Patent Publication Number: US-2021167746-A1

Title: Distributed varactor network with expanded tuning range

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/660,216, filed on Apr. 19, 2018, and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     A varactor is a variable capacitance diode whose capacitance varies with an applied reverse bias voltage. By changing the value of the applied voltage, the capacitance of the varactor is changed over a given range of values. Varactors are used in many different circuits and applications, including, for example, advanced millimeter wave applications in wireless communications and Advanced Driver Assistance Systems (“ADAS”) that demand higher bandwidth and data rates. The millimeter wave spectrum covers frequencies between 30 and 300 GHz and is able to reach data rates of 10 Gbits/s or more with wavelengths in the 1 to 10 mm range. The shorter wavelengths have distinct advantages, including better resolution, high frequency reuse and directed beamforming that are critical in wireless communications and autonomous driving applications. The shorter wavelengths are, however, susceptible to high atmospheric attenuation and have a limited range (just over a kilometer). 
     Millimeter wave applications, although attracting heightened interest, present significant challenges for device and circuit designers. In particular, the design of varactors for millimeter wave applications suffer from quality factor and tuning range limitations, with the quality factor falling well below desired levels. Varactors having a broad tuning range in millimeter wave are therefore hard to achieve, thereby limiting their use in millimeter wave applications that may require a 360° phase shift to realize their full potential. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present application may be more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, which are not drawn to scale and in which like reference characters refer to like parts throughout, and wherein: 
         FIG. 1  illustrates a schematic diagram of a circuit for increasing the tuning range and phase coverage of an ideal varactor in accordance with various examples; 
         FIG. 2  shows the Smith charts at each reference plane illustrated in the distributed varactor network of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a distributed varactor network for millimeter wave applications in accordance with various examples; 
         FIG. 4  shows the Smith charts at each reference plane illustrated in the distributed varactor network of  FIG. 3 ; 
         FIG. 5  shows a phase shift network incorporating the distributed varactor network of  FIG. 3  to achieve up to a full 360° phase shift; 
         FIG. 6  is a schematic diagram of an example millimeter wave antenna system utilizing the phase shift network of  FIG. 5 ; and 
         FIG. 7  shows a schematic diagram of an array of MTS cells for use in the antenna system of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     A distributed varactor network with an expanded tuning range and phase shift coverage is disclosed. The distributed varactor network is implemented with multiple varactors and other components and is suitable for many different applications, including those in the millimeter wave spectrum. In various examples, the distributed varactor network can be incorporated in a phase shift network design to achieve a full 360° phase shift. The phase shift network integrates multiple distributed varactor networks with Radio Frequency (“RF”) switches to provide any desired phase shift up to a full 360° at considerably lower loss than conventional phase shift networks. 
     In various examples, the phase shift network is implemented in advanced millimeter wave applications in wireless communications, ADAS, and autonomous driving, and in particular, in those applications making use of radiating structures to generate wireless and radar signals having improved directivity and reduced undesired radiation patterns, e.g., side lobes. Such radiating structures may include novel meta-structures (“MTS”) with unprecedented capabilities in manipulating electromagnetic waves as desired. An MTS structure is an engineered structure with electromagnetic properties not found in nature, where the index of refraction may take any value. An MTS structure may be aperiodic, periodic, or partially periodic (semi-periodic.) MTS structures manipulate electromagnetic waves&#39; phase as a function of frequency and spatial distribution and may have a variety of shapes and configurations. MTS structures may be designed to meet certain specified criteria, including, for example, desired beam characteristics. 
     In various examples, the phase shift network is integrated into an MTS-based antenna system that provides smart beam steering and beam forming using MTS radiating structures in a variety of configurations. The phase shift network described herein enables fast scans of up to 360° of an entire environment in a fraction of time of current systems, and supports autonomous driving with improved performance, all-weather/all-condition detection, advanced decision-making and interaction with multiple vehicle sensors through sensor fusion. 
     Autonomous driving applications are enhanced with the phase shift network described herein incorporated in an MTS-based antenna system, enabling long-range and short-range visibility. In an automotive application, short-range is considered to be within 30 meters of a vehicle, such as to detect a person in a cross walk directly in front of the vehicle, while long-range is considered to be 250 meters or more, such as to detect approaching vehicles on a highway. The MTS-based antenna system incorporating the phase shift network enables automotive radars capable of reconstructing the world around them and are effectively a radar “digital eye,” having true 3D vision and human-like interpretation of the surrounding environment. The ability to capture environmental information early aids control of a vehicle, allowing anticipation of hazards and changing conditions. 
     In various examples, an MTS-based antenna system steers a highly-directive RF beam that can accurately determine the location and speed of road objects regardless of weather conditions or clutter in an environment. The MTS-based antenna system can be used in a radar system to provide information for 2D image capability as it measures range and azimuth angle, and to provide distance to an object and azimuth angle identifying a projected location on a horizontal plane. 
     The examples described herein provide enhanced phase shifting of a transmitted RF signal to achieve transmission in the autonomous vehicle range, which in the US is approximately 77 GHz and has a 5 GHz range, specifically, 76 GHz to 81 GHz. The examples described herein also reduce the computational complexity of a radar system and increase its transmission speed. The examples provided accomplish these goals by taking advantage of the properties of MTS structures coupled with novel feed structures. 
     It is appreciated that, in the following description, numerous specific details are set forth to provide a thorough understanding of the examples. However, it is appreciated that the examples may be practiced without limitation to these specific details. In other instances, well-known methods and structures may not be described in detail to avoid unnecessarily obscuring the description of the examples. Also, the examples may be used in combination with each other. 
     Referring now to  FIG. 1 , a schematic diagram of a circuit for increasing the tuning range and phase coverage of an ideal varactor in accordance with various examples is described. Consider an ideal varactor  102 , i.e., a lossless non-linear reactance, with a given capacitance range (e.g., 20 to 80 fF) and no loss (Rs=0Ω). The ideal varactor  102  can provide a phase shift in the range of about 52 to 126 degrees. Note that as an ideal varactor, this phase shift can occur in different spectrums, including a millimeter wave spectrum in the 30 to 300 GHz. In various applications where a full 360° phase shift is desired, this phase shift of the ideal case is not sufficient. 
     Circuit  100  provides a solution to this problem by introducing a distributed varactor network. Distributed varactor network  100  starts by adding a uniform (Z 0 ) transmission line  104  of a quarter of a wavelength, denoted by λ/4, connecting ideal varactor  102  to inductor  106  in parallel with varactor  102 . This creates a variable LC parallel circuit that can result in a purely inductive or purely capacitance reactance based on the value of varactor  102 . At reference plane P 2 , the variable capacitance of ideal varactor  102  is transformed to a variable inductance with inductor  106 . 
     The distributed varactor network  100  continues with the addition of another ideal varactor, varactor  108 , that is identical to ideal varactor  102 . This results in a parallel LC tank circuit, such that at reference plane P 3 , the tank circuit can behave either purely inductive, purely capacitive or have a resonance that depends on the value of the inductance L of inductor  106  and the capacitance C of varactors  102  and  108 . 
     With the addition of another varactor to the distributed varactor network  100 , varactor  110 , in series with the parallel tank LC circuit formed by varactors  102  and  108  and inductor  104 , at reference plane P 4 , the distributed varactor network  100  behaves as either purely inductive or purely capacitive. The resulting network  100  forms a series LC or series CC circuit that results in a full 360° phase coverage in a Smith chart as well as a large variable reactance range. 
       FIG. 2  shows the Smith charts at each reference plane illustrated in the distributed varactor network of  FIG. 1 . Smith charts  200  include a Smith chart  202  corresponding to reference plane P 1  of  FIG. 1 , a Smith chart  204  corresponding to reference plane P 2  of  FIG. 1 , a Smith chart  206  corresponding to reference plane P 3  of  FIG. 1 , and a Smith chart  208  corresponding to reference plane P 4  of  FIG. 1 . Note that the phase coverage range shown in Smith chart P 1  corresponds to the phase coverage range of the varactor  102 , an ideal varactor with approximate phase coverage in the 52 to 126 degrees range. At P 2 , the inductor  106  introduces a phase shift as shown in Smith chart  204 . The addition of the ideal varactor  108  in parallel with LC circuit  102 - 106  results in an expanded phase coverage shown in Smith chart  206 . With the varactor  110  placed in series with the LC tank circuit, the phase coverage of the distributed varactor network  100  corresponds to a full 360° as shown in Smith chart  208 . As described above, this is highly desirable for many new millimeter wave applications, including autonomous driving applications where a full 360° phase shift enables object detection in a full field of view from the vehicle. 
     Note, however, that the distributed varactor network  100  achieves the full 360° phase shift in the ideal varactor case. Actual varactors designed for millimeter wave applications suffer from quality factor and tuning range limitations. The tuning range of a millimeter wave varactor is in reality much smaller than that of ideal varactors  102 ,  108  and  110 . In the case of millimeter wave varactors, a different design for a distributed varactor network is needed to achieve broader phase shifts. 
     Attention is now directed to  FIG. 3 , which shows a schematic diagram of a distributed varactor network for millimeter wave applications. Distributed varactor network  300  is designed with varactors that have limited tuning range and quality factors at millimeter waves. In various examples, the varactors are GaAs varactors. In other examples, the varactors can be silicon varactors or other such material. The goal of the distributed varactor network  300  is to expand the tuning range and phase coverage that can be achieved by varactors in millimeter wave applications. 
     Distributed varactor network  300  achieves this by having distributed phase shifting elements interspersed with varactors and quarter wave transmission line sections. The network  300  starts with varactors  302   a - b , which have, for example, low quality factors Q of around 5-6 and a capacitance range of around 37-72 fF in millimeter wave applications. This low Q is a limiting factor in achieving broader phase shifts in millimeter wave applications. 
     To address this challenge, a 3 dB, 90° hybrid line coupler  304  having wave sections  306   a - b  of λ/4 is coupled to varactors  302   a - b . The hybrid line coupler  304  is a four-port device (labelled as ports  1 - 4  in  FIG. 3 ) that can split a signal equally into two output ports having a 90° phase shift between them, or that can combine two signals while maintaining high isolation between the ports. The hybrid line coupler  304  together with varactors  302   a - b  results in a parallel LC circuit. 
     Adding another hybrid line coupler coupled to two more varactors, this time a 3 dB, 45° hybrid line coupler  308  with wave sections  310   a - b  of λ/8 coupled to varactors  312   a - b  with a capacitance range of around 18-33 fF, results in a further increase of phase coverage as it provides another additional series LC-network with the parallel LC circuit formed by coupler  304  and varactors  302   a - b.    
     The behavior of distributed varactor network  300  can be further understood with reference to  FIG. 4 , which shows the Smith charts at each reference plane illustrated in  FIG. 3 . Smith charts  400  include a Smith chart  402  corresponding to reference plane P 1  of  FIG. 3 , a Smith chart  404  corresponding to reference plane P 2  of  FIG. 3 , and a Smith chart  406  corresponding to reference plane P 3  of  FIG. 3 . Smith chart  402  shows the limited phase range of varactors  302   a - b  with hybrid coupler  304 . The phase range achieved from the hybrid coupler  304  is only about 20°. Adding varactors  312   a - b  increases the phase shift range to about 55° at reference plane P 2 , as shown in Smith chart  404 . With hybrid coupler  308 , the phase shift range increases at reference plane P 3  by another 55°, thereby resulting in an overall phase shift range achieved by distributed varactor network of about 110°, as shown in Smith chart  406 . 
     It is appreciated that distributed varactor network  300  can be cascaded with other distributed varactor networks  300  to expand the phase shift range from about 120° to even higher values. However, doing so will result in further loss, which may not be desirable in millimeter wave applications. Distributed varactor network  300  has a loss of up to 6 dB. Cascading another distributed varactor network to it will add another 6 dB. 
     It is also appreciated that differences in varactor and hybrid coupler implementations (e.g., use of ¼ wave section instead of the ⅛ wave section in coupler  308 ), may result in variations in their specifications, which may result in variations in the phase shift range achievable by distributed varactor network  300 . For example, simulations have shown that phase shift ranges of 120° or more may be achievable with distributed varactor network  300 . 
     Attention is now directed at  FIG. 5 , which shows a phase shift network incorporating the distributed varactor network of  FIG. 3  to achieve up to a full 360° phase shift. Phase shift network system  500  has a phase shift network  502  composed of three distributed varactor networks  504   a - c . Each one of the distributed varactor networks  504   a - c  is capable of achieving phase shift ranges of up to 120° and may be implemented, for example, as the distributed varactor network  300  of  FIG. 3 . In various examples, distributed varactor network  504   a  is capable of achieving phase shifts from 0 to 120°, distributed varactor network  504   b  is capable of achieving phase shifts from 120° to 240°, and distributed varactor network  504   c  is capable of achieving phase shifts from 240° to 360°. 
     The phase shift network  502  can be integrated with two 3-way RF switches, such as for example, SP3T switches  506  and  508 . The switches  506 - 508  can be designed to have a loss of up to approximately 2.5 dB each. Since each distributed varactor network  504   a - c  has a loss of up to 6 dB at a frequency of 77 GHz, the phase shift network circuit  500  has a loss of up to 10-11 dB, which is significantly lower than the 18-20 dB loss typically experienced by conventional phase shift networks. The phase shift network circuit  500  is therefore capable of providing a full 360° phase shift range at a low loss in the millimeter wave spectrum, which as described above, is required to realize the full potential of many millimeter wave applications, including in autonomous driving where accurate object detection and classification are imperative. 
     Referring now to  FIG. 6 , a schematic diagram of an example millimeter wave antenna system utilizing the phase shift network of  FIG. 5  is described. Antenna system  600  includes modules such as radiating structure  632  coupled to an antenna controller  614 , a central processor  602 , and a transceiver  612 . A signal is provided to antenna system  600  and the transmission signal controller  610  may act as an interface, translator or modulation controller, or otherwise as required for the signal to propagate through antenna system  600 . 
     In various examples, the transmission signal controller  610  generates a transmission signal, such as a Frequency Modulated Continuous Wave (“FMCW”), which is used for example, in radar or other applications as the transmitted signal is modulated in frequency, or phase. The FMCW signal enables radar to measure range to an object by measuring the phase differences in phase or frequency between the transmitted signal and the received signal, or the reflected signal. Other modulation types may be incorporated according to the desired information and specifications of a system and application. Within FMCW formats, there are a variety of modulation patterns that may be used within FMCW, including triangular, sawtooth, rectangular and so forth, each having advantages and purposes. For example, sawtooth modulation may be used for large distances to a target; a triangular modulation enables use of the Doppler frequency, and so forth. The received information is stored in a memory storage unit  608 , wherein the information structure may be determined by the type of transmission and modulation pattern. 
     In operation, the antenna controller  614  receives information from other modules in antenna system  600  indicating a next radiation beam, wherein a radiation beam may be specified by parameters such as beam width, transmit angle, transmit direction and so forth. The antenna controller  614  determines a voltage matrix to apply to capacitance control mechanisms coupled to the radiating structure  632  to achieve a given phase shift. The transceiver  612  prepares a signal for transmission, such as a signal for a radar device, wherein the signal is defined by modulation and frequency. The signal is received by each element of the radiating structure  632  and the phases of radiating patterns generated by the radiating array structure  626  is controlled by the antenna controller  614 . 
     In various examples, transmission signals are received by a portion, or subarray, of the radiating array structure  626 . These radiating array structures  626  are applicable to many applications, including radar in autonomous vehicles to detect objects in the environment of the car, or in wireless communications, medical equipment, sensing, monitoring, and so forth. Each application type incorporates designs and configurations of the elements, structures and modules described herein to accommodate their needs and goals. 
     Radiating structure  632  includes a feed distribution module  618  coupled to a transmission array structure  624  for transmitting signals through radiating array structure  626 , which generates controlled radiation beams that may then be reflected back and ultimately analyzed by an AI module  606  and other sensor modules (not shown) in antenna system  600  for object detection and identification (e.g., in an autonomous driving application). An interface to sensor fusion module  604  interfaces with other sensor modules in antenna system  600  and a sensor fusion module (not shown) that processes the data from antenna system  600  and other sensors to detect and locate objects and provide an understanding of the surrounding environment. It is appreciated that antenna controller  614  may receive signals in response to processing of previous signals by AI module  606  or interface to sensor fusion module  604 , or it may receive signals based on program information from memory storage unit  608 . 
     The feed distribution module  618  has an impedance matching element  620  and a reactance control element  622 . The impedance matching element  620  and the reactance control element  622  may be positioned within the architecture of feed distribution module  618 . Alternatively, one or both of impedance matching element  620  and reactance control element  622  may be external to the feed distribution module  618  for manufacture or composition as an antenna or radar module. The impedance matching element  620  works in coordination with the reactance control element  622  to provide phase shifting of the radiating signal(s) from radiating array structure  626 . In various examples, reactance control element  622  includes a reactance control mechanism controlled by antenna controller  614 , which may be used to control the phase of a radiating signal from radiating array structure  16 . Reactance control module may, for example, include a phase shift network system such as phase shift network system shown in  FIG. 5  to provide any desired phase shift up to 360°. 
     As illustrated, radiating structure  632  includes the radiating array structure  626 , composed of individual radiating cells such as cell  630  and discussed in more detail herein below with reference to  FIG. 7 . The radiating array structure  626  may take a variety of forms and is designed to operate in coordination with the transmission array structure  624 , wherein individual radiating cells (e.g., cell  630 ) correspond to elements within the transmission array structure  624 . As illustrated, the radiating array structure  626  is an array of unit cell elements, wherein each of the unit cell elements has a uniform size and shape; however, some examples may incorporate different sizes, shapes, configurations and array sizes. When a transmission signal is provided to the radiating structure  632 , such as through a coaxial cable or other connector, the signal propagates through the feed distribution module  618  to the transmission array structure  624  and then to radiating array structure  626  for transmission through the air. 
     Attention is now directed at  FIG. 7 , which shows a schematic diagram of an array of MTS cells such as array  628  of  FIG. 6 . Array  700  contains multiple MTS cells positioned in one or more layers of a substrate and coupled to other circuits, modules and layers, as desired and depending on the application. In some examples, the MTS cells are metamaterial cells in a variety of conductive structures and patterns, such that a received transmission signal is radiated therefrom. Each metamaterial cell may have unique properties. These properties may include a negative permittivity and permeability resulting in a negative refractive index; these structures are commonly referred to as left-handed materials (“LHM”). The use of LHM enables behavior not achieved in classical structures and materials, including interesting effects that may be observed in the propagation of electromagnetic waves, or transmission signals. Metamaterials can be used for several interesting devices in microwave and terahertz engineering such as antennas, sensors, matching networks, and reflector used in telecommunications, automotive and vehicular, robotic, biomedical, satellite and other applications. For antennas, metamaterials may be built at scales much smaller than the wavelengths of transmission signals radiated by the metamaterial. Metamaterial properties come from the engineered and designed structures rather than from the base material forming the structures. Precise shape, dimensions, geometry, size, orientation, arrangement and so forth result in the smart properties capable of manipulating electromagnetic waves by blocking, absorbing, enhancing, or bending waves. The MTS cells in array  700 , such as MTS cell  702  may be arranged as shown or in any other configuration, such as, for example, in a hexagonal lattice. 
     MTS cell  702  is illustrated having a conductive outer portion or loop  704  surrounding a conductive area  706  with a space in between. Each MTS cell  702  may be configured on a dielectric layer, with the conductive areas and loops provided around and between different MTS cells. A voltage controlled variable reactance device  708 , e.g., a varactor, provides a controlled reactance between the conductive area  706  and the conductive loop  704 . The controlled reactance is controlled by an applied voltage, such as an applied reverse bias voltage in the case of a varactor. The change in capacitance changes the behavior of the MTS cell  702 , enabling the MTS array  700  to provide focused, high gain beams directed to a specific location. It is appreciated that additional circuits, modules and layers may be integrated with the MTS array  700 . 
     It is appreciated that antenna system  600  of  FIG. 6  (with, for example, MTS array  700  as radiating array structure  628  and phase shift network system  500  incorporated in reactance control element  622 ) is applicable in wireless communication and radar applications, and in particular in MTS structures capable of manipulating electromagnetic waves using engineered radiating structures. It is also appreciated that antenna system  600  is capable of generating wireless signals, such as radar signals, having improved directivity, reduced undesired radiation patterns aspects, such as side lobes. Further, antenna system  600  is able to scan an entire environment in a fraction of the time of current systems. Antenna system  600  provides smart beam steering and beam forming using MTS radiating structures in a variety of configurations, wherein electrical changes to the antenna are used to achieve phase shifting and adjustment reducing the complexity and processing time and enabling fast scans of up to approximately 360° field of view for long range object detection. 
     It is further appreciated that antenna system  600  supports autonomous driving with improved sensor performance, all-weather/all-condition detection, advanced decision-making algorithms and interaction with other sensors through sensor fusion. These configurations optimize the use of radar sensors, as radar is not inhibited by weather conditions in many applications, such as for self-driving cars. The ability to capture environmental information early aids control of a vehicle, allowing anticipation of hazards and changing conditions. Antenna system  600  enables automotive radars capable of reconstructing the world around them and are effectively a radar “digital eye,” having true 3D vision and capable of human-like interpretation of the world, aided by the 360° phase shift provided by phase shift network system  500  of  FIG. 5  integrated into antenna system  600 . 
     It is appreciated that the previous description of the disclosed examples is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these examples will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other examples without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the examples shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.