Patent Publication Number: US-8981868-B2

Title: Balun printed on substrate

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to baluns, and more particularly to a balun printed on a substrate. 
     2. Description of Related Art 
     A balun is a device operable to convert between balanced and unbalanced lines for input and output in an electrical system and may be used in a communication apparatus, such as a set top box, to transform unbalanced broadcasting signals into balanced broadcasting signals. A typical prior art balun, such as low-temperature co-fired ceramic balun, is discretely mounted on a substrate. This balun is both bulky and expensive. Therefore, designing a balun, which has a small size and low cost, is a question for discussion. 
     Therefore, a need exists in the industry to overcome the described limitations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The details of the disclosure, both as to its structure and operation, can best be understood by referring to the accompanying drawing, in which like reference numbers and designations refer to like elements. 
         FIGS. 1-2  are schematic plan views of one embodiment of a balun formed on a first surface of a substrate in accordance with the present disclosure; 
         FIG. 3  is a schematic plan view of the balun of  FIG. 1  formed on a second surface of a substrate; 
         FIGS. 4-14  are schematic plan views illustrating dimensions of the balun of  FIG. 1 ; 
         FIG. 15  is a graph of test result showing an input return loss of the balun of  FIG. 1 ; 
         FIG. 16  is a graph of test results showing insertion losses of a first output port and a second output port of the balun of  FIG. 1 ; and 
         FIG. 17  is a graph of test results showing phase balance of a first output port and a second output port of the balun of  FIG. 1 . 
         FIG. 18  is a schematic plan view of a hardware design of a communication apparatus including the balun in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , one embodiment of a balun  100  in accordance with the present disclosure is printed on a substrate  10  in a communication device and operable to transform an unbalanced signal into a first balanced signal and a second balanced signal. The balun  100  includes an input port  20  operable to input the unbalanced signal, a coupling microstrip group  30 , a first output port  40  operable to output the first balanced signal and a second output port  50  operable to output the second balanced signal. The substrate  10  includes a first surface  101  and a second surface  102  (shown in  FIG. 3 ). The substrate  10  may be formed as a single layer PCB or a multi-layer PCB. In this embodiment, the input port  20 , the coupling microstrip group  30 , the first output port  40  and the second output port  50  are coplanar with each other and printed on the first surface  101  of the substrate  10 . A ground layer  70  and a clearance area  80  are formed on the first surface  101  of the substrate  10 . The clearance area  80  is an insulative area surrounded by the ground layer  70 . The input port  20 , the coupling microstrip group  30 , the first output port  40  and the second output port  50  are arranged in the clearance area  80 . 
     The coupling microstrip group  30  is connected between the input port  20  and the first and second output ports  40  and  50  to transform the unbalanced signal into the first and second balanced signals. The input port  20  is located at one side of the coupling microstrip group  30 , and the first and second output ports  40 ,  50  are located at the other side of the coupling microstrip group  30  and opposite the input port  20 . The coupling microstrip group  30  includes an input line  31 , a first output line  32  connected to the first output port  40 , a first coupling line  33 , a second output line  35  connected to the second output port  50  and a second coupling line  36 . 
     In other embodiments, the coupling microstrip group  30  may be distributed on different layers of the substrate  10  formed as the multi-layer PCB. For example, the substrate  10  includes a first layer, on which the input line  31  is arranged, a second layer, on which the first and second output lines  32 ,  35  are arranged, and a third layer, on which the first and second coupling lines  33 ,  36  are arranged. 
     In  FIG. 2 , the input line  31  includes a first coupling section  318 , a second coupling section  319  opposite to the first coupling section  318  and a connecting section  314  connected between the first coupling section  318  and the second coupling section  319 . The first coupling section  318  is connected between the input port  20  and the connecting section  314 . The first output line  32  and the first coupling line  33  are respectively located at two sides of the first coupling section  318 . One end of the first coupling line  33  is electrically connected to the first output line  32 . The second output line  35  and the second coupling line  36  are respectively located at two sides of the second coupling section  319 . One end of the second coupling line  36  is electrically connected to the second output line  35 . 
     The first output line  32 , the second output line  35 , the first coupling line  33  and the second coupling line  36  are grounded by extending from the ground layer  70  into the clearance area  80 . The coupling microstrip group  30  includes four grounding junctions G 1 , G 2 , G 3  and G 4  distributed close to the input port  20 . The grounding junction G 1  is formed between the first output line  32  and the ground layer  70 . The grounding junction G 2  is formed between the first coupling line  33  and the ground layer  70 . The grounding junction G 3  is formed between the second coupling line  36  and the ground layer  70 . The grounding junction G 4  is formed between the second output line  35  and the ground layer  70 . The connecting section  314  is arranged close to the first and second output ports  40 ,  50 . That is, the four grounding junctions G 1 , G 2 , G 3  and G 4  are opposite to the connecting section  314 . 
     The first output line  32  and the second output line  35  are respectively located at two opposite sides of the input line  31 . The first output line  32  extends beside and along the first coupling section  318 . The second output line  35  extends beside and along the second coupling section  319 . The first coupling line  33  and the second coupling line  36  are located between the first coupling section  318  and the second coupling section  319 . The first coupling line  33  extends beside and along the first coupling section  318 . The second coupling line  36  extends beside and along the second coupling section  319 . In this embodiment, the first coupling line  33 , the first output line  32  and the first coupling section  318  are parallel with each other. The second coupling line  36 , the second output line  35  and the second coupling section  319  are parallel with each other. 
     In  FIG. 3 , in this embodiment, the balun  100  further includes a first connecting part  34  and a second connecting part  37  formed on the second surface  102  of the substrate  10  and close to the connecting section  314  of the input line  31 . The first connecting part  34  is electrically connected to the first coupling line  33  and the first output line  32  by two conducting holes extending from the first surface  101  to the second surface  102 . The second connecting part  37  is operable to be electrically connected between the second coupling line  36  and the second output line  35  by two conducting holes extending between the first surface  101  and the second surface  102 . 
     The first output line  32  is electrically connected to the first output port  40 . 
     The second output line  35  is electrically connected to the second output port  50 . The input line  31  is operable to receive and transmit the unbalanced signal. The unbalanced signal is transformed into the first balanced signal via coupling among the first coupling section  318 , the first output line  32  and the first coupling line  33 . The first balanced signal is transmitted to the first output port  40  from the first output line  32 . The unbalanced signal is transformed into the second balanced signal via coupling among the second coupling section  319 , the second output line  35  and the second coupling line  36 . The second balanced signal is transmitted to the second output port  50  from the second output line  35 . 
     The first coupling section  318  includes a first straight portion  311 , a first wandering portion  312  and a second straight portion  313  connecting with each other in turn. The first straight portion  311  is adjacent and connected to the input port  20 . The second straight portion  313  is adjacent and connected to the connecting section  314 . The second coupling section  319  includes a third straight portion  315 , a second wandering portion  316  and a fourth straight portion  317  connecting with each other in turn. The third straight portion  315  is adjacent and connected to the connecting section  314 . The fourth straight portion  317  includes a free end  3172  formed away from the connecting section  314 . The first wandering portion  312  is opposite to the fourth straight portion  317 . The second wandering portion  316  is opposite to the second straight portion  313 . The first straight portion  311  and the second straight portion  313  are collinear and lie on a same straight line A 1 . The third straight portion  315  and the fourth straight portion  317  are collinear and lie on a same straight line A 2  parallel with the straight line A 1  and perpendicular to the connecting section  314 . 
     The balun  100  further includes an input matching circuit  90  formed between the input port  20  and the input line  31  and operable to improve an input return loss for obtaining a better stability for the balun  100 . The input matching circuit  90  includes a first capacitor  92 , a second capacitor  94  and an inductor  96 . The first capacitor  92  and the second capacitor  94  are connected with each other in series and located between the input port  20  and the input line  31 . The input end of the first capacitor  92  is connected to the input port  20 , and the output end of the first capacitor  92  is connected to the second capacitor  94 . The inductor  96  has two ports, one of which is connected to the output end of the first capacitor  92 , and the other one is connected to the ground layer  70 . 
     The first output port  40  includes a first connecting end  41 , a first output capacitor  42 , a first middle section  44  and a first output section  46  in turn. The first connecting end  41  is connected to the first output line  32 . The first output capacitor  42  is connected between the first connecting end  41  and the first middle section  44 . The first connecting end  41  and the first middle section  44  are collinear, and the first output section  46  is perpendicular to the first middle section  44 . The second output port  50  includes a second connecting end  51 , a second output capacitor  52 , a second middle section  54  and a second output section  56  in turn. The second connecting end  51  is connected to the second output line  35 . The second output capacitor  52  is connected between the second connecting end  51  and the second middle section  54 . The second connecting end  51  and the second middle section  54  are collinear, and the second output section  56  is perpendicular to the second middle section  54 . The first output capacitor  42  and the second output capacitor  52  are operable to improve insertion losses of the first output port  40  and the second output port  50 . The first output port  40  is formed as L-shaped and mirror-symmetrical to the second output port  50 . The first connecting end  41 , the first middle section  44 , the second connecting end  51  and the second middle section  54  are collinear and lie on a same straight line A 3 , shown in  FIG. 7 . 
       FIGS. 4-14  are schematic plan views illustrating dimensions of the balun  100 . In  FIG. 4 , distance between the input line  31  and the first output line  32  is 5 mil. Distance between the input line  31  and the first coupling line  33  is 5 mil. Distance between the input line  31  and the second output line  35  is 5 mil. Distance between the input line  31  and the second coupling line  36  is 5 mil. A smallest distance between the free end  3172  of the input line  31  and the ground layer  70  is 5 mil. In  FIG. 5 , widths of the input line  31 , the first output line  32  and the first coupling line  33  are all 10 mil. A smallest distance between the first coupling line  33  and the connecting section  314  is 10 mil. Widths of the first connecting end  41 , the first middle section  44  and the first output section  46  are all 20 mil. 
     In  FIG. 6 , distance between the first middle section  44  of the first output port  40  and the connecting section  314  and distance between the second middle section  54  of the second output port  40  and the connecting section  314  are all 10 mil. Distance between the first output section  46  and the second output section  56  is 10 mil. Distance between the first middle section  44  and the ground layer  70  and distance between the second middle section  54  and the ground layer  70  are all 20 mil. Distance between the first output section  46  and the ground layer  70  and distance between the second output section  56  and the ground layer  70  are all 10 mil. 
     In  FIG. 7 , length of the first connecting end  41  along the straight line A 3  is 20 mil and equal to that of the second connecting end  51 . The first connecting end  41  and the second connecting end  51  are formed in square shapes. Distance between the first connecting end  41  and the first middle section  44  and distance between the second connecting end  51  and the second middle section  54  are all 200 mil. Length of the first middle section  44  along the straight line A 3  is 60 mil and equal to that of the second middle section  54 . 
     In  FIG. 8 , length of the first straight portion  311  is 100 mil. Length of the second straight portion  313  is 220 mil. Length of the connecting section  314  is 163 mil. The first wandering portion  312  includes a first section  3122 , a second section  3124  and a third section  3126  in turn. The second section  3124  is parallel with the first and second straight portions  311 ,  313  and perpendicularly connected between the first section  3122  and the second section  3124  parallel with each other. Length of the first section  3122  is 80 mil and equal to that of the third section  3126 . Length of the second section  3124  is 120 mil. 
     In  FIG. 9 , the first output line  32  includes a first straight portion  321 , a first wandering portion  322  and a second straight portion  323  connecting with each other in turn. The first output line  32  has a similar shape with the input line  31 . The first wandering portion  322  includes a first section  3222 , a second section  3224  and a third section  3226  in turn. The second section  3224  is parallel with the first and second straight portions  321 ,  323  and perpendicularly connected between the first section  3222  and the second section  3224  parallel with each other. Length of the first straight portion  321  is 75 mil. Length of the second straight portion  323  is 255 mil. Length of the first section  3222  is 80 mil and equal to that of the third section  3226 . Length of the second section  3224  is 90 mil. 
     In  FIG. 10 , the first coupling line  33  includes a first straight portion  331 , a first wandering portion  332  and a second straight portion  333  connecting with each other in turn. The first coupling line  33  has a similar shape with the first coupling section  318  of the input line  31 . The first wandering portion  332  includes a first section  3322 , a second section  3324  and a third section  3326  in turn. The second section  3324  is parallel with the first and second straight portions  331 ,  333  and perpendicularly connected between the first section  3322  and the second section  3324 . Length of the first straight portion  331  is 50 mil. Length of the second straight portion  333  is 195 mil. Length of the first section  3322  is 80 mil and equal to that of the third section  3326 . Length of the second section  3324  is 150 mil. 
     In  FIG. 11 , length of the third straight portion  315  of the second coupling section  319  is 70 mil. Length of the fourth straight portion  317  is 245 mil. The second wandering portion  316  includes a first section  3162 , a second section  3164  and a third section  3166  in turn. The second section  3164  is parallel with the third and fourth straight portions  315 ,  317  and perpendicularly connected between the first section  3162  and the second section  3164  parallel with each other. Length of the first section  3162  is 65 mil and equal to that of the third section  3166 . Length of the second section  3164  is 85 mil. 
     In  FIG. 12 , the second coupling line  36  includes a third straight portion  365 , a second wandering portion  366  and a fourth straight portion  367  connecting with each other in turn. The second coupling line  36  has a similar shape with the second coupling section  319  of the input line  31 . The second wandering portion  366  includes a first section  3662 , a second section  3664  and a third section  3666  in turn. The second section  3664  is parallel with the third and fourth straight portions  365 ,  367  and perpendicularly connected between the first section  3662  and the second section  3664  parallel with each other. Length of the third straight portion  365  is 45 mil. Length of the fourth straight portion  367  is 235 mil. Length of the first section  3662  is 65 mil and equal to that of the third section  3666 . Length of the second section  3664  is 115 mil. 
     In  FIG. 13 , the second output line  35  includes a third straight portion  355 , a second wandering portion  356  and a fourth straight portion  357  connecting with each other in turn. The second output line  35  has a similar shape with the second coupling section  319  of the input line  31 . The second wandering portion  356  includes a first section  3562 , a second section  3564  and a third section  3566  in turn. The second section  3564  is parallel with the third and fourth straight portions  355 ,  357  and perpendicularly connected between the first section  3562  and the second section  3564  parallel with each other. Length of the third straight portion  355  is 105 mil. Length of the fourth straight portion  357  is 265 mil. Length of the first section  3562  is 65 mil and equal to that of the third section  3566 . Length of the second section  3564  is 55 mil. 
     In  FIG. 14 , the clearance area  80  is surrounded by the ground layer  70 . The ground layer  70  includes a first ground area  72  opposite to the first output line  32 , a second ground area  74  opposite to the second output line  35  and a third ground area  76  close to the input port  20 . A smallest distance between the first ground area  72  and the first output line  32  is 40 mil. A smallest distance between the second ground area  74  and the second output line  35  is 40 mil. A smallest distance between the first section  3322  of the first wandering portion  332  of the first coupling line  33  and the third ground area  76  is 40 mil. A smallest distance between the first coupling line  33  and the fourth straight portion  367  of the second coupling line  36  is 40 mil. A smallest distance between the third section  3326  of the first wandering portion  332  of the first coupling line  33  and the second wandering portion  366  of the second coupling line  36  is 35 mil. A smallest distance between the second straight portion  333  of the first coupling line  33  and the second wandering portion  366  is 55 mil. A smallest distance between the second wandering portion  366  and the connecting section  314  is 45 mil. 
     The present disclosure enables the balun  100  to cover radio frequency bands between 0.95 GHz-2.15 GHz. Lengths of the first coupling section  318 , the second coupling section  319 , the first output line  32 , the first coupling line  33 , the second output line  35  and the second coupling line  36  are substantially one-fourth (¼) of a working wavelength of the balun  100 . 
     In  FIG. 15 , an input return loss of the balun  100  is below −20 dB, when the balun  100  covers radio frequency bands between 0.95 GHz-2.15 GHz. 
     In  FIG. 16 , insertion losses of a first output port  40  and a second output port  50  of the balun  100  are shown. The insertion losses of a first output port  40  and a second output port  50  are greater than −5 dB, when the balun  100  covers radio frequency bands between 0.95 GHz-2.15 GHz. 
     In  FIG. 17 , a phase difference at the first output port  40  and the second output port  50 . The phase difference between the first output port  40  and the second output port  50  are all close to 180 degrees, when the balun  100  covers radio frequency bands between 0.95 GHz-2.15 GHz. Therefore, the balun  100  has a good balanced input and output signal. 
       FIG. 18  is a schematic plan view of a hardware design of a communication apparatus including the balun  100  in accordance with the present disclosure. The communication apparatus includes a high pass filter, a low noise amplifier, a DVB-S tuner, a demodulator and a CPU. A Radio Frequency (RF) signal is filtered by passing through the high pass filter, and enlarged by passing through the low noise amplifier. The RF signal after being enlarged is an unbalanced signal. Then, the unbalanced signal is transformed to two balanced signals. The DVB-S tuner receives and processes the two balanced signals and output a signal to the demodulator. Finally, the demodulator transmits a TV signal to the CPU. 
     The balun  100  is printed on the substrate  10  and not protruding from the substrate  10 . The balun  100  has a small size and low cost. 
     While various embodiments and methods of the present disclosure have been described above, it should be understood that they have been presented by way of example only and not by way of limitation. Thus the breadth and scope of the present disclosure should not be limited by the above-described embodiments, but should be defined only in accordance with the following claims and their equivalents.