PATENT DOCUMENT

Publication Number: US-11888504-B2
Application Number: US-202217675983-A
Country: US
Kind Code: B2

Title: Electronic devices with output load independent detection capabilities

Abstract:
An electronic device may include signal transmission circuitry such as wireless circuitry having a signal source, a signal path, and an output node coupled to an output load. The signal source may transmit a signal to the output load over the signal path. The output load may have an impedance characterized by a first reflection coefficient. A signal coupler may be disposed on the signal path. A power detector coupled to a coupled node of the signal coupler may measure a voltage at the third node. A termination coupled to an isolated node of the signal coupler may include components that cause the termination to exhibit a second reflection coefficient. The second reflection coefficient may be selected to configure the voltage at the third node to track a power wave at the output load to within a constant that is invariant as the first reflection coefficient changes over time.

Claims:
What is claimed is: 
     
       1. An electronic device comprising:
 a signal source; 
 an output load coupled to the signal source over a signal path, the signal source being configured to transmit a signal to the output load over the signal path and the output load having a first reflection coefficient; 
 a signal coupler disposed on the signal path, the signal coupler having a first node communicably coupled to the signal source, a second node communicably coupled to the output load, a third node, and a fourth node, and the signal coupler being characterized by scattering parameters S 21 , S 32 , S 22 , and S 31 ; 
 a power detector communicably coupled to the third node and configured to measure a voltage at the third node; and 
 a termination communicably coupled to the fourth node, the termination having circuit components that configure the termination to exhibit a second reflection coefficient that is directly proportional to S 21 *S 32 −S 22 *S 31 . 
 
     
     
       2. The electronic device of  claim 1 , wherein the second reflection coefficient is a function of an S 44  scattering parameter value of the signal coupler, an S 34  scattering parameter value of the signal coupler, and an S 42  scattering parameter value of the signal coupler. 
     
     
       3. The electronic device of  claim 2 , wherein the second reflection coefficient is a function of an S 41  scattering parameter value of the signal coupler and an S 24  scattering parameter value of the signal coupler. 
     
     
       4. The electronic device of  claim 3 , wherein the second reflection coefficient has a denominator that is a function of the S 44  scattering parameter value, the S 34  scattering parameter value, the S 42  scattering parameter value, the S 41  scattering parameter value, and the S 24  scattering parameter value. 
     
     
       5. The electronic device of  claim 1 , wherein the signal coupler is further characterized by scattering parameters S 44 , S 34 , S 42 , S 41 , and S 24  scattering parameter, the second reflection coefficient being inversely proportional to S 21 *(S 32 *S 44 −S 34 *S 42 )+S 22 *(S 34 *S 41 −S 31 *S 44 )+S 24 *(S 31 *S 42 −S 32 *S 41 )−. 
     
     
       6. The electronic device of  claim 1 , wherein the signal source comprises a power amplifier, the signal path comprises a radio-frequency transmission line, the output load comprises an antenna, and the signal comprises a radio-frequency signal. 
     
     
       7. The electronic device of  claim 6 , further comprising:
 one or more processors configured to adjust the antenna based at least in part on the voltage at the third node measured by the power detector. 
 
     
     
       8. The electronic device of  claim 6 , further comprising:
 one or more processors configured to reduce a transmit power level of the power amplifier based at least in part on the voltage at the third node measured by the power detector. 
 
     
     
       9. A method of operating an electronic device, the method comprising:
 transmitting, using a power amplifier, a radio-frequency signal to an output node over a transmission line, the output node being coupled to an antenna; 
 coupling, using a signal coupler disposed on the transmission line, at least some of the radio-frequency signal onto a coupled node of the signal coupler while an isolated node of the signal coupler is coupled to a termination, the signal coupler being characterized by scattering parameters S 21 , S 32 , S 22 , and S 31 , and the termination having circuit components that configure the termination to exhibit a reflection coefficient that is directly proportional to S 21 *S 32 −S 22 *S 31 ; and 
 detecting, using a power detector, a voltage of the radio-frequency signal coupled onto the coupled node. 
 
     
     
       10. The method of  claim 9 , the method further comprising:
 adjusting the antenna based at least in part on the voltage detected by the power detector. 
 
     
     
       11. The method of  claim 9 , the method further comprising:
 reducing a transmit power level of the power amplifier based at least in part on the voltage detected by the power detector. 
 
     
     
       12. The method of  claim 9 , further comprising:
 coupling, using the signal coupler, at least some of a reverse wave (RW) signal onto the isolated node while the coupled node is coupled to an additional termination causing an infinite reverse wave directivity; and 
 detecting, using the power detector, a voltage of the RW signal coupled onto the isolated node. 
 
     
     
       13. The method of  claim 9 , wherein the signal coupler is further characterized by scattering parameters S 44 , S 34 , S 42 , S 41 , and S 24 , the reflection coefficient being inversely proportional to S 21 *(S 32 *S 44 −S 34 *S 42 )+S 22 *(S 34 *S 41 −S 31 *S 44 )+S 24 *(S 31 *S 42 −S 32 *S 41 ). 
     
     
       14. An electronic device comprising:
 a power amplifier; 
 an output node coupled to an antenna; 
 a transmission line that couples the power amplifier to the output node; 
 a signal coupler disposed on the transmission line and having a coupled node and an isolated node, the signal coupler being configured to transmit a first portion of a signal wave to the output node while coupling a second portion of the signal wave onto the coupled node, and the signal coupler being characterized by scattering parameters S 21 , S 32 , S 44 , S 34 , S 42 , S 22 , S 41 , S 31 , and S 24 ; 
 a power detector communicably coupled to the coupled node; and 
 a termination communicably coupled to the isolated node, wherein the termination has circuit components that configure the termination to exhibit a reflection coefficient that is inversely proportional to S 21 *(S 32 *S 44 −S 34 *S 42 )+S 22 *(S 34 *S 41 −S 31 *S 44 )+S 24 *(S 31 *S 42 −S 32 *S 41 ). 
 
     
     
       15. The electronic device of  claim 14 , further comprising:
 a first switch that couples the coupled node to an additional termination and to the power detector; and 
 a second switch that couples the isolated node to the termination and to the power detector. 
 
     
     
       16. The electronic device of  claim 14 , the reflection coefficient being directly proportional to S 22 *S 32 −S 22 *S 31 .

Description:
FIELD 
     This disclosure relates generally to electronic devices and, more particularly, to electronic devices with circuitry for transmitting signals. 
     BACKGROUND 
     Electronic devices are often provided with signal transmission capabilities in which a signal is transmitted onto an output load. Electronic devices with signal transmission capabilities include wireless electronic devices having a wireless transmitter that transmits radio-frequency signals onto an output load such as an antenna. 
     It is often desirable to be able to measure one or more characteristics of the output load by measuring the output power of the transmitted signal at the output load. However, the presence of other circuitry at or around the output load can make it difficult to accurately measure output power at the output load, particularly as the impedance of the output load changes over time. 
     SUMMARY 
     An electronic device may include signal transmission circuitry having a signal source, a signal path, and an output node coupled to an output load. For example, the signal transmission circuitry may be part of wireless circuitry in the electronic device. The signal source may transmit a signal to the output load over the signal path. The output load may have an impedance characterized by a first reflection coefficient. 
     A signal coupler may be disposed on the signal path. The signal coupler may have a first node coupled to the signal source, a second node coupled to the output path leading to an output load, a coupled node, and an isolated node. A power detector may be coupled to the coupled node. The power detector may measure a voltage at the third node. A termination may be coupled to the isolated node. The termination may include circuit components that configure the termination to exhibit an impedance characterized by a second reflection coefficient. The impedance and the second reflection coefficient of the termination may be selected to configure the voltage at the third node to track a power wave into the output load to within a constant that is invariant as the first reflection coefficient changes over time (e.g., due to loading from external objects). Control circuitry may process the voltage measured by the power detector to accurately perform other operations such as antenna tuning or switching, backing off transmit power level, etc. 
     An aspect of the invention provides an electronic device. The electronic device can include a signal source. The electronic device can include an output load coupled to the signal source over a signal path, the signal source being configured to transmit a signal to the output load over the signal path and the output load having a first reflection coefficient. The electronic device can include a signal coupler disposed on the signal path, the signal coupler having a first node communicably coupled to the signal source, a second node communicably coupled to the output load, a third node, and a fourth node. The electronic device can include a power detector communicably coupled to the third node and configured to measure a voltage at the third node. The electronic device can include a termination communicably coupled to the fourth node, the termination having a second reflection coefficient that causes the voltage at the third node to track a power wave at the output load to within a constant that is invariant as the first reflection coefficient changes over time. 
     An aspect of the disclosure provides a method of operating an electronic device. The method can include with a power amplifier, transmitting a radio-frequency signal to an output node over a transmission line, the output node being coupled to an antenna. The method can include with a signal coupler disposed on the transmission line, coupling at least some of the radio-frequency signal onto a coupled node of the signal coupler while an isolated node of the signal coupler is coupled to a termination that causes a power wave of the radio-frequency signal coupled onto the coupled node to track a voltage of the radio-frequency signal at the output node to within a constant that is invariant as a voltage standing wave ratio (VSWR) of the antenna changes over time. The method can include with a power detector, detecting the voltage of the radio-frequency signal coupled onto the coupled node. 
     An aspect of the disclosure provides an electronic device. The electronic device can include a power amplifier. The electronic device can include an output node coupled to an antenna. The electronic device can include a transmission line that couples the power amplifier to the output node. The electronic device can include a signal coupler disposed on the transmission line and having a coupled node and an isolated node, the signal coupler being configured to transmit a first portion of a signal wave to the output node while coupling a second portion of the signal wave onto the coupled node. The electronic device can include a power detector communicably coupled to the coupled node. The electronic device can include a termination communicably coupled to the isolated node, wherein the termination has an impedance that causes a ratio of a magnitude of the second portion of the signal wave coupled onto the coupled node to a magnitude of the first portion of the signal wave transmitted to the output node to be a constant value as an impedance of the output node varies over time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a functional block diagram of an illustrative electronic device in accordance with some embodiments. 
         FIG.  2    is a diagram of illustrative signal transmission circuitry having a signal coupler and having an isolated node termination that configures a receiver to measure transmit signal power wave levels at an output load that are invariant to impedance changes at the output load in accordance with some embodiments. 
         FIG.  3    is a diagram showing how an illustrative signal coupler of the type shown in  FIG.  2    may be simplified and implemented without switches in accordance with some embodiments. 
         FIG.  4    is a circuit diagram of an illustrative isolated node termination in accordance with some embodiments. 
         FIG.  5    is a plot of voltage standing wave ratio (VSWR) variation vs. reflection coefficient of an isolated node termination showing how the isolated node termination of  FIGS.  2 - 5    may produce signal power level measurements that are invariant to VSWR variations in accordance with some embodiments. 
         FIG.  6    is a flow chart of illustrative operations that may be performed by circuitry of the type shown in  FIGS.  2 - 4    for measuring signal power levels at an output load that are invariant to impedance changes at the output load in accordance with some embodiments. 
         FIG.  7    is a diagram of an illustrative coupled node termination that may be coupled to a signal coupler for gathering forward and reverse wave measurements in accordance with some embodiments. 
         FIG.  8    is a flow chart of illustrative operations that may be performed by circuitry of the type shown in  FIG.  2    to perform forward and reverse wave measurements with minimal distortion in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     An electronic device such as device  10  of  FIG.  1    may be a computing device such as a laptop computer, a desktop computer, a computer monitor containing an embedded computer, a tablet computer, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wristwatch device, a pendant device, a headphone or earpiece device, a device embedded in eyeglasses or other equipment worn on a user&#39;s head, or other wearable or miniature device, a television, a computer display that does not contain an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, a wireless internet-connected voice-controlled speaker, a home entertainment device, a remote control device, a gaming controller, a peripheral user input device, a wireless base station or access point, a networking device, equipment that implements the functionality of two or more of these devices, or other electronic equipment. User equipment device  10  may sometimes be referred to herein as electronic device  10  or simply as device  10 . 
     As shown in the functional block diagram of  FIG.  1   , device  10  may include components located on or within an electronic device housing such as housing  12 . Housing  12 , which may sometimes be referred to as a case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, metal alloys, etc.), other suitable materials, or a combination of these materials. In some situations, parts or all of housing  12  may be formed from dielectric or other low-conductivity material (e.g., glass, ceramic, plastic, sapphire, etc.). In other situations, housing  12  or at least some of the structures that make up housing  12  may be formed from metal elements. 
     Device  10  may include control circuitry  14 . Control circuitry  14  may include storage such as storage circuitry  16 . Storage circuitry  16  may include hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid-state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Storage circuitry  16  may include storage that is integrated within device  10  and/or removable storage media. 
     Control circuitry  14  may include processing circuitry such as processing circuitry  18 . Processing circuitry  18  may be used to control the operation of device  10 . Processing circuitry  18  may include on one or more processors, microprocessors, microcontrollers, digital signal processors, host processors, baseband processor integrated circuits, application specific integrated circuits, central processing units (CPUs), graphics processing units (GPUs), etc. Control circuitry  14  may be configured to perform operations in device  10  using hardware (e.g., dedicated hardware or circuitry), firmware, and/or software. Software code for performing operations in device  10  may be stored on storage circuitry  16  (e.g., storage circuitry  16  may include non-transitory (tangible) computer readable storage media that stores the software code). The software code may sometimes be referred to as program instructions, software, data, instructions, or code. Software code stored on storage circuitry  16  may be executed by processing circuitry  18 . 
     Control circuitry  14  may be used to run software on device  10  such as satellite navigation applications, internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, etc. To support interactions with external equipment, control circuitry  14  may be used in implementing communications protocols. Communications protocols that may be implemented using control circuitry  14  include internet protocols, wireless local area network (WLAN) protocols (e.g., IEEE 802.11 protocols—sometimes referred to as Wi-Fi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol or other wireless personal area network (WPAN) protocols, IEEE 802.11 ad protocols (e.g., ultra-wideband protocols), cellular telephone protocols (e.g., 3G protocols, 4 G (LTE) protocols, 3 GPP Fifth Generation (5 G) New Radio (NR) protocols, etc.), antenna diversity protocols, satellite navigation system protocols (e.g., global positioning system (GPS) protocols, global navigation satellite system (GLONASS) protocols, etc.), antenna-based spatial ranging protocols, or any other desired communications protocols. Each communications protocol may be associated with a corresponding radio access technology (RAT) that specifies the physical connection methodology used in implementing the protocol. 
     Device  10  may include input-output circuitry  20 . Input-output circuitry  20  may include input-output devices  22 . Input-output devices  22  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  22  may include user interface devices, data port devices, and other input-output components. For example, input-output devices  22  may include touch sensors, displays (e.g., touch-sensitive and/or force-sensitive displays), light-emitting components such as displays without touch sensor capabilities, buttons (mechanical, capacitive, optical, etc.), scrolling wheels, touch pads, key pads, keyboards, microphones, cameras, buttons, speakers, status indicators, audio jacks and other audio port components, digital data port devices, motion sensors (accelerometers, gyroscopes, and/or compasses that detect motion), capacitance sensors, proximity sensors, magnetic sensors, force sensors (e.g., force sensors coupled to a display to detect pressure applied to the display), temperature sensors, etc. In some configurations, keyboards, headphones, displays, pointing devices such as trackpads, mice, and joysticks, and other input-output devices may be coupled to device  10  using wired or wireless connections (e.g., some of input-output devices  22  may be peripherals that are coupled to a main processing unit or other portion of device  10  via a wired or wireless link). 
     Input-output circuitry  20  may include wireless circuitry  24  to support wireless communications. Wireless circuitry  24  (sometimes referred to herein as wireless communications circuitry  24 ) may include two or more antennas  30 . Antennas  30  may be formed using any desired antenna structures for conveying radio-frequency signals. For example, antennas  30  may include antennas with resonating elements that are formed from loop antenna structures, patch antenna structures, inverted-F antenna structures, slot antenna structures, planar inverted-F antenna structures, helical antenna structures, monopole antennas, dipoles, hybrids of these designs, etc. Filter circuitry, switching circuitry, impedance matching circuitry, and/or other antenna tuning components may be adjusted to adjust the frequency response and wireless performance of antennas  30  over time. If desired, two or more of antennas  30  may be integrated into a phased antenna array (sometimes referred to herein as a phased array antenna) in which each of the antennas conveys radio-frequency signals with a respective phase and magnitude that is adjusted over time so the radio-frequency signals constructively and destructively interfere to produce a signal beam in a given pointing direction. 
     The term “convey radio-frequency signals” as used herein means the transmission and/or reception of the radio-frequency signals (e.g., for performing unidirectional and/or bidirectional wireless communications with external wireless communications equipment). Antennas  30  may transmit the radio-frequency signals by radiating the radio-frequency signals into free space (or to free space through intervening device structures such as a dielectric cover layer). Antennas  30  may additionally or alternatively receive the radio-frequency signals from free space (e.g., through intervening devices structures such as a dielectric cover layer). The transmission and reception of radio-frequency signals by antennas  30  each involve the excitation or resonance of antenna currents on an antenna resonating element in the antenna by the radio-frequency signals within the frequency band(s) of operation of the antenna. 
     Wireless circuitry  24  may include one or more radios  26 . Radio  26  may include circuitry that operates on signals at baseband frequencies (e.g., baseband circuitry) and radio-frequency transceiver circuitry such as one or more radio-frequency transmitters  28  and one or more radio-frequency receivers  31 . Transmitter  28  may include signal generator circuitry, modulation circuitry, mixer circuitry for upconverting signals from baseband frequencies to intermediate frequencies and/or radio frequencies, amplifier circuitry such as one or more power amplifiers, digital-to-analog converter (DAC) circuitry, control paths, power supply paths, switching circuitry, filter circuitry, and/or any other circuitry for transmitting radio-frequency signals using antennas  30 . Receiver  31  may include demodulation circuitry, mixer circuitry for downconverting signals from intermediate frequencies and/or radio frequencies to baseband frequencies, amplifier circuitry (e.g., one or more low-noise amplifiers (LNAs)), analog-to-digital converter (ADC) circuitry, control paths, power supply paths, signal paths, switching circuitry, filter circuitry, and/or any other circuitry for receiving radio-frequency signals using antennas  30 . The components of radio  26  may be mounted onto a single substrate or integrated into a single integrated circuit, chip, package, or system-on-chip (SOC) or may be distributed between multiple substrates, integrated circuits, chips, packages, or SOCs. 
     Each radio  26  may be coupled to one or more antennas  30  over one or more radio-frequency transmission lines  32 . Radio-frequency transmission lines  32  may include coaxial cables, microstrip transmission lines, stripline transmission lines, edge-coupled microstrip transmission lines, edge-coupled stripline transmission lines, transmission lines formed from combinations of transmission lines of these types, etc. Radio-frequency transmission lines  32  may be integrated into rigid and/or flexible printed circuit boards if desired. One or more radio-frequency lines  32  may be shared between multiple radios  26  if desired. Radio-frequency front end (RFFE) modules may be interposed on one or more radio-frequency transmission lines  32 . The radio-frequency front end modules may include substrates, integrated circuits, chips, or packages that are separate from radios  26  and may include filter circuitry, switching circuitry, amplifier circuitry, impedance matching circuitry, radio-frequency coupler circuitry, and/or any other desired radio-frequency circuitry for operating on the radio-frequency signals conveyed over radio-frequency transmission lines  32 . 
     Radio  26  may transmit and/or receive radio-frequency signals within corresponding frequency bands at radio frequencies (sometimes referred to herein as communications bands or simply as “bands”). The frequency bands handled by radio  26  may include wireless local area network (WLAN) frequency bands (e.g., Wi-Fi® (IEEE 802.11) or other WLAN communications bands) such as a 2.4 GHz WLAN band (e.g., from 2400 to 2480 MHz), a 5 GHz WLAN band (e.g., from 5180 to 5825 MHz), a Wi-Fi® 6E band (e.g., from 5925-7125 MHz), and/or other Wi-Fi® bands (e.g., from 1875-5160 MHz), wireless personal area network (WPAN) frequency bands such as the 2.4 GHz Bluetooth® band or other WPAN communications bands, cellular telephone frequency bands (e.g., bands from about 600 MHz to about 5 GHz, 3 G bands, 4 G LTE bands, 5 G New Radio Frequency Range 1 (FR1) bands below 10 GHz, 5G New Radio Frequency Range 2 (FR2) bands between 20 and 60 GHz, etc.), other centimeter or millimeter wave frequency bands between 10-300 GHz, near-field communications frequency bands (e.g., at 13.56 MHz), satellite navigation frequency bands (e.g., a GPS band from 1565 to 1610 MHz, a Global Navigation Satellite System (GLONASS) band, a BeiDou Navigation Satellite System (BDS) band, etc.), ultra-wideband (UWB) frequency bands that operate under the IEEE 802.15.4 protocol and/or other ultra-wideband communications protocols, communications bands under the family of 3 GPP wireless communications standards, communications bands under the IEEE 802.XX family of standards, and/or any other desired frequency bands of interest. 
     The example of  FIG.  1    is merely illustrative. While control circuitry  14  is shown separately from wireless circuitry  24  in the example of  FIG.  1    for the sake of clarity, wireless circuitry  24  may include processing circuitry (e.g., one or more processors) that forms a part of processing circuitry  18  and/or storage circuitry that forms a part of storage circuitry  16  of control circuitry  14  (e.g., portions of control circuitry  14  may be implemented on wireless circuitry  24 ). As an example, control circuitry  14  may include baseband circuitry (e.g., one or more baseband processors), digital control circuitry, analog control circuitry, and/or other control circuitry that forms part of radio  26 . The baseband circuitry may, for example, access a communication protocol stack on control circuitry  14  (e.g., storage circuitry  16 ) to: perform user plane functions at a PHY layer, MAC layer, RLC layer, PDCP layer, SDAP layer, and/or PDU layer, and/or to perform control plane functions at the PHY layer, MAC layer, RLC layer, PDCP layer, RRC, layer, and/or non-access stratum layer. If desired, the PHY layer operations may additionally or alternatively be performed by radio-frequency (RF) interface circuitry in wireless circuitry  24 . 
     Electronic devices such as device  10  may include circuitry that transmits signals. This circuitry includes a signal source, which can be modeled as an arbitrary source impedance having a source power, that is coupled to an output node over a signal path. The output node may be coupled to an output load having an output impedance. In signal transmission systems such as these, it may be desirable to be able to perform measurements of the transmit signals at the output node. For example, measurements of the output power level of the transmitted signals at the output node can be used to characterize the performance of the output load, which can then be used to calibrate subsequent signal transmissions, to adjust circuitry in device  10 , or to perform other actions. 
       FIG.  2    is a diagram of illustrative signal transmission circuitry  35  in device  10 . As shown in  FIG.  2   , signal transmission circuitry  35  may include a signal source  36  having a source impedance and a source power. Signal source  36  may be coupled to output node N over signal path  34 . Output node N may be coupled to an output load  42  over signal path  34 . Signal source  36  may transmit signals to output node N over signal path  34 . It may be desirable to be able to measure the output power level Pour of the transmitted signals at output node N (e.g., at output load  42 ). Measuring output power level Pour may, for example, allow control circuitry  14  in device  10  ( FIG.  1   ) to characterize the performance of output load  42 , which can then be used to calibrate subsequent signal transmissions, to adjust circuitry in device  10 , or to perform other actions, as examples. 
     Signal transmission circuitry  35  may, for example, form a part of wireless circuitry  24  ( FIG.  1   ). In this example, signal source  36  may be a power amplifier (e.g., in transmitter  28  of  FIG.  1   ), signal path  34  may be a radio-frequency transmission line (e.g., radio-frequency transmission line  32  of  FIG.  1   ), and output load  42  may be a corresponding antenna (e.g., antenna  30  of  FIG.  1   ). Signal source  36  may therefore sometimes be referred to herein as power amplifier (PA)  36  and signal path  34  may sometimes be referred to herein as transmission line  34 . Power amplifier  36  may transmit radio-frequency signals over transmission line  34  and antenna  30 . While implementations in which signal transmission circuitry  35  forms a part of wireless circuitry  24  for transmitting radio-frequency signals over antenna  30  are described herein as an example, signal transmission circuitry  35  may, in general, include any desired signal transmission circuitry in device  10  (e.g., for transmitting signals at any frequencies between different boards, packages, nodes, chips, integrated circuits, processors, components, accessories, devices such as device  10 , etc.). The systems and methods for measuring output power levels and otherwise characterizing the performance of output node N when signal transmission circuitry  35  forms a part of wireless circuitry  24  for transmitting radio-frequency signals over antenna  30  as described herein may be similarly applied in any of these signal transmission contexts. 
     Output load  42  may have an impedance. The impedance of output load  42  may vary (e.g., at a given frequency) due to changes in environmental conditions around output load  42 , such as when an external object  54  approaches the output load. In examples where output load  42  is an antenna, external object  54  (e.g., a user&#39;s hand or other body part) may externally load the antenna, causing the antenna to become detuned and producing an impedance discontinuity between output node N and transmission line  34 . This impedance discontinuity may cause a relatively large amount of the transmitted signal power to be reflected back towards power amplifier  36  from output node N, reducing the overall efficiency of the antenna. By measuring output power level P OUT , signal transmission circuitry  35  may measure (e.g., detect) the impedance of output load  42  (e.g., as subject to external loading by external object  54 ) and may use this information to adjust impedance matching circuitry for the antenna, to adjust tuning of the antenna, to reduce transmit power level of power amplifier  36  (e.g., to comply with regulatory limits on radio-frequency energy exposure or absorption), and/or to perform any other desired operations to characterize the performance of output load  42  or to mitigate loading by external object  54 . In general, the impedance of output load  42  is a complex value and may be characterized by the complex reflection coefficient Γ L . Reflection coefficient Γ L  may have a relatively high magnitude when a relatively large impedance discontinuity at output node N causes a relatively large amount of the transmitted signal power to be reflected back towards power amplifier  36 , for example. 
     Ideally, signal transmission circuitry  35  can measure output power level Pour by placing a power detector at the location of output node N. However, in practice, there may be one or more circuit blocks  40  interposed along transmission line  34  between power amplifier  36  and output node N (e.g., circuit blocks for performing one or more functions of device  10  that may or may not be associated with the transmission of signals at output node N). Circuit blocks  40  may include, for example, passive devices, capacitors, inductors, resistors, impedance matching circuitry, antenna tuning circuitry, routing circuitry, transmission lines, switches, filters, other couplers coupled to radio-frequency front end circuitry, transmit/receive (TR) switches connected to other radio-frequency front end circuitry, etc. The presence of circuit blocks  40  may make it infeasible or impractical to place a power detector at output node N. 
     Signal transmission circuitry  35  may therefore include a signal coupler such as signal coupler  38  interposed on transmission line  34  for detecting output power level P OUT . Signal coupler  38  may be a directional coupler or any other desired coupler that couples signals off of transmission line  34  and towards receiver  48  (e.g., a dedicated feedback receiver, a part of receiver  31  of  FIG.  1   , etc.). If desired, signal coupler  38  may include transmission line structures, transformers, or other signal coupling structures. Receiver  48  may include a power detector such as power detector (PDECT)  50  and/or any other desired circuitry for receiving and/or measuring signals coupled off of transmission line  34  by signal coupler  38 . Power detector  50  need not be integrated within a receiver such as receiver  48 , if desired. The signal coupled off of transmission line  34  may exhibit a voltage V PDET  at power detector  50 . Power detector  50  may measure voltage V PDET  and/or the power associated with voltage V PDET  (e.g., power detector  50  may convert a radio-frequency voltage waveform into a DC voltage). Control circuitry  14  ( FIG.  1   ) may process the voltage and/or power measured by power detector  50  to measure (e.g., estimate, determine, identify, compute, calculate, generate, sense, etc.) the signal or power wave at output node N, which may be characteristic of output power level Pour (e.g., without placing a power detector at output node N, thereby accommodating the presence of circuit blocks  40  along transmission line  34 ). 
     Signal coupler  38  may have a first port P1 communicably coupled to the output of power amplifier  36  over a first portion of transmission line  34 , a second port P2 communicably coupled to output node N over a second portion of transmission line  34  (e.g., where port P2 is between circuit blocks  40  and output node N or, alternatively, where the signal coupler has an internal port P2_internal at the input of circuit blocks  40 ), a third port P3 communicably coupled to receiver  48 , and a fourth port P4 communicably coupled to a termination impedance such as isolated node termination  46 . If desired, additional circuit blocks or components (not shown in  FIG.  2    for the sake of clarity) may be disposed along transmission line  34  between port P1 and the output of power amplifier  36 . Port P3 represents the coupled node of signal coupler  38  and may therefore sometimes be referred to herein as coupled node P3. Port P4 represents the isolated node of signal coupler  38  (e.g., the port/node isolated from the signal source) and may therefore sometimes be referred to herein as isolated node P4. Isolated node termination  46  may have a complex impedance characterized by a corresponding complex reflection coefficient Γ T,ISOL . Isolated node termination  46  may include one or more resistive, capacitive, inductive, and/or switching components that configure isolated node termination  46  to exhibit the impedance characterized by reflection coefficient Γ T,ISOL . 
     In the example of  FIG.  2   , signal coupler  38  is a switch coupler having switching circuitry such as switch SW1 and switch SW2. As shown in  FIG.  2   , switch SW1 may have a first terminal coupled to coupled node P3, a second terminal coupled to a termination impedance such as coupled node termination  44 , and a third port coupled to power detector  50  in receiver  48 . Coupled node termination  44  may have a complex impedance characterized by a corresponding complex reflection coefficient Γ T,COUP . Coupled node termination  44  may include one or more resistive, capacitive, inductive, and/or switching components that configure coupled node termination  44  to exhibit the impedance characterized by reflection coefficient Γ T,COUP  Switch SW2 may have a first terminal coupled to isolated node P4, a second terminal coupled to isolated node termination  46 , and a third terminal coupled to power detector  50 . 
     When configured in this way, portions of transmission line  34 , signal coupler  38 , switches SW1 and SW2, terminations  44  and  46 , circuit blocks  40 , and receiver  48  may collectively form a reflectometer  52  for transmission line  34 . During signal transmission, power amplifier  36  may transmit radio-frequency signals on transmission line  34 . These signals may sometimes be referred to as forward wave (FW) signals. The energy of the FW signals into port P1 may be characterized by coefficient al (e.g., in a four-port network model of the system). The energy (power wave) of the FW signals out of port P2 and into the output load may be characterized by a coefficient b 2  (e.g., the magnitude of the signal wave of the FW signals in the four-port network model). 
     Switches SW1 and SW2 may have a first state in which switch SW1 couples coupled node P3 to power detector  50  and switch SW2 couples isolated node P4 to isolated node termination  46 . In the first state, reflectometer  52  may perform FW measurements. Signal coupler  38  may couple some of the FW signals off of transmission line  34  and may pass the FW signals (as well as a portion of the RW signal bouncing off the isolated node termination) to power detector  50  via coupled node P3 and switch SW1. Power detector  50  may measure the amplitude and/or phase of the FW signals. 
     During signal transmission, some of the FW signals will reflect off of output node N and back towards signal coupler  38  (e.g., due to the impedance discontinuity between transmission line  34  and output node N, which may change based on the presence of external object  54 ). These reflected signals may sometimes be referred to as reverse wave (RW) signals. Switches SW1 and SW2 may also have a second state in which switch SW1 couples coupled node P3 to coupled node termination  44  and switch SW2 couples isolated node P4 to power detector  50 . In the second state, reflectometer  52  may perform RW signal measurements. Signal coupler  38  may couple some of the RW signals off of transmission line  34  and may pass the RW signals to power detector  50  via isolated node P4 and switch SW2. Power detector  50  may measure the amplitude and/or phase of the RW signals. Control circuitry  14  may process the FW signal measurements and/or the RW signal measurements to characterize (e.g., identify, determine, detect, compute, calculate, measure, etc.) the impedance of the antenna (e.g., reflection coefficient Γ L ) for performing subsequent processing operations, for example. 
     The example of  FIG.  2    in which signal coupler  38  is integrated into reflectometer  52  is merely illustrative. In general, signal coupler  38  need not be a switch coupler integrated into a reflectometer. In other words, switches SW1 and SW2 and coupled node termination  44  may be omitted if desired.  FIG.  3    is a diagram showing a simplest case of how signal coupler  38  may be interposed onto transmission line  34 . 
     As shown in  FIG.  3   , coupled node P3 may be coupled to power detector  50  and isolated node P4 may be coupled to isolated node termination  46  (e.g., without intervening switches). The simplified case of  FIG.  3    also models the reflectometer arrangement shown in  FIG.  2    when performing FW signal measurements, for example. Implementing signal coupler  38  within reflectometer  52  of  FIG.  2    may allow for additional measurements such as RW signal measurements relative to the simplified case of  FIG.  3   , which may allow signal coupler  38  to be used as part of a vector network analyzer (VNA), for example. The energy of the signals passed to power detector  50  from coupled node P3 may be characterized by coefficient b 3  (e.g., in the four-port network model). Coefficient b 3  may characterize the energy (power wave) passing to power detector  50  as shown in  FIG.  3    or may characterize the energy of the FW signals and the RW signals passing to power detector  50  as shown in  FIG.  2   . 
     Returning to  FIG.  2   , using signal coupler  38  to measure the power wave at output node N may accommodate the presence of other circuit blocks  40  at or near output node N. However, if care is not taken, measurements made using signal coupler  38  may undesirably vary as the reflection coefficient Γ L  of output load  42  varies (e.g., due to changes in the presence of external object  54 ). In other words, the ratio b 3 /b 2  may vary over different reflection coefficients Γ L  due to changes in external object  54 , which is generally outside of the control of the circuitry on device  10 . These variations can reduce the accuracy of the power measurements made by power detector  50 , thereby reducing accuracy in how device  10  characterizes the impedance of output load  42 . In examples where output load  42  is an antenna, this may reduce the accuracy of the antenna impedance as measured by the device, thereby causing the antenna to not be properly matched or tuned in different operating conditions, leading to reduced antenna efficiency. 
     To mitigate these issues, isolated node termination  46  of  FIGS.  2  and  3    may be configured to exhibit a particular complex impedance that is characterized by reflection coefficient Γ OPT,ISOL . In other words, isolated node termination  46  may include capacitive, resistive, switching, inductive, and/or other circuit components arranged in a manner (e.g., in series, in parallel, with respect to ground, etc.) that configure isolated node termination  46  to exhibit a reflection coefficient Γ T,ISOL =Γ OPT,ISOL . 
     The reflected and incident power waves at the ports of reflectometer  52  may be characterized by sixteen scattering parameters or S-parameters for a fixed frequency, which are complex numbers associated with the four-port network model of signal coupler  38  (e.g., where the first (input) port is defined by port P 1 , the second (output) port is defined by port P2, the third port is defined by coupled node P 3 , and the fourth port is defined by isolated node P4). The S-parameters include: S 11  (e.g., a reflection coefficient at the input port), S 12  (e.g., characterizing reverse voltage gain), S 13 , S 14 , S 21  (e.g., characterizing forward voltage gain), S 22  (e.g., a reflection coefficient at the output port), S 23 , S 24 , S 31 , S 32 , S 33 , S 34 , S 41 , S 42 , S 43 , and S 44 . The circuit components of isolated node termination  46  may be selected such that isolated node termination  46  exhibits an impedance characterized by a reflection coefficient Γ T,ISOL =Γ OPT,ISOL , where Γ OPT,ISOL  is given by equation 1. 
     
       
         
           
             
               
                 
                   
                     Γ 
                     
                       
                         O 
                         ⁢ 
                         P 
                         ⁢ 
                         T 
                       
                       , 
                       
                         I 
                         ⁢ 
                         S 
                         ⁢ 
                         O 
                         ⁢ 
                         L 
                       
                     
                   
                   = 
                   
                     
                       
                         
                           s 
                           
                             2 
                             ⁢ 
                             1 
                           
                         
                         ⁢ 
                         
                           s 
                           
                             3 
                             ⁢ 
                             2 
                           
                         
                       
                       - 
                       
                         
                           s 
                           
                             2 
                             ⁢ 
                             2 
                           
                         
                         ⁢ 
                         
                           s 
                           
                             3 
                             ⁢ 
                             1 
                           
                         
                       
                     
                     
                       
                         
                           s 
                           
                             2 
                             ⁢ 
                             1 
                           
                         
                         ( 
                         
                           
                             
                               s 
                               
                                 3 
                                 ⁢ 
                                 2 
                               
                             
                             ⁢ 
                             
                               s 
                               
                                 4 
                                 ⁢ 
                                 4 
                               
                             
                           
                           - 
                           
                             
                               s 
                               34 
                             
                             ⁢ 
                             
                               s 
                               42 
                             
                           
                         
                         ) 
                       
                       + 
                       
                         
                           s 
                           
                             2 
                             ⁢ 
                             2 
                           
                         
                         ( 
                         
                           
                             
                               s 
                               
                                 3 
                                 ⁢ 
                                 4 
                               
                             
                             ⁢ 
                             
                               s 
                               
                                 4 
                                 ⁢ 
                                 1 
                               
                             
                           
                           - 
                           
                             
                               s 
                               
                                 3 
                                 ⁢ 
                                 1 
                               
                             
                             ⁢ 
                             
                               s 
                               
                                 4 
                                 ⁢ 
                                 4 
                               
                             
                           
                         
                         ) 
                       
                       + 
                       
                         
                           s 
                           
                             2 
                             ⁢ 
                             4 
                           
                         
                         ( 
                         
                           
                             
                               s 
                               31 
                             
                             ⁢ 
                             
                               s 
                               42 
                             
                           
                           - 
                           
                             
                               s 
                               
                                 3 
                                 ⁢ 
                                 2 
                               
                             
                             ⁢ 
                             
                               s 
                               
                                 4 
                                 ⁢ 
                                 1 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In other words, reflection coefficient Γ OPT,ISOL  is a function of each of the S-parameters except for S 11 , S 12 , S 13 , S 14 , S 23 , S 33 , and S 43 . The numerator of reflection coefficient Γ OPT,ISOL  is a function of S 21 , S 32 , S 22 , and S 31 . The denominator of reflection coefficient Γ OPT,ISOL  is a function of S 21 , S 32 , S 44 , S 34 , S 42 , S 22 , S 41 , S 31 , S 24 , and S 42 . When configured to exhibit this impedance, isolated node termination  46  configures the ratio of coefficients b 3 /b 2  to be invariant or constant as the impedance of output load  42  (reflection coefficient Γ L ) changes over time. This in turn configures the voltage V PDET  at coupled node P3 and into power detector  50  and its corresponding power (e.g., power level P PDET  of  FIG.  3   ) to track the amplitude (magnitude) of the FW signal (power wave) at output node N (e.g., the power wave having output power level Pour) to within a constant value that does not change as the impedance of output load  42  (reflection coefficient Γ L ) changes over time (e.g., as external object  54  moves closer to or farther from output load  42 ). Put differently, isolated node terminating impedance has an impedance that allows b 3 /b 2  to be constant (e.g., equal to a constant k) independent of reflection coefficient Γ L , phase and/or magnitude of the signal at output node N, and/or the voltage standing wave ratio (VSWR) at output node N. The power levels measured by power detector  50  will therefore properly and accurately track the power wave at output node N even as the impedance (e.g., reflection coefficient Γ L , phase and/or magnitude, and/or VSWR) of output load  42  changes due to changes in the presence of external object  54  or other factors. Control circuitry  14  ( FIG.  1   ) may then use the accurate power level measurements to perform other processing operations with a high degree of accuracy and confidence across a wide variety of external loading conditions. 
       FIG.  4    is a circuit diagram of isolated node termination  46 . As shown in  FIG.  4   , isolated node termination  46  may include one or more circuit components coupled between isolated node P4 and ground. For example, isolated node termination  46  may include one or more capacitors  56  (e.g., switchable capacitors), one or more resistors  58  (e.g., switchable resistors), and/or one or more inductors  60  (e.g., switchable inductors) coupled between isolated node P4 and ground. If desired, isolated node termination  46  may include multiple chains of components that can be switched into use for covering different frequencies or channels. Capacitor(s)  56 , resistor(s)  58 , and inductor(s)  60  may be selected to configure isolated node termination  46  to exhibit an impedance as characterized by the reflection coefficient Γ OPT,ISOL  of equation 1. The example of  FIG.  4    is merely illustrative. In general, isolated node termination  46  may include any desired combination of non-switched components, switched components, switches, resistors, capacitors, inductors, etc., arranged in parallel, in series, or in any desired manner between isolated node P4 and ground that configure isolated node termination  46  to have an impedance as given by reflection coefficient Γ OPT,ISOL  of equation 1. 
       FIG.  5    is a plot (Smith chart) of VSWR variation vs. different values of reflection coefficient Γ T,ISOL  for isolated node termination  46 . Regions  62 - 80  may be populated by points, where each point is a different value of reflection coefficient Γ T,ISOL  for isolated node termination  46 . As shown in  FIG.  5   , points within region  62  exhibit a large amount of VSWR variation and therefore correspond to reflection coefficients Γ T,ISOL  for isolated node termination  46  that produce relatively inaccurate power level measurements as the impedance of output load  42  changes. Points within region  64  exhibit less VSWR variation than points within region  64 , points within region  66  exhibit less VSWR variation than points within region  64 , points within region  68  exhibit less VSWR variation than points within region  66 , points within region  70  exhibit less VSWR variation than points within region  68 , points within region  72  exhibit less VSWR variation than points within region  70 , points within region  74  exhibit less VSWR variation than points within region  72 , points within region  76  exhibit less VSWR variation than points within region  74 , points within region  78  exhibit less VSWR variation than points within region  76 , and points within region  80  exhibit less VSWR variation than points within region  78 . 
     As shown by the relative size of regions  62 - 80 , the amount of VSWR variation steadily decreases as the reflection coefficient Γ T,ISOL  for isolated node termination  46  approaches (e.g., collapses onto) point  82 . Point  82  corresponds to the reflection coefficient Γ T,ISOL =Γ OPT,ISOL , as given by equation  1 . In other words, by configuring reflection coefficient Γ T,ISOL  to equal Γ OPT,ISOL , the power level measured by power detector  50  ( FIGS.  2  and  3   ) becomes VSWR insensitive, as the power level perfectly tracks the power wave at the output load regardless of changes in the reflection coefficient Γ L  of the output load. 
       FIG.  6    is a flow chart of illustrative operations that may be performed by device  10  for gathering and processing measurements made by power detector  50  of  FIGS.  2  and  3   . At operation  84 , power amplifier  36  may transmit radio-frequency signals on transmission line  34 . Signal coupler  38  may couple some of the signals off of transmission line  34  and onto coupled node P3. Power detector  50  may measure the voltage V PDET  and/or the corresponding power P PDET  ( FIG.  3   ) on coupled node P3. Configuring isolated node termination  46  to exhibit an impedance given by Γ T,ISOL =Γ OPT,ISOL  may configure voltage V PDET , power P PDET , and coefficient b 3  to perfectly track the power wave at output node N (e.g., characterizing output power Pour when the magnitude of Γ L  is known) even as reflection coefficient Γ L  of output load  42  changes. In general, V PDET , P PDET , and b 3  will track the power wave at output node N, which is characterized by coefficient b 2  and is sometimes referred to herein as power wave b 2 , since V PDET , P PDET , and b 3  are all related to one another by a constant (whereas voltage/power at output node N is dependent on the output load itself). By measuring voltage V PDET  and/or power P PDET , power detector  50  may thereby accurately measure (e.g., generate, compute, gather, sense, detect, identify, etc.) power wave or signal wave at the output node, which is characteristic of output power level P OUT , regardless of the current reflection coefficient Γ L  of output load  42 . Equivalently, V PDET , P PDET , and b 3  will perfectly track the output power P OUT  at output node N for a fixed output load magnitude and thus may be used to measure output power level Pour for the fixed output load magnitude (e.g., independent of the load reflection coefficient phase). 
     At operation  86 , control circuitry  14  may perform any desired operations based on the measured power or signal wave (e.g., output power level Pour) detected by power detector  50  via coupled node P 3 . For example, control circuitry  14  may tune antenna  30  (e.g., to compensate for detuning due to loading by external object  54 ), may adjust impedance matching circuitry for antenna  30  (e.g., to allow transmission line  34  to match the impedance of antenna  30  to maximize energy transfer), may switch a different antenna into use or reduce the transmit power level of power amplifier  36  (e.g., to satisfy regulatory requirements on radio-frequency energy absorption and/or exposure given the presence of a body part adjacent antenna  30 ), may display or otherwise issue a notification, and/or may perform any other desired operations based on the detected power level. Processing may subsequently loop back to operation  84 . 
     When signal coupler  38  is integrated into a reflectometer such as reflectometer  52  of  FIG.  2   , coupled node termination  44  may be coupled to coupled node P3 during RW signal measurements.  FIG.  7    is a diagram of coupled node termination  44  in these implementations. As shown in  FIG.  7   , coupled node termination  44  may have circuit components such as one or more (switchable) capacitors C, one or more (switchable) resistors R, and/or one or more (switchable) inductors L that configure coupled node termination  44  to have an impedance characterized by an infinite reverse directivity, such that reflection coefficient Γ T,COUP =Γ OPT,COUP . The components of coupled node termination  44  may configure coupled node termination  44  to have a reflection coefficient Γ T,COUP =Γ OPT,COUP =S 41 /(S 41 S 33 −S 43 S 31 ), for example. In these arrangements, configuring isolated node termination  46  to have a reflection coefficient Γ T,ISOL =Γ OPT,ISOL  (e.g., as given by equation 1) may serve to minimize distortion in the measurements gathered using signal coupler  38 . 
       FIG.  8    is a flow chart of illustrative operations that may be performed by signal transmission circuitry  35  of  FIG.  2    to perform forward and reverse wave measurements with minimal distortion. The operations of  FIG.  8    may, for example, be performed while processing operation  84  of  FIG.  6   . At operation  88 , power amplifier  36  may begin transmitting signals. 
     At operation  90 , signal coupler  38  may perform forward path measurements (e.g., to measure the FW signal) while switch SW1 couples coupled node P3 to receiver  48  and switch SW2 couples isolated node P4 to isolated node termination  46  having reflection coefficient Γ T,ISOL =Γ OPT,ISOL  (e.g., as given by equation 1). Power detector  50  may measure the FW signal on coupled node P3 (as well as a portion of the RW signal bouncing off the isolated node termination). 
     At operation  92 , signal coupler  38  may perform reverse path measurements (e.g., to measure the RW signal) while switch SW2 couples isolated node P4 to receiver  48  and switch SW1 couples coupled node P3 to coupled node termination  44  having reflection coefficient Γ T,COUP =Γ OPT,COUP . Power detector  50  may measure the RW signal on isolated node P4. 
     At operation  94 , control circuitry  14  ( FIG.  1   ) may process the forward and/or reverse path measurements (e.g., to characterize the impedance of output load  42 ). Configuring isolated node termination  46  to have reflection coefficient Γ T,ISOL =Γ OPT,ISOL  (e.g., as given by equation 1) may minimize distortion in the measurements. The example of  FIG.  8    is merely illustrative. If desired, operation  90  or operation  92  may be omitted. Operation  92  may be performed prior to operation  90  if desired. 
     Configuring isolated node termination  46  to have reflection coefficient Γ T,ISOL =Γ OPT,ISOL  (e.g., as given by equation 1) may minimize distortion in the measurements made using signal coupler  38  of  FIG.  2   . When estimating reflection coefficient Γ L  using signal coupler  38  while isolated node termination  46  is terminated with infinite forward directivity (e.g., where Γ T,ISOL =Γ ∞,F ), the estimated reflection coefficient Γ L  will exhibit substantial distortion, producing errors in the estimated reflection coefficient. However, when isolated node termination  46  is terminated with reflection coefficient Γ T,ISOL =Γ OPT,ISOL  (e.g., as given by equation 1), there may be almost no distortion in the estimated reflection coefficient Γ L  (e.g., producing just a constant multiplier on the value of Γ L ), thereby producing an accurate estimated reflection coefficient. This may, for example, configure signal coupler  38  to form an accurate VNA for measuring the impedance of a particular point (e.g., output node N). 
     The examples of  FIGS.  2 - 8    are merely illustrative. While described in connection with radio-frequency signal transmission as an example, signal coupler  38  may be used to gather measurements at any desired frequencies for characterizing the impedance, phase, magnitude, power wave level, and/or VSWR of any desired output load  42  and output node N accurately, even as reflection coefficient Γ L  of the output load changes over time. 
     Device  10  may gather and/or use personally identifiable information. It is well understood that the use of personally identifiable information should follow privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining the privacy of users. In particular, personally identifiable information data should be managed and handled so as to minimize risks of unintentional or unauthorized access or use, and the nature of authorized use should be clearly indicated to users. 
     The methods and operations described above in connection with  FIGS.  1 - 9    may be performed by the components of device  10  using software, firmware, and/or hardware (e.g., dedicated circuitry or hardware). Software code for performing these operations may be stored on non-transitory computer readable storage media (e.g., tangible computer readable storage media) stored on one or more of the components of device  10  (e.g., storage circuitry  16  of  FIG.  1   ). The software code may sometimes be referred to as software, data, instructions, program instructions, or code. The non-transitory computer readable storage media may include drives, non-volatile memory such as non-volatile random-access memory (NVRAM), removable flash drives or other removable media, other types of random-access memory, etc. Software stored on the non-transitory computer readable storage media may be executed by processing circuitry on one or more of the components of device  10  (e.g., processing circuitry  18  of  FIG.  1   , etc.). The processing circuitry may include microprocessors, central processing units (CPUs), application-specific integrated circuits with processing circuitry, or other processing circuitry. 
     The foregoing is merely illustrative and various modifications can be made to the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20220218
Publication Date: 20240130
Grant Date: 20240130
Priority Date: 20220218
Inventors: CALDERIN, LUCAS
OZGUN, MEHMET T
Assignee: APPLE INC
CPC Classifications: [{"code": "H04B1/0475", "inventive": true, "first": true, "tree": "[]"}, {"code": "H03F3/245", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F2200/451", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B2001/0416", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R21/00", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04B17/103", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04B1/0475", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R27/06", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01R21/01", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F3/245", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F2200/451", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04B2001/0416", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 85036085