Abstract:
The present invention provides a ninety degree coupler for a frequency range of 100 mHz to gigaHertz ranges and including a minimum number of components employing a four port device which receives an input signal and splits it between a transformer coil and another reactor and couples it to another port and splits it between the transformer and a capacitor, then cross-connecting to a capacitor where phase shift is created by the inductors and capacitors resulting in phase shift between the signal ports remaining at 90 degrees over a wide frequency band.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to ninety degree couplers suited for use in integrated circuits operating between 100 mHz to GHz ranges.  
         BACKGROUND OF THE INVENTION  
         [0002]    Ninety degree couplers are used in many radio frequency integrated circuit (RFIC) applications. Couplers split or combine two orthogonal signals. This kind of coupler is used in balanced amplifiers, mixers, particularly Gilbert cell mixers, balanced attenuators, signal splitters and signal combiners. Particularly popular use for ninety degree couplers is in cell phones. Cell phone signals are multiplexed into In-phase, I and Quadrature panels. Ninety degree couplers are used to bind the signals for transmission and separate them for reception. Currently, the predominant form on ninety degree coupler for RFIC applications is an RC network. Couplers have been provided in MMIC technology and GaAs processing. However, greater simplicity can yet be achieved. It is desirable to minimize the number of components in a ninety degree coupler. This allows a smaller IC area. Reducing area permits reduce costs and insensitivity to spurious signals.  
         SUMMARY OF THE INVENTION  
         [0003]    In accordance with the present invention, a ninety degree coupler is provided for a frequency range of 100 mHz to 2500 mHz and beyond to accommodate new frequency bands and including a minimum number of components.  
           [0004]    Briefly stated, in accordance with the present invention, there is provided a four port device which receives an input at port  1 . The signal is split between a transformer coil and another reactor. In one form, the signal entering port  1  splits between the transformer and a capacitor. Part of the signal is coupled to port  2  through the transformer by mutual inductance. Another part of the signal couples through cross-connected capacitors. Phase shift created by the inductors and capacitors result in signals adding at ports  2  and  3  and signal cancellation at point  4 . Phase of the signal shift from port  1  to port  2  is 45 degrees and from port  3  is −45 degrees. Thus the phase shift between the signal and ports  2  and  3  is 90 degrees and remains 90 degrees over a wide frequency band. In another embodiment, a signal passes around from port  1  to port  2  to port  3  to port  4  with a ninety degree phase delay at each port. The power from port  1  will be equally split between port  2  and port  3  and the phase of the signal will be 90 degrees and 180 degrees respectively, providing a 90 degree phase shift across ports  2  and  3 . 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The means by which the invention is embodied may be further understood with reference to the following description taken in connection with the following drawings.  
         [0006]    Of the drawings:  
         [0007]    [0007]FIG. 1 is a schematic of a first embodiment of the present invention;  
         [0008]    [0008]FIG. 2 is a schematic drawing of the second embodiment of the present invention;  
         [0009]    [0009]FIG. 3 is a mechanical diagram of the layout of the transformers of FIG. 2;  
         [0010]    [0010]FIG. 4, FIG. 5, comprising FIGS. 5A and 5B, and FIG. 6, comprising FIGS. 6A, 6B and  6 C are charts illustrating performance of the embodiments of FIGS. 1 and 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    The prior art IC couplers utilize lumped components wherein it is desirable to provide a coupler which takes less surface area on the IC chip.  
         [0012]    The present invention provides advantages both in performance and in manufacturability. Closely coupled bifilar transformers are utilized. This form of transformer, more specifically illustrated in FIG. 3 below takes reduced space on an IC chip compared to prior art arrangements.  
         [0013]    Referring now to FIG. 1, there is illustrated in schematic form a 90 degree coupler  10  constructed in accordance with the present invention. The 90 degree coupler  10  comprises a transformer  12  having a primary winding  14  and a secondary winding  16 . The windings  14  and  16  are coupled in-phase. The winding  14  is connected across ports  1  and  3 . The winding  16  is connected across ports  2  and  4 .  
         [0014]    First and second capacitors  18  and  19  are cross-coupled across the transformer  12 . More specifically, a capacitor  18  is connected from point  1  to point  4 , and a capacitor  19  is connected from port  3  to port  2 . Each of the capacitors  18  and  19  are connected from one respective end of the primary winding  14  to respectively opposite ends of the secondary winding  16 .  
         [0015]    In operation, a signal entering port  1  splits between the transformer winding  14  and the capacitor  18 . The mutual inductance of the transformer  12  couples part of the signal across the winding  14  to the secondary winding  16 . At the signal is coupled to port  4  from port  1  through the primary coil  14  of the transformer  12 . Another part of the signal applied to port  1  is coupled through the cross-connected capacitors  18  and  19 . Due to the phase shifts created by inductors  14  and  16  and capacitors  18  and  19 , signals at port  2  receive from the signal applied to port  1  and to each other. Similarly, signal adding occurs at port  3 . Due to the relative polarities, there is signal cancellation at port  4 .  
         [0016]    The phase shift from port one to port  2  is 45 degrees. The phase shift from port  1  to port  3  is −45 degrees. Consequently, the phase shift between signals at ports  2  and  3  is ninety degrees. Phase shift remains very very close to 90 degrees in a wide frequency band.  
         [0017]    It is important to translate signals via the transformer  12 . The transformer  12  should have very high coupling between the primary coil  14  and the secondary coil  16 . More specifically, mutual inductance, M should be equal to the winding inductance, L, or almost L. It is important to use very tightly coupled inductors. The inductors may be preferably bifilar or multilayer balanced. Two capacitors used with the transformers form a narrow band equivalent to a quarter-wave directional coupler. As an example, in an embodiment for frequency f=100 mHz, component values were used L=40 nH and C=15.5 pF.  
         [0018]    For frequency f=2 GHz, component values in a particular embodiment are L=1.5 nH and c=0.8 pF.  
         [0019]    [0019]FIG. 2 illustrates another embodiment of the 90 degree coupler  10  having ports  1 ,  2 ,  3  and  4 . FIG. 3 is a plan view of a physical embodiment of the circuit of FIG. 2. Capacitors  20  and  21  are connected between ports  1  and  4  respectively and ground. Similarly, capacitors  30  and  31  are respectively connected across ports  2  and  3   a  and ground. The capacitors  20 ,  21   30  and  31  are transmission line capacitors. Note that in this embodiment, ports  1  and  4  are illustrated at the left of the diagram, and ports  2  and  3  are illustrated at the right of the diagram. This embodiment comprises a first transformer  22  having a primary winding  24  connected between the ports  1  and  2  and a secondary winding  26  connected between the ports  4  and  3 . The windings are closely coupled to have a mutual inductance M 1 . Each winding has an inductance value L1. The windings  24  and  26  are coupled in-phase.  
         [0020]    Similarly, a second transformer  32  is provided with a primary winding  34  and a secondary winding  36  coupled in-phase. The windings  34  and  36  have mutual inductance M 2  and each have an inductance value of L2. The winding  34  is connected across ports  1  and  4 . The winding  36  is connected across ports  2  and  3 .  
         [0021]    A signal entering port  1  is divided between two paths. The first path is through the winding  24  of the port  2  and the second path is through the winding  34  to the port  4 . A signal from port  2  passes through the winding  36  to port  3  and from port  3  through the winding  26  to port  4 . The shunt capacitors  20 ,  21 ,  30  and  31  act in conjunction with transformer windings  24 ,  26 ,  34  and  36  to form transmission lines.  
         [0022]    In order to balance this circuit, each path between ports will have to shift the phase of the signal 90 degrees. This means that the signal passing from port  1  will be delayed in-phase 90 degrees at port  2  180 degrees at port  3  270 degrees at port  4 . Signals passing in the other direction from port  4  to point  1  will be shifted in-phase 90 degrees and will be cancelled with the signal passing around from port  1  to port  2  to port  3  to port  4 . Consequently no power will be available at port  4 .  
         [0023]    Power from port  1  will be equally split between port  2  and port  3  and the phase of this signal will be 90 degrees and 180 degrees respectively. Any reflections from the terminations of port  2  and/or port  3  will be transferred to port  4 . In this case, power will appear at port  4  and be dissipated in determination of port  4 .  
         [0024]    The topology of this coupler has a low-pass filter response. Consequently, further filtering which removes higher frequency components of signals is provided. The close coupling between windings  24  and  26  of transformer  22  and between the windings  34  and  36  of the transformer  32  permits the compact physical arrangement seen in FIG. 3.  
         [0025]    FIGS.  4 - 6  illustrate the relations of the embodiments of FIGS. 1 and 2, and the performance in each simulation, following parameters are measured:  
         [0026]    s 11 , input reflection co-efficient with the output port terminated by a matched load,  
         [0027]    s 22  output reflection co-efficient with the input terminated by a matched load;  
         [0028]    s 21  forward transmission(s) insertion gain with the output port terminated in a matched load;  
         [0029]    s 31  a forward transmission co-efficient;  
         [0030]    s 23  forward transmission co-efficient.  
         [0031]    ang-s 21  and ang-s 23  are measured in degrees.  
         [0032]    These are the phased differences between terminal  2  and terminal  1  and the phase differences between port  3  and port  1  respectively and measured in degrees. Forty-five degrees is ideal since it is half of 90 degrees.  
       EXAMPLE I  
       [0033]    Signal C coupler equivalent to a quarter-wave direction is provided in an embodiment designed to operate in the are of f=100 mHz with L=40 nH and c=15.5 pF and tested over a range of 10 mHz to 200 mHz results are represented in FIG. 4.  
       EXAMPLE II  
       [0034]    A coupler intended to operate at the higher range, for example 2 GHz has component values of L=1.98 nH and c=0.8 pF and is tested over each of 100 mHz to 3 GHz.  
       EXAMPLE III  
       [0035]    A hybrid branch coupler with mutually coupled conductors was simulated according to the embodiment of FIG. 2 with the values L1=9 nH, L2=18 nH, C=7 pF, M 1 =0.95 (L) and M 2 =0.95 (L). In this embodiment, for angle s 21  and angle s 23  there is ideally a difference of 90 degrees between them. As stated in the text of Example III this is over range of one frequency to another 10 mHz through 750 mHz.  
         [0036]    The present invention makes effective use of tightly coupled conductors connected between ports. The present disclosure will enable those skilled in the art to provide many different constructions apart from the specific illustrations of the present examples in accordance with the present invention.