Abstract:
Disclosed are systems and methods which provide phase shifting of signals while minimizing signal attenuation associated with providing such phase shifting. Preferred embodiments provide a compact polyphase filter structure in which the geometry of the polyphase filter both minimizes parasitic capacitance and signal transmission paths therein. A polyphase filter of a preferred embodiment utilizes a radial pinwheel structure providing a geometry in which successive circuit components are disposed very near one another and in an orientation to accommodate very short connections therebetween. Embodiments may include the use of a buffer at the outputs of the polyphase filter circuit to reduce the amount of parasitics associated with the polyphase filter.

Description:
BACKGROUND OF THE INVENTION 
     It is often desirable to provide a controlled phase shift with respect to a signal. For example, radio frequency (RF) circuits often utilize 90° phase differentials with respect to a signal, e.g., in-phase (I) and quadrature (Q) signal components, to provide desired signal processing. One circuit using such 90° phase differentials is a single sideband mixer, typically using 2 90° phase shifters in order provide I and Q components with respect to a carrier signal and I and Q components with respect to a local oscillator (LO) signal. Through proper phase shifting and combining of the signals (destructive combining of undesired frequencies e.g., sidebands and/or images), such a single sideband mixer can be utilized to cancel undesired frequencies. 
     To suppress undesired frequencies through destructive combining, the 90° phase differentials utilized by the single sideband mixer should be very accurate. Likewise, the amplitudes of the signal components for which destructive combining is desired should be very close. 
     The aforementioned 90° phase differentials have often been provided using a polyphase filter circuit. Such circuits are typically embodied as a resistor and capacitor (RC) network, where the number of stages of resistors and capacitors determines the accuracy of the phase shift provided. However, there is parasitic capacitance associated with every node in such a circuit. At high frequencies, parasitic capacitance will cause attenuation of an RF signal as it passes through the network. For example, for a polyphase filter operating with an RF signal having a characteristic frequency of about 2 GHz, 7 dB of attenuation is not uncommon. 
     Moreover, the amount of parasitic capacitance and transmission delays associated with various signal paths within the polyphase filter must be carefully matched in order to provide an accurate phase shift output. Such matching concerns have typically been addressed in the prior art by providing excess signal path portions, thereby resulting in a complex and large polyphase filter structure. 
     Accordingly, a need exists in the art for polyphase filters adapted to provide a desired phase shift with a minimal amount of signal attenuation. Moreover, there is a need in the art for the structure of such polyphase filters to be compact. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to systems and methods which provide phase shifting of signals while minimizing signal attenuation associated with providing such phase shifting. A most preferred embodiment of the present invention provides a compact polyphase filter structure in which the geometry of the polyphase filter both minimizes parasitic capacitance and signal transmission paths therein. 
     A preferred embodiment polyphase filter of the present invention utilizes a radial pinwheel structure. This preferred embodiment structure provides a geometry in which successive circuit components, e.g., resistors and capacitors, are disposed very near one another and in an orientation to accommodate very short connections therebetween. Accordingly, this embodiment of the invention does not require large routing signal paths going from component to component. 
     According to a preferred embodiment pinwheel structure polyphase filter, input signals are routed into the center of the pinwheel. The signals therefore start at the center of the pinwheel structure and propagate outward through the polyphase filter components disposed in a radial configuration. Accordingly, the last stage of the polyphase filter of this embodiment is preferably at or near the circumference of the pinwheel structure. 
     The preferred embodiment radial pinwheel polyphase filter structure provides several advantages over a typical polyphase filter structure. For example, the structure results in much less attenuation because the signal paths from device to device become much smaller. Additionally, such an embodiment provides synergism with respect to circuit geometry, facilitating a lay out in which parasitics are not only reduced but are also matched. Accordingly, preferred embodiments of the present invention have been able to provide phase shift differentials of 90° with respect to signals in the 2 GHz frequency range resulting in approximately 1 dB of attenuation associated with the polyphase filter circuit, as compared to approximately 7 dB of attenuation as is common with a typical prior art polyphase filter of conventional design. 
     An embodiment of the present invention includes the use of a buffer at the outputs of the polyphase filter circuit. For example, an emitter follower buffer may be placed at the 4 corners of a preferred embodiment pinwheel structure polyphase filter structure to provide buffering of the signals output therefrom. Such buffers may be utilized to reduce the amount of parasitics associated with the polyphase filter. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  shows a block diagram of a conventional single sideband mixer; 
         FIG. 2  shows a schematic diagram of a polyphase filter according to the prior art; 
         FIG. 3  shows a block diagram of a prior art implementation of a polyphase filter according to the prior art; 
         FIG. 4  shows a block diagram of a polyphase filter having a radial configuration according to an embodiment of the present invention; 
         FIG. 5  shows a block diagram of a polyphase filter having a radial configuration according to an alternative embodiment of the present invention; and 
         FIG. 6  shows a block diagram of a polyphase filter having a radial configuration according to another alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Directing attention to  FIG. 1 , a single sideband mixer is shown as mixer  100 . As is well known in the art, a single sideband mixer utilizes 90° phase differentials to cancel a signal sideband and/or image associated with the mixing of signals of different frequencies. An input signal, here Vin having a first characteristic frequency, may be down-converted or up-converted to provide an output signal, here Vout having a second characteristic frequency, while undesired frequencies associated with the mixing of Vin with the local oscillator signal (LO) are canceled. 
     Accordingly, single sideband mixer  100  includes first 90° phase shifter  121  providing in-phase (I) and quadrature (Q) components of the input signal Vin. Similarly, second 90° phase shifter  122  provides I and Q components of the LO signal. Mixer  111  combines the I component of Vin with the I component of LO to thereby provide both fLO+fVin and fLO−fVin, wherein fLO is the characteristic frequency of the LO signal and fvin is the characteristic frequency of Vin as input to mixer  100 . Similarly, mixer  112  combines the Q component of Vin with the Q component of LO to thereby provide both ELO+fVin and fLO−fVin. However, because of the phase differential between the signals input to mixer  111  and  112 , the undesired sideband signal LO−fVin in the output stream of mixers  111  and  112  destructively combine in mixer  131  because they have a 180° phase differential therebetween. Accordingly, the output of combiner  131  is a constructive combination of the desired signals fLO+fVin from mixers  111  and  112  because these signal components are in phase with one another. 
     From the above discussion, it should be readily appreciated that providing a relatively precise phase differential, to thereby provide accuracy with respect to the aforementioned 180° phase differential as well as to provide accuracy with respect to the phase of the signals to constructively combine, suggests that the phase shifts provided by phase shifters  121  and  122  should be precise. That is, the quality of the 90° phase shift has to be very good in order to have acceptable side bands for suppression. 
     One method for providing the aforementioned 90° phase shifts has been to utilize a polyphase filter. Directing attention to  FIG. 2 , a schematic diagram of a conventional polyphase filter is shown as polyphase filter  200 . Polyphase filter  200  comprises a multi stage resistor and capacitor (RC) network. Specifically, resistors  211 – 214  and capacitors  231 – 234  comprise a first stage of multi stage polyphase filter  200 . Similarly, resistors  221 – 224  and capacitors  241 – 244  comprise a second stage of multi stage polyphase filter  200 . It should be appreciated that, as more stages are added to the network, the resulting polyphase filter will provide a more accurate phase shift. Accordingly, although only 2 stages are shown, a polyphase filter may include more such stages, depending upon the desired accuracy of the phase shift provided. However, as discussed below, conventional implementations of polyphase filters result in substantial attenuation of signals and, therefore, may practically limit the number of stages which may be implemented. 
     Although conventional polyphase filters may be utilized to provide phase shifting suitable for use in many applications, such as the aforementioned single sideband mixer, their use is not without problems. For example, typical implementations of the polyphase filter circuitry of  FIG. 2  results in introduction of a significant amount of attenuation with respect to a signal applied thereto. Such signal attenuation is particularly undesirable as it both reduces the performance of the system as well as results in a more noisy circuit. 
     Signal attenuation associated with polyphase filter  200  results at least in part due to parasitic capacitance attached to every node in the circuit from any point in the circuit to ground. At high frequencies, such parasitic capacitance will cause attenuation through the network. Moreover, techniques that have been used to layout such a filter network in the past have caused the potential for an appreciable amount of attenuation in the network from parasitic capacitance associated with signal path geometry. 
     Directing attention to  FIG. 3 , a typical polyphase filter circuit configuration is shown. Specifically, the stages of multi stage polyphase filter  200  are disposed in columns, wherein a first column includes the resistors and capacitors of the first stage and a second column includes the resistors and capacitors of the second stage. Before, after, and between these columns of resistors and capacitors are routing channels (routing channels  301 – 303 ) that consist of signal feed busses. The number of signal feed busses utilized in each routing channel corresponds to the number of RC circuits (rows) in each stage, here  4 . From these routing channels it is possible to connect the resistors and capacitors in the circuit configuration of  FIG. 2 . However, it should be appreciated that such routing channels contained a high amount of parasitic capacitance which, as described above, results in attenuation at high frequencies. 
     An additional disadvantage of the polyphase filter circuit configuration shown in  FIG. 3  is that circuit must be matched, i.e., the signal paths for each signal component should have a same signal propagation delay and impedance associated therewith, in order to achieve better phase and amplitude matching. An imbalance in signal paths from row to row will degrade the desired 90° phase shift. The routing channels shown in  FIG. 3  consist of four wires wherein the inner wires experience one parasitic environment from line to line, but the outer two lines have a different parasitic environment. Specifically, the internal lines experience parasitic capacitance associated with two neighboring wires whereas the external lines experience parasitic capacitance associated with one neighboring wire each. Accordingly, implementations of the configuration shown in  FIG. 3  include excess routing in order to achieve good matching. 
     Directing attention to  FIG. 4 , an embodiment of a polyphase filter configuration according to the present invention is shown as multi stage polyphase filter  400 . Polyphase filter  400  is configured to provide an RC network, such as may be utilized in single sideband mixer  100 , substantially as that shown in  FIG. 2 . Accordingly, polyphase filter  400  includes resistors  411 – 414  and capacitors  431 – 434  comprising a first stage and resistors  421 – 424  and capacitors  441 – 444  comprising a second stage. However, it should be appreciated that the components of polyphase filter  400  are disposed in a radial pinwheel configuration. Preferred embodiments of such a radial configuration provide symmetry with respect to the polyphase filter circuit and, therefore, facilitate signal path matching. That is, in the preferred embodiment pinwheel structure, each radial segment may be matched such that all radial segments experience the same parasitic environment as each of its neighbors. 
     Moreover, the preferred embodiment radial configuration provides flexibility with respect to positions of nodes at which particular components are coupled, providing flexibility with respect to implementing a desired configuration. For example, it may be desired to provide a slightly different layout than that shown in  FIG. 4 , such as providing a slightly different placement of the resistors of one or more stages, in order to get a better fit. The particular layout chosen may be based upon such consideration such as the geometries of the individual circuit components, e.g., resistors and capacitors, used etcetera. 
     According to a preferred embodiment of the present invention, signals input into the polyphase filter are brought to the center of the radial structure. For example, in the embodiment of  FIG. 4 , input signals Vp and Vn are brought into the center of polyphase filter  400  where they are coupled to resistors  411  and  412  and resistors  413  and  414 , respectively, of the first stage. Similarly, capacitors  431  and  434  and  432  and  433  of the first stage are coupled to the Vp and Vn input signal paths, respectively. In each succeeding stage of capacitors of the preferred embodiment, the output of the capacitors are routed radially, thereby establishing a pinwheel configuration. The signals propagate through the components of the polyphase filter toward the periphery of the structure where signal components I+, Q+, I−, and Q− are output. 
     Preferred embodiment polyphase filter structures dispose input signals toward the center of the radial structure to accommodate a compact design utilizing geometries of the preferred embodiment components. For example, it may be desired to minimize the amount of voltage loss associated with the use of the polyphase filter and, therefore, a circuit geometry may be adopted in which the impedance of circuit components of each successive stage of the polyphase filter are increased. Accordingly, disposing input signals toward the center of the radial structure accommodates a geometry in which the larger impedance capacitors (expected to be the largest components of the filter circuit) are disposed toward the center of the radial structure with smaller impedance capacitors disposed more toward the circumference of the radial structure. Such a configuration is illustrated in  FIG. 4 , wherein larger ones of the components are placed at the center of the structure. It should be appreciated that the illustrated embodiment of  FIG. 4  utilizes the geometries of the components to provide an efficient circuit foot print. 
     Of course, other layouts may be used according to the present invention, if desired. For example, where current loss is to be minimized, a circuit of the present invention may utilize decreased impedance of components of successive stages. Such an alternative embodiment may, for example, dispose the polyphase filter inputs toward the circumference of the radial structure and the outputs towards the center. 
     The radial or spiraling routing is utilized according to the preferred embodiment to provide for small parasitics in the polyphase filter circuit. For example, the distances signals are routed are reduced according to the preferred embodiment radial structure as compared a conventional implementation. Moreover, preferred embodiments of the present invention eliminate the use of routing channels which have substantial parasitics associated therewith due to their parallel line configuration. Accordingly, polyphase filters implementing a radial configuration such as that illustrated in  FIG. 4  have resulting in providing a desired phase shift with only approximately 1 dB of attenuation associated with the polyphase filter circuit, as compared to approximately 7 dB of attenuation as is common with a typical prior art polyphase filter design. 
     It should be appreciated that polyphase filters of the present invention are not limited to the configuration illustrated in  FIG. 4 . For example, there is no limitation to a polyphase filter of the present invention including 2 stages. Directing attention to  FIG. 5 , an alternative embodiment polyphase filter is shown having 3 stages. Specifically, polyphase filter  500  of  FIG. 5  is configured substantially as polyphase filter  400  of  FIG. 4 , having a third stage defined by resistors  511 – 514  and capacitors  521 – 524 . The additional stage of polyphase filter  500  may be utilized to provide desired precision with respect to the phase shift provided thereby where such precision may not be possible with a conventional implementation due to attenuation of the signals propagating through the network. 
     Also shown in the embodiment of  FIG. 5  are buffers  531 – 534 . Specifically, buffers  531 – 534  of the illustrated embodiment are emitter followers placed at the 4 corners of the radial structure polyphase filter to provide buffering of the signals output therefrom. Such buffers may be utilized to reduce the amount of parasitics associated with the last stage of the polyphase filter. Specifically, once the signal passes through the output buffers, a very long signal path may be driven because the impedance on the output line has been reduced. 
     Another alternative embodiment of the present invention is shown in  FIG. 6 . Polyphase filter  600  of  FIG. 6  is configured to provide an RC network in a radial configuration substantially as that shown in  FIG. 4 . Accordingly, polyphase filter  600  includes resistors  611 – 614  and capacitors  631 – 634  comprising a first stage and resistors  621 – 624  and capacitors  641 – 644  comprising a second stage. However, it should be appreciated that the components of polyphase filter  600  are disposed somewhat differently than those of  FIG. 4 , although still providing a radial configuration according to the present invention. Specifically, in the configuration of  FIG. 6 , the resistors of the first and second stages are disposed between the relatively large capacitors of the first stage. Accordingly, it should readily be appreciated that a number of particular layouts may be utilized in providing a radial structure according to the present invention. 
     Although preferred embodiments have been described herein with respect to input signals being provided toward the center of a radial structure, it should be appreciated that the present invention is not so limited. Accordingly, embodiments of the present invention may utilize a radial structure in which inputs are introduced to the circuit at the periphery with outputs thereof appearing toward the center of the radial structure. For example, the circuitry of  FIG. 4  may be utilized in providing a phase combiner, in which case input signals would be introduced along the periphery and the phase combined signals made available toward the center of the radial structure. 
     It should be appreciated that polyphase filters of the present invention may be implemented using a variety of technologies. For example, discrete components may be utilized in providing an RC network according to the present invention. However, a most preferred embodiment of the present invention provides an integrated circuit implementation polyphase filter in which resistors and capacitors thereof are disposed on a common substrate. For example, capacitors utilized in a polyphase filter of the present invention may be provided as conductive plates disposed in strata of a monolithic substrate. The values of such capacitors may be determined through such attributes as size, spacing, and/or coupling a plurality of capacitors in series or parallel, as is well known in the art. Resistors utilized in a polyphase filter of the present invention may be provided, for example, as doped strips disposed in the monolithic substrate. The value of such resistors may be determined through such attributes as the amount of doping, the length of the strip, and/or coupling a series of resistors in series or parallel. 
     Although preferred embodiments of the present invention have been described above with reference to resistors and capacitors of an RC network, there is no limitation to the use of such components according to the present invention. For example, radial circuit structure of the present invention may be utilized with resistors and inductors to provide a pinwheel configuration LC network, if desired. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.