Patent Publication Number: US-9406991-B2

Title: Quadrature hybrid

Description:
TECHNICAL FIELD 
     The present invention discloses an improved quadrature hybrid with improved bandwidth and reduced size. 
     BACKGROUND 
     A quadrature hybrid is a component which has one input port and two output ports, and which is arranged to use an input RF signal at the input port to generate two output signals, one at each output port but with a ninety degree phase difference between them, so called I and Q signals. The amplitude of the I and Q signals will (in an ideal quadrature hybrid) be the same and will be half of the amplitude of the input signal, for which reason a quadrature hybrid is also sometimes referred to as a “3 dB quadrature hybrid”. 
     Quadrature hybrids are widely used in microwave applications such as, for example, amplifiers and mixers, as well as in phase shifters and other such microwave applications. In broadband circuits, there is naturally a need for broadband quadrature hybrids. 
     One way of designing traditional quadrature hybrids is by means of so called “lumped components”, e.g. inductors and capacitors. A drawback to such quadrature hybrids is that they have quite a narrow operational bandwidth as well as a rather low relative bandwidth. 
     Another traditional way of designing quadrature hybrids is to use microstrip lines. Such quadrature hybrids have a broad bandwidth and a good relative bandwidth, but are also of a large size. 
     Techniques to design quadrature hybrids by means of capacitors in microstrip based quadrature hybrids. So called metatransmission lines have also been utilized to obtain quadrature hybrids. 
     However, the relative bandwidth of the quadrature hybrids enumerated above remain limited, particularly since, in some broadband applications, a relative bandwidth of more than 100% is required, a performance which these known quadrature hybrids cannot provide. 
     SUMMARY 
     It is an object of the invention to provide a quadrature hybrid which obviates at least some of the disadvantages of known quadrature hybrids, in particular when it comes to size and bandwidth. 
     This object is achieved by means of a quadrature hybrid which comprises a first and a second open waveguide which are electrically coupled to each other. Each of the open waveguides comprises a first and a second port. One of the ports in the first open waveguide is arranged to be used as input port for an input signal which the quadrature hybrid is arranged to use to generate I and Q output signals, and the other port in the first open waveguide is arranged to be used to output the Q signal, and one of the ports in the second waveguide is arranged to be used to output the I signal. The other of the ports in the second open waveguide is arranged to be an isolated port, and the quadrature hybrid additionally comprises a first differential amplifier with a positive and a negative port. The positive port is connected to the first open waveguide and the negative port is connected to the second open waveguide. 
     In embodiments of the quadrature hybrid, the first differential amplifier has its connections to a point in the open waveguides which is at a centre point of the open waveguides. 
     In embodiments, the quadrature hybrid comprises a second differential amplifier with a positive and a negative port, where the positive port is connected to the first open waveguide and the negative port is connected to the second open waveguide. 
     In embodiments of the quadrature hybrid, the second differential amplifier has its connections to the open waveguides at a distance of L/2 from the connections of the first differential amplifier, where L is the lengths of the open waveguides. 
     In embodiments, the quadrature hybrid comprises a third differential amplifier with a positive and a negative port, with the positive port being connected to the first open waveguide and the negative port being connected to the second open waveguide. 
     In embodiments of the quadrature hybrid, the third differential amplifier has its connections to the open waveguides at a distance of L/4 from the connections of the first differential amplifier and at a distance of 3L/4 from the connections of the second differential amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail in the following, with reference to the appended drawings, in which 
         FIG. 1  shows a prior art quadrature hybrid, and 
         FIG. 2  shows a first embodiment of a quadrature hybrid, and 
         FIG. 3  shows a performance graph of the embodiment of  FIG. 2 , and 
         FIG. 4  shows a second embodiment of a quadrature hybrid, and 
         FIG. 5  shows a third embodiment of a quadrature hybrid, and 
         FIGS. 6-8  show performance graphs of the embodiment of  FIG. 5 , and 
         FIG. 9  shows an embodiment of a differential amplifier. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like numbers in the drawings refer to like elements throughout. 
     The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the invention. 
       FIG. 1  shows an example of a prior art quadrature hybrid  100 . As shown in  FIG. 1 , the quadrature hybrid  100  comprises a first  110  and a second  105  open waveguide, which are electrically coupled to each other. The first open waveguide comprises a first  107  and a second  109  port, and the second open waveguide also comprises a first  106  and a second  108  port. The waveguides should be of equal length, which should be a quarter of the desired operational centre wavelength of the quadrature hybrid, shown as λ/4 in  FIG. 1 . 
     As shown in  FIG. 1 , the port  107  in the open waveguide  110  is arranged to be used as input port for an RF (Radio Frequency) signal. The other port  109  in the open waveguide  110  is arranged to output a so called Quadrature Phase signal, commonly referred to as a Q signal, and the port  106  in the open waveguide  105  is arranged to output a so called In Phase signal, commonly referred to as an I signal. The I and Q signals are generated from the RF signal, and have, in an ideal circuit, the same amplitude but a phase difference of ninety degrees. The I and Q signals ideally have the same amplitude, which ideally will be half the amplitude of the input RF signal. 
     In addition, the port  108  in the open waveguide  105  is arranged to be a so called “isolated port”, usually connected to ground via a  500  resistor, which in an ideal case causes no RF energy to be lost from the circuit  100  via the isolated port  108 . 
     Thus, in the quadrature hybrid  100  of  FIG. 1 , we have an input RF signal at port  107  and at ports  106  and  109  we obtain I and Q output signals. 
       FIG. 2  shows a first embodiment of a quadrature hybrid  200  of the invention. In addition to the features of the quadrature hybrid  100  of  FIG. 1 , the quadrature hybrid  200  comprises a differential amplifier  205 , which has a positive and a negative port, marked with plus and minus signs in  FIG. 2 , and, as shown in  FIG. 2 , the positive port is connected to the open waveguide  110  and the negative port is connected to the open waveguide  105 . The connections could also be the opposite, i.e. it does not matter which of the ports of the amplifier  205  that is connected to which of the open waveguides. Suitably, the distances between the connection points for the negative and the positive ports of the differential amplifier  205  and the ends of the two open waveguides are the same, and are preferably L/2, where L is the total length of the open waveguides. In other words, preferably, the positive and negative ports of the amplifier  205  are both connected to the middle of their respective open waveguide. 
     Regarding the open waveguides, they can be designed according to a number of different techniques for open waveguides, for example the following:
         microstrip lines   strip lines   coplanar waveguide,
 
and will be arranged to be coupled to each other by means of having a common ground plane, as well as by means of electromagnetic interaction between the two waveguides. It should be pointed out that one of the open waveguide can be designed in one of the techniques listed above and the other open waveguide can be designed from one of the other techniques, although usually, both of the open waveguides will be designed in the same technique.
       

       FIG. 3  shows graphs which illustrate and advantage gained by means of the quadrature hybrid  200  of  FIG. 2  as compared to the prior art quadrature hybrid  100  of  FIG. 1 : we see that the amplitude of the I signal is improved, particularly in the frequency range of 10 GHz to 40 GHz. At frequencies above 40 GHz, the amplitude of the I signal decreases, but this is due to the fact that the quadrature hybrid for which the graph was generated is optimized for frequencies below 40 GHz by means of the length of the coupled open waveguides. If it is desired to obtain higher amplitudes at frequencies above 40 GHz, the length of the coupled open waveguides could be shortened accordingly, i.e. given a length which corresponds to λ/4, where λ is the operational (centre) frequency of the quadrature hybrid. 
     However, as can also be seen in the graphs of  FIG. 3 , the amplitude of the Q signal is not boosted in the same manner as that of the I signal. If it is desired to remedy this, the quadrature hybrid can be equipped with an additional differential amplifier  405 , as shown in  FIG. 4 , by means of which a quadrature hybrid  400  is obtained. In this case, as shown in  FIG. 4 , the additional, second differential amplifier  405  should have its connections to the open waveguides at a distance from the connections of the first differential amplifier  205  which equals L/2, i.e. the second differential amplifier is connected to one end of the open waveguides, while the first differential amplifier  205  is connected to the centre of the open waveguides. 
     In a further embodiment, the quadrature hybrid is also equipped with a third differential amplifier  505 , by means of which a quadrature hybrid  500  is obtained, as shown in  FIG. 5 . As indicated in  FIG. 5 , the position for the first  205  and second differential amplifiers  405  are maintained as described above, while the third differential amplifier  505 , is connected with both of its ports to a point of the open waveguides  105 ,  110 , which is L/4 from the connections of the first differential amplifier  205 , and this 3L/4 from the connections of the second differential amplifier  405 . 
       FIG. 6  shows a graph of the amplitudes of the I and Q signals of the embodiment  500  as a function of frequency: we see that the amplitude of the Q signal approaches that of the I signal, and that the amplitude of the I signal has increased slightly from the embodiment  200  in  FIG. 2 . This means that more power is transferred from the Q port to the I port by means of the three differential amplifiers  205 ,  405  and  505 . As a consequence of this, balanced output amplitudes is achieved in a frequency range of 6.3 to 39 GHz. 
       FIG. 7  shows the phase of the I and Q signals: we see that the phase difference of 90 degrees is essentially maintained in the interval of 6.3 to 39 GHz. 
       FIG. 8  shows both the phase difference between the I and Q signals (solid line) and the amplitude difference between the I and Q signals (dashed line). A conclusion that can be drawn from the graph of  FIG. 9  is that acceptably balanced output amplitudes are achieved within the frequency range of 6.3 to 39 GHz, with the amplitude difference between the I and Q ports being less than 1 dB. In addition, the maximum phase error is no more than 7 degrees, which is almost equal to that of other designs. 
       FIG. 9  shows an embodiment of a differential amplifier such as the one  205  which has been used in the embodiments of  FIGS. 2, 4 and 5 . In  FIG. 9 , we see the positive and negative ports, shown by means of a plus and a minus sign. Actually, the ports can be used in either combination, i.e. if one port is used as the negative port, the other port will serve as the positive port. 
     The differential amplifier  205  comprises bipolar transistors, but can also be designed using FET transistors, in which case the following substitutions should be made in the text below: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Bipolar 
                 FET 
               
               
                   
                   
               
             
            
               
                   
                 Base 
                 Gate 
               
               
                   
                 Collector 
                 Drain 
               
               
                   
                 Emitter 
                 Source 
               
               
                   
                   
               
            
           
         
       
     
     Each of the ports is connected to the collector of a respective transistor  235 ,  210  via respective first capacitors. The emitters of the first  210  and second  235  transistors are connected to each other, and are connected to a current source  230  which is comprised in the differential amplifier  205 . The base of each of the transistors  210 ,  235  is “cross-connected” to the collector of the other transistor  235 ,  210  via respective second capacitors. 
     As mentioned, the transistors  210 ,  235  have their emitters connected to each other, and are via this connection connected to a current source  230  comprised in the differential amplifier  205 . The current source  230  comprises a third  220  and a fourth  225  transistor, which have their bases connected to each other and have their emitters connected to ground. The base of the third transistor  220  is connected to the transistor&#39;s collector, which is also connected to the emitter of a fifth transistor  215 , which is also comprised in the current source  230 . In addition, the collector of the fourth transistor  225 , is connected to the base of the fifth transistor  215 . The collector of the fifth transistor  215  serves as the “connection point” between the current source  230  and the rest of the differential amplifier  205 . 
     As can be seen in  FIG. 9 , the differential amplifier  900  is arranged to have a number of voltages applied to it. Using the notations of  FIG. 9 , these are as follows:
         Vb: DC bias voltage applied at the base of the first  210  and the second  235  transistor,   Vm: DC bias voltage for the current source  230 , applied at the collector of the transistor  225  and the base of the transistor  215 ,   Vc: DC supply voltage at the collectors of the transistors  210  and  235 , suitably via equally sized resistors.       

     In the drawings and specification, there have been disclosed exemplary embodiments of the invention. However, many variations and modifications can be made to these embodiments without substantially departing from the principles of the present invention. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation. 
     The invention is not limited to the examples of embodiments described above and shown in the drawings, but may be freely varied within the scope of the appended claims.