Patent Publication Number: US-11050497-B2

Title: Wide band radio-frequency localization devices and associated systems and methods

Description:
RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 120 and is a continuation of U.S. patent application Ser. No. 16/537,822, titled “WIDE BAND RADIO-FREQUENCY LOCALIZATION DEVICES AND ASSOCIATED SYSTEMS AND METHODS”, filed Aug. 12, 2019, which claims the benefit under 35 U.S.C. § 120 and is a continuation of U.S. patent application Ser. No. 16/121,422, titled “WIDE BAND RADIO-FREQUENCY LOCALIZATION DEVICES AND ASSOCIATED SYSTEMS AND METHODS”, filed Sep. 4, 2018, which claims the benefit under 35 U.S.C. § 120 and is a continuation of U.S. patent application Ser. No. 15/954,968, titled “WIDE BAND RADIO-FREQUENCY LOCALIZATION DEVICES AND ASSOCIATED SYSTEMS AND METHODS”, filed on Apr. 17, 2018, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application Ser. No. 62/629,581, titled “WIDE BAND RADIO-FREQUENCY LOCALIZATION DEVICES AND ASSOCIATED SYSTEMS AND METHODS”, filed on Feb. 12, 2018, each of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The ability to accurately determine the location of an object or target has potential benefits for numerous applications. Some exemplary applications benefitting from object localization include motion tracking, virtual reality, gaming, autonomous systems, robotics, etc. A number of technologies have been pursued that seek to provide localization, including global positioning system (GPS) technology, received signal strength indicator (RSSI) measurements, optical image data processing techniques, infrared ranging, etc. Generally, these conventional approaches are limited in application due to one or more deficiencies, including relatively poor or insufficient accuracy and/or precision, computational complexity resulting in relatively long refresh rates, environmental limitations (e.g., operation limited to outdoors, cellular or network access requirements and/or vulnerability to background clutter or noise), cost, size, etc. 
     SUMMARY 
     Some embodiments provide for a device, comprising: a radio frequency (RF) coupler comprising an input port, an output port and a coupled port; an antenna capable of receiving RF signals having a first center frequency and transmitting RF signals having a second center frequency different from the first center frequency, the antenna connected to the RF coupler to provide received RF signals to the input port of the RF coupler and to transmit RF signals received at the input port via coupling to signals received at the coupled port; and signal transformation circuitry having an input connected to the output port of the RF coupler to receive RF signals provided by the antenna to the input port and an output connected to the coupled port, the signal transformation circuitry configured to transform first RF signals having the first center frequency received from the output port to second RF signals having the second center frequency and to provide the second RF signals to the coupled port. 
     Some embodiments provide for a method performed by a device comprising a radio frequency (RF) antenna, an RF coupler having an input port, and output port, and a coupled port, and signal transformation circuitry. The method comprises: receiving, via the RF antenna, a wireless RF signal having a first center frequency; providing, from the RF antenna to the input port of the RF coupler, a first RF signal generated based on the wireless RF signal and having the first center frequency; and generating, using the RF coupler and based on the first RF signal, a second RF signal having the first center frequency and providing the second RF signal to the signal transformation circuitry via the output port of the RF coupler; generating, using the signal transformation circuitry and based on the second RF signal, a third RF signal having the second center frequency and providing the third RF signal to the coupled port of the RF coupler; and generating, using the RF coupler, a fourth RF signal having the second center frequency and providing the fourth RF signal to the RF antenna via the input port of the RF coupler. 
     In some embodiments, the RF coupler comprises: a main line having the input port at a first end and the output port at a second end; and a coupled line having the coupled port at a first end and an isolated port at a second end. 
     In some embodiments, the antenna is connected to the RF coupler to provide received RF signals to the input port of the RF coupler and to transmit RF signals received from the input port via coupling of the main line and the coupled line. 
     In some embodiments, the main line and the coupled line are coupled transmission lines. In some embodiments, the main line and the coupled line comprise striplines. In some embodiments, the main line and the coupled line comprise microstrips. In some embodiments, the main line and the coupled line have a coupling loss of 5-15 dB. 
     In some embodiments, the antenna is configured to receive, from an interrogator device different from the device, RF signals having the first center frequency and to transmit, to the interrogator device, RF signals having the second center frequency different from the first center frequency. 
     In some embodiments, the antenna is configured to receive RF signals in a range of 4.0-7.5 GHz and transmit RF signals in a range of 8.0-15 GHz. 
     In some embodiments, the antenna is configured to receive C-band RF signals and transmit X-band RF signals. In some embodiments, the antenna is configured to receive RF signals in a range of 50-70 GHz and transmit RF signals in a range of 100-140 GHz. 
     In some embodiments, the antenna comprises an Archimedean spiral antenna, an exponential spiral antenna, a sinuous antenna, or a log-periodic antenna. In some embodiments, the antenna comprises a circularly polarized antenna. In some embodiments, the antenna is a single-port antenna. 
     In some embodiments, the signal transformation circuitry comprises at least one frequency multiplier. In some embodiments, the signal transformation circuitry is configured to transform first RF signals having the first center frequency received to second RF signals having the second center frequency that is a harmonic of the first center frequency. In some embodiments, the harmonic is the first harmonic of the first center frequency. 
     Some embodiments, provide for an interrogator device, comprising: a radio frequency (RF) coupler comprising an input port, an output port and a coupled port; an antenna capable of transmitting RF signals having a first center frequency and transmitting RF signals having a second center frequency different from the first center frequency, the antenna connected to the RF coupler to provide received RF signals to the input port of the RF coupler and to transmit RF signals received at the input port via coupling to signals received at the coupled port; transmit circuitry having an output connected to the coupled port of the RF coupler, the transmit circuitry configured to generate the RF signals having the first center frequency for transmission by the antenna; and receive circuitry having an input connected to the output port of the RF coupler, the receive circuitry configured to obtain, via the output port of the RF coupler, a version of the RF signals having the second center frequency received by the antenna. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Various aspects and embodiments will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. 
         FIG. 1  shows an illustrative system  100  that may be used to implement radio frequency (RF) localization techniques, in accordance with some embodiments of the technology described herein. 
         FIG. 2  shows illustrative components of an interrogator device and a target device, which are part of the illustrative system  100  shown in  FIG. 2 , in accordance with some embodiments of the technology described herein. 
         FIG. 3A  is a diagram of an illustrative target device, in accordance with some embodiments of the technology described herein. 
         FIG. 3B  is a diagram showing illustrative components of signal transformation circuitry of the target device shown in  FIG. 3A , in accordance with some embodiments of the technology described herein. 
         FIG. 4A  is a diagram of an illustrative interrogator device, in accordance with some embodiments of the technology described herein. 
         FIG. 4B  is a diagram showing illustrative components of transmit and receive circuitry of the interrogator device shown in  FIG. 4A , in accordance with some embodiments of the technology described herein. 
         FIG. 4C  is a diagram of another illustrative interrogator device, in accordance with some embodiments of the technology described herein. 
         FIG. 5  is a diagram of an illustrative RF coupler, in accordance with some embodiments of the technology described herein. 
         FIG. 6  is a flowchart of an illustrative method, performed by a target device, of providing an RF signal having a second center frequency to an interrogator device in response to receiving an RF signal having a first center frequency from the interrogator device, in accordance with some embodiments of the technology described herein. 
         FIG. 7A  is a diagram of an illustrative mount for mounting an RF antenna to a substrate, in accordance with some embodiments of the technology described herein. 
         FIG. 7B  is a cutaway view of the illustrative mount of  FIG. 7B , in accordance with some embodiments of the technology described herein. 
         FIG. 7C  is a diagram of an illustrative alignment device for attaching an RF antenna to the illustrative mount of  FIG. 7A , in accordance with some embodiments of the technology described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Determining the location of an object or target (also referred to herein as localization) has an array of applications in a number of fields. For example, the ability to locate and/or track an object at very small scales (i.e., at high resolutions) facilitates advancement of numerous applications, and has wide spread applicability to a number of different fields. For example, the ability to accurately and precisely track an object or target in real-time has numerous benefits in the gaming industry and, in particular, for interactive video games. Object localization also has many applications in autonomous systems, such as autonomous vehicle navigation, exploration, robotics and human machine interaction. Virtual reality, motion tracking and capture and the like also benefit from the ability to locate and/or track object(s) accurately, robustly and in real-time or near real-time. Details of using localization techniques in a variety of applications are described in U.S. patent application Ser. No. 15/181,930 titled “High-Precision Time of Flight Measurement Systems,” filed on Jun. 14, 2016, U.S. patent application Ser. No. 15/181,956 titled “High Precision Motion Tracking with Time of Flight Measurement Systems,” filed on Jun. 14, 2016, U.S. patent application Ser. No. 15/181,978 titled “High Precision Subsurface Imaging and Location Mapping with Time of Flight Measurement Systems,” filed on Jun. 14, 2016, and U.S. Pat. No. 15,181,999 titled “High-Precision Time of Flight Measurement System for Industrial Automation,” filed on Jun. 14, 2016, each of which is herein incorporated by reference in its entirety. 
     For many applications, high localization accuracy (resolution) is an important capability and is a limiting factor for many conventional localization techniques. The inventors have developed localization techniques with the ability to resolve the location of a target in the millimeter and sub-millimeter range, referred to herein generally as micro-localization. According to some embodiments, a micro-localization system comprises an interrogator configured to transmit a radio frequency (RF) signal (e.g., a microwave or millimeter wave RF signal) and a target device configured to, in response to receiving the RF signal, transmit an RF signal to be received by the interrogator. The RF signal received from the target device by the interrogator is used to determine the distance between the interrogator and the target device. 
     In some micro-location systems, the interrogator and target device may each include a pair of RF antennas to communicate with the other device. For example, the interrogator may have a transmit and a receive antenna and the target device may have its own transmit and receive antenna. The interrogator may use its transmit antenna to transmit an RF signal, which the target device may receive using its receive antenna. In response to receiving the RF signal, the target device may use its transmit antenna to transmit a responsive RF signal to be received by the interrogator&#39;s receive antenna. 
     Multi-path errors present a significant challenge to achieving micro-localization in many, if not most, of the environments where micro-location systems are utilized. To mitigate multi-path errors, the bandwidth of the system may be increased. However, increasing the bandwidth brings its own set of challenges that must be addressed. The inventors have developed techniques to facilitate wide band micro-localization. In particular, the inventors have developed antenna, transmit and receive systems configured for wide band operation, thus allowing for significant mitigation of multi-path errors, examples of which are described in further detail below. 
     The inventors have recognized that micro-location systems may be improved if the target device and the interrogator device were each to utilize a single RF antenna to communicate with the other device. Utilizing a single RF antenna on a target device instead of multiple RF antennas has a number of advantages. (Using a single RF antenna on an interrogator device instead of multiple RF antennas has similar advantages). First, using a single RF antenna reduces the cost of manufacturing the target device. Second, using a single RF antenna improves the sensitivity of the target device because it eliminates any interference and/or coupling that would have existed between the receive and transmit antennas. For example, in some embodiments where the target device receives RF signals having a first center frequency and transmits RF signals having a second center frequency that is a harmonic center frequency, there may be harmonic coupling between separate transmit and receive antennas, which reduces their sensitivity. Reducing the sensitivity of the target device&#39;s receive sensitivity due to the presence of interference (e.g., due to harmonic coupling) would reduce the range at which distance to and/or the location of the target device could be precisely determined. This problem would be eliminated with the use of a single RF antenna on the target device. 
     Third, using a single RF antenna on the target device would facilitate high-precision localization. After transmitting an RF signal from an interrogator device to the single RF antenna on the target device and receiving a responsive RF signal from that single RF antenna, the transmitted RF signal and the responsive RF signal may be used to determine the distance from the interrogator device to the phase center of the single RF antenna. Since an RF antenna may be much smaller than the size of the target device, this facilitates determining the distance(s) from one or more interrogators to the phase center of the single RF antenna with high accuracy, which in turn allows for the determination of the location of the RF antenna&#39;s phase center with high resolution. 
     Accordingly, some embodiments provide for a target device having a single RF antenna configured to receive RF signals from one or more interrogator devices and transmit responsive RF signals to the interrogator device(s). Additionally or alternatively, in some embodiments, the interrogator device may also have a single RF antenna for communicating with one or more target devices. 
     In order to reduce the effect of clutter on the performance of the micro-localization system and to increase the system&#39;s sensitivity, in some embodiments, a target device may be configured to receive RF signals having a first center frequency and respond with RF signals having a second center frequency different from the first center frequency. The second center frequency may be a harmonic of (e.g., a first harmonic) of the first center frequency. For example, a target device may be configured to receive RF signals having a center frequency in the range of 4-7.5 GHz and respond with RF signals having a center frequency in the range of 8-15 GHz. To this end, a target device may have signal transformation circuitry configured to transform RF signals having the first center frequency to RF signals having the second center frequency. 
     In some embodiments, in order to use a single wideband RF antenna with the above-described signal transformation circuitry, the target device may use an RF coupler comprising an input port, an output port, and a coupled port. 
     In such embodiments, the target device may comprise: (1) an RF coupler comprising an input port, an output port, and a coupled port; (2) an RF antenna (e.g., a single-port RF antenna) capable of receiving RF signals having a first center frequency (e.g., a C-band signal) and transmitting RF signals having a second center frequency different from the first center frequency (e.g., an X-band signal), the antenna connected to the RF coupler to provide received RF signals to the input port of the RF coupler and to transmit RF signals received at the input port via coupling to signals received at the coupled port; and (3) signal transformation circuitry having an input connected to the output port of the RF coupler to receive RF signals provided by the antenna to the input port and an output connected to the coupled port, the signal transformation circuitry configured to transform first RF signals having the first center frequency received from the output port to second RF signals having the second center frequency and to provide the second RF signals to the coupled port. 
     In the above described configuration, which is further illustrated and discussed in detail below including with reference to  FIGS. 3A and 3B , no loss is incurred by the coupler when receiving RF signals from one or more interrogator devices, which maximizes the range at which the micro location system can locate the target device with high resolution. Any losses that are incurred through the coupling may be addressed through amplification on the target device, as described herein, and would not impact the range of the micro-location system. By contrast, using a splitter instead of an RF coupler, would incur at least a 3 dB loss on the received RF signals thereby reducing the maximum range at which the micro location system could locate the target device with high resolution. 
     It should be appreciated that the techniques introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the techniques are not limited to any particular manner of implementation. Examples of details of implementation are provided herein solely for illustrative purposes. Furthermore, the techniques disclosed herein may be used individually or in any suitable combination, as aspects of the technology described herein are not limited to the use of any particular technique or combination of techniques. 
       FIG. 1  shows an illustrative micro-localization system  100  that may be used to implement radio frequency (RF) localization techniques, in accordance with some embodiments of the technology described herein. Micro-localization system  100  comprises a plurality of interrogator devices  102 , one or more of which are configured to transmit an RF signal  103  (e.g., RF signals  103   a ,  103   b ,  103   c , etc.). System  100  also comprises one or more target devices  104  configured to receive RF signals  103  and, in response, transmit RF signals  105  (e.g., RF signals  105   a ,  105   b  and  105   c , etc.). Interrogator devices  102  are configured to receive RF signals  105  that are then used to determine distances between respective interrogator and target devices. The computed distances may be used to determine the location of one or more target devices  104 , a number of techniques of which are described in further detail below. It should be appreciated that while multiple target devices  104  are illustrated in  FIG. 1A , a single target device may be utilized. More generally, it should be appreciated that any number of interrogator devices  102  and target devices  104  may be used, as the aspects of the technology described herein are not limited in this respect. 
     Micro-localization system  100  may also include a controller  106  configured to communicate with interrogator devices  102  and target devices  104  via communication channel  108 , which may include a network, device-to-device communication channels, and/or any other suitable means of communication. Controller  106  may be configured to coordinate the transmission and/or reception of RF signals  103  and  105  between desired interrogator and target devices via communication channels  107   a ,  107   b  and  108 , which may be a single communication channel or include multiple communication channels. Controller  106  may also be configured to determine the location of one or more target devices  104  from information received from interrogator devices  102 . Controller  106  may be implemented as a standalone controller or may be implemented in full or in part by one or more interrogator devices  102  and/or target devices  104 . Different exemplary configurations and implementations for micro-localization system  100  are described in further detail below, but are not limited to the configurations discussed herein. 
     Resolving the location of a target with a high degree of accuracy depends in part on receiving the RF signals transmitted by the target with high fidelity and, in part, on the ability to distinguish the RF signals transmitted by a target device from RF signals transmitted by an interrogator device, background clutter, and/or noise. The inventors have developed techniques for improving the signal-to-noise ratio of the signals received by the interrogator and target devices to facilitate micro-localization of one or more target devices. As one example, the inventors recognized that by configuring the interrogator and target devices to transmit at different frequencies, localization performance could be improved. According to some embodiments, one or more interrogator devices transmit first RF signals (e.g., RF signals  103 ) having a first center frequency and, in response to receiving the first RF signals, one or more target devices transmit second RF signals (e.g., RF signals  105 ) having a second center frequency different from the first center frequency. In this manner, receive antennas on the one or more interrogator devices can be configured to respond to RF signals about the second center frequency, improving the ability of the interrogator devices to receive RF signals from target devices in cluttered and/or noisy environments. 
     The inventors further recognized that relatively simple and/or cost effective circuitry could be implemented at the target device to transform RF signals having a first center frequency received from an interrogator device to RF signals having a second center frequency different from the first center frequency for transmission. According to some embodiments, the second center frequency is harmonically related to the first center frequency. For example, in system  100  illustrated in  FIG. 1A , a target device  104  may be configured to transform RF signals  103  and transmit RF signals  105  at a harmonic (e.g., a first harmonic, a second harmonic, etc.) of the center frequency of the received RF signal  103 . According to other embodiments, a target device transforms RF signals having a first center frequency received from an interrogator device to RF signals having second center frequency that is different from, but not harmonically related to the first center frequency. In other embodiments, a target device is configured to generate RF signals at a second center frequency different from the first center frequency, either harmonically or not harmonically related, rather than transforming RF signals received from an interrogator device. Exemplary techniques for transmitting RF signals, from interrogator and target devices, having different respective center frequencies are discussed in more detail herein. 
       FIG. 2  shows illustrative components of an illustrative interrogator device  102  and a illustrative target device  104 , which are part of the illustrative system  100  shown in  FIG. 1 , in accordance with some embodiments of the technology described herein. As shown in  FIG. 2 , illustrative interrogator device  102  includes waveform generator  110 , transmit and receive circuitry  112 , RF coupler  114 , antenna  115 , control circuitry  118 , and external communications module  120 . It should be appreciated that, in some embodiments, an interrogator device may include one or more other components in addition to or instead of the components illustrated in  FIG. 2 . 
     In some embodiments, waveform generator  110  may be configured to generate RF signals to be transmitted by the interrogator  102  using antenna  115 . Waveform generator  110  may be configured to generate any suitable type(s) of RF signals. In some embodiments, waveform generator  110  may be configured to generate frequency modulated RF signals, amplitude modulated RF signals, and/or phase modulated RF signals. Non-limiting examples of modulated RF signals, any one or more of which may be generated by waveform generator  110 , include linear frequency modulated signals (also termed “chirps”), non-linearly frequency modulated signals, binary phase coded signals, signals modulated using one or more codes (e.g., Barker codes, bi-phase codes, minimum peak sidelobe codes, pseudo-noise (PN) sequence codes, quadri-phase codes, poly-phase codes, Costas codes, Welti codes, complementary (Golay) codes, Huffman codes, variants of Barker codes, Doppler-tolerant pulse compression signals, impulse waveforms, noise waveforms, and non-linear binary phase coded signals. Waveform generator  110  may be configured to generate continuous wave RF signals or pulsed RF signals. Waveform generator  110  may be configured to generate RF signals of any suitable duration (e.g., on the order of microseconds, milliseconds, or seconds). 
     In some embodiments, waveform generator  110  may be configured to generate microwave and/or millimeter wave RF signals. For example, waveform generator  110  may be configured to generate RF signals having a center frequency in a given range of microwave and/or millimeter frequencies (e.g., 4-7.5 GHz, 50-70 GHz). It should be appreciated that an RF signal having a particular center frequency is not limited to containing only that particular center frequency (the RF signal may have a non-zero bandwidth). For example, waveform generator  110  may be configured to generate a chirp having a center frequency of 60 GHz whose instantaneous frequency varies from a lower frequency (e.g., 59 GHz) to an upper frequency (e.g., 61 GHz). Thus, the generated chirp has a center frequency of 60 GHz and a bandwidth of 2 GHz and includes frequencies other than its center frequency. 
     As another example, waveform generator  110  may be configured to generate a chirp having a center frequency of 5.75 GHz whose instantaneous frequency varies from a lower frequency (e.g., 4 GHz) to an upper frequency (e.g., 7.5 GHz). Thus, the generated chirp has a center frequency of 5.75 GHz and a bandwidth of 3.5 GHz and includes frequencies other than its center frequency. 
     In some embodiments, waveform generator  110  may be configured to generate RF signals having a lower center frequency than the RF signals transmitted by antenna  115 . In such embodiments, the transmit and receive circuitry may transform the RF signals generated by the waveform generator (e.g., using one or more frequency multipliers in conjunction with one or more amplifiers to counteract losses incurred by the multipliers) to have the desired frequency content upon transmission. For example, waveform generator  110  may generate RF signals having a center frequency of 7.5 GHz, which may be processed by three frequency doublers to obtain RF signals having a center frequency of 60 GHz. 
     In some embodiments, transmit and receive circuitry  112  may be configured to provide RF signals generated by waveform generator  110  to RF coupler  114 , which in turn provides the RF signals to antenna  115 . For example, transmit and receive circuitry  112  may provide RF signals to the coupled port of RF coupler  114  and the RF coupler may provide these RF signals to antenna  115  through its input port via electromagnetic coupling. In this manner, antenna  115  transmits RF signals received by the antenna  115  at the input port of the RF coupler  114  via coupling to RF signals received (from the transmit and receive circuitry  114 ) at the coupled port of RF coupler  114 . Such a configuration is described in more detail herein including with reference to  FIGS. 4A-4C . 
     Additionally, transmit and receive circuitry  112  may be configured to obtain and process RF signals received by the antenna  115  and provided to the transmit and receive circuitry  112  through the RF coupler  114 . For example, antenna  115  may receive wireless RF signals and provide them to the input port of RF coupler  114 , which in turn forwards these RF signals to the transmit and receive circuitry via its output port. Such a configuration is described in more detail herein including with reference to  FIGS. 4A-4C . 
     In some embodiments, transmit and receive circuitry  112  may be configured to: (1) provide, via RF coupler  114 , a first RF signal (e.g., RF signal  111 ) to the antenna  115  for transmission to target device  104 ; (2) obtain, via RF coupler  114 , a responsive second RF signal (e.g., RF signal  113 ) received by the antenna  115  and generated by the target device  104  in response to receiving the transmitted first RF signal; and (3) process the received second RF signal by mixing it (e.g., using a frequency mixer) with a transformed version of the first RF signal to obtain a mixed RF signal. The resulting mixed RF signal may be indicative of a time-of-flight between the interrogator  102  and the target device  104 . As such, the resulting mixed RF signal may be indicative of the distance between the interrogator  102  and the target device  104 ; that distance may be obtained from the mixed RF signal as described herein. The transmit and receive circuitry  112  may be configured to provide mixed RF signals to control circuitry  118 , which may (with or without performing further processing the RF signals obtained from circuitry  112 ) provide the RF signals to external communications module  120 . 
     In some embodiments, the transformed version of the first RF signal may be obtained by the transmit and receive circuitry  112  by transforming the first RF signal in a manner analogous to the processing performed on received RF signals by the target device  104  to generate the responsive RF signals (e.g., when the target device doubles the frequency of the receive RF signals prior to transmitting them back, the transformed version of the first RF signal may be obtained by passing the first RF signal through a frequency doubler). In this way, the center frequency of the transformed version of the first RF signal may be the same as the center frequency of the responsive second RF signal received by the antenna  115 . 
     In some embodiments, antenna  115  may be a wideband RF antenna configured to receive and transmit RF signals across a wide range of frequencies. Antenna  115  may be a single-port antenna. Antenna  115  may be capable of transmitting RF signals having a first center frequency and receiving RF signals having a second center frequency different from the first center frequency. In some embodiments, the second center frequency may be a harmonic (e.g., a first harmonic) of the first center frequency. For example, antenna  115  may be configured to transmit RF signals having a center frequency in the range of 4-7.5 GHz and receive RF signals having a center frequency in the range of 8-15 GHz. As another example, antenna  115  may be configured to transmit C-band RF signals and receive X-band RF signals. As another example, antenna  115  may be configured to transmit RF signals having a center frequency in the range of 50-140 GHz and receive RF signals having a center frequency in the range of 100-140 GHz. 
     In some embodiments, the antenna  115  may be a circularly polarized antenna such that it is configured to transmit and receive circularly-polarized RF signals. For example, antenna  115  may be configured to transmit and receive RF signals having right-handed circular polarization. As another example, antenna  115  may be configured to transmit and receive RF signals having left-handed circular polarization. 
     Circularly polarized RF signals may be referred to as having right-handed circular polarization or left-handed circular polarization depending on the direction in which the electric field rotates from the perspective of the source. Accordingly, a circularly polarized RF signal has right-handed circular polarization when, upon pointing the right thumb away from the source in the same direction that the circularly polarized signal is propagating, the electric filed rotates in the direction of the curled fingers of the right hand. Right-handed circular polarization may also be referred to as “clockwise” circular polarization. On the other hand, a circularly polarized RF signal has left-handed circular polarization when, upon pointing the left thumb away from the source in the same direction that the circularly polarized signal is propagating, the electric filed rotates in the direction of the curled fingers of the left hand. Left handed circular polarization may also be referred to as “counter-clockwise” circular polarization. As may be appreciated from the foregoing, RF signals having right-handed (or clockwise) circular polarization and RF signals having left-handed (or counter-clockwise) circular polarization are circularly polarized in different and opposing directions. 
     In some embodiments, antenna  115  may have a tight phase center. An antenna may have a “tight phase center” when its phase center does not vary by more than 1 mm across all frequencies (at which the antenna operates) and across all angles within 60 degrees of the antenna&#39;s boresight. For example, antenna  115  may be an Archimedean spiral antenna, an exponential spiral antenna, a sinuous antenna, a log-periodic antenna, or any other suitable type of RF antenna having a tight phase center. 
     In some embodiments, RF coupler  114  may be a transmission line type coupler. For example, the RF coupler may have a main line, connecting its input and output ports and a coupled line connecting its couple and isolated ports. The coupled line and the main line may be electromagnetically coupled to one another. This is described in greater detail with reference to  FIG. 5 . In other embodiments, RF coupler  114  may be a waveguide directional coupler, a transformer-based coupler, or any other suitable type of coupler, as aspects of the technology described herein are not limited in this respect. 
     In some embodiments, control circuitry  118  may be configured to trigger the waveform generator  110  to generate an RF signal for transmission by the antenna  115 . The control circuitry  118  may trigger the waveform generator in response to a command to do so received by external communications interface  120  and/or based on logic part of control circuitry  118 . 
     In some embodiments, control circuitry  118  may be configured to receive RF signals from transmit and receive circuitry  112  and forward the received RF signals to external communications interface  120  for sending to control system  106 . In some embodiments, control circuitry  118  may be configured to process the RF signals received from transmit and receive circuitry  112  and forward the processed RF signals to external communications interface  120 . Control circuitry  118  may perform any of numerous types of processing on the received RF signals including, but not limited to, converting the received RF signals to from analog to digital (e.g., by sampling using an ADC), performing a Fourier transform to obtain a time-domain waveform, estimating a time of flight between the interrogator and the target device from the time-domain waveform, and determining an estimate of distance between the interrogator  102  and the target device that the interrogator  102  interrogated. The control circuitry  118  may be implemented in any suitable way and, for example, may be implemented as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a combination of logic circuits, a microcontroller, or a microprocessor. 
     Techniques for estimating a time of flight between the interrogator and the target devices using the transmitted and received RF signals are described in further detail in U.S. Pat. App. Pub. No. 2017/0181118, titled “RADIO-FREQUENCY LOCALIZATION TECHNIQUES AND ASSOCIATED SYSTEMS, DEVICES, AND METHODS”, which is incorporated by reference herein in its entirety. 
     External communications module  120  may be of any suitable type and may be configured to communicate according to any suitable wireless protocol(s) including, for example, a Bluetooth communication protocol, an IEEE 802.15.4-based communication protocol (e.g., a “ZigBee” protocol), and/or an IEEE 802.11-based communication protocol (e.g., a “WiFi” protocol). 
     As shown in  FIG. 2 , target device  104  includes antenna  125 , RF coupler  126 , signal transformation circuitry  124 , control circuitry  128 , and external communications module  130 . It should be appreciated that, in some embodiments, a target device may include one or more other components in addition to or instead of the components illustrated in  FIG. 2 . 
     In the illustrated embodiment, antenna  125  is configured to provide received wireless RF signals having a first center frequency to (e.g., an input port of) RF coupler  126 , which in turn is configured to forward the RF signals to signal transformation circuitry  124  (e.g., via its output port). The signal transformation circuitry  124  may be configured to transform the RF signals having the first center frequency to RF signals having a second center frequency different from (e.g., a harmonic of) the first center frequency. The signal transformation circuitry is configured to provide the transformed RF signals to (e.g., a coupled port of) RF coupler  126 , which in turn is configured to provide them for transmission to antenna  125  (e.g., via the input port of the RF coupler). In turn, the antenna  125  is configured transmit transformed RF signals having the second center frequency. This configuration is described in more detail herein including with reference to  FIGS. 3A and 3B . 
     In some embodiments, the antenna  125  may be a wideband RF antenna configured to receive and transmit RF signals across a wide range of frequencies. Antenna  125  may be a single-port antenna. Antenna  125  may be capable of receiving RF signals having a first center frequency and transmitting RF signals having a second center frequency different from the first center frequency. In some embodiments, the second center frequency may be a harmonic (e.g., a first harmonic) of the first center frequency. For example, antenna  125  may be configured to receive RF signals having a center frequency in the range of 4-7.5 GHz and transmit RF signals having a center frequency in the range of 8-15 GHz. As another example, antenna  125  may be configured to receive C-band RF signals and transmit X-band RF signals. As another example, antenna  125  may be configured to receive RF signals having a center frequency in the range of 50-140 GHz and transmit RF signals having a center frequency in the range of 100-140 GHz. 
     In some embodiments, the antenna  125  may be a circularly polarized antenna such that it is configured to transmit and receive circularly-polarized RF signals. For example, antenna  125  may be configured to transmit and receive RF signals having right-handed circular polarization. As another example, antenna  125  may be configured to transmit and receive RF signals having left-handed circular polarization. 
     In some embodiments, antenna  125  may have a tight phase center. For example, antenna  115  may be an Archimedean spiral antenna, an exponential spiral antenna, a sinuous antenna, a log-periodic antenna, or any other suitable type of RF antenna having a tight phase center. 
     In some embodiments, RF coupler  126  may be a transmission line coupler or any other suitable type of RF coupler described herein. In some embodiments, RF coupler  126  and RF coupler  124  may be the same type of coupler, though in other embodiments this need not be the case. 
     In some embodiments, the interrogator antenna  115  may be a directional antenna. This may be useful in applications where some information is known about the region of space in which the target device is located (e.g., the target device is located in front of the interrogator, to the front left of the interrogator, etc.). Even if the target device is attached to a moving object (e.g., an arm of an industrial robot, a game controller), the movement of the target device may be constrained so that the target device is always within a certain region of space relative to the interrogator so that using a directional antenna to focus on that region of space increases the sensitivity of the interrogator to signals generated by the target device. In turn, this increases the distance between the interrogator and target device at which the micro-localization system may operate with high accuracy. However, it should be appreciated that in some embodiments, the interrogator antenna may be isotropic (omnidirectional), as aspects of the technology described herein are not limited in this respect. 
     In some embodiments, the target device antenna  125  may be isotropic so that the target device may be configured to receive signals from and/or provide RF signals to an interrogator located in any location relative to the target device. This is advantageous because, in some applications of micro-localization, the target device may be moving and its relative orientation to one or more interrogators may not be known in advance. However, in some embodiments, the target device antenna may be directional (anisotropic), as aspects of the technology described herein are not limited in this respect. 
     In some embodiments, control circuitry  128  may be configured to turn the target device  104  on or off (e.g., by powering off one or more components in signal transformation circuitry  124 ) in response to a command to do so received via external communications interface  130 . The control circuitry  128  may be implemented in any suitable way and, for example, may be implemented as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a combination of logic circuits, a microcontroller, or a microprocessor. External communications module  130  may be of any suitable type including any of the types described herein with reference to external communications module  120 . 
     As discussed above with reference to  FIG. 1 , multiple interrogator devices may be utilized in order to determine a location of a target device. In some embodiments, each of the interrogator devices may be configured to transmit an RF signal to a target device, receive a responsive RF signal from the target device (the responsive signal may have a different polarization and/or a different center frequency from the signal that was transmitted), and process the transmitted RF signal together with the received RF signal to obtain an RF signal indicative of the distance between the interrogator device and the target device. The RF signals indicative of the distances between the interrogator devices and the target device may be processed (e.g., by the interrogators or another processor) to obtain estimates of the distances between the target device and each of the interrogators. In turn, the estimated distances may be used to determine the location of the target device in 2D space and/or in 3D space. 
       FIG. 3A  is a diagram of an illustrative target device  300 , in accordance with some embodiments of the technology described herein. In the illustrative embodiment of  FIG. 3A , target device  300  includes antenna  302 , RF coupler  304 , and signal transformation circuitry  306 . RF coupler  304  includes input port  305   a , output port  305   b , coupled port  305   c , and isolated port  305   d . It should be appreciated that, in some embodiments, target device  300  may include one or more other components in addition to or instead of the components illustrated in  FIG. 3A . For example, an external communications module, such as external communications module  130  described with reference to  FIG. 2 ), may be included. 
     As shown in  FIG. 3A , the antenna  302  is connected, via line  303 , to the input port  305   a  of RF coupler  304 . The RF coupler  304  is connected to signal transformation circuitry  306 . In particular, the output port  305   b  of RF coupler  304  is connected, via line  307   a , to an input of signal transformation circuitry  306 . In addition, an output of signal transformation circuitry  306  is connected, via line  307   b , to the coupled port  305   c  of RF coupler  304 . 
     In the illustrative embodiment of  FIG. 3A , antenna  302  provides received wireless RF signals, via line  303 , to the input port  305   a  of RF coupler  304 . The received RF signals may have a first center frequency. In turn, RF coupler  304  provides RF signals having the first center frequency through output port  305   b , via line  307   a , to an input of signal transformation circuitry  306 . Signal transformation circuitry  306  transforms the RF signals having the first center frequency to RF signals having a second center frequency different from the first center frequency. For example, the second center frequency may be a harmonic (e.g., a first harmonic) of the first center frequency. The RF signals, having the second center frequency and output from signal transformation circuitry  306  are provided, via line  307   b , to the coupled port  305   c  of RF coupler  304 . The RF signals received via the coupled port  305   c  induce, in the RF coupler  304 , corresponding RF signals having the second center frequency, which are then provided via the input port  305   a  to antenna  302 . In turn, antenna  302  transmits RF signals having the second center frequency. 
     For example, antenna  302  may receive a wireless RF signal having a first center frequency and provide, via line  303  to the input port  305   a  of RF coupler  304 , a first RF signal having the first center frequency and generated using the received wireless RF signal. The RF coupler  304  may generate a second RF signal having the first center frequency using the first RF signal and provide the second RF signal, via output port  305   b  and line  307   a , to an input of signal transformation circuitry  306 . The signal transformation circuitry may generate a third RF signal having a second center frequency different from the first center frequency (e.g., a harmonic of the first center frequency) using the second RF signal and provide the third RF signal, via line  307   b , to the coupled port  305   c  of the RF coupler  304 . The RF coupler  304  may generate a fourth RF signal having the second center frequency using the third RF signal and provide the fourth RF signal to the antenna  302  via input port  305   a . For example, the fourth RF signal may be induced on the main line of the RF coupler as a result of the third RF signal being provided as input to the coupled port, which is connected to the coupled line of the RF coupler. In turn, the antenna  302  may transmit a wireless RF signal having the second center frequency and generated using the fourth RF signal. 
     In this way, in response to receiving, from an interrogator device, RF signals having a first center frequency, the target device  300 , transmits to the interrogator device, RF signals having a second center frequency. For example, in some embodiments, the target device  300  may receive RF signals having a center frequency in the range of 4-7.5 GHz and transmit RF signals having a center frequency in the range of 8-15 GHz. As another example, in some embodiments, the target device  300  may receive RF signals having a center frequency in the range of 50-70 GHz and transmit RF signals having a center frequency in the range of 100-140 GHz. 
     In some embodiments, antenna  302  may be a wideband RF antenna configured to receive and transmit RF signals across a wide range of frequencies. Antenna  302  may be capable of receiving RF signals having a first center frequency and transmitting RF signals having a second center frequency different from the first center frequency. In some embodiments, the second center frequency may be a harmonic (e.g., a first harmonic) of the first center frequency. For example, antenna  302  may be configured to receive RF signals having a center frequency in the range of 4-7.5 GHz and transmit RF signals having a center frequency in the range of 8-15 GHz. As another example, antenna  302  may be configured to receive C-band RF signals and transmit X-band RF signals. As another example, antenna  302  may be configured to receive RF signals having a center frequency in the range of 50-140 GHz and transmit RF signals having a center frequency in the range of 100-140 GHz. 
     In some embodiments, the antenna  302  may be a circularly polarized antenna such that it is configured to transmit and receive circularly-polarized RF signals. For example, antenna  302  may be configured to transmit and receive RF signals having right-handed circular polarization. As another example, antenna  302  may be configured to transmit and receive RF signals having left-handed circular polarization. 
     In some embodiments, antenna  302  may have a tight phase center. For example, antenna  302  may be an Archimedean spiral antenna, an exponential spiral antenna, a sinuous antenna, a log-periodic antenna, or any other suitable type of RF antenna having a tight phase center. 
     In some embodiments, RF coupler  304  may be a transmission line type coupler. For example, the RF coupler may have a main line, connecting the input port  305   a  and the output  305   b , and a coupled line connecting the coupled port  305   c  and the isolated port  305   d . The coupled line and the main line may be electromagnetically coupled to one another. This is described in greater detail with reference to  FIG. 5 . In other embodiments, RF coupler  304  may be a waveguide directional coupler, a transformer-based coupler, or any other suitable type of coupler, as aspects of the technology described herein are not limited in this respect. 
     In some embodiments, signal transformation circuitry  306  may be configured to transform RF signals having a first center frequency to RF signals having a second center frequency. Additionally, in some embodiments, signal transformation circuitry  306  may be configured to amplify received RF signals (e.g., to counteract losses to be imparted to the RF signals as a result of frequency multiplication and/or coupling losses incurred in the RF coupler). Additionally or alternatively, the signal transformation circuitry  306  may be configured to filter the RF signals using one or more filters (e.g., one or more low-pass, band-pass, or high-pass filters). 
     For example, in the illustrative embodiment shown in  FIG. 3B , signal transformation circuitry  306  includes amplifier  309 , frequency multiplier  311 , and amplifier  313 . Signals output by RF coupler  304  through output port  305  may be provided to amplifier  309  via line  307   a . The amplifier  309  amplifies signals received via line  307   a  and provides the amplified signals to frequency multiplier  311 . The frequency multiplier  311  generates output signals whose frequency is a multiple of the frequency of the input signals received from amplifier  309  and provides the output signals to amplifier  313 . The amplifier  313  amplifies signals received from frequency multiplier  311  and provides the amplified signals to the coupled port  305   c  of the RF coupler  304  via line  307   b.    
     In some embodiments, frequency multiplier  311  may receive input signals having a center frequency of f 0  and generate output signals having a center frequency that is an integer multiple of f 0 . For example, frequency multiplier  311  may receive input signals having a center frequency of f 0  and generate output signals having a center frequency of 2f 0 , 3f 0 , 4f 0 , 8f 0 , 10f 0 , or any other suitable integer multiple of f 0 , as aspects of the technology described herein are not limited in this respect. As one example, in some embodiments, frequency multiplier  908  may receive input signals having a center frequency in a range of 50-70 GHz (e.g., 61.25 GHz) and generate output signals having a center frequency in a range of 100-140 GHz (e.g., 122.5 GHz). As another example, in some embodiments, frequency multiplier  311  may receive input signals having a center frequency in a range of 4-7.5 GHz (e.g., 5 GHz) and generate output signals having a center frequency of 8-14 GHz (e.g., 10 GHz). 
     In some embodiments, frequency multiplier  311  may be implemented as a cascade of frequency multipliers, which cascade may include one or more amplifiers and/or one or more filters between successive frequency multipliers. For example, in some embodiments, frequency multiplier  311  may be configured to output signals having a center frequency that is four times the center frequency of the input signals provided to frequency multiplier  311 . In such an example, frequency multiplier may be implemented as a single “4×” frequency multiplier or a sequence of two “2×” frequency multipliers, each of which is configured to output signals having a center frequency that is double the center frequency of the input signals provided to it. One or more amplifiers and/or filters may be provided between the “2×” frequency multipliers. 
     In some embodiments, frequency multiplier  311  may include a non-linear circuit. The non-linear circuit may distort input signals provided to frequency multiplier  311  to generate signals having center frequencies that are multiples of the center frequencies of the input signals. Additionally, frequency multiplier  311  may include one or more (e.g., bandpass) filters for selecting a desired center frequency for the output signals (e.g., a desired harmonic frequency) and removing the fundamental and/or one or more other harmonic frequency components from the non-linearly distorted signals. 
     In some embodiments, the non-linear circuit in a frequency multiplier  311  may be a diode. Frequency multiplier  311  may be any suitable type of diode frequency multiplier. For example, in some embodiments, frequency multiplier  311  may be a Schottky diode, a silicon diode, a varistor-type diode frequency multiplier, a varactor-type frequency multiplier, a step recover diode frequency multiplier, or a PIN diode frequency multiplier, any (e.g., all) of which may or may not be biased with a quiescent bias current. 
     In the illustrated embodiment of  FIG. 3B , each of amplifiers  309  and  313  may be of any suitable type and may be used to induce any suitable amount of gain to the input signals. In some embodiments, the gain of amplifier  309  may be greater than the gain of amplifier  313 , and, in some instances, may be significantly greater than the gain of amplifier  313 . For example, the gain of amplifier  309  may be at least 10, 20, 30, 40, 50, or 100 times the gain of amplifier  313 . Having amplifier  309  induce a greater amount of gain than amplifier  313  provides as much drive power as possible to the frequency multiplier. In addition, less power is needed to induce power gain at lower frequencies (before the signals pass through a frequency multiplier) than to induce the same amount of power gain at higher frequencies (after the signals pass through the frequency multiplier). Thus, inducing a greater amount of gain via amplifier  309 , reduces the overall power consumption requirements of the signal transformation circuitry, which is advantageous. 
     In some embodiments, amplifier  313  may be used to amplify the RF signals to counteract the coupling to be incurred in the RF coupler  304 . That coupling loss may be 5-15 dB in some embodiments. Accordingly, amplifier  313  and one or more other amplifiers in the signal transformation circuitry may be used to achieve a gain sufficient (e.g., at least 5-20 dB) to compensate for the coupling loss that will be incurred in the RF coupler (when the antenna  302  picks up signals induced by the RF signals provided via the coupled port). 
     As may be appreciated from  FIG. 3B , the inclusion of frequency multiplier  311  in the signal transformation circuitry  306  causes RF signals provided to coupled port  305   c , via line  307   b , to have higher frequencies than the RF signals output from output port  305   b , via line  307   a . Accordingly, in some embodiments, antenna  302  may be configured to receive RF signals having a first center frequency transmit RF signals having a second center frequency different from the first center frequency. For example, the second center frequency may be a harmonic of the first frequency. As a specific example, the second center frequency may be twice the first center frequency (e.g., when the frequency multiplier  311  receives input signals having a center frequency of f 0  and generates output signals having a center frequency of 2f 0 ). 
     The target device  300  may be manufactured in any of numerous ways. For example, in some embodiments, the target device  300  may comprise a substrate, with antenna  302 , RF coupler  304 , and signal transformation circuitry  306  realized as discrete components mounted on the substrate. In other embodiments, the target device  300  may comprise a substrate (e.g., a printed circuit board) having the antenna  302  fabricated thereon, and a semiconductor die mounted to the substrate, coupled to the antenna  302 , and having the RF coupler  304  and the signal transformation circuitry  306  fabricated thereon. In such embodiments, the RF coupler  304  and signal transformation circuitry  306  may be integrated circuitry monolithically integrated with the semiconductor die. In some embodiments, the semiconductor die may be flip-chip bonded to the substrate. In some embodiments, the substrate may comprise a printed circuit board having at least one conductive layer, and the antenna  302  may be fabricated on the substrate by patterning the at least one conductive layer. 
     It should be appreciated that the embodiment illustrated in  FIGS. 3A and 3B  is illustrative and that there are variations. For example, although in the illustrated embodiment a single amplifier  309  is shown between RF coupler  304  and frequency multiplier  311 , in other embodiments there may be zero, two, three, four, five or more amplifiers between RF coupler  304  and frequency multiplier  311 . As another example, although in the illustrated embodiment, a single amplifier  313  is shown between frequency multiplier  311  and RF coupler  304 , in other embodiments there may be zero, two, three, four, five or more amplifiers between frequency multiplier  311  and RF coupler  304 . In some embodiments, the number and gain of the amplifiers in the signal chain from output port  305   b  to the coupled port  305   c  may be determined based on an overall amount of gain desired to induce to the signal and in view of an amount of loss induced by the frequency multiplier  311  and the coupling loss in the RF coupler  304 . For example, in some embodiments, one or multiple amplifiers (e.g., amplifiers  309  and  313 ) that provide at least 30 dB (or at least 40 dB, at least 50 dB, at least 60 dB, etc.) of gain overall may be introduced into the signal chain between the output port  305   b  and the coupled port  305   c . As another example of a variation of the embodiments shown in  FIG. 3B , there may be one or more filters between any pair of circuits along the signal path from the output port  305   b  and the coupled port  305   c.    
       FIG. 4A  is a diagram of an illustrative interrogator device  400 , in accordance with some embodiments of the technology described herein. In the illustrative embodiment of  FIG. 4A , interrogator device  400  includes antenna  402 , RF coupler  404 , transmit and receive circuitry  406 , waveform generator  410 , and control circuitry  412 . RF coupler  404  includes input port  405   a , output port  405   b , coupled port  405   c , and isolated port  405   d . It should be appreciated that, in some embodiments, interrogator device  400  may include one or more other components in addition to or instead of the components illustrated in  FIG. 4A . For example, an external communications module, such as external communications module  120  described with reference to  FIG. 2 ), may be included. 
     As shown in  FIG. 4A , the antenna  402  is connected, via line  403 , to the input port  405   a  of RF coupler  404 . The RF coupler  404  is connected to transmit and receive circuitry  406 . In particular, the output port  405   b  of RF coupler  404  is connected, via line  407   a , to an input of transmit and receive circuitry  406 . In addition, an output of transmit and receive circuitry  406  is connected, via line  407   b , to the coupled port  405   c  of RF coupler  404 . 
     In some embodiments, transmit and receive circuitry  406  may include transmit circuitry that, using signals provided by waveform generator  410 , generates RF signals for transmission by antenna  402 . The RF signals generated in the transmit and receive circuitry  406  are provided, via line  407   b , to the coupled port  405   c  of the RF coupler  404 . In turn, antenna  402  transmits, to a target device, RF signals received at the input port  405   a  via coupling to signals received at the coupled port  405   c.    
     In some embodiments, antenna  402  receives wireless RF signals from a target device and provides the received RF signals, via line  403 , to the input port  405   a  of the RF coupler  404 . In turn, RF signals, which are generated based on the RF signals received through the input port  405   a , are provided to transmit and receive circuitry  406 , through output port  405   b  and via line  407   a . The transmit and receive circuitry  406  may use the received RF signals to generate signals indicative of a time-of-flight and/or distance between the interrogator device and the target device. 
     For example, in some embodiments, the transmit and receive circuitry  406  may be configured to: (1) provide, via RF coupler  404 , a first RF signal to the antenna  402  for transmission to a target device (e.g., target device  104 ); (2) obtain, via RF coupler  404 , a responsive second RF signal received by antenna  402  and generated by the target device in response to receiving the transmitted first RF signal; and (3) process the received RF signal by mixing it with a transformed version of the first RF signal to obtain a mixed RF signal. The resulting mixed RF signal may be indicative of a time-of-flight between the interrogator  400  and the target device. As such, the resulting mixed RF signal may be indicative of the distance between the interrogator  400  and the target device; that distance may be obtained from the mixed RF signal as described herein. In some embodiments, the transmit and receive circuitry  406  may be configured to provide mixed RF signals to control circuitry  412 , which may (with or without performing further processing the RF signals obtained from circuitry  406 ) provide the RF signals to another component (e.g., an external communications module). 
       FIG. 4B  is a diagram showing an illustrative embodiment of transmit and receive circuitry  406 . Each of the illustrated components of transmit and receive circuitry  406  may be of any suitable type, as aspects of the technology described herein are not limited in this respect. As shown in  FIG. 4B , transmit and receive circuitry  406  includes a transmit signal chain comprising frequency multiplier  414 , which is connected to amplifier  416 , which is connected to splitter  418 . In the illustrated embodiment, the transmit signal chain is configured to provide, via RF coupler  404 , a first RF signal to the antenna  402  for transmission to a target device. For example, RF signals generated by the waveform generator  410  may be transformed by frequency multiplier  414 , amplified by amplifier  416  and then provided, via splitter  418 , to the coupled port  405   c  of RF coupler  404  to be transmitted by antenna  402 . For example, RF signals having a center frequency of 3 GHz may be generated by waveform generator  410 , processed by frequency multiplier  414  to obtain RF signals having a center frequency of 6 GHz, which are then provided, via amplifier  416 , splitter  418 , and RF coupler  404 , to antenna  402  for transmission. As another example, RF signals having a center frequency of 7.5 GHz may be generated by waveform generator  410 , processed by frequency multiplier  414  to obtain RF signals having a center frequency of 60 GHz (frequency multiplier  414  need not be a doubler), which are then provided, via amplifier  416 , splitter  418 , and RF coupler  404 , to antenna  402  for transmission. In other embodiments, the waveform generator  410  may be configured to generate RF signals at the frequency at which they are to be transmitted by antenna  402 . In such embodiments, frequency multiplier  414  (and, optionally, amplifier  416 ) may be omitted from the transmit signal chain. 
     In some embodiments, amplifier  416  may be used to amplify the RF signals in order to counteract the coupling to be incurred in the RF coupler  404 . That coupling loss may be 5-15 dB in some embodiments. Accordingly, amplifier  414  and/or one or more other amplifiers may be employed to achieve a gain sufficient (e.g., at least 5-20 dB) to compensate for the coupling loss that will be incurred in the RF coupler (when the antenna  402  picks up signals induced by the RF signals provided via the coupled port). 
     As shown in  FIG. 4B , transmit and receive circuitry also includes a receive signal chain comprising amplifier  426 , mixer  424 , amplifier  428 , filter  429 , and analog-to-digital (ADC) converter  431 . In the illustrated embodiment, the receive signal chain is configured to, obtain, via RF coupler  404 , a responsive second RF signal received by antenna  402  and generated by the target device in response to receiving the transmitted first RF signal. In the illustrated embodiment, the receive signal chain is also configured to process the responsive second RF signal by mixing it with a transformed version of the first RF signal to obtain a mixed RF signal. As shown in  FIG. 4B , the mixer  424  is configured to mix the responsive second RF signal (after it is amplifier by amplifier  426 ) with a transformed version of the first RF signal obtained by transforming the RF signal provided from splitter  410  by using frequency multiplier  420  and amplifier  422 . In the illustrated embodiment, the resulting mixed RF signal provided at the output of mixer  424  is amplified by amplifier  428 , filtered by filter  429 , and digitized using ADC  431 . 
     In some embodiments, the mixed RF signal generated by mixer  424  may be indicative of the time of flight between the interrogator  400  and the target device that provided the responsive signal. Thus, the mixed RF signal may be used to determine the time of flight and/or distance between the interrogator  400  and the target device. For example, the mixed RF signal may be used to determine the distance between the phase center of antenna  402  on the interrogator and the phase center of the antenna on the target device. 
     In the illustrative embodiment of  FIG. 4B , RF signals transmitted to a target device are transformed using frequency multiplier  420  to obtain transformed RF signals having the same center frequency (e.g., 2f 0 ) as the RF signals received by antenna  402  from the target device. Thus, the RF signals being mixed by frequency mixer  424 , and subsequently digitized by ADC  431 , have the center frequency of 2f 0 . In other embodiments, instead of transforming transmitted RF signals, the received RF signals may be transformed instead by a frequency divider circuit to obtain transformed RF signals having the same center frequency (e.g., f 0 ) as the transmitted RF signals. In such embodiments, the RF signals being mixed by frequency mixer  424 , and subsequently digitized by ADC  431 , have the center frequency of f 0 , which may reduce the cost and/or improve the performance of the frequency mixer  424  and/or ADC  431 . 
     In some embodiments, antenna  402  may be a wideband RF antenna configured to receive and transmit RF signals across a wide range of frequencies. Antenna  402  may be a single-port antenna. Antenna  402  may be capable of transmitting RF signals having a first center frequency and receiving RF signals having a second center frequency different from the first center frequency. In some embodiments, the second center frequency may be a harmonic (e.g., a first harmonic) of the first center frequency. For example, antenna  402  may be configured to transmit RF signals having a center frequency in the range of 4-7.5 GHz and receive RF signals having a center frequency in the range of 8-15 GHz. As another example, antenna  402  may be configured to transmit C-band RF signals and receive X-band RF signals. As another example, antenna  402  may be configured to transmit RF signals having a center frequency in the range of 50-140 GHz and receive RF signals having a center frequency in the range of 100-140 GHz. 
     In some embodiments, the antenna  402  may be a circularly polarized antenna configured to transmit and receive circularly-polarized RF signals. For example, antenna  402  may be configured to transmit and receive RF signals having right-handed or left-handed circular polarization. 
     In some embodiments, antenna  402  may have a tight phase center. For example, antenna  115  may be an Archimedean spiral antenna, an exponential spiral antenna, a sinuous antenna, a log-periodic antenna, or any other suitable type of RF antenna having a tight phase center. 
     In some embodiments, RF coupler  404  may be a transmission line type coupler. For example, the RF coupler may have a main line, connecting its input and output ports and a coupled line connecting its coupled and isolated ports. The coupled line and the main line may be electromagnetically coupled to one another. This is described in greater detail with reference to  FIG. 5 . In other embodiments, RF coupler  404  may be a waveguide directional coupler, a transformer-based coupler, or any other suitable type of coupler, as aspects of the technology described herein are not limited in this respect. 
     In some embodiments, waveform generator  410  may be configured to generate any RF signals of any suitable type including frequency-modulated waveforms. For example, waveform generator  410  may be configured to generate linear frequency modulated waveforms and/or any other waveform described with reference to waveform generator  110  shown in  FIG. 2 . 
     The interrogator  400  may be manufactured in any of numerous ways. For example, in some embodiments, the interrogator  400  may comprise a substrate, with antenna  402 , RF coupler  404 , and transmit and receive circuitry  406  realized as discrete components mounted on the substrate. In some embodiments, the antenna may be on a separate substrate from the substrate on which the RF coupler  404  and transmit and receive circuitry  406  are mounted. In other embodiments, the interrogator  400  may comprise a substrate (e.g., a printed circuit board) having the antenna  402  fabricated thereon, and a semiconductor die mounted to the substrate, coupled to the antenna  402 , and having the RF coupler  404  and the transmit and receive circuitry  406  fabricated thereon. In such embodiments, the RF coupler  404  and the circuitry  406  may be integrated circuitry monolithically integrated with the semiconductor die. In some embodiments, the semiconductor die may be flip-chip bonded to the substrate. In some embodiments, the substrate may comprise a printed circuit board having at least one conductive layer, and the antenna  402  may be fabricated on the substrate by patterning the at least one conductive layer. 
     It should be appreciated that the embodiment illustrated in  FIGS. 4A and 4B  is illustrative and that there are variations. For example, in some embodiments, there may be zero, one, two, three, or any other suitable number of amplifiers of any suitable type between any pair of circuits shown in  FIG. 4B . As another example, there may be zero, one, two, three, or any other suitable number of filters of any suitable type between any pair of circuits shown in  FIG. 4B . As another example, although in the illustrated embodiment ADC  431  is part of transmit and receive circuitry  406  so that digitized waveforms are output from the transmit and receive circuitry  406  to control circuitry  412 , in other embodiments ADC  431  may not be part of transmit and receive circuitry  406  and, instead, may be further downstream in the processing chain. In such embodiments, analog waveforms may be output from the transmit and receive circuitry  406  to control circuitry  412 . 
     In the embodiment illustrated in  FIGS. 4A and 4B , the interrogator device  400  includes a single RF antenna  402 . However, in other embodiments, an interrogator device may include multiple RF antennas, as aspects of the technology described herein are not limited in this respect. One such embodiment is illustrated in  FIG. 4C , which shows an interrogator device  450  having three antennas  432 ,  442 , and  452 . It should be appreciated that an interrogator may include any suitable number of antennas (e.g., one, two, three, four, five, six, seven, etc.), as aspects of the technology described herein are not limited in this respect. 
     As shown in  FIG. 4C , the antenna  432  is connected, via line  433 , to the input port  435   a  of RF coupler  434 . The RF coupler has input port  435   a , output port  435   b , coupled port  435   c , and isolated port  435   d . The RF coupler  434  is connected to transmit and receive circuitry  430 . In particular, the output port  435   b  of RF coupler  434  is connected, via line  437   a , to an input of transmit and receive circuitry  430 . In addition, an output of transmit and receive circuitry  430  is connected, via line  437   b , to the coupled port  435   c  of RF coupler  434 . 
     As also shown in  FIG. 4C , the antenna  442  is connected, via line  443 , to the input port  445   a  of RF coupler  444 . The RF coupler has input port  445   a , output port  445   b , coupled port  445   c , and isolated port  445   d . The RF coupler  444  is connected to transmit and receive circuitry  430 . In particular, the output port  445   b  of RF coupler  444  is connected, via line  447   a , to an input of transmit and receive circuitry  430 . In addition, an output of transmit and receive circuitry  430  is connected, via line  447   b , to the coupled port  445   c  of RF coupler  444 . 
     As also shown in  FIG. 4C , the antenna  452  is connected, via line  453 , to the input port  455   a  of RF coupler  454 . The RF coupler has input port  455   a , output port  455   b , coupled port  455   c , and isolated port  455   d . The RF coupler  454  is connected to transmit and receive circuitry  430 . In particular, the output port  455   b  of RF coupler  454  is connected, via line  457   a , to an input of transmit and receive circuitry  430 . In addition, an output of transmit and receive circuitry  430  is connected, via line  457   b , to the coupled port  455   c  of RF coupler  454 . 
     In some embodiments, the transmit and receive circuitry  430  may generate RF signals and provide them to one or more of RF couplers  434 ,  444 , and  454  for transmission by antennae  432 ,  442 , and  452 , respectively. 
     In some embodiments, the transmit and receive circuitry  430  may obtain, from one or more of RF couplers  434 ,  444 , and  454 , RF signals received by antennas  432 ,  442 , and  452  respectively. In this way, the transmit and receive circuitry  430  can obtain one or more mixed RF signals indicating the distance between the phase centers of each of one or more of the antennas  432 ,  442 , and  452  and the phase center of the antenna on the target device. In turn, such distances, when used together with the locations on of the antennas  432 ,  442 , and  452 , may be used to determine the 2D and/or 3D location of the target device. 
       FIG. 5  is a diagram of an illustrative RF coupler  500 , in accordance with some embodiments of the technology described herein. RF coupler  500  includes input port  502 , output port  504 , coupled port  506 , and isolated port  508 . RF coupler  500  further includes a main line  505  having the input port  502  at its first end and the output port  504  at its second end, and a coupled line having the coupled port  506  at its first end and isolated port  508  at its second end. 
     In some embodiments, the main line  505  and the coupled line  507  may be electromagnetically coupled such that RF signals on the coupled line  507  induce corresponding RF signals on the main line  505 . As a result, RF signals provided as inputs to coupled port  506  (e.g., from signal transformation circuitry, such as signal transformation  306 ) may induce corresponding RF signals on the main line  505 . In some embodiments in which an RF antenna is connected to input port  502 , the RF antenna may be configured to provide received RF signals to the input port  502  and to transmit RF signals received from the input port  502  via coupling of the main line  505  and the coupled line  507 . 
     In some embodiments, the main line  505  and the coupled line  507  may be coupled transmission lines. For example, the main line  505  and coupled line  507  may be edge coupled. As another example, the main line  505  and coupled line  507  may be broadside coupled. 
     In some embodiments, the main line  505  may be implemented using striplines and/or microstrips. In some embodiments, the coupled line  507  may be implemented using striplines and/or microstrips. For example, in some embodiments, the main line and the coupled line comprise striplines. As another example, in some embodiments, the main line and the coupled line comprise microstrips. 
     In some embodiments, the coupling loss between the main line  505  and the coupled line  507  is between 5 and 15 dB. In some embodiments, the coupling loss between the main line  505  and the coupled line  507  is between 8 and 12 dB. For example, the coupling loss between the main line  505  and the coupled line  507  may be substantially (e.g., within 5%) of 10 dB. 
     It should be appreciated that although in the illustrative embodiment of  FIG. 5 , RF coupler  500  is shows as being implemented using two coupled lines, other types of RF couplers may be used in other embodiments. For example, an RF coupler implemented using one or more transformers or in any other suitable way, as aspects of the technology described herein are not limited in this respect. 
       FIG. 6  is a flowchart of an illustrative method  600 , performed by a target device, of providing an RF signal having a second center frequency to an interrogator device in response to receiving an RF signal having a first center frequency from the interrogator device, in accordance with some embodiments of the technology described herein. The method  600  may be performed by any suitable target device described herein including, for example, target device  104  or target device  300 . 
     Process  600  begins at act  602 , where the target device receives, using its RF antenna, a first wireless RF signal having a first center frequency. The target device may receive the first wireless RF signal from an interrogator (e.g., interrogator  102  or interrogator  400 ). In some embodiments, the first center frequency may be any suitable microwave frequency. For example, the first wireless RF signal may have a center frequency in the range of 4-7.5 GHz. As another example, the first wireless RF signal may have a center frequency in the range of 50-70 GHz. In some embodiments, the first RF signal may be a linear frequency modulated signal. 
     Next, at act  604 , the RF antenna generates a first RF signal having the first center frequency based on the received first wireless RF signal and provides the first RF signal to an input port of an RF coupler part of the target device. 
     Next, at act  606 , the RF coupler generates a second RF signal having the first center frequency based on the first RF signal and provides the second RF signal to signal transformation circuitry part of the target device. In some embodiments, the RF coupler may comprise a main line connecting the input port the output port, and the second RF signal may be obtained at the output port as a result of inputting the first RF signal at the input port. 
     Next, at act  608 , the signal transformation circuitry generates a third RF signal using the second RF signal and provides the generated third RF signal to a coupled port of the RF coupler. The third RF signal has a second center frequency different from the first center frequency. For example, the second center frequency is a harmonic (e.g., a first harmonic) of the first center frequency. As one example, the first center frequency may be in the range of 4.0-7.5 GHz and the second center frequency may be in the range of 8.0-15 GHz. As another example, the first center frequency may be in the range of 50-70 GHz and the second center frequency may be in the range of 100-140 GHz. 
     Next, at act  610 , the RF coupler generates a fourth RF signal having the second frequency based on the third RF signal and provides the generated fourth RF signal to the RF antenna via the input port of the RF coupler. In some embodiments, the fourth RF signal is generated via coupling between the coupled port and the input port in the RF coupler. For example, in some embodiments, the RF coupler may have a main line connecting the input and output ports and a coupled line connecting the coupled and isolated ports, and the fourth RF signal may be generated on the main line as a result of electromagnetic coupling between the main line and the coupled line. 
     Next, at act  612 , the RF antenna transmits a second wireless RF signal having the second center frequency and generated using the fourth RF signal provided to the RF antenna through the input RF port. The transmitted second wireless RF signal may be received by an interrogator that had transmitted the first wireless RF signal, and may be used by the interrogator to determine the distance between the interrogator and the target device using the techniques described herein. 
     As described herein, an interrogator may transmit an RF signal to a target device and receive a responsive RF signal from the target device. The transmitted RF signal together with the responsive RF signal may be used by the interrogator and/or other circuitry to determine the distance between the phase center of the interrogator&#39;s RF antenna and the phase center of the target device&#39;s RF antenna. In turn, distances between the phase center of each of multiple RF interrogator antennas (disposed on one interrogator or on multiple interrogators) and the phase center of the target device&#39;s RF antenna may be used to determine a precise (2D or 3D) location of the phase center of the target device&#39;s RF antenna. 
     In many applications, however, the location of interest may not be location of the phase center of the target device&#39;s RF antenna. For example, the target device may be mounted on an object, but the location of a specific point on that object may be of interest rather than the location of the phase center of the RF antenna on the target device. For example, in some applications, it may be important to determine the location of a specific point on the end of a robotic arm with high precision (e.g., within a millimeter). However, given how a target device is mounted on the robotic arm, that specific point of interest may be sufficiently far away from the phase center of the target device&#39;s RF antenna such that the location of the phase center itself is not a good proxy for the location of the specific point of interest. 
     Accordingly, the inventors have recognized that it is important, for some applications, to not only determine the precise location the phase center of a target device&#39;s RF antenna, but also to determine where that phase center is located relative to one or more points of interest on the target device and/or object to which the target device is mounted. 
     For example, if the phase center of the RF antenna on a target device is at point P 1  and the location of interest on the target device and/or object on which the target device is mounted is point P 2 , then: (1) one or more interrogators may be used to determine the precise location of point P 1 ; and (2) the location of point P 2  may be determined using the location of point P 1  and information indicating the relative location of points P 1  and P 2 . 
     Accordingly, in some embodiments, the RF antenna of the target device may be mounted on a target device in a way that allows the location of the RF antenna&#39;s center to be determined relative to a reference location on the target device. In turn, the relative location of the phase center to the reference location and the relative location of that reference location to any point of interest on the target device (or on the object to which the target device is mounted) may be used to determine the location of the point of interest. 
     Accordingly, in some embodiments, the RF antenna of a target device may be precision-mounted to the target device using an alignment tool that allows for the determination of the relative locations of the phase center of the RF antenna and a reference point on the target device.  FIGS. 7A-7C  illustrate one embodiment for how to an antenna may be mounted using such an alignment tool. 
     As shown in  FIG. 7A , RF antenna  706  having phase center located at point  708  may be attached to mount  704 , which is disposed on a substrate  702  of a target device, using mounting holes  705  and  707 . Although, in this example, RF antenna  706  is a spiral antenna, any of the other types of tight phase center antennas described herein may be utilized, as aspects of the technology described herein are not limited in this respect. 
     In some embodiments, the alignment tool  750 , shown in  FIG. 7C , may be used to perform the mounting process in a calibrated way such that the relative location of the phase center  708  of the RF antenna  706  and mounting hole  705  may be determined. In the illustrated embodiment, the alignment tool  750  is composed of two concentric rings: inner ring  752  having mounting pegs  754  and outer ring  756  comprising mounting peg  758 . As may be seen from  FIGS. 7A and 7C , the alignment tool  750  may be used to mount RF antenna  706  to mount  704  by inserting the inner ring pegs  754  into mounting holes  707  and the outer ring peg  758  into mounting hole  705 . The inner and outer rings may be rotated relative to one another in order to place RF antenna  706  into a desired rotational orientation. After the antenna is positioned using alignment tool  750 , the alignment tool  750  may be removed and the antenna may be covered with cover  710 , as shown in  FIG. 7A . 
     Since the relative locations of the mounting holes  707  and the antenna&#39;s phase center  708  are known (from manufacture), the alignment tool  750  may be used to determine the distance between the phase center  708  and mounting hole  705  (e.g., depending on the widths of the inner ring  752  and outer ring  756 ) and the angle of the line connecting phase center  708  and mounting hole  705  (e.g., depending on the degree of rotation of the inner ring  752  relative to the outer ring  756 ). 
       FIG. 7B  shows a cutaway view of the illustrative mount of  FIG. 7A , with reference numeral  703  indicating a mounting hole used for connecting mount  704  to substrate  702 . As may be appreciated from  FIGS. 7A-7C : (1) the relative locations of the antenna&#39;s phase center and mounting holes  707  may be known from manufacture of RF antenna  706 ; (2) the relative locations of the mounting holes  707  and mounting hole  705  may be determined using alignment tool  750 ; (3) the relative locations of mounting hole  705  and mounting hole  703  may be known from manufacture of mount  704 ; and (4) the relative locations of mounting hole  703  and any point of interest on substrate  702  may be known in advance. This information may be used to determine the relative locations of phase center  708  and a point of interest on substrate  702  of the target device (or any point of interest on the object to which the target device is mount) may be determined. 
     According, some embodiments provide for a method of manufacturing an RF interrogator device and/or an RF target device. The method comprising mounting an RF antenna to a substrate of the interrogator device (or to a substrate of the target device) using the alignment tool described herein. The alignment tool may be used to rotate the RF antenna into position such that one or more pegs may be used to couple the RF antenna to the substrate. 
     It should be appreciated that the above-described techniques are not limited to being applied for precision-mounting of an RF antenna on a target device, as the above-described techniques may be used for precision-mounting an RF antenna on an interrogator device, in some embodiments. 
     Having thus described several aspects some embodiments, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the present disclosure. Accordingly, the foregoing description and drawings are by way of example only. 
     The above-described embodiments of the present disclosure can be implemented in any of numerous ways. For example, the embodiments may be implemented using hardware, software or a combination thereof. When implemented in software, the software code can be executed on any suitable processor or collection of processors, whether provided in a single computer or distributed among multiple computers. 
     Also, the various methods or processes outlined herein may be coded as software that is executable on one or more processors that employ any one of a variety of operating systems or platforms. Additionally, such software may be written using any of a number of suitable programming languages and/or programming or scripting tools, and also may be compiled as executable machine language code or intermediate code that is executed on a framework or virtual machine. 
     In this respect, the concepts disclosed herein may be embodied as a non-transitory computer-readable medium (or multiple computer-readable media) (e.g., a computer memory, one or more floppy discs, compact discs, optical discs, magnetic tapes, flash memories, circuit configurations in Field Programmable Gate Arrays or other semiconductor devices, or other non-transitory, tangible computer storage medium) encoded with one or more programs that, when executed on one or more computers or other processors, perform methods that implement the various embodiments of the present disclosure discussed above. The computer-readable medium or media can be transportable, such that the program or programs stored thereon can be loaded onto one or more different computers or other processors to implement various aspects of the present disclosure as discussed above. 
     The terms “program” or “software” are used herein to refer to any type of computer code or set of computer-executable instructions that can be employed to program a computer or other processor to implement various aspects of the present disclosure as discussed above. Additionally, it should be appreciated that according to one aspect of this embodiment, one or more computer programs that when executed perform methods of the present disclosure need not reside on a single computer or processor, but may be distributed in a modular fashion amongst a number of different computers or processors to implement various aspects of the present disclosure. 
     Computer-executable instructions may be in many forms, such as program modules, executed by one or more computers or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments. 
     Also, data structures may be stored in computer-readable media in any suitable form. For simplicity of illustration, data structures may be shown to have fields that are related through location in the data structure. Such relationships may likewise be achieved by assigning storage for the fields with locations in a computer-readable medium that conveys relationship between the fields. However, any suitable mechanism may be used to establish a relationship between information in fields of a data structure, including through the use of pointers, tags or other mechanisms that establish relationship between data elements. 
     Various features and aspects of the present disclosure may be used alone, in any combination of two or more, or in a variety of arrangements not specifically discussed in the embodiments described in the foregoing and is therefore not limited in its application to the details and arrangement of components set forth in the foregoing description or illustrated in the drawings. For example, aspects described in one embodiment may be combined in any manner with aspects described in other embodiments. 
     Also, the concepts disclosed herein may be embodied as a method, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
     Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.