Patent Publication Number: US-6992546-B2

Title: Electronic phase shifter with enhanced phase shift performance

Description:
RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/691,198, filed Oct. 22, 2003, which is a continuation of U.S. application Ser. No. 09/774,534, filed Jan. 31, 2001. The entire teachings of the above applications are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   An emerging class of consumer electronic devices are wireless data access units that permit, for example, a portable laptop computer to be connected to a data network using radio waves. Ideally, such access devices take the form factor of a small handheld unit, much in the nature of the well-known cellular mobile telephone handsets. Because the users of such systems demand the highest data rate possible, given a specific available bandwidth for providing the service, these units are increasingly being designed to take advantage of sophisticated antenna techniques. 
   These techniques involve typically the use of antenna arrays that permit the radio link between the access unit and a centralized network base station to be made over a directional or diverse connection. The directivity provided by an antenna array reduces interference generated by a given radio connection with connections made to other access units operating within the same region, or cell, serviced by a particular base station. In order to accomplish the required directivity of the antenna array a number of components may be used to create the antenna beam. This may include switches, delay circuits, or phase shifters; the phase shifters provide the maximum control over the direction and shape of the resulting beam. 
   It becomes desirable therefore to provide for phase shifters that are as efficient, low-loss, and provide as wide a phase shift range as possible. Ideally, such phase shifter circuits are constructed using planar circuit techniques so that they may be as small and as inexpensive as possible. These requirements are critical if such phase shifters are to be effectively and economically deployed in portable access unit equipment. 
   At operating frequencies in the Very High Frequency (VHF) and higher frequency bands, one such circuit design makes use of a four port directional coupler. This design uses one or more varactors coupled to quadrature ports of the directional coupler. If the directional coupler is a half power, i.e., three decibel (dB) coupler, the reflections from the quadrature port(s) are equally recombined at the fourth output port. The signals combined at the output port will have a phase that is quasi-proportional to the impedance phase angle of the varactor(s). Thus, the amount of phase shift provided is a monotonic function that varies as the inverse of the line impedance. 
   SUMMARY OF THE INVENTION 
   The present invention is an improvement to a class of varactor based phase shifters that provides an increase in phase shift range and a reduction in the circuit requirements of the varactor components. 
   Briefly, the invention makes use of the property that a lower line impedance will provide greater phase shift, relying a unique technique to realize the lower line impedance. The technique used to achieve lower impedance is to embed a quarter-wave impedance transformer into the directional coupler, without adding extra signal path line lengths. 
   For example, if the input to output impedance is 50 ohms, which is the standard instrumentation line impedance, the impedance transformer implements a 50 ohm to 20 ohm transformation. In this embodiment, the impedance transformer may take the form of a pair of circuit traces. The first circuit trace runs from the input port to a quadrature port, and has a width that presents a 22 ohm impedance and a length that approximates one-quarter wavelength at the operating frequency. The 22 ohms is determined from the equation
 
√{square root over ( Z   O1   Z   O2 )} /F   QC 
 
where Z 01  is the input-output port impedance (50 ohms), Z 02  is the quadrature port impedance (20 ohms), and F QC  is a quadrature hybrid coupler factor. In the case of a branch line coupler, F QC  is equal to √{square root over ( )}2.
 
   The second circuit trace, running from the second quadrature port to the output port, is similarly formed from a conductive path that presents the 22 ohm transform impendence, and a length also of the desired one-quarter wavelength. 
   The quadrature ports each have attached thereto a varactor diode. The varactor diodes are biased by an input control voltage applied to the quadrature ports. 
   Coupling between the input/output port and between the quadrature ports may be provided by a circuit trace a quarter wave long connected between the respective ports. In the case of the input to output port, the circuit trace carries the characteristic desired 50 ohm impedance. Between the quadrature ports, the circuit trace provides the 20 ohm impedance desired across the quadrature ports. 
   In an alternative arrangement, quarter wave long face-coupled lines may provide the desired coupling between the input and output ports as well as between the coupling between quadrature ports. 
   The invention improves the available phase shift range by a factor of approximately 70% when compared to a standard 50 ohm to 50 ohm design, with comparable loading such as a single varactor coupled to each quadrature port. 
   Although the basic application of the invention is described in connection with the use of phase shifters, the technique can be used in a broader range of devices as well. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a portable access unit, such as may be used to provide wireless internet connectivity, with the unit having one more phase shifters implemented according to the invention. 
       FIG. 2  is a circuit diagram for a varactor based quadrature port phase shifter implemented according to the invention. 
       FIG. 3  is a circuit layout for one implementation of the phase shifter showing the impedance transformers coupled between the input and quadrature port and quadrature port and output. 
       FIGS. 4A and 4B , are respectively, Smith chart diagrams for respectively a prior art phase shifter and the present invention, showing the increase in available phase shift range. 
       FIG. 5  is a circuit layout for an alternate embodiment of the invention using coupled lines. 
   

   The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
   DETAILED DESCRIPTION OF THE INVENTION 
   A description of preferred embodiments of the invention follows. 
   Turning attention first to  FIG. 1 , there is shown a block diagram of one particular application of a phase shifter having improved phase shift range according to the invention. This device is a subscriber access unit  10  for a wireless communication system, and is seen to include an antenna array  12 , antenna Radio Frequency (RF) sub-assembly  20 , and an electronics sub-assembly  30 . The subscriber access unit  10  may be used to provide wireless data connectivity such as between the user of a laptop computer  60  and data networks such as the Internet. A wireless base station unit (not shown in  FIG. 1 ) provides network connectivity through internetwork switches or routers. In the typical scenario, a number of subscriber access units  10  are located within the area surrounding a base station and are serviced by the common base station. However, other arrangements are possible. 
   Before, turning attention to the phase shifter  25  in particular, it will be instructive to understand how the subscriber access unit  10  operates in general. Wireless signals arriving from the base station are first received at the antenna array  12  which consists of a number of antenna elements  14 - 1 ,  14 - 2 , . . . ,  14 -N. The signals arriving at each antenna element are fed to an RF subassembly  20 , including, for example, a phase shifter  25 , delay  24 , and/or switch  23 . There is an associated phase shifter  25 , delay  24 , and/or switch  23  associated with each antenna element  14 . 
   The signals are then fed through a combiner divider network  22  which typically adds the vector voltages in each signal chain providing the summed signal to the electronics sub-assembly  30 . 
   In the transmit direction, radio frequency signals provided by the electronic sub-assembly  30  are fed to the combiner divider network  22 . The signals to be transmitted follow through the signal chain, including the switch  23 , delay  24 , and/or phase shifter  25  to a respective one of the antenna elements  14 , and from there are transmitted back towards the base station. 
   In the receive direction, the electronics sub-assembly  30  receives the radio signal at the duplexer/filter  32  which provides the received signals to the receiver  35 . The radio receiver  35  provides a demodulated signal to a decoder circuit  37  that removes the modulation coding. For example, such decoder may operate to remove Code Division Multiple Access (CDMA) type encoding which may involve the use of pseudorandom codes and/or Walsh codes to separate the various signals intended for particular subscriber units, in a manner which is known in the art. The decoded signal is then fed to a data buffering circuit  40  which then feeds the decoded signal to a data interface circuit  50 . The interface circuit  50  may then provide the data signals to a typical computer interface such as may be provided by a Universal Serial Bus (USB), PCMCIA type interface, serial interface or other well-known computer interface that is compatible with the laptop computer  60 . A controller  46  may receive and/or transmit messages from the data interface to and from a message interface circuit  44  to control the operation of the decoder  37 , encoder  36 , the tuning of the transmitter  34  and receiver  35 . This may also provide the control signals  62  associated with controlling the state of the switches  23 , delays  24 , and/or phase shifters  25 . For example, a first set of control signals  62 - 3  may control the phase shifter states such that each individual phase shifter  25  imparts a particular desired phase shift to one of the signals received from or transmitted by the respective antenna element  14 . This permits the steering of the entire antenna array  12  to a particular desired direction, thereby increasing the overall available data rate that may be accomplished with the equipment. For example, the access unit  10  may receive a control message from the base station commanded to steer its array to a particular direction and/or circuits associated with the receiver  35  and/or decoder  37  may provide signal strength indication to the controller  46 . The controller  46  in turn, periodically sets the values for the phase shifter  25 . 
   As mentioned above, of particular interest to the present invention is the construction of the phase shifter  25 . 
   Turning now to  FIG. 2 , there is shown a more detailed circuit diagram of the preferred embodiment of the phase shifter  25  as a four port device. In particular, the phase shifter  25  includes an input port (IN)  100 , an output port (OUT)  200 , a first quadrature port (Q 1 )  150 , and a second quadrature port (Q 2 )  160 . The input port  100  and output port  200  have an associated characteristic impedance Z O1 . Similarly, the quadrature ports  150  and  160  have associated with them a characteristic impedance Z O2 . Coupled between the input port  100  and quadrature port  150  is an impedance transformer  120 . The impedance transformer provides for a transformation from the characteristic impedance Z O1  between the input port  100  and the output port  200  to the characteristic impedance Z O2  between the quadrature ports  150  and  160 . As will be understood shortly, in connection with the description of  FIG. 3 , the impedance transformer  120  is implemented using a strip of transmission line of the appropriate length. Similarly, an impedance transformer  130  is connected between the second quadrature port  160  and the output port  200 . It is these impedance transformers  120  and  130  that provide for increased phase range in connection with the novel aspects of the present invention. 
   A varactor diode  180  is connected between the first quadrature port  150  and a ground reference potential; similarly, a second varactor diode  190  is connected between the second quadrature port  160  and the ground reference as well. A bias input voltage representing the signal  62 - 3  which was provided in the description of  FIG. 1  to control the phase shift imported by the phase shifter  25  is applied to the quadrature ports  150  and  160 . An RF blocking inductor  195  may be typically disposed in the bias input. In addition, blocking capacitors  112  and  114  may be applied to the input port  100  and output port  200  to prevent the introduction of direct current signals beyond the phase shifter circuit  25 . In the preferred embodiment, the four port coupler arrangement is a one-quarter wave device having a line length of λ/4. One implementation of such a coupler is a so-called branch line coupler, as shown in  FIG. 3 .  FIG. 3  is a circuit layout diagram illustrating a planar implementation of the invention. Particular circuit elements, including the input blocking capacitors  112  and  114 , varactor diodes  180  and  190 , and RF blocking inductor  195  are implemented using known planar circuit techniques. In this implementation, the impedance transformer circuits  120  and  130  are provided by sections of transmission line  121  and  131  having a length equal to one-quarter wavelength of the desired operating frequency. The distance λ/4 associated with the impedance transformer  120  and  130  is as measured from a center line of the center line C of each end of the circuit structure. 
   The width, w 1 , associated with the impedance transformers  120  and  130  is selected to provide the appropriate transformation from the characteristic input impedance Z O1  across the input port  100  and output port  200  to the characteristic impedance Z O2  associated across the quadrature ports  150  and  160 . The formula is
 
 Z   OT   =√{square root over (Z     O1     Z     O2     )}   /F   QC 
 
where F QC  is a quadrature hybrid factor value that depends upon the hybrid coupler design. In the case of a branch line coupler, the F QC  factor is known to the practitioners to be √{square root over ( )}2.
 
   In this embodiment, the impedance transformers  120  and  130  have a width, w 1 , that approximately provides a 22 ohm impedance to current flow. 
   Coupling between the input port  100  and output port  200  is provided by a straight branch line  155 , in this embodiment. The branch line  155  has a width, w 0 , that provides the desired characteristic impedance; here this impedance is 50 ohms. Also in this embodiment, another one quarter wavelength branch line  158  provides coupling between the quadrature ports  150  and  160 . This branch line  158  has a width, W 2 , that provides the desired characteristic impedance between the quadrature ports of 20 ohms. The branch lines  155  and  158  may be straight or follow a serpentine path as is illustrated. The serpentine path permits the overall dimension of the phase shifter  25  to be less than would otherwise be required; for in the preferred embodiment, the overall length of each of the branch lines  155  and  158  is λ/4. 
   By changing the voltage applied to the bias terminal, the reactance of the varactors  180  and  190  changes. This provides a change in the phase shift imparted by the pair of varactors  180  and  190 , in turn effecting a phase change at the quadrature ports  150  and  160 . This results in an insertion phase shift being evident in the signal going from the input port to the output port. 
   A dramatic increase in the amount of available phase shift range is available with the introduction of the impedance transformers  120  and  130 . This difference is illustrated by the Smith charts in  FIGS. 4A and 4B .  FIG. 4A  represents a Smith chart for a prior art phase shifter in which the characteristic impedance between the input and output ports and across the quadrature ports are each set at 50 ohms. Such an implementation provides a phase shift range as illustrated, for example, of approximately 80°, going from the inductive zone to the capacitive zone. The prior art circuit implementation made the assumption that matching the characteristic impedance at both ends of the four port device provides for the best performance. However, with the present invention, it is clear that by dropping the characteristic impedance across the quadrature ports to 20 ohms, as shown in  FIG. 4B , the overall available phase shift range has been marketedly increased such as, for example, to a range of approximately 200°. 
   The narrow line widths on either side of each varactor are designed in to provide added inductance to the varactors, so that when the varactors are under bias, they can exhibit both inductive and capacitive properties. This allows the phase shift to vary over a broader range of degrees in both the capacitive and inductive zones about the 180° point, as shown in  FIG. 4B . 
     FIG. 5  illustrates an alternative arrangement for the invention making use of a so-called cross line face-coupled coupler. In this embodiment, coupling between the input and output ports is provided by a pair of transmission lines in a cross coupled orientation, as shown at  225  between the 50 ohm input port  100  and 50 ohm output port  200 . Similarly, a pair of cross coupled lines may be provided to implement the coupling between the 20 ohm quadrature ports  150  and  160 , as illustrated at  258 . Cross-coupling is implemented by forming one set of the circuit traces and components on a first layer of a printed circuit board, as shown with the solid lines, and a second set of traces and components on another layer of the printed circuit board, as shown with the dashed lines. As is know to those of skill in the art, each pair of cross-coupled lines provides a 6 dB directional coupler. Two pairs of these coupled lines in tandem make up a 3 dB coupler, or a hybrid, which has the same properties as the branch line coupler. 
   The transformers  120  and  130  are one quarter wavelength long. The characteristic impedance of the transformers are 32 ohms, which is different from the previous branch line example. The difference is due to the fact that the quadrature hybrid factor, F QC , in the case of the crossed line coupler is one (1), instead of √{square root over ( )}2. 
   While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.