Patent Publication Number: US-2020295428-A1

Title: High directivity compact size inter layer coupler

Description:
BACKGROUND 
     Field 
     This disclosure relates generally to a high directivity compact size inter-layer coupler and, more particularly, to a high directivity compact size inter-layer coupler that includes a main signal line formed on a top dielectric layer and a coupled line formed on an inner dielectric layer, where the coupled line includes narrow line portions connecting block portions to improve the directivity of the coupler. 
     Discussion 
     Telecommunications systems continue to improve in efficiency and throughput, such as through fourth generation (4G) to fifth generation (5G) products and services. In a typical telecommunications system, the transmitter employs digital components that convert digital bits to be transmitted at a particular point in time into in-phase and quadrature-phase symbol constellations for transmission. These symbols are mapped to corresponding analog signals that modulate a carrier, and the signal is amplified by a power amplifier (PA) to be transmitted over some distance, where the power amplifier is printed on a printed circuit board (PCB) having a number of board layers along with other system components. Power amplifiers often operate at or near their saturation level to provide high throughput and increased efficiency, which causes signal non-linearities that distort the transmitted signal. Therefore, some type of signal correction is typically required in the transmitter. Telecommunications systems thus often provide digital pre-distortion (DPD) of the digital signal before it is converted to an analog signal to be amplified by the power amplifier that reshapes the signal using a digital signal processing operation to reverse the distortion caused by the power amplifier so that the transmitted signal is not distorted. 
     The transmitter circuitry in a telecommunications system of the type discussed above generally includes a coupler that couples off a portion of the amplified main signal to be transmitted to be used as a feedback signal to the DPD circuit. These couplers need to operate at high power, have a low insertion loss and be separated from the main signal line. These couplers come in a number of designs depending on the specific application. Generally, these designs include couplers that are discrete components, which requires added space and cost, couplers that are printed on a top layer of the PCB for the power amplifier where many other signal lines need to be provided, and couplers that include a main signal line printed on a top layer of the PCB and a coupled line printed on an inter-layer of the PCB. As telecommunications system improve and increase in performance these couplers also need to become increasingly smaller in size and with a reduced cost. Therefore, improvements to the known designs are needed. 
     SUMMARY 
     The following discussion discloses and describes a high directivity compact size inter-layer coupler that has application for coupling off a pre-distortion coupling signal from a main signal at an output of a power amplifier and provide it to a pre-distortion circuit. The coupler includes a plurality of dielectric layers having at least a first dielectric layer, a second dielectric layer adjacent to the first dielectric layer and third dielectric layer adjacent to the second dielectric layer and opposite to the first dielectric layer. The coupler also includes a main signal line formed on a surface of the first dielectric layer opposite to the second dielectric layer, where the main signal line receives the main signal, and a coupling line formed between the second and third dielectric layers, where the coupling line is electromagnetically coupled to the main line so as to generate the coupling signal thereon. The coupling line has a general U-shape including a first leg portion, a second leg portion and a transverse portion between the first and second leg portions. The transverse portion includes a plurality of spaced apart blocks coupled together by a narrow line portion. 
     Additional features of the disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified block diagram of a communications system employing a high power amplifier and a coupler; 
         FIG. 2  is an isometric line drawing of a coupler that can be used as the coupler shown in  FIG. 1 ; 
         FIG. 3  is a side view of the coupler shown in  FIG. 2 ; and 
         FIG. 4  is a top view of the coupler shown in  FIG. 2  with the first two layers removed. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the disclosure directed to a directional inter-layer coupler is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses. For example, the coupler is described as having particular application for providing a feedback signal from a power amplifier for providing pre-distortion of a digital signal in a telecommunications system. However, as will be appreciated by those skilled in the art, the coupler described herein will have other applications. 
       FIG. 1  is a simplified block diagram of a telecommunications system  10  employing a power amplifier (PA)  12 . The system  10  is intended to represent any suitable telecommunications system consistent with the discussion herein including, for example, 4G and 5G systems. The system  10  includes a digital circuit  14  that generates a digital signal to be transmitted that is then pre-distorted by a DPD  16 . The pre-distorted digital signal is converted to an analog signal by a digital-to-analog (D/A) converter  18  before being sent to the PA  12  for amplification. The system  10  also includes a coupler  20  that couples off a sample portion of the amplified signal provided as a feedback signal that is sent to the DPD  16  and is used as a reference signal for the digital pre-distortion, where the pre-distortion can be performed in any suitable manner known to those skilled in the art. Therefore, the pre-distortion in the analog signal from the D/A converter  18  is removed by the non-linear effects caused by saturation of the PA  12  so that the signal transmitted by an antenna  22  is not distorted. This type of digital pre-distortion using a coupler to provide a feedback signal for use as a pre-distortion reference signal is well understood by those skilled in the art. 
     As mentioned above, it is desirable to reduce the size and cost of the couplers used in telecommunications system for this purpose. As will be discussed in detail below, the present disclosure is directed to such a coupler that includes a main signal line that receives the amplified signal provided on a top surface of a top dielectric layer of a printed circuit board (PCB) on which the PA  12  is fabricated and a coupler signal line fabricated on an inner layer of the PCB, where the PCB includes four component layers. 
       FIG. 2  is an isometric line drawing,  FIG. 3  is a side view and  FIG. 4  is a top view of a coupler  28  of this type that can be used as the coupler  20  in the system  10 . The coupler  28  is fabricated on a PCB  30  having a top dielectric layer  32 , a middle dielectric layer  34 , and a bottom dielectric layer  36 . The coupler  28  includes a main signal layer  38  formed on a top surface of the top dielectric layer  32 , a first ground layer  40  between the top dielectric layer  32  and the middle dielectric layer  34  and formed either on the bottom surface of the top dielectric layer  32  or the top surface of the middle dielectric layer  34 , a stripline layer  42  between the middle dielectric layer  34  and the bottom dielectric layer  36  and formed either on the bottom surface of the middle dielectric layer  34  or the top surface of the bottom dielectric layer  36 , and a second ground layer  44  formed on a bottom surface of the bottom dielectric layer  36 . Although not specifically shown, the PCB  30  is also the board on which the PA  12  is fabricated and possibly the circuit  14 , the DPD  16  and the D/A converter  18  along with other system components, where the components for those devices would also be formed on the various surfaces of the dielectric layers  32 ,  34  and  36 . The dielectric layers  32 ,  34  and  36  can be of any material and thickness for the purposes discussed herein. In this non-limiting embodiment, the dielectric layer  32  is R4350 and is about 20 mils thick, the dielectric layer  34  is FR4, a glass-reinforced epoxy laminate material, and is about 8.4 mils thick, and the dielectric layer  36  is FR4 and is about 30 mils thick. 
     The main signal layer  38  includes a main signal line  50  that receives the amplified signal to be transmitted from the PA  12  at the left side. In this non-limiting embodiment, the signal line  50  is a 500 microstrip line of a suitable metal deposited on the top surface of the top dielectric layer  32  and having a width of about 44 mils. The stripline layer  42  includes a microstrip coupling line  52  of a suitable metal fabricated in the configuration shown. The first ground layer  40  includes a metalized ground plane  54  having a cut-out section  56  so as to allow electromagnetic signal coupling between the main signal line  50  and the coupling line  52 . In this non-limiting embodiment, the cut-out section  56  has a rectangular configuration to improve the coupling between the main line  50  and the coupling line  52  and has, for example, a length of 5 mm and a width of 25 mils. The second ground layer  44  includes a metalized ground plane  58 . 
       FIG. 4  shows the dielectric layers  32  and  34  removed to better illustrate the coupling line  52 , where the main signal line  50  and the cut-out section  56  are shown in phantom. The coupling line  52  has a general U-shape including a first leg portion  62 , a transverse portion  64  and a second leg portion  66 . The transverse portion  64  includes a number of blocks  68  separated by narrow line portions  70 . The first leg portion  62  includes a narrow line portion  72  coupled to a narrow line portion  74  at one end of the transverse portion  64  and coupled to a wide line portion  76  at one port  78  of the coupler  28 . The second leg portion  66  includes a narrow line portion  80  coupled to a narrow line portion  82  at an opposite end of the transverse portion  64  and coupled to a wide line portion  84  at another port  86  of the coupler  28 . The dimensions of the various sections of the coupling line  52  referred to above will be application specific depending on the particular system and the frequencies of the signals. In one non-limiting embodiment, the blocks  68  have a width of 33 mils and a length of 0.8 mm, and have a space between the blocks  68  of 0.4 mm. Further, the number of the blocks  68  is four, although any suitable number can be used. The narrow line portions  70 ,  72 ,  74 ,  80  and  82  have a width of 6 mils and the narrow line portions  72  and  80  have a length of 1.5 mm. The wide line portions  76  and  84  have a width of 12 mils. 
     The amplified signal from the PA  12  is provided and impedance matched to input port of the main signal line  50  and is output to the antenna  22  at output port. Port  86  of the coupling line  52  is impedance matched to a  500  load  94  formed on the top surface of the top dielectric layer  32  by a suitable through via  96  and port  78  is connected to the DPD  16 , so that a minimal amount of the coupling signal is provided to the port  86  and most of the coupling signal flows from the port  78 . The measure of the proportion of the coupling signal provided to the port  78  defines the directivity of the coupler  28 . The configuration of the blocks  68  and the narrow line portions  70  as described above limits the amount of signal provided at the port  86 , thus improving the directivity of the coupler  28 . The narrow line portions  72  and  80  between the end narrow line portions  74  and  82 , respectively, and the wide line portions  76  and  84  at the ports  78  and  86 , respectively, improve the voltage standing wave ratio (VSWR) of the coupler  28 , which is a measure of its impedance matching. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.