Patent Publication Number: US-7911292-B2

Title: Mode transition between a planar line and a waveguide with a low loss RF substrate and a high loss low frequency substrate

Description:
TECHNICAL FIELD 
     The present invention relates to a mode transition circuit for transferring a radio frequency (RF) signal and a transceiver module having the same; and more particularly, to a mode transition circuit for transferring a Radio Frequency signal generated from a RF substrate to a module such as an antenna through a planar transmission line and a waveguide including vias and metal patches formed inside the RF substrate and a hole formed inside a low frequency substrate, and a transceiver module having the same. 
     BACKGROUND ART 
     A mode transition circuit for transferring a radio frequency (RF) signal and a transceiver module having the same are generally formed of a substrate having a predetermined dielectric constant that allows it to be operated in a RF domain. Particularly, the mode transition circuit is generally embodied using a micro-strip line to waveguide transition, which shows quasi TEM to TE01 mode transition. Since the micro-strip line must include a conductive element such as a back-short, the micro-strip line is not proper in a view of miniaturizing a mode transition circuit. 
     In order to overcome such a shortcoming of the conventional mode transition circuit, there were many research projects in progress for embodying a mode transition circuit using a metal waveguide filled with an air or a waveguide filled with a dielectric. However, a transmission line such as micro-strip line is still required to connect active elements and a mode transition circuit so the signal loss is unescapable due to the transmission line. 
     A conventional technology to embody a RF module using a mode transition circuit was introduced in U.S. Pat. No. 5,982,250 entitled “MILLIMETER-WAVE LTCC PACKAGE.” The conventional technology of U.S. Pat. No. 5,982,250 discloses a single-layer substrate made of alumina, or a multi-layered substrate formed by a Low Temperature Co-Fired Ceramics (LTCC) process. 
     However, the conventional technology of U.S. Pat. No. 5,982,250 still has a shortcoming of a high manufacturing cost and a difficulty of integration if it is integrated with circuits operated in a low frequency circuit, such as a baseband processor or a power integration chip (IC). 
     Generally, a low frequency substrate made of inexpensive material such as FR4 is used to integrate a baseband processor or a power IC. However, FR4 has a great substrate loss, and it is not easy to form a RF module using FR4. 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     It is, therefore, an object of the present invention to a mode transition circuit for transferring a Radio Frequency (RF) signal generated from a RF substrate to a module such as an antenna through a planar transmission line and a waveguide including vias and metal patches formed inside the RF substrate and a hole formed inside a low frequency substrate, and a transceiver module having the same. 
     Technical Solution 
     In accordance with one aspect of the present invention, there is provided a mode transition circuit for transferring a radio frequency (RF) signal including: a planar transmission line mounted at a RF substrate for receiving a RF signal from a RF signal generating means; a via formed inside the RF substrate and connected to one side of the planar transmission line for receiving the RF signal from the planar transmission line; at least one of metal patches formed inside the RF substrate and connected to the one side of the via for receiving the RF signal from the via; and a hole formed inside a low frequency substrate and connected to one side of the metal patch for receiving the RF signal from the metal patch. 
     In accordance with another aspect of the present invention, there is provided a transceiver module having a mode transition circuit including: a mode transition circuit for transferring a RF signal and having a planar transmission line mounted at a RF substrate, a via formed inside the RF substrate, at least one of metal patches, and a hole formed inside a low frequency substrate; an RF signal generating means mounted on the RF substrate; and a low frequency signal processing means mounted on the low frequency substrate, wherein the RF signal generated from the RF signal generating means is transferred to the low frequency signal processing means or to a RF signal processing means connected at the bottom of the low frequency substrate through the planar transmission line, the via, the metal patch and the hole. 
     ADVANTAGEOUS EFFECTS 
     In the present invention, a waveguide is formed for integrating a radio frequency (RF) substrate and a low frequency substrate. Therefore, the signal loss generated while transferring a RF signal can be minimized according to the present invention. 
     According to the present invention, a baseband processor or a power IC of a low frequency substrate can be effectively integrated with a RF substrate. Therefore, a transceiver module can be miniaturized, and the manufacturing cost thereof can be reduced. 
     According to the present invention, a mode transition circuit and a transceiver module using the same can be formed of inexpensive material and simple manufacturing processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a top view illustrating a transition circuit for transferring a RF signal in accordance with an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of  FIG. 1  taken along the line A-A′; 
         FIG. 3  is a bottom view of  FIG. 1 ; and 
         FIG. 4  is a cross-sectional view of a transceiver module having a mode transition circuit for transferring a RF signal in accordance with another embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. 
       FIG. 1  is a top view illustrating a transition circuit for transferring a RF signal in accordance with an embodiment of the present invention,  FIG. 2  is a cross-sectional view of  FIG. 1  taken along the line A-A′ shown in  FIG. 1 , and  FIG. 3  is a bottom view of  FIG. 1 . 
     The present invention proposes a mode transition circuit for transferring a Radio Frequency (RF) signal generated from a RF substrate  5  ( FIGS. 1 ,  2 ) to a module such as an antenna through a planar transmission line  14  ( FIG. 2 ) and a waveguide including vias  16  ( FIG. 2 ) and metal patches  15  ( FIGS. 2 ,  3 ) formed inside the RF substrate  5  and a hole  11  ( FIG. 2 ) formed inside a low frequency substrate  2 , and a transceiver module having the same. 
     As described above, the present invention proposes the mode transition circuit for transferring a RF signal and a transceiver module having the same. Hereinafter, the mode transition circuit for transferring a RF signal and the transceiver module having the same will be described with reference to  FIGS. 1 to 3 . 
       FIGS. 1 to 3  show the transceiver module having the transition circuit for transferring a RF signal according to an exemplary embodiment of the present invention. The transceiver module according to the present embodiment includes a multi-layered low frequency substrate  2 , a plurality of baseband processors mounted on the low frequency substrate  2 , a plurality of power integrated chips (IC)  4  ( FIGS. 1 ,  2 ), a multi-layered RF substrate  5 , a monolithic microwave integrated chip (MMIC)  6  ( FIGS. 1 ,  2 ) mounted on the RF substrate  5 , and a mode transition circuit for transferring a RF signal. That is, the transceiver module includes a unit for transferring a RF signal from the planar transmission line  14  to the other module such as an antenna through a RF substrate  5  and low frequency substrate  2 , which are formed in a shape of a waveguide. The transceiver module may include only one of the baseband processor  3  ( FIGS. 1 ,  2 ) and the power IC  4 , or include both of them on the low frequency substrate  2 . 
     Furthermore, in the present invention, a RF signal generated from the RF substrate  5  is transferred to the other high frequency module through a waveguide and a low frequency substrate  2 . That is, the present invention is characterized in transferring the RF signal through a low frequency substrate without any loss. 
     The mode transition circuit for transferring a RF signal according to the present embodiment includes a planar transmission line  14  for transferring a RF signal generated from a MMIC  6  of a RF substrate  5 , a via  16  ( FIG. 2 ) formed inside the RF substrate having one end connected to the planar transmission line  14 , at least one metal patch  15  formed inside the RF substrate  5 , and a hole  11  formed inside the low frequency substrate  2 . 
     Generally, a wireless communication device must transfer a RF signal generated from a RF substrate  5  to other RF substrates or antennas so as to process the RF signal. In order to effectively transfer the RF signal, it is desirable to form a waveguide as a signal transferring path on a transceiver module. However, a low frequency substrate  2  generates a great signal loss while transferring the RF signal. 
     In order to overcome the shortcoming of the low frequency substrate  2 , the hole  11  is formed inside the low frequency substrate  2 , and the edge of the hole  11  is coated with a conductor, thereby forming the hole  11  to have a waveguide shape in the present embodiment. As described above, a signal transfer path for transferring a RF signal with a very small loss is embodied by forming the hole  11  having the conductor coated edge in the waveguide shape. Also, the baseband process  3  or the power IC  4  can be integrally formed with the high frequency substrate  5 , thereby miniaturizing the transceiver module and reducing the manufacturing cost thereof. 
     The RF signal transferring path in the transceiver module according to the present embodiment will be described hereinafter. 
     A RF signal generated or amplified from the MMIC  6  of the RF substrate  5  is transferred to an antenna connected to the low frequency substrate  2  through a bonding wire  13  ( FIGS. 1 ,  2 ), a planar transmission line  14 , a via, a metal patch and a hole  11  formed inside the low frequency substrate  2 . Also, the RF signal is transferred to the baseband processor  3  or the Power IC  4  mounted on the low frequency substrate  2 . 
     The planar transmission line  14  may be formed as a micro-strip transmission line, strip line or a coplanar waveguide (CPW). At least one dielectric layers  21  ( FIG. 2 ) is formed inside the RF substrate  5 . 
     The via  16  is formed inside the high frequency substrate  5 . The via  16  is formed between the metal patch  15  of the RF substrate  5 , which is connected to the hole  11  of the low frequency substrate  2 , and the planar transmission line  14  of the RF substrate  5 , which is connected to the MMIC  6  that generates the RF signal, thereby transferring the RF signal from the RF substrate  5  to the low frequency substrate  2 . 
     The metal patches  15  can be formed on the RF substrate  5  as a stacked structure. Although the metal patch  15  is not stacked on the RF substrate  5 , a RF signal can be transferred without any problems. If the metal patch  15  is formed on the RF substrate  5  as a stacked structure, a bandwidth for transferring the RF signal can be expanded. 
     As shown in  FIG. 2 , the low frequency substrate  2  includes a first dielectric  7  disposed at an upper portion thereof as a layer, a second dielectric  8  disposed at a lower portion thereof as a layer, and a conductor  9  interposed between the first and second dielectrics  7  and  8 . A plurality of vias  10  are used to connect the conductor  9 . The layers in the low frequency substrate  2  may be formed repeatedly. 
     As described above, a metal waveguide is formed of the hole  11  formed in the low frequency substrate  2  with the via  16  and the metal patches  15  formed inside the RF substrate  5 . It is preferable to form a conductor  12  on the surface of the low frequency substrate  2 , which forms the hole  11 , through a plating process in order to enable the hole  11  to perform a metal waveguide function. That is, the signal is transferred through the via  16  and the metal patch  15  of the RF substrate  5  without signal loss by forming the conductor  12  on the surface of the low frequency substrate  2 . 
       FIG. 3  shows a bottom view of a transceiver module in accordance with an embodiment of the present invention. As shown in  FIG. 3 , a metal layer  22  surrounds the bottom of the low frequency substrate  2 , and a metal patch is formed at the bottom of the radio frequency substrate  5 . A metal surface  20  for a RF substrate ground is formed one side of the metal patch  15  of the RF substrate  5 . 
     The bottom of the transceiver module is connected to the other module. For example, the hole  11  of the low frequency substrate  2  is connected to a horn antenna, a planar antenna, or a second RF substrate  60  shown  FIG. 4 . 
     As shown in  FIG. 2 , the metal surface  20  is formed on one side of the metal patch  15  of the RF substrate, which is adjacent to the hole  11  formed at the low frequency substrate  2 , for the RF substrate ground. It is preferable that the size of the hole  11  formed in the low frequency substrate  2  is separated from the edge of the metal surface  20  at a predetermined distance d in order to optimize for transferring a signal. Also, it is preferable to control the distance d in consideration of a frequency band and a power of a RF signal generated from the transceiver module. 
     While the transceiver module processes a signal, for example, transferring a RF signal, the large amount of heat is generated from a substrate. In order to evacuate heat generation, it is preferable that the metal layer  22  is formed on the bottom of the low frequency substrate  2  for heat sink. 
     A hole is formed at the metal layer  22  surrounding the bottom of the low frequency substrate  2  to have a same size of the hole  11  formed at the low frequency substrate  2  to effectively transfer a RF signal to an antenna. 
     Although the metal layer  22  is described to surround the entire bottom of the low frequency substrate  2  with metal, a predetermined portion of the bottom of the low frequency substrate  2  may be only surrounded with metal. 
     In the RF substrate  5 , the MMIC  6  and the planar transmission line  14  are connected through a bonding wire  13 . Such a boding wire  13  can be used to connect the MMIC  6  and the metal pad  30  ( FIG. 1 ). The metal pad  30  directly transfers the RF signal generated from the MMIC  6  to a predetermined module. Herein, a plurality of MMICs  6  may be formed on the RF substrate  5 . 
     The top of the RF substrate  5  can be covered by a cover  17  formed of metal or difference material. Also, it is preferable that an electromagnetic wave absorber  18  is formed on the MMIC  6  of the RF substrate  5  to prevent the electromagnetic wave from being reflected. 
     Furthermore, it is preferable to connect the RF substrate  5  on the low frequency  2  using adhesive such as soldering without performing a ball grid array (BGA) process or a u-BGA process. 
     In the present embodiment, it is preferable that the RF substrate  5  and the low frequency substrate  2  have different dielectric constant for effectively transiting a RF signal to a low frequency signal and transferring the low frequency signal. For example, the low frequency substrate is formed of a material having a great dielectric loss such as FR4, and the RF substrate  5  may be made of a material having a small dielectric loss such as LTCC, or alumina. 
       FIG. 4  is a cross-sectional view of a transceiver module having a mode transition circuit for transferring a RF signal in accordance with another embodiment of the present invention. 
     Referring to  FIG. 4 , the transceiver module according to the another embodiment of the present invention includes a low frequency substrate  2 , a first RF substrate  50  mounted on the low frequency substrate  2 , and a second RF substrate  60  mounted on the bottom of the low frequency substrate  2 . 
     The present application contains subject matter related to Korean patent application No. 2005-119620, and No. 2006-72417, filed in the Korean Intellectual Property Office on Dec. 8, 2005, and Jul. 31, 2006, the entire contents of which is incorporated herein by reference. 
     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.