package  12 B may have a receiving part  12   c  and a demodulating part  12   d  as an RE circuit. 
     A serial signal (baseband signal) from the serializer  11   a  is input to the modulating part  12   a . The modulating part  12   a  modulates the input serial signal and then outputs the signal. The transmitting part  12   b  includes a voltage controlled oscillator (VCO), a mixer, a power amplifier, and the like. Furthermore, the transmitting part  12   b  multiplies the modulated signal and the output signal of the voltage controlled oscillator, generates (up-converts) a high frequency RF signal (RF signal with a millimeter wave frequency), and outputs this to the antenna  12   e  as a RF signal. 
     The antenna  12   e  of the semiconductor package  12 A converts the RF signal (electrical signal) input from the transmitting part  12   b  into radio waves and emits this towards the dielectric waveguide  21 . In addition, the antenna  12   e  of the semiconductor package  12 B converts the electrical signal received from the dielectric waveguide  21  to an RF signal (electrical signal) and outputs this towards the receiving part  12   c . As described below, the antenna  12   e  may be formed on the surface (package surface) of the semiconductor packages  12 A and  12 B. 
     The receiving part  12   c  includes an amplifier, a bandpass filter, a mixer, and a voltage controlled oscillator (VCO), amplifies the RF signal input from the antenna  12   e , and multiplies the output signal of the voltage controlled oscillator and the RF signal to lower (down convert) the frequency of the high frequency RF signal. Furthermore, the receiving part  12   c  then outputs the RF signal with lowered frequency to the demodulating part  12   d . The demodulating part  12   d  demodulates the RF signal and outputs a serial signal (baseband signal). 
     Unlike the example illustrated in  FIG.  1   , the SerDes part  1 I A of the first circuit board  10 A may have a deserializer and the SerDes part  11 B of the second circuit board  10 B may have a serializer. In this case, the semiconductor package  12 B of the second circuit board  10 B may have a modulating part  12   a  and a transmitting part  12   b  in addition to the receiving part  12   c  and the like. In addition, the semiconductor package  12 A of the first circuit board  10 A may have a receiving part  12   c  and a demodulating part  12   d  in addition to the transmitting part  12   b  and the like. In addition, unlike the example illustrated in  FIG.  1   , the semiconductor package  12 A may have SerDes part  11 A. In other words, an RF circuit containing a modulating part  12   a  and transmitting part  12   b  may be packaged with the serializer  11   a  (and deserializer). Similarly, the semiconductor package  12 B may have SerDes part  11 B. In other words, an RF circuit containing the receiving part  12   c  and the demodulating part  12   d  may be packaged with the deserializer  11   b  (and serializer). 
     The dielectric waveguide  21  may be formed of, for example, liquid crystal polymer resin (LCP resin), polyphenylene sulfide resin (PPS resin), polyamide, polybutylene terephthalate, or the like resin. The dielectric waveguide  21  may be flexible. In this case, a degree of freedom in the positions of the two circuit boards  10 A and  10 B can be ensured. In addition, by using a dielectric as the waveguide  21  as compared to, for example, a metal waveguide (or waveguide tube), there is an increased degree of freedom of the positions of circuit boards  10 A and  10 B as well as a reduced manufacturing cost. The thickness of the dielectric waveguide  21  is adapted to the millimeter wave frequency that is transmitted and received between the semiconductor packages  12 A and  12 B. The cross section of the dielectric waveguide  21  is, for example, rectangular. 
     The arrangement and support structure of the dielectric waveguide will be described below. In the following description that is applicable to both the two semiconductor packages  12 A and  12 B, the explanatory code  12  will be used for these semiconductor packages  12 A and  12 B. In addition, in the following description that is applicable to both the two circuit boards  10 A and  10 B, the explanatory code  10  will be used for these circuit boards  10 A and  10 B. 
       FIG.  2 A  to  FIG.  2 D  are figures that illustrate a signal transmission system  1   a  that is an example of the signal transmission system  1  described above.  FIG.  2 A  is a front view.  FIG.  2 B  is a perspective view illustrating a connecting part  14  and the semiconductor package  12  mounted on the circuit board  10  ( 10 A). These figures illustrate a dielectric waveguide  21 E as an example of the dielectric waveguide  21  described above.  FIG.  2 C  is a perspective view illustrating the dielectric waveguide  21 E, and a first circuit board  10  ( 10 A); and a second circuit board  10  ( 10 B) is omitted.  FIG.  2 D  is a cross-sectional view of a transmission system obtained from the cross-section illustrated by the IId-IId line illustrated in  FIG.  2 C   
     In the description below, the directions illustrated by Z 1  and Z 2  in  FIG.  2 A  are respectively called upward and downward. In addition, the directions illustrated using Y 1  and Y 2  in  FIG.  2 C  are respectively called forward and backward and the directions illustrated by X 1  and X 2  are respectively called right and left. 
     As illustrated in  FIG.  2 A , with the signal transmission system  1   a , the two circuit boards  10  may be arranged facing each other. Semiconductor packages  12  are mounted on each of the circuit boards  10 . The two semiconductor packages  12  are facing each other in a direction perpendicular to the circuit boards  10 . Furthermore, the dielectric waveguide  21 E is arranged between the two semiconductor packages  12 . 
     As illustrated in  FIG.  2 B  and  FIG.  2 D , the antenna  12   e  is formed on the surface of the semiconductor package  12  (package surface). As the example illustrates in the figure, the antenna  12   e  is formed in the center of the package surface  12   f  but is not limited to this position. The antenna  12   e  may be formed, for example, near a corner of package surface  12   f . The semiconductor package  12  has an IC chip  12   h  (see  FIG.  2 D ) on which an RF circuit is formed, and a mold resin  12   g  covering the IC chip  12   h . The antenna  12   e  is formed on the package surface  12   f  that is the surface of this mold resin  12   g . Note that a protective layer for protecting the antenna  12   e  may be present on the surface of the antenna  12   e  to the extent losses are not affected. 
     As illustrated in  FIG.  2 D , the dielectric waveguide  21 E has a surface  21   a  (waveguide end surface) facing the antenna  12   e . An air gap G is ensured between the waveguide end surface  21   a  and the antenna  12   e . Compared to a structure with the antenna  12   e  in contact with the waveguide end surface  21   a , for example, this air gap G enables reducing energy losses for signals between the antenna  12   e  and the waveguide end surface  21   a.    
       FIG.  3    is a graph illustrating the relationship between an air gap and insertion losses. The horizontal axis is the air gap and the vertical axis is insertion losses. This graph indicates higher insertion losses in the downward direction of the vertical axis. In addition, “air gap: 0 mm” on the horizontal axis of the graph indicates that the antenna  12   e  is in contact with the waveguide end surface  21   a.    
     Even if the size (cross-sectional area) of the dielectric waveguide is designed in conjunction with a signal frequency (for example 60 GHz) to be transmitted via the dielectric waveguide, the frequency where actual reflection losses are minimized may be at a frequency slightly offset from that frequency (for example, 61 GHz or 62 GHz). Here, first the inventors measured the frequency where the reflection losses in the signal transmission system  1   a  were minimized. Furthermore, the relationship between the air gap G and insertion losses was calculated using a simulation for the case of the frequency that minimizes reflection losses being transmitted from the first semiconductor package  12  to the second semiconductor package  12  via the dielectric waveguide  21 E. The inventors performed this manner of simulation on a plurality of frequencies within the range of 60 GHz to 300 GHz.  FIG.  3    schematically illustrates the results thereof. In this figure, the horizontal axis is the air gap. The vertical axis represents insertion losses, with the smallest insertion loss being 0 dB. Insertion losses increase going downward on the vertical axis. As illustrated in this figure, it can be seen that insertion losses are relatively large at “air gap: 0 mm”. In addition, insertion losses abruptly decrease as the air gap G is increased from 0 mm and it can be seen that insertion losses are minimized in the air gap range of from 0.025 mm to 0.1 mm. In addition, in the range of the air gap being increased beyond 0.1 mm, insertion losses gradually increase as the air gap G gets larger. The same trend was found in the relationship of the air gap G and the insertion losses for all frequencies. 
     Therefore, the air gap G is preferably 0.025 mm or more. This can reduce insertion losses. The air gap G may more preferably be 0.05 mm or more. Thus, the air gap G is more reliably ensured enabling reducing the effect of tolerance of the dielectric waveguide and the circuit board on insertion losses. The air gap G may even more preferably be 0.1 mm or more. Thus, the effect of tolerance of the dielectric waveguide and the circuit board on insertion losses can be reliably reduced. In addition, the air gap G is preferably 0.8 mm or less. Thus relative positioning accuracy between the antenna  12   e  and the waveguide end surface  21  can be ensured and increase in insertion losses due to an excessive air gap G can be suppressed. The air gap G is even more preferably 0.5 mm or less. Thus, relative positioning accuracy of the antenna  12   e  and the waveguide end surface  21  can be enhanced and increase in insertion losses can effectively be suppressed. 
     As illustrated in  FIG.  2 B  and  FIG.  2 D , the dielectric waveguide  21 E may include the support part  21   b  for supporting the dielectric waveguide  21 E on the circuit board  10 . The support part  21   b  is directly or indirectly attached to the circuit board  10  and ensures the air gap G between the antenna  12   e  and the waveguide end surface  21   a.    
     The support part  21   b  may be integrally formed, for example, with the dielectric waveguide  21 E. In other words, for example, the support part  21   b  is not mutually connected to the dielectric waveguide  21 E or another portion (waveguide main body  21   f  positioned between the two antennas  12   e ) by screws or the like but is mutually connected based on the chemical properties of the materials. The support part  21   b  and the waveguide main body  21   f  may be formed using a mold process of supplying molten material to a mold corresponding to the shapes thereof. Compared to a structure of mutual connection using screws or the like, this manner of structure of the dielectric waveguide  21 E enables reducing the component count as well as simplifying the manufacturing process of the signal transmission system  1   a.    
     The dielectric waveguide  21 E may, for example, include two support parts  21   b . The two support parts  21   b  may be arranged in a direction along the circuit board  10  on mutually opposite sides of the semiconductor package  12 . Thus, support stability of the dielectric waveguide  21 E can be ensured. 
     Note that unlike the example illustrated in the figure, the dielectric waveguide  21 E may include four support parts  21   b . Furthermore, two support parts  21   b  may be arranged on mutually opposite sides in a first direction (for example, left-right direction) and the remaining two support parts  21   b  may be arranged on mutually opposite sides in a second direction (for example, front-to-back direction). In still another example, the dielectric waveguide  21 E may include a wall part surrounding the entire periphery of the semiconductor package  12  as the support part  21   b . Furthermore, this wall part may be secured to the circuit board  10 . 
     As illustrated in  FIG.  2 B  and  FIG.  2 C , a connecting part  14  that connects to the dielectric waveguide  21 E is mounted on the circuit board  10 . In the examples illustrated in these figures, on the circuit board  10 , there are two connecting parts  14  mounted in positions on mutually opposite sides of the semiconductor package  12 . The dielectric waveguide  21 E is mated between these two connecting parts  14  and is retained by these connecting parts  14 . 
     As illustrated in  FIG.  2 D , an engaging part  14   a  may be formed on the connecting part  14 . The engaging parts  14   a  may be formed on the inside of the two connecting parts  14 , in other words, facing the semiconductor package  12 . The engaging part  14   a  engages with the support part  21   b  of the dielectric waveguide  21 E and retains the dielectric waveguide  21 E on the circuit board  10 . The engaging parts  14   a  are formed elastically deformable, for example, in the directions that the two connecting parts  14  face. The two support parts  21   b  of the dielectric waveguide  21 E may be retained by the elasticity of the engaging parts  14   a . In addition, a protruding part may be formed on the engaging part  14   a . A recessed part in which the protruding part of the engaging part  14   a  is mated may be formed on the support part  21   b.    
     As illustrated in  FIG.  2 C , the connecting part  14  may include a mounting fitting  14   b  formed, for example, of metal. This mounting fitting  14   b  may be soldered to the circuit board  10 . The connecting part  14  may include a resin part  14   c  (see  FIG.  2 D ). The mounting fitting  14   b  may be secured to the resin part  14   c.    
     As illustrated in  FIG.  2 D , when the support part  21   b  of the dielectric waveguide  21 E is mated with the engaging part  14   a  of the connecting part  14 , the end surface of the support part  21   b  may be in contact with the surface of the circuit board  10 . In this case, the length of the support part  21   b  (distance from the waveguide end surface  21   a  to the end surface of the support part  21   b ) may be set such that an appropriate air gap G is obtained. 
     Unlike the example illustrated in  FIG.  2 D , when the support part  21   b  of the dielectric waveguide  21 E is mated with the engaging part  14   a  of the connecting part  14 , a gap may be present between the end surface of the support part  21   b  and the front surface of the circuit board  10 . In this case, the distance in the vertical direction from the portion coupled with the engaging part  14   a  (recessed part in example illustrated in the figure) to the waveguide end surface  21   a  may be set such that an appropriate air gap G is obtained. 
     Unlike the examples illustrated in  FIG.  2 A  to  FIG.  2 D , for the signal transmission system  1   a , the two connecting parts  14  may be integrally molded so as to surround the semiconductor package  12 . In other words, the connecting part  14  may have a form of wholly surrounding the semiconductor package  12 . Furthermore, two or four engaging parts  14   a  may be formed on portions of the connecting part  14  mutually facing the semiconductor package  12 . 
       FIG.  4 A  to  FIG.  4 C ,  FIG.  5   ,  FIG.  6   ,  FIG.  7 A , and  FIG.  7 B  are figures illustrating signal transmission systems that are Modified Examples of the signal transmission system. In these figures, the same elements as those in the signal transmission system described with reference to  FIG.  1    and  FIG.  2 A  are given the same code. Hereinafter, primarily differences with the signal transmission system and the dielectric waveguide described so far will be described. Points in the examples illustrated in  FIG.  4 A  to  FIG.  4 C ,  FIG.  5   ,  FIG.  6   ,  FIG.  7 A , and  FIG.  7 B  without a description may be the same as the examples described so far. 
     In the description below, the directions illustrated as Z 1  and Z 2  are respectively called upward and downward, the directions illustrated as Y 1  and Y 2  are respectively called forward and backward, and the directions illustrated as X 1  and X 2  are respectively called right and left. 
       FIG.  4 A  is a perspective view illustrating a dielectric waveguide  21 F that is an example of the dielectric waveguide  21  and the first circuit board  10  ( 10 A). In this figure, the second circuit board  10  ( 10 B) is omitted.  FIG.  4 B  is a cross-sectional view of the transmission system obtained from the cross-section indicated by the IVb-IVb line illustrated in  FIG.  4 A . 
     As illustrated in  FIG.  4 A , the dielectric waveguide  21 F has two support parts  21   b  similar to the dielectric waveguide  21 E illustrated in  FIG.  2 A  and the like. The support part  21   b  has a mating part  21   c  protruding toward the circuit board  10 . A connecting hole  10   b  having a size corresponding to the mating part  21   c  is formed in the circuit board  10 . The mating part  21   c  fits inside and is retained in the connecting hole  10   b  (see  FIG.  4 B ). The mating part  21   c  may be press fit into the connecting hole  10   b.    
     As illustrated in  FIG.  4 A , the support part  21   b  may have an end surface  21   d  adjacent to the base part of the mating part  21   c . When the mating part  21   c  is mated inside the connecting hole  10   b , the end surface  21   d  faces the front surface of the circuit board  10 . The end surface  21   d  may be in contact with the front surface of the circuit board  10 . In this case, the distance from the waveguide end surface  21   a  to the end surface  21   d  may be set so as to obtain an appropriate air gap G. 
       FIG.  5    is a cross-sectional view of a dielectric waveguide of yet another example of a signal transmission system, and the cross-section is the same as that of  FIG.  4 B . In the signal transmission system  1   c  illustrated in  FIG.  5   , a dielectric waveguide  21 G includes two support parts  21   b  similar to the dielectric waveguide  21 E illustrated in  FIG.  2 A  and the like. Adhesive may be coated on an end surface  21   e  of the support part  21   b  or the front surface of the circuit board  10  to adhere the two together. Note that this manner of adhesive may be applied to the support part  21   b  of the dielectric waveguide  21 F illustrated in, for example,  FIG.  4 A  and  FIG.  4 B . 
       FIG.  6    is a cross-sectional view of a dielectric waveguide of yet another example of a signal transmission system, and the cross-section is the same as that of  FIG.  4 B . In the signal transmission system  1   d  illustrated in  FIG.  6   , a dielectric waveguide  21 H includes two support parts  21   b  similar to the dielectric waveguide  21 E illustrated in  FIG.  2 A  and the like. A mounting fitting  22  formed of metal may be attached to the support part  21   b . The mounting fitting  22  may be soldered to the circuit board  10 . The mounting fitting  22  includes a first portion  22   a  mounted to the support part  21   b  and a second portion  22   b  soldered to the circuit board  10 . A hole may be formed in the support part  21   b  and the first portion  22   a  may be press fit into this hole. In addition, as another example, the mounting fitting  22  may be integrally molded with the dielectric waveguide  21 H. In other words, in a molding process of the dielectric waveguide  21 H, the mounting fitting  22  may be arranged in a mold and thereafter molten resin that is the material of the dielectric waveguide  21 H may be formed. With this method, the resin that is the material of the dielectric waveguide  21 H enters the hole formed in the first portion  22   a  of the mounting fitting  22  of the recessed part formed at the edge of the first portion  22   a . Thus, the mounting fitting  22  can be retained by the dielectric waveguide  21 H. The first portion  22   a  and the second portion  22   b  may be formed, for example, in an L shape. The second portion  22   b  may be arranged parallel to the circuit board  10  and then soldered. 
       FIG.  7 A  is a perspective view illustrating a dielectric waveguide  21 J that is an example of the dielectric waveguide  21  and the first circuit board  10  ( 10 A). In this figure, the second circuit board  10  ( 10 B) is omitted.  FIG.  7 B  is a cross-sectional view of a transmission system  1   d  obtained from the cross-section indicated by the VIIb-VIIb line illustrated in  FIG.  7 A . 
     Similar to the example illustrated in  FIG.  2 A , on the circuit board  10 , two connecting parts  14  are mounted facing each other with the semiconductor package  12  interposed therebetween. A dielectric waveguide  21 J is fitted between the two connecting parts  14 . The spacing W 2  of the two connecting parts  14  (see  FIG.  7 B ) corresponds to the width of the dielectric waveguide  21 J. The dielectric waveguide  21 J is press fit between the two connecting parts  14  and is retained by these two connecting parts  14 . 
     As illustrated in  FIG.  7 A , the two connecting parts  14  include a mating part  14   e  on the inside thereof, in other words, on the side thereof facing the semiconductor package  12 . The support part  21   b  of each dielectric waveguide  21 J is mated into the mating part  14   e . The distance W 2  of the inner surface of the mating part  14   e  (see  FIG.  7 B ) corresponds to the width of the two support parts  21   b . As a result, the support part  21   b  is press fit inside the two mating parts  14   e  and is retained by the connecting parts  14 . In this manner, the position of the dielectric waveguide  21 J is defined by the directions that the two connecting parts  14  face. In addition, the dielectric waveguide  21 J in the direction orthogonal relative to the direction in which the two connecting parts  14  face each other is defined by the edge  14   d  of the mating part  14   e.    
     The connecting part  14  illustrated in this figure may also include the mounting fitting  14   b  formed, for example, of metal. This mounting fitting  14   b  may be soldered to the circuit board  10 . The connecting part  14  may include a resin part  14   c . The mounting fitting  14   b  may be secured to the resin part  14   c.    
     As illustrated in  FIG.  7 B , when the support part  21   b  of the dielectric waveguide  21 J is retained by the two connecting parts  14 , the end surface  21   e  of the support part  21   b  may be in contact with the front surface of the circuit board  10 . In this case, the length of the support part  21   b  (distance in the vertical direction from the waveguide end surface  21   a  to the end surface  21   e  of the support part  21   b ) may be set such that an appropriate air gap G is obtained. 
       FIG.  8    is a cross-sectional view illustrating a Modified Example of the dielectric waveguide and the cross-section is similar to that of  FIG.  4 B . A dielectric waveguide  21 M illustrated in  FIG.  8    includes two support parts  21   b  positioned on mutually opposite sides of the semiconductor package  12  similar to the dielectric waveguide  21 E illustrated in  FIG.  2 A  and the like. As illustrated in  FIG.  8   , a width W 3  of the waveguide main body  21   f  is different than a width W 4  of the two support parts  21   b . More specifically, the width W 3  of the waveguide main body  21   f  may be smaller than the width W 4  of the two support parts  21   b . With this structure, the width W 4  of the support part  21   b  can be adapted to the size of the semiconductor package  12  while the width W 3  of the waveguide main body  21   f  can be a size that is adapted to the frequency of the electromagnetic waves transmitted and received between the two semiconductor packages  12 . 
       FIG.  9 A  to  FIG.  9 D  illustrate yet another example of a signal transmission system. The same codes are assigned to the same elements in the signal transmission systems described in the figures so far. Hereinafter, primarily differences with the signal transmission system described so far will be described. 
       FIG.  9 A  is a side view of the signal transmission system if.  FIG.  9 B  is a perspective view of a signal transmission system  1   f  and in this figure, the second circuit board  10 B is omitted.  FIG.  9 C  is a perspective view of the arrangement of the relay fittings  32 A and  32 B included in the connectors  30 A and  30 B included in the signal transmission system if.  FIG.  9 D  is a cross-sectional view of a transmission system obtained from the cross-section illustrated by the VIIId-VIIId line illustrated in  FIG.  9 B  Points in the examples illustrated in  FIG.  9 A  to  FIG.  9 D  without a description may be the same as the examples described so far. 
     As illustrated in  FIG.  9 A , the signal transmission system  1   f  may include two connectors  30 A and  30 B mounted respectively on the circuit boards  10 A and  10 B. The first connector  30 A is mounted on the first circuit board  10 A arranged on the lower side. The first connector  30 A may include housing  31 A, relay fitting  32 A (see  FIG.  9 C ), and semiconductor package  12 A. The second connector  30 B is mounted on the second circuit board  10 B arranged on the upper side. Similar to the first connector  30 A, the second connector  30 B may also include housing  31 B, relay fitting  32 B (see  FIG.  9 C ), and semiconductor package  12 B. A dielectric waveguide  21 K is arranged between and retained by the two connectors  30 A and  30 B. 
     As illustrated in  FIG.  9 A , the housings  31 A and  31 B include pedestals  31   a  and  31   b , respectively. The semiconductor package  12 A is arranged on the upper surface (surface facing the second circuit board  10 B) of the pedestal  31   a  in the first connector  30 A. The semiconductor package  12 B is arranged on the lower surface (surface facing the first circuit board  10 A) of the pedestal  31   b  in the second connector  30 B. 
     As illustrated in  FIG.  9 A , a dielectric waveguide  21 K is arranged between the two pedestals  31   a  and  31   b . The dielectric waveguide  21 K includes two support parts  21   b  on the upper end and on the lower end respectively (see  FIG.  9 D ) similar to the dielectric waveguide  21 E. The two support parts  21   b  formed on the end part (lower end) on the first connector  30 A are arranged on opposite sides of the semiconductor package  12 A. The two support parts  21   b  formed on the end part (upper end) of the second connector  30 B are also arranged on opposite sides of the semiconductor package  12 B. The support part  21   b  on the upper end and the support part  21   b  on the lower end may be formed so as to surround the entire circumference of the semiconductor packages  12 A and  12 B. 
     As illustrated in  FIG.  9 D , the support part  21   b  on the first connector  30 A side is supported on the front surface of the pedestal  31   a . The support part  21   b  on the second connector  30 B is supported on the front surface of the pedestal  31   b . Various structures can be used as the fixed structure of the support part  21   b  and the pedestals  31   a  and  31   b . For example, the end surface of the support part  21   b  can be adhered to the front surface of the pedestals  31   a  and  31   b  using adhesive. Alternatively, the mounting fitting  22  illustrated in  FIG.  6    may be mounted on the support part  21   b  and this mounting fitting  22  may be soldered to a metal part formed on the pedestals  31   a  and  31   b.    
     As illustrated in  FIG.  9 D , an air gap G is formed between the waveguide end surface  21   a  of the dielectric waveguide  21 K and the antenna  12   e  of the semiconductor package  12 A. This air gap G may be defined by the length of the support part  21   b.    
     The housing  31 A of the first connector  30 A and the housing  31 B of the second connector  30 B may be formed to mutually mate and to restrict relative movement. Thereby, the relative position between the dielectric waveguide  21 K and the antenna  12   e  can also be appropriately maintained. 
     As an example, an insertion part  31   d  may be formed on the housing  31 B of the second connector  30 B extending downward toward the first circuit board  10 A as illustrated in  FIG.  9 A . The housing  31 B of the second connector  30 B may include two insertion parts  31   d  positioned on mutually opposite sides of the semiconductor package  12 B (in other words, opposite sides of the pedestal  31   b ). In addition, the housing  31 A of the first connector  30 A may have an opening receiving part  31   c  formed facing the second circuit board  10 B. The housing  31 A of the first connector  30 A may have two receiving parts  31   c  positioned on mutually opposite sides of the semiconductor package  12 A (in other words, opposite sides of the pedestal  31   a ). The insertion part  31   d  mates with the receiving part  31   c . This restricts relative movement of the two connectors  30 A and  30 B. More specifically, relative movement of the connectors  30 A and  30 B in the front-to-back direction is restricted by the insertion part  31   d  and the receiving part  31   c.    
     In the example illustrated in  FIG.  9 A , regarding the receiving part  31   c , the gap formed between an outer wall part  31   e  and the side surface of the pedestal  31   a  functions as the receiving part  31   c . The receiving part  31   c  is also open in the left-right direction (direction aligned with the relay fittings  32 A and  32 B) rather than upward. The shape of the receiving part  31   c  is not limited to this. The receiving part  31   c  may be formed so as to surround the entirety of each insertion part  31   d . In other words, the left end and right end of the receiving part  31   c  may be closed. 
     As illustrated in  FIG.  9 D , the semiconductor package  12 A mounted on the first connector  30 A includes a plurality of connecting terminals  12   i  on the bottom surface thereof (surface opposite the package surface  12   f ). The connecting terminals  12   i  connect to the IC chip  12   h  (see  FIG.  2 D ) incorporated in the semiconductor package  12 A. The connecting terminals  12   i  of the semiconductor package  12 A are, for example, soldered to a first connecting part  32   a  of the relay fitting  32 A (see  FIG.  9 C ). In addition, the semiconductor package  12 B of the second connector  30 B also includes a plurality of connecting terminals  12   i  (see  FIG.  9 D ) on the upper surface thereof (surface opposite the package surface  12   f ). These connecting terminals  12   i  are, for example, soldered to a first connecting part  32   d  (see  FIG.  9 C ) of the relay fitting  32 B retained by the pedestal  31   b  of the housing  31 B. 
     The first connecting part  32   a  of the relay fitting  32 A is exposed on the front surface of the pedestal  31   a . As illustrated in  FIG.  9 C , the relay fitting  32 A includes a second connecting part  32   b  soldered to a conductive pad (not shown) formed on the first circuit board  10 A. In addition, the relay fitting  32 A includes a center section  32   c  extending from the first connecting part  32   a  in the mating direction of the two connectors  30 A and  30 B, in other words, the direction orthogonal to the circuit boards  10 A and  10 B in the example illustrated in the figure. 
     The first connecting part  32   d  of the relay fitting  32 B is exposed on the front surface of the pedestal  31   b . As illustrated in  FIG.  9 C , the relay fitting  32 B includes a second connecting part  32   e  soldered to a conductive pad (not shown) formed on the second circuit board  10 B. In addition, the relay fitting  32 B includes a center section  32   f  extending from the first connecting part  32   d  in the mating direction of the two connectors  30 A and  30 B, in other words, the direction orthogonal to the circuit boards  10 A and  10 B in the example illustrated in the figure. 
     The relay fittings  32 A and  32 B are retained by the housings  31 A and  31 B. In a first example, the relay fittings  32 A and  32 B are integrally formed with the housings  31 A and  31 B. In other words, the relay fittings  32 A and  32 B may be arranged in the mold during the molding process of the housings  31 A and  31 B. Thereafter, the molten resin that is the material of the housings  31 A and  31 B may be formed. This method enables the material of housings  31 A and  31 B material to enter holes formed in the center sections  32   c  and  32   f  of the relay fittings  32 A and  32 B or recessed parts formed on the edges of the center sections  32   c  and  32   f . Thus, the relay fittings  32 A and  32 B are retained by the housings  31 A and  31 B. Alternatively, the relay fittings  32 A and  32 B may be press fit into the housings  31 A and  31 B. 
     The relay fittings  32 A and  32 B are formed of metal. The material of the relay fittings  32 A and  32 B may be, for example, brass or phosphor bronze. The relay fittings  32 A and  32 B may be formed using press forming such as bending or punching. The housings  31 A and  31 B may be formed of resin. The material of the housings  31 A and  31 B may be, for example, polybutylene terephthalate (PBT) or polyamide (PA). 
     High frequency signals are transmitted and received between electronic components and sensors mounted on the two circuit boards  10 A and  10 B via the relay fittings  32 A and  32 B, semiconductor packages  12 A and  12 B, the antenna  12   e , and dielectric waveguide  21 K. 
     As illustrated in  FIG.  9 C , the first connector  30 A may include a connecting terminal  33 A and the second connector  30 B may include a connecting terminal  33 B. The two connecting terminals  33 A and  33 B may be in direct contact with each other. With the connecting terminals  33 A and  33 B, low frequency signals and direct current can be transmitted and received between the two circuit boards  10 A and  10 B without passing through the dielectric waveguide  21 K. 
     The connecting terminal  33 A includes a connecting part  33   a  soldered on the first circuit board  10 A and a plate spring contact part  33   b . On the other hand, the connecting terminal  33 B includes a connecting part  33   c  soldered on the second circuit board  10 B and a contact part  33   d  extending from the connecting part  33   c  toward the first circuit board  10 A. The connecting terminals  33 A and  33 B may be mutually in contact with the contact parts  33   b  and  33   d . The connecting terminal  33 A is retained by the housing  31 A together with the relay fitting  32 A. In addition, the connecting terminal  33 B is retained by the housing  31 B together with the relay fitting  32 B. 
       FIG.  10 A  and  FIG.  10 B  illustrate yet another example of a signal transmission system. The same codes are assigned to the same elements in the signal transmission systems described in the figures so far. Hereinafter, primarily differences with the signal transmission system  1   f  described with reference to  FIG.  9 A  to  FIG.  9 D  will be described. 
       FIG.  10 A  is a side view of a signal transmission system  1   g .  FIG.  10 B  is a perspective view of the arrangement of the relay fittings  32 A and  32 B included in the connectors  30 A and  30 B of the signal transmission system  1   g . Points in the examples illustrated in  FIG.  10 A  and  FIG.  10 B  without a description may be the same as the examples described so far. 
     As illustrated in  FIG.  10 A , the first connector  30 A may include an interposer board  34 A. The interposer board  34 A is arranged on the pedestal  31   a  of the housing  31 A. The semiconductor package  12 A is arranged on the interposer board  34 A. A conductive pattern (circuit) for electrically connecting the first connecting part  32   a  of the relay fitting  32 A and the connecting terminals  12   i  of the semiconductor package  12 A is formed on the interposer board  34 A. Use of the interposer board  34 A enables raising the degree of freedom of the relay fitting  32 A arrangement as well as the degree of freedom of arranging the connecting terminals  12   i  of the semiconductor package  12 A. 
     In addition, as illustrated in  FIG.  10 A , the second connector  30 B may also have an interposer board  34 B. The interposer board  34 B is arranged below the pedestal  31   a  on the housing  31 B. The semiconductor package  12 B may be attached to this interposer board  34 B. A conductive pattern (circuit) for electrically connecting the first connecting part  32   d  of the relay fitting  32 B and the connecting terminals  12   i  of the semiconductor package  12 B is formed on the interposer board  34 B. Use of the interposer board  34 B enables raising the degree of freedom of the relay fitting  32 B arrangement as well as the degree of freedom of arranging the connecting terminals  12   i  of the semiconductor package  12 B. 
     A dielectric waveguide  21 L may be supported by the interposer boards  34 A and  34 B. For example, the support part  21   b  of the dielectric waveguide  21 L formed on the first circuit board  10 A may be connected to the front surface of the interposer board  34 A by adhesive, or the mounting fitting  22  illustrated in  FIG.  6    may be attached to the support part  21   b  and this mounting fitting  22  may be soldered to the interposer board  34 A. In a similar manner, the support part  21   b  of the dielectric waveguide  21 L formed on the second circuit board  10 B may be connected to the front surface of the interposer board  34 B by adhesive, or the mounting fitting  22  illustrated in  FIG.  6    may be attached to the support part  21   b  and this mounting fitting  22  may be soldered to the interposer board  34 B. 
     Note that unlike the examples illustrated in  FIG.  10 A  and  FIG.  10 B , of the two connectors  30 A and  30 B, an interposer board may be included on only one of the connectors. For example, the interposer board  34 A may be included on only the first connector  30 A. In this case, the support part  21   b  of the dielectric waveguide  21 L formed on the first circuit board  10 A may be the one attached to the interposer board  34 A and the support part  21   b  of the dielectric waveguide  21 L formed on the second circuit board  10 B may be attached to the pedestal  31   b  of the housing  31 B. 
     As has been described above, the signal transmission systems  1   a  to  1   g  include the circuit boards  10 A and  10 B, and semiconductor packages  12 A and  12 B containing an RF circuit and the dielectric waveguides  21 ,  21 E,  21 F,  21 G,  21 H,  21 J,  21 K, and  21 L mounted on the circuit boards  10 A and  10 B. The semiconductor packages  12 A and  12 B include the package surface  12   f  and the antenna  12   e  formed on the package surface  12   f . The dielectric waveguides  21  and  21 E to  21 M include a waveguide end surface  21   a  facing the antenna  12   e . An air gap G is ensured between the waveguide end surface  21   a  and the antenna  12   e . With this system, the air gap G can reduce insertion losses. 
     Note that the signal transmission systems proposed in the present disclosure are not limited to those described above. 
     For example, a plurality of dielectric waveguides may be arranged between the circuit boards  10 A and  10 B. Furthermore, the signal transmission system may include a member securing the relative position of the plurality of dielectric waveguides.