Abstract:
A directional coupler having a broadband frequency response comprises a pair of parallel TEM transmission lines having an input port, a thru port, a coupled port, and an isolation port. A quarter-wave, short circuited transmission line coupled to the thru port, and a half-wave, open circuited transmission line coupled to the isolation port functions to flatten the frequency response between the input port and the thru port and between the input port and the coupled port and to increase the operating bandwidth of the directional coupler.

Description:
This is a continuation of application Ser. No. 08/235,922, filed May 2, 1994, abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to directional couplers, and more particularly to a directional coupler including compensating networks for increasing operational bandwidth. 
     BACKGROUND OF THE INVENTION 
     The basic directional coupler is a linear, passive, four port network, consisting of a pair of coupled transmission lines. A first transmission line defines an input port and a thru port, and a second transmission line defines a coupled port and an isolation port. Propagation of a signal applied to the input port along the first transmission line induces the propagation of a coupled signal along the second transmission line. Maximum signal coupling between the pair of coupled transmission lines is achieved when the length of the coupling region is an odd multiple of a quarter wavelength. Because signal coupling is dependent on the signal wavelength, existing directional couplers are narrowly limited to a specific bandwidth. The ability to increase the operational bandwidth of a directional coupler would greatly increase the benefits of presently existing couplers and broaden their applications into other areas. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the foregoing and other problems with directional couplers by connecting a pair of compensation networks to the coupler. The first compensation network comprises a closed circuited, quarter-wave transmission line coupled to the thru port of the coupler. The second compensation network comprises an open circuited, half-wave transmission line coupled to the isolation port of the coupler. The included compensation networks function to flatten the frequency response of the coupler between the input port and the coupled port, and between the input port and the thru port. This allows the directional coupler to have a broader operational bandwidth than was previously available with prior art directional couplers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following Detailed Description taken in conjunction with the accompanying Drawings in which: 
     FIG. 1 is a schematic diagram of a prior art uncompensated directional coupler; 
     FIG. 2 is a diagram of the frequency response of the prior art uncompensated, equal power split directional coupler; 
     FIG. 3 is a schematic diagram of a compensated directional coupler of the present invention; 
     FIG. 4 is a diagram of the frequency response of the compensated directional coupler of FIG. 3; 
     FIG. 5 is an alternative embodiment of a compensated directional coupler; and 
     FIGS. 6A and 6B are illustrations of a T-shaped and a π-shaped lumped constant network, respectively, used in the alternative embodiment of FIG. 5. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the Drawings, and more particularly to FIG. 1, there is shown a schematic representation of a prior art uncompensated directional coupler. The uncompensated directional coupler comprises two parallel, adjacent transverse-electromagnetic mode (TEM) transmission lines (8 and 10) defining four ports. The input port 12 receives an input signal from an external source (not shown) for propagation along transmission line 8 to the thru port 18. The coupled port 14 emits a coupled signal induced along the transmission line 10. The coupled signal is induced within the coupling region 16 of the directional coupler. The signal emitted from the thru port 18 has a power value equal to the power value of the signal received at the input port 12, minus the power value of the coupled signal emitted from the coupled port 14. This power value relationship at thru port 18 signal assumes an ideal, lossless structure for the coupler. In reality, the power value at the thru port 18 would also be reduced by line losses within the transmission lines (8 and 10). The isolation port 20 at the opposite end of the transmission line 10 from the coupled port 14 emits no signal. Reflected energy, due to impedance mismatches at either output port, appears at the isolation port 20. This isolation port 20 is normally terminated by the characteristic coupler impedance of 50 ohms. 
     Referring now to FIG. 2, there is illustrated the frequency response for the uncompensated directional coupler of FIG. 1 designed to have a midband coupling of 3.0 dB at 1 GHz. Assuming the coupler allowed a coupling deviation between the two output ports of only ±0.2 dB (0.4 dB), the relative frequency response would only extend from approximately 0.83 GHz to approximately 1.18 GHz. 
     Referring now to FIG. 3, there is shown a schematic drawing of a compensated directional coupler of the present invention. Two parallel TEM transmission lines, 30 and 32, are coupled together over a coupling region 34. The input port 36, coupled port 38, thru port 40 and isolation port 42 are the same as those described with respect to FIG. 1. The compensated directional coupler further includes two compensating networks. The first compensating network 43 comprises a quarter-wave, short circuited transmission line 44 coupled to the thru port 40. This first compensation network principally affects the input port 36 to thru port 40 coupling. The second compensating network 45 comprises a half-wave, open circuited transmission line 46 connected to the isolation port 42. The termination resistor 48 is normally attached to the directional coupler isolation port 42 to absorb mismatch energy. This second compensation network serves to flatten the coupling response between the input port 36 and the coupled port 38. The net result of the two compensation networks is illustrated in FIG. 4, wherein the relative frequency response demonstrates equal coupling over a greater frequency range from the compensated coupler as compared to the uncompensated coupler. 
     Referring now to TABLE 1, there is illustrated a comparison of the relative frequency response of a compensated directional coupler of the present invention (FIG. 3) versus an uncompensated directional coupler (FIG. 1) as a function of allowable output port amplitude imbalance. The response at the UHF band (225 to 400 MHz, F c  =312.5 MHz) is also shown. As can be seen from TABLE 1, at a typical design imbalance of 0.25 dB, the compensated coupler frequency response is flat from 233 to 392 MHz, whereas the conventional coupler only performs between 266 and 359 MHz. 
     
                                           TABLE 1__________________________________________________________________________Port-to-PortAmplitude  Conventional Directional                 Compensated DirectionalImbalance  Coupler (FIG. 1)                 Coupler (FIG. 3)(Coupled-to-  Relative         UHF     Relative                        UHFThru Port  Frequency         Frequency                 Frequency                        FrequencyDifference  Range  Range   Range  Range__________________________________________________________________________0.05 dB  0.932 to 1.068         291 to 334 MHz                 0.800 to 1.200                        250 to 375 MHz0.10 dB  0.905 to 1.095         283 to 342 MHz                 0.780 to 1.220                        244 to 381 MHz0.15 dB  0.880 to 1.120         275 to 350 MHz                 0.765 to 1.235                        239 to 386 MHz0.20 dB  0.865 to 1.135         270 to 355 MHz                 0.755 to 1.245                        236 to 389 MHz0.25 dB  0.850 to 1.150         266 to 359 MHz                 0.745 to 1.255                        233 to 392 MHz0.30 dB  0.834 to 1.166         261 to 364 MHz                 0.736 to 1.264                        230 to 395 MHz0.35 dB  0.820 to 1.180         256 to 369 MHz                 0.729 to 1.271                        228 to 397 MHz0.40 dB  0.809 to 1.191         253 to 372 MHz                 0.723 to 1.277                        226 to 399 MHz0.45 dB  0.896 to 1.204         249 to 376 MHz                 0.716 to 1.284                        224 to 401 MHz0.50 dB  0.786 to 1.214         246 to 379 MHz                 0.710 to 1.290                        222 to 403 MHz__________________________________________________________________________ 
    
     Referring now to FIG. 5, there is illustrated an alternative embodiment of a compensated directional coupler of the present invention utilizing lumped constant equivalent circuits in place of the quarter-wave and half-wave transmission lines. The basic directional coupler parallel transmission line and input ports are the same as those described with respect to FIG. 3, and similar reference numerals have been utilized. Instead of a quarter-wave short-circuited transmission line 44, a short-circuited, lumped constant equivalent network 50 is connected to the thru port 40. In place of the half-wave, open-circuited transmission line 46, an open circuited, lumped constant equivalent network 52 is connected to the isolation port 42. The lumped constant equivalent networks comprise two port networks composed of inductors and capacitors that emulate a transmission line. 
     FIGS. 6A and 6B illustrate simple lumped constant equivalent networks using a T-shaped or a π-shaped network. In each of the π-shaped and T-shaped networks, the blocks Z 1 , Z 2  and Z 3  will be comprised of inductors or capacitors to achieve the desired transmission line representation. To determine the values for Z 1 , Z 2  and Z 3 , the input characteristic impedance R 1  is set equal to the output characteristic impedance R 2 , such that the network is symmetric and R 1  =R 2  =R. For most applications the characteristic impedance will equal 50 ohms. 
     For a T-shaped artificial transmission line equivalent network: ##EQU1## where: θ equals the equivalent electrical length of the transmission line. 
     For a π-shaped artificial transmission line equivalent network: 
     
         Z.sub.1 =Z.sub.2 =j(R.sup.2 sin Θ)/(R cos Θ-R) (1) 
    
     
         Z.sub.3 =jR sin θ                                    (2) 
    
     Each of the above described π-shaped and T-shaped artificial transmission line equivalent networks create an equivalent quarter-wave transmission line. To achieve the equivalent half-wave, open circuited transmission line two π-shaped or T-shaped networks would be cascaded together. 
     Although preferred embodiments of the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions of parts and elements without departing from the spirit of the invention.