Patent Publication Number: US-9413053-B2

Title: Galvanically isolated, directional coupler

Description:
TECHNICAL FIELD 
     The invention relates to a galvanically isolated, directional coupler, especially for in- and out-coupling of high-frequency measurement signals of a radar fill-level measuring device. Furthermore, the invention relates to a transmitting/receiving separator for a radar fill-level measuring device, in which the directional coupler of the invention is applied. 
     BACKGROUND DISCUSSION 
     Directional couplers are circuits of high frequency technology, which have the property that they divide a signal of predetermined frequency, which is fed into an input port, in a defined manner on two output ports. The dividing of the signal components onto the two output ports must, in such case, not occur equally. In the case of a directional coupler with four ports, one port is “decoupled”, i.e., in the ideal case, no signal components are output on this port. In the case of an individually considered port, the dividing onto the remaining ports depends on the direction of the signal, respectively the waves, through the considered port. One speaks, consequently, of a directional coupler. 
     There are many different forms of directional couplers in diverse technologies. A basic type in microstrip conductor technology is a coupler formed of coupled lines. This is based, among other things, on the physical property that two wave signals with a phase difference of 180° cancel destructively. With reference to high frequency waves, this means a canceling at a phase difference of a half wavelength (λ/2) at the considered frequency. 
     The functional principle of a directional coupler of coupled lines can be described in simplified manner as follows. The coupler of coupled lines is composed of two lines lying next to one another along a distance of a quarter wavelength (λ/4) of the considered frequency. A corresponding directional coupler is shown in  FIG. 1  and is described in greater detail in the description of the figures. 
     Interferences occur in regions, in which the lines lie near enough to one another. In the case of a simple coupling of two lines of a quarter wavelength, a part of the power fed into a port is transmitted from one line to the other. The transmission of the power occurs, for example, in the region of a quarter wavelength. The rest of the power goes to the remaining port. Further explanations of this follow in the description of the figures. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a directional coupler and a transmitting/receiving separator, which are distinguished by increased bandwidth and simple construction. 
     The object is achieved by features including that two mutually engaging, oppositely bent, conductive traces are provided, wherein the two oppositely bent, conductive traces are so arranged that they couple with one another over a region of a quarter wavelength (λ/4) of the wavelength associated with the center frequency of the measuring signals and form two groups of laterally coupled, conductive traces, and that curved conductive trace portions adjoin each of the two groups of laterally coupled, conductive traces, in each case, over a region, which is less than an eighth wavelength (λ/8) of the wavelength associated with the center frequency. 
     In the case of two conductive traces, the essentially “round”, directional coupler has four ports. Preferably, the directional coupler is constructed rotationally symmetrically, whereby none of the ports of the directional coupler is preferred. The meaning of term “port” will be clear from the above description of the state of the art. 
     It has been found that, starting from an eighth wavelength, the properties of the coupler of the invention improve with declining length down to about 1/16 wavelength of the center frequency. A more extensive shortening of the length begins then to offer only slight improvement. 
     An advantageous embodiment of the directional coupler of the invention provides that the directional coupler is constructed of at least one SMD component. The SMD component is either a resistor or a capacitor or two equally constructed resistors or capacitors. 
     The at least one component (capacitor or equally constructed resistors) is arranged in a horizontal plane of a circuit board, or it is provided in at least two parallel planes of a circuit board. 
     An advantageous embodiment of the directional coupler of the invention provides that the two transitions between the laterally coupled, conductive traces and the bent conductive traces are so embodied that the high-frequency measuring signals are transmitted with an as great as possible bandwidth. Preferably, the conductive traces have a toothed structure. 
     Furthermore, it is provided that the directional coupler is so dimensioned that it acts as a 3 dB coupler. The terminology, 3 dB coupler, means that the coupler is so dimensioned that a uniform power division occurs on two ports. 
     The transmitting/receiving separator of the invention for a radar, fill level, measuring device is composed of an above described, directional coupler and a terminating element, respectively a high frequency sink, respectively a matched termination, which is provided on one of the at least four ports of the directional coupler. 
     An embodiment of the transmitting/receiving separator of the invention provides that the terminating element, respectively the high frequency sink, is composed of a resistor, which has double the resistance of the line wave resistance, respectively impedance, and a match structure. 
     Moreover, it is provided that the match structure is composed of three mutually connected, conductive trace portions of defined length and width and two vias to the reference potential ply of the circuit card. 
     Furthermore, it is provided in connection with the transmitting/receiving separator of the invention that the three remaining ports of the directional coupler are connected to an antenna, a transmitting unit and a receiving unit of the radar fill-level measuring device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained in greater detail based on the appended drawing, the figures of which show as follows: 
         FIG. 1  is a plan view onto a known directional coupler with two coupled lines; 
         FIG. 2  is a representation of how a signal in the directional coupler shown in  FIG. 1  is divided into individual signal components; 
         FIG. 3  is a representation of the decoupling of a port in the case of the directional coupler illustrated in  FIG. 1 ; 
         FIG. 4  is a representation for ascertaining bandwidth of a directional coupler; 
         FIG. 5  is a plan view onto a known Lange coupler; 
         FIG. 6  is a plan view onto a known Lange coupler having six fingers; 
         FIG. 7  is a representation of the segment portions of a preferred embodiment of the directional coupler of the invention; 
         FIG. 8  is a clarification of the operation of the directional coupler illustrated in  FIG. 7 ; 
         FIG. 9  is another clarification of the operation of the directional coupler illustrated in  FIG. 7 ; 
         FIG. 10  is a clarification of the operation of the directional coupler illustrated in  FIG. 7  as regards achievable broadbandedness; 
         FIG. 11  is a representation of the transitions of line impedance, 
         FIG. 12  is a representation of a matched terminating element for a directional coupler; 
         FIG. 13  is a representation of a preferred embodiment of the transmitting/receiving separator of the invention; 
         FIG. 14  is a representation of an embodiment of the transmitting/receiving separator of the invention, wherein the matched connection element is provided at a first port; 
         FIG. 15  is a representation of an embodiment of the transmitting/receiving separator of the invention, wherein the matched connection element is provided at a second port; and 
         FIG. 16  is a representation of an embodiment of the transmitting/receiving separator of the invention, wherein the matched connection element is provided at a third port. 
     
    
    
     DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS 
       FIG. 1  shows a plan view onto a known linear directional coupler having two coupled lines  1 ,  2  and four ports  3 ,  4 ,  5 ,  6 . The two coupled lines  1 ,  2  extend parallel to one another along a distance of a quarter wavelength λ/4 of the considered frequency. 
     Interferences occur in the regions, in which the lines  1 ,  2  lie near enough to one another. In the case of a simple coupling of two lines  1 ,  2  of a quarter wavelength λ/4, a part of the power fed into a port  3  is transmitted from a line  1  to the other line  2 . The transfer of the power occurs in the region of a quarter wavelength λ/4. A part of the power arrives at port  4 , while the rest of the power reaches the remaining port  6 . 
       FIG. 2  shows a representation of the dividing of a signal in the case of the directional coupler shown in  FIG. 1 .  FIG. 3  shows a representation of the decoupling of the port  5  in the case of the directional coupler illustrated in  FIG. 1 . Especially,  FIG. 2  shows how the signal of port  3  is divided between the ports  4 ,  6 , wherein the greater signal portion is available at port  4 . 
     In the region of the sharp bends  7 ,  8  on both sides of the coupled lines  1 ,  2 , in each case, a small part of the signal is reflected. Thus, there arises between the ports  3 ,  5  a direct, though weak, coupling in the region of the sharp bends  7 ,  8 . Via the signal path from the sharp bend  8  via the sharp bend  7  back to the sharp bend  8  there results a signal path of a half wavelength λ/2 in the region, in which the two lines  1 ,  2  lie near to one another. As a result of destructive interference, a canceling of the signal occurs, whereby port  5  is decoupled. 
     Due to the symmetric embodiment of the directional coupler, the earlier described behavior shows up likewise when the signal is introduced not into port  3 , but, instead, into one of the other ports  4 ,  5 ,  6 . Then, the decoupled port and the port with the largest of the other two power portions change correspondingly. 
     Of course, for optimized dimensioning, also multiple reflections and different propagation velocities of the signals can, respectively must, be taken into consideration. 
     In the following, the properties of a coupler with coupled lines  1 ,  2  will now be described. The conditions for coupling and for destructive interference hold, in each case, for the considered frequency. For frequencies outside of the center frequency, these conditions are only uncleanly fulfilled, so that the coupler characteristics worsen strongly with increasingly deviating frequency. Thus, this is a narrow band coupler. 
     For direct voltage and signal components of very low frequency, there is no coupling in the region where the lines  1 ,  2  lie near to one another. The ports  3 ,  4  are galvanically isolated from the ports  5 ,  6 . No galvanic isolation is present between the ports  3 ,  4  and  5 ,  6 , respectively. 
     A disadvantage of the linear coupler is that the decoupling of the respectively decoupled port (port  5  in  FIG. 3 ) is in the case of this coupler type relatively poor. 
       FIG. 4  shows a schematic representation of ascertaining the bandwidth of a directional coupler. These considerations hold both for the known directional coupler as well as also for the directional coupler of the invention. 
     In the case of a coupler, certain criteria, such as the measure for the decoupling and or the ratio of the power distributions to the individual ports can be specified. As already mentioned above, the properties of a directional coupler worsen, the more strongly the frequency deviates from the center frequency. Theoretically, a frequency range can be determined, in which acceptable sizes for the individual criteria are still just fulfilled. This frequency range is referred to as the bandwidth of a directional coupler and designates, thus, a certain frequency range. The cutoffs of the bandwidth are the upper limit frequency and lower limit frequency. 
     The broadbandedness of a directional coupler is defined as the ratio the above defined bandwidth to the center frequency and is usually given in percent. The center frequency of a component or of a frequency-statically behaving assembly corresponds to the linear mean (frequencies can also be presented logarithmically) between the upper and the lower limit frequencies. The broadbandedness can, thus, lie in a range between &gt;0% and &lt;200%. 
       FIG. 5  is a plan view onto a simple embodiment of a Lange coupler  9 . The Lange coupler  9  represents an improvement compared with the simple, linear, directional coupler illustrated in  FIGS. 1-3 . The Lange coupler  9  is, moreover, also referred to as an interdigital coupler. For improving the coupling characteristics, a number of coupling structures  12 ,  13  of a quarter wavelength are connected in parallel. Furthermore, the coupling of most of the line elements—with the exception of the outer line elements—occurs on both sides, i.e. a line element is located, in each case, near to two additional line elements. In this way, a desired power distribution can be achieved. Furthermore, the regions  10 ,  11  are further developed in comparison to the regions  7 ,  8  in  FIGS. 1-3 . 
     With the Lange coupler  9 , good coupling is still possible in the range of wavelengths, which deviate slightly from the center frequency. Additionally, in the case of the Lange coupler  9 , the multiple reflections can be best made use of. 
     For additionally improving the bandwidth, the decoupling and/or the power distribution (dimensionally dependently and partially in opposition to one another), the number of coupling structures  12 ,  13 , which, in each case, have a length of, for instance, a quarter wavelength of the center frequency, can be multiply expanded. This embodiment is shown in  FIG. 6 , by way of example. 
     A four fingered Lange coupler  9  as in  FIG. 5  permits achieving a broadbandedness of, for example, 80%. Increasing the number of coupling structures  12 ,  13  does, indeed, improve the broadbandedness, but leads, however, also to increasingly narrower line portions, narrower line separations and—as regards manufacture—to an increase in the number of connecting, or bond, wires. Moreover, more finely structured circuit board structures are more complex and more expensive to manufacture, provided that such is technically feasible at all. For processing bond wires, additionally, particularly expensive machines are necessary. The bond wires needed for high frequency structures are very fine and very sensitive as regards handling and transport and can, moreover, be repaired manually only by the expenditure of much time. 
       FIG. 7  shows a representation of a preferred embodiment of the directional coupler  14  of the invention. Especially to be seen here are the individual segment portions  15 ,  16 ,  34 ,  35  of the directional coupler of the invention. The directional coupler of the invention  14  has a “round” shape, is galvanically isolated and serves preferably for in- and out-coupling of high-frequency measurement signals of a radar fill-level measuring device. Also a mixer can be implemented in similar manner. 
     According to the invention, two mutually engaging, oppositely bent, conductive traces are provided, wherein the two oppositely bent, conductive traces are so arranged that
         they couple with one another over a region ( 23 ,  24  to  25 ,  26 , respectively  27 ,  28  to  29 ,  30 ) of a quarter wavelength λ/4 of the wavelength associated with the center frequency of the measuring signals and form two groups of laterally coupled, conductive traces  15 ,  16 , and   curved conductive trace portions  34 ,  35  adjoin each of the two groups of laterally coupled, conductive traces  15 ,  16 , in each case, over a region, which is less than an eighth wavelength λ/8 of the wavelength associated with the center frequency.       

     The round directional coupler  14  of the invention virtually combines the interference characteristics of adjoining and mutually following line portions of the length of a quarter wavelength with the interference characteristics of a wave traveling around a circle, such as is used, for example, already in hybrid couplers or branch line couplers. However, the known hybrid couplers have no laterally coupled structures. 
     The invention, thus, combines laterally coupled structures and the interferences occurring as a result of a “closed” ring. The terminology, closed ring, refers, in such case, to the high frequency signal path. According to the invention, two groups of laterally coupled line pairs  15 ,  16  of the length of a quarter wavelength of the center frequency are coupled with one another by two other bent line portions  34 ,  35  of length clearly smaller than an eighth wavelength of the center frequency. Instead of the sharp bends in the separation of the lines occurring in the case of the state of the art, the solution of the invention has soft, flowing transitions (see transition  36  in  FIG. 11 ). In this way, point  23  shifts as boundary between the coupling structure  32  and the connecting line  33  (see  FIG. 10 ). At the center frequency, the corresponding length lies, for example, at about 1/30 wavelength and changes within the bandwidth in the range of about 1/64 (lower frequencies) to about 1/16 (higher frequencies) non-linearly with the frequency. A dimensioning in the range, for example, 1/10 to 1/40 is, however, likewise possible. 
     Operation of the directional coupler  14  of  FIG. 7  will now be explained in greater detail based on  FIGS. 8 and 9 . It is assumed here that the signal is fed into port  20 . There is between the region  24 ,  23  and  25 ,  26  an effect similar to that produced in the coupler of coupled lines shown in  FIG. 1 . The wave incoming at point  24  (incoming signal) is divided at the points  25 ,  26 , while no power fractions reach point  23 . First, a part of the output power arrives at port  22 , and another part of the wave travels on at point  25 . 
     This wave outgoing via point  25  in a very short travel time reaches point  27  (see  FIG. 9 ). At point  27  there begins a structure, which, again, is similar to the structure of the known coupler of coupled lines shown in  FIG. 1 . This structure extends in the region from the points  28 ,  27  to the points  29 ,  30 . 
     As in the case of a known coupler of two coupled lines, first of all, no power fractions reach the point  28  adjoining port  21 . The power of the wave is divided between the points  29  and  30 . The power fractions at point  29  are partially reflected and partially led via the SMD component  31  to the port  19 . 
     The reflected power fractions are, in turn, according to the principle of coupled lines, divided between the points  27  and  28 . The power fractions at point  28  go to the port  21 . The power fractions at point  27  travel to point  25 . The signal path behaves in the same way. The power fractions at point  30 , however, travel to the point  23 . In comparison to the next period of the wave reaching point  24  via port  20 , there is a phase difference of 180°. Since the waves are, however, conveyed on different lines, the region  24 ,  23  produces a destructive interference. Since, such as above described, a part of the wave of point  24  reaches point  26 , there occurs in the total region from  23  to  26  a destructive interference. Port  22  is accordingly very well decoupled. Through a suitable dimensioning, moreover, a uniform power distribution to the ports  19  and  21  can be achieved for a large broadbandedness. 
     Frequencies deviating from the center frequency lead to differing wavelengths. Drawn in  FIG. 10 , by way of example, are a longer wavelength (longer line in  32 ; lower frequency) and a shorter wavelength (shorter line in  32 ; higher frequency). Due to the “soft” transition in the region around the points  23 ,  24  relative to the separation of the coupled lines, the operation of this coupler is possible both for the somewhat lower as well as also for the somewhat higher frequency; the two wavelengths are still “suitable” in this structure of marked broadbandedness. 
     In the case of the coupler of coupled lines shown in  FIG. 1 , there is a “sharp bend” at both ends. There is thus a sharp bounding of the laterally coupled structures (see also  FIG. 11 ). As a consequence of this abrupt transition, the coupler is narrow banded. 
     Due to the frequency dependence of the length of the laterally coupled structures  32 , different lengths of the bent connecting lines  33  result. The lengths are, however, clearly shorter than an eighth of the associated wavelengths, so that this influence is small. 
     More exactly considered, the two line lengths  34  and  35  lead, moreover, to a compensation of the continually arising travel time difference of the in, and out, of phase modes arising in a laterally coupled structure. This is mentioned here for reasons of completeness. It contributes nothing, however, to a basic understanding of the operation of the coupler of the invention. Due to the different field spreading of the different wave modes, a different, mode dependent, effective coupling length of the bent line portions results, which differs from the different, mode dependent, effective coupling length of straight, coupled line portions. 
     Likewise, the different modes must be taken into consideration in the case of the destructive interference in the region from point  23  to point  26 , and, indeed, especially in the case of frequencies, which deviate from the center frequency. Various modes occur, moreover, also in the case of the coupler of coupled lines and other types of couplers. 
     In connection with  FIG. 11 , an abrupt transition of the line impedance will now be described. A medium, in which a physical wave propagates, has a wave impedance. This is also called wave resistance or—with reference to connecting lines for signals of high frequency fractions such as in the case of the here applied, microstrip lines—line wave resistance, respectively impedance. Basically, this describes the stiffness, which the medium presents to the wave (compare physical flow resistance). A microstrip line is composed of a connecting line on board material with traversing copper backing layer without interruptions in the surrounding region. The line impedance of a microstrip line depends on the width of the line on the upper side of the board. 
     There are a number of options for connecting two lines of different line impedances, for example, a hardened transition, a conically extending line segment or other partially complex structures. 
     The known coupler of coupled lines as well as the Lange coupler use a flowing transition  17 ,  18 , while, in the case of the new, round coupler  36 , an abrupt, so-called “impedance jump” is necessary. More exactly considered, a certain field distribution results at this impedance jump. The influence of impedance jumps in general is state of the art. Especially, the different modes of laterally coupled, conductive traces have an influence, so that here such impedance jumps are necessary. 
     In the case of the coupler types based on circulating waves, such as hybrid ring couplers and branch line couplers, likewise a hard impedance jump is necessary. A coupler already exists, based only on a laterally coupled structure, in the case of which a soft transition is necessary. This is called the “Tapered Coupled Line Hybrid Coupler in the 180° Embodiment”. 
     The operation of the coupler  14  of the invention with coupling elements and circulating wave is based on effects of coupling via a metallically non-connected separation (the “lateral coupling” such as in the case of the coupler of coupled lines or the Lange coupler) and a wave traveling a closed loop, such as, for example, in the case of the coupler types: Hybrid ring coupler, branch line coupler and rat race coupler. 
     Alternatively to lateral coupling, also other distributed coupling structures are possible for the coupler  14  of the invention, for example, lines superimposed as different conductive traces, provision of lines with teeth, application of multiple, fine, connecting lines and material of defined resistivity in the region between the lines. 
       FIG. 12  shows a representation of a matched termination  37  for a directional coupler  14 . If the coupler  14  of the invention is dimensioned in such a manner that a uniform power distribution arises on two ports—one speaks then of a “3 dB coupler”—there results a power distribution within the bandwidth according to Table 1 and a decoupling according to Table 2. As already stated, decoupling is present when as little as possible power transmission occurs between the ports. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Power distribution in the case of symmetric dimensioning 
               
            
           
           
               
               
               
            
               
                   
                 signal output 
                 signal output 
               
               
                 signal input 
                 ½ output power 
                 ½ output power 
               
               
                   
               
               
                 port (22) 
                 port (21) 
                 port (19) 
               
               
                 port (19) 
                 port (20) 
                 port (22) 
               
               
                 port (20) 
                 port (19) 
                 port (21) 
               
               
                 port (21) 
                 port (20) 
                 port (22) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Decoupling 
               
            
           
           
               
               
               
            
               
                   
                 signal input 
                 no signal output 
               
               
                   
                   
               
               
                   
                 port (21) 
                 port (19) 
               
               
                   
                 port (19) 
                 port (21) 
               
               
                   
                 port (20) 
                 port (22) 
               
               
                   
                 port (22) 
                 port (20) 
               
               
                   
                   
               
            
           
         
       
     
     Those ports, at which power division occurs in the case of in-coupling through a third port, are, in the case of respective in-feeding, in each case, decoupled from one another. The remaining fourth port is, in turn, decoupled from the port used for in-feeding the power to be divided. 
       FIG. 13  shows a preferred embodiment of the transmitting/receiving separator of the invention  40  for e.g. a radar fill-level measuring device. With a matched termination  37 , respectively terminating element, the coupler  14  of the invention becomes a transmitting/receiving separator  40 . The matched termination  37  includes a connecting piece  39  between the matched termination  37  and the coupler  14 . This connecting line portion  39  is a line portion of defined length, on whose end  38  the coupler  14  can be joined. Of course, also other forms of a matched termination  37  than the one shown here can be used. The connections remain the same. Other connections are evident from the characteristics of the coupler  14  according to the Tables 1 and 2. They are listed completely in Table 3. The different connections in the case of a radar fill-level measuring device are shown in  FIGS. 13 to 16 . 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 connection per 
                 port (20) 
                 port (21) 
                 port (22) 
                 port (19) 
               
               
                   
               
             
            
               
                 FIG. 13 
                 transmitter 
                 antenna 
                 receiver 
                 matched 
               
               
                   
                   
                   
                   
                 termination 
               
               
                 FIG. 14 
                 antenna 
                 transmitter 
                 matched 
                 receiver 
               
               
                   
                   
                   
                 termination 
                   
               
               
                 FIG. 15 
                 transmitter 
                 matched 
                 receiver 
                 antenna 
               
               
                   
                   
                 termination 
                   
                   
               
               
                 FIG. 16 
                 matched 
                 transmitter 
                 antenna 
                 receiver 
               
               
                   
                 termination 
                   
                   
                   
               
               
                 FIG. 13 
                 receiver 
                 antenna 
                 transmitter 
                 matched 
               
               
                   
                   
                   
                   
                 termination 
               
               
                 FIG. 15 
                 receiver 
                 matched 
                 transmitter 
                 antenna 
               
               
                   
                   
                 termination  
                   
                   
               
               
                 FIG. 14 
                 antenna 
                 receiver 
                 matched 
                 transmitter 
               
               
                   
                   
                   
                 termination 
                   
               
               
                 FIG. 16 
                 termination 
                 receiver 
                 antenna 
                 transmitter 
               
               
                   
                 matched