PATENT DOCUMENT

Publication Number: US-9070968-B2
Application Number: US-201213437804-A
Country: US
Kind Code: B2

Title: Methods for characterizing tunable radio-frequency elements in wireless electronic devices

Abstract:
A wireless electronic device may contain an antenna tuning element for tuning the device&#39;s operating frequency range. The antenna tuning element may include radio-frequency switches, continuously/semi-continuously adjustable components such as tunable resistors, inductors, and capacitors, etc. A test system may be used to measure the radio-frequency characteristics associated with the tuning element assembled with an electronic device. The test system may include a test host, a test chamber, a signal generator, power meters, and radio-frequency testers. The electronic device under test (DUT) may be placed in the test chamber. The signal generator may generate radio-frequency test signals for energizing the antenna tuning element. The power meters and radio-frequency testers may be used to measure conducted and radiated signals emitted from the DUT while the DUT is placed in different desired orientations. A phantom object is optionally placed in the vicinity of the DUT to simulate actual user scenario.

Claims:
What is claimed is: 
     
       1. A radio-frequency test system for testing an electronic device having an antenna, comprising:
 a signal generator configured to provide radio-frequency test signals to the antenna via an input line, wherein the antenna is disposed within a housing of the electronic device; 
 a first radio-frequency tester configured to measure signals that are reflected back from the antenna on the input line as a result of providing the radio-frequency test signals with the signal generator; and 
 a second radio-frequency tester configured to measure signals that are radiated from the antenna as a result of providing the radio-frequency test signals with the signal generator. 
 
     
     
       2. The radio-frequency test system defined in  claim 1  wherein the antenna comprises a tunable antenna having an adjustable antenna tuning element. 
     
     
       3. The radio-frequency test system defined in  claim 2  wherein the adjustable antenna tuning element comprises at least one adjustable circuit selected from the group consisting of: a radio-frequency switch, a tunable resistive component, a tunable capacitive component, and a tunable inductive component. 
     
     
       4. The radio-frequency test system defined in  claim 3 , further comprising:
 an anechoic test chamber within which the electronic device is placed during testing. 
 
     
     
       5. The radio-frequency test system defined in  claim 4 , further comprising:
 a phantom object placed within the anechoic test chamber, wherein the phantom object serves to simulate a user body part; 
 a positioner in the anechoic test chamber operable to place the electronic device in different positions relative to the phantom object during testing. 
 
     
     
       6. The radio-frequency test system defined in  claim 4 , further comprising:
 a test antenna placed within the anechoic test chamber, wherein the test antenna is configured to receive the signals radiated from the antenna; 
 a radio-frequency power amplifier, 
 at least one radio-frequency switch; and 
 at least one filter circuit, wherein the radio-frequency power amplifier, the at least one radio-frequency switch, and the at least one filter circuit are coupled between the test antenna and the second radio-frequency tester. 
 
     
     
       7. The radio-frequency test system defined in  claim 6 , wherein the at least one filter circuit comprises a high pass filter circuit. 
     
     
       8. The radio-frequency test system defined in  claim 3 , further comprising:
 a radio-frequency power amplifier, 
 at least one radio-frequency switch; and 
 at least one filter circuit, wherein the radio-frequency power amplifier, the at least one radio-frequency switch, and the at least one filter circuit are interposed in the input line between the signal generator and the antenna. 
 
     
     
       9. The radio-frequency test system defined in  claim 8 , wherein the at least one filter circuit comprises a low pass filter circuit. 
     
     
       10. The radio-frequency test system defined in  claim 3 , further comprising:
 a first power meter configured to measure power levels for signals that are reflected back from the antenna on the input line as a result of providing the radio-frequency test signals with the signal generator; and 
 a second power meter configured to measure power levels for signals that are being delivered to the antenna on the input line. 
 
     
     
       11. A method for using a radio-frequency test system to test an electronic device under test, wherein the electronic device under test contains a tunable antenna having an adjustable antenna tuning element and the radio-frequency test system includes a signal generator and a test chamber, the method comprising:
 placing the adjustable antenna tuning element in respective states; 
 with the signal generator, providing radio-frequency test signals to the tunable antenna via a radio-frequency test probe that is in direct contact with the tunable antenna; 
 simulating signal interference from a user body part by placing a phantom object in the vicinity of the electronic device during testing; and 
 while the phantom object is placed in the vicinity of the electronic device, measuring antenna performance for the tunable antenna when the adjustable antenna tuning element is placed in each of the respective states. 
 
     
     
       12. The method defined in  claim 11  wherein the adjustable antenna tuning element comprises at least one adjustable circuit selected from the group consisting of: a radio-frequency switch, a tunable resistive component, a tunable capacitive component, and a tunable inductive component. 
     
     
       13. The method defined in  claim 12 , further comprising:
 with a positioner in the test chamber, placing the electronic device under testing in different orientations while measuring the antenna performance. 
 
     
     
       14. The method defined in  claim 12 , wherein providing radio-frequency test signals to the tunable antenna comprises providing radio-frequency test signals transmitted at a given frequency to the tunable antenna, and wherein measuring the antenna performance for the tunable antenna comprises:
 with a radio-frequency tester, measuring signal levels at harmonic frequencies that are equal to integer multiples of the given frequency for signals that are reflected back from the tunable antenna on the input line as a result of providing the radio-frequency test signals with the signal generator. 
 
     
     
       15. The method defined in  claim 12 , wherein providing radio-frequency test signals to the tunable antenna comprises providing radio-frequency test signals transmitted at a given frequency to the tunable antenna, and wherein measuring the antenna performance for the tunable antenna comprises:
 with a radio-frequency tester, measuring signal levels at harmonic frequencies that are equal to integer multiples of the given frequency for signals that are radiated from the tunable antenna as a result of providing the radio-frequency test signals with the signal generator. 
 
     
     
       16. A method for using a test system to test an electronic device having a tunable antenna, wherein the test system includes a signal generator and a plurality of radio-frequency testers, the method comprising:
 with the signal generator, providing radio-frequency test signals to the tunable antenna via an input line; 
 with a first radio-frequency tester in the plurality of radio-frequency testers, measuring signals that are reflected back from the tunable antenna on the input line as a result of providing the radio-frequency test signals with the signal generator; and 
 with a second radio-frequency tester in the plurality of radio-frequency testers, measuring signals that are radiated from the tunable antenna as a result of providing the radio-frequency test signals with the signal generator, wherein providing the radio-frequency test signals to the tunable antenna comprises providing the radio-frequency test signals transmitted at a given frequency to the tunable antenna, the first radio-frequency tester comprises a spectrum analyzer, and measuring the signals that are reflected back from the tunable antenna comprises measuring signal levels at harmonic frequencies that are equal to integer multiples of the given frequency for the signals that are reflected back from the tunable antenna with the spectrum analyzer. 
 
     
     
       17. The method defined in  claim 16 , wherein the first radio-frequency tester comprises a power meter, and wherein measuring the signals that are reflected back from the tunable antenna comprises obtaining reflection coefficient data with the power meter. 
     
     
       18. The method defined in  claim 16 , wherein the tunable antenna includes an adjustable antenna tuning element, the method further comprising:
 placing the adjustable antenna tuning element in respective states; and 
 simulating signal interference from a user body part by placing a phantom object in the vicinity of the electronic device during testing when the adjustable antenna tuning element is placed in each of the respective states. 
 
     
     
       19. A method for using a test system to test an electronic device having a tunable antenna, wherein the test system includes a signal generator and a plurality of radio-frequency testers, the method comprising:
 with the signal generator, providing radio-frequency test signals to the tunable antenna via an input line; 
 with a first radio-frequency tester in the plurality of radio-frequency testers, measuring signals that are reflected back from the tunable antenna on the input line as a result of providing the radio-frequency test signals with the signal generator; and 
 with a second radio-frequency tester in the plurality of radio-frequency testers, measuring signals that are radiated from the tunable antenna as a result of providing the radio-frequency test signals with the signal generator, wherein providing the radio-frequency test signals to the tunable antenna comprises providing the radio-frequency test signals transmitted at a given frequency to the tunable antenna, the second radio-frequency tester comprises a spectrum analyzer, and measuring the signals that are radiated from the tunable antenna comprises measuring signal levels at harmonic frequencies that are equal to integer multiples of the given frequency for the signals that are radiated from the tunable antenna with the spectrum analyzer.

Description:
BACKGROUND 
     This relates generally to wireless communications circuitry, and more particularly, to electronic devices that have wireless communications circuitry. 
     Electronic devices such as portable computers and cellular telephones are often provided with wireless communications capabilities. For example, electronic devices may use long-range wireless communications circuitry such as cellular telephone circuitry to communicate using cellular telephone bands. Electronic devices may use short-range wireless communications circuitry such as wireless local area network communications circuitry to handle communications with nearby equipment. Electronic devices may also be provided with satellite navigation system receivers and other wireless circuitry. 
     To satisfy consumer demand for small form factor wireless devices, manufacturers are continually striving to implement wireless communications circuitry such as antenna components using compact structures. However, it can be difficult to fit conventional antenna structures into small devices. For example, antennas that are confined to small volumes often exhibit narrower operating bandwidths than antennas that are implemented in larger volumes. If the bandwidth of an antenna becomes too small, the antenna will not be able to cover all communications bands of interest. 
     In view of these considerations, it would be desirable to provide antenna tuning elements that allow the antenna to cover a wider range of frequency bands. Moreover, it may be desirable to provide ways for characterizing the performance of such types of tuning elements when assembled within a wireless electronic device. 
     SUMMARY 
     A wireless electronic device may include storage and processing circuitry and wireless communications circuitry. The wireless communications circuitry may include a baseband processor, transceiver circuitry, and at least one antenna. The antenna may include an antenna resonating element and at least one antenna tuning element. The antenna tuning element may be used to help the antenna cover a wider range of communications frequencies than would otherwise be possible. 
     The tunable element may include radio-frequency switches, continuously or semi-continuously tunable resistive/inductive/capacitive components forming using integrated circuits, discrete surface mount components, or other suitable conductive structures, and other load circuits configured to provide desired impedance characteristics for the antenna at selected frequencies. 
     In accordance with an embodiment of the present invention, a test system may be provided that includes a test host, a test chamber, a signal generator, power meters, radio-frequency testers, and other test equipment (e.g., radio-frequency switches, power amplifiers, isolators, filters, etc.). An electronic device under test (DUT) containing an antenna tuning element may be placed in the test chamber. The test host may configure the signal generator to provide radio-frequency test signals at a selected fundamental frequency to the antenna tuning element in the DUT. A first of the power meters may be used to measure the power level at which the test signals are being delivered to the DUT. A second of the power meters may be used to measure the power kevel of signals that are reflected back from the DUT. A first of the testers (e.g., a first spectrum) may be used to measure the harmonic distortion levels (i.e., 2 nd  order harmonic distortion, 3 rd  order harmonic distortion, 4 th  order harmonic distortion, etc.) for the signals being reflected back from the DUT. 
     The test host may control a positioner in the test chamber to vary the position/orientation of the DUT within the test chamber during testing. A phantom object may be placed in the vicinity of the DUT to facilitate in simulating actual user scenarios in which a user&#39;s hand or body part may impact the performance of the DUT. 
     In response to receiving the radio-frequency test signals from the signal generator, the antenna tuning element may radiate corresponding radio-frequency signals. A test antenna (e.g., a horn antenna) within the test chamber may be used to receive the radiated radio-frequency signals from the DUT. The radiated signals may be separated accordingly to their polarization orientation. A first portion of the radiated signals that are horizontally polarized may be fed to a second of the testers (e.g., a second spectrum analyzer), whereas a second portion of the radiated signals that are vertically polarized may be fed to a third of the testers (e.g., a third spectrum analyzer). The second and third spectrum analyzers may be used to measure the harmonic distortion levels for the radiated radio-frequency signals. 
     The test host may retrieve the radio-frequency measurement results obtained using the power meters and the spectrum analyzers and may compare the test results to predetermined threshold levels to determine whether the antenna tuning element and the antenna of the DUT satisfy design criteria. In the scenario in which the test results fail to satisfy design criteria, the antenna tuning element may be removed and inspected for possible manufacturing defects, the position of the antenna tuning element within the DUT may be changed to help minimize radio-frequency interference, and/or other changes can be made in an effort to obtain satisfactory test results. 
     Further features of the present invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device with wireless communications circuitry in accordance with an embodiment of the present invention. 
         FIG. 2  is a diagram showing how radio-frequency transceiver circuitry may be coupled to one or more antennas within an electronic device of the type shown in  FIG. 1  in accordance with an embodiment of the present invention. 
         FIG. 3  is a circuit diagram showing how an antenna in the electronic device of  FIG. 1  may be coupled to radio-frequency transceiver circuitry in accordance with an embodiment of the present invention. 
         FIGS. 4A ,  4 B, and  4 C are schematic diagrams of an illustrative inverted-F antenna containing antenna tuning elements in accordance with an embodiment of the present invention. 
         FIGS. 5A and 5B  are plots showing how antennas containing tunable elements may be used to cover multiple communications bands of interest in accordance with an embodiment of the present invention. 
         FIGS. 6A and 6B  are circuit diagrams of illustrative switchable load circuits that may be used as antenna tuning elements in accordance with an embodiment of the present invention. 
         FIG. 6C  is a circuit diagram of an illustrative variable capacitor circuit that may be used as an antenna tuning element in accordance with an embodiment of the present invention. 
         FIG. 7  is a diagram of an illustrative test system for characterizing an electronic device having an antenna tuning element in accordance with an embodiment of the present invention. 
         FIG. 8  is a flow chart of illustrative steps for using the test system of  FIG. 7  in accordance with an embodiment of the present invention. 
         FIG. 9  is a plot showing measured power level versus input power level illustrating harmonic distortion in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as device  10  of  FIG. 1  may be provided with wireless communications circuitry. The wireless communications circuitry may be used to support long-range wireless communications such as communications in cellular telephone bands. Examples of long-range (cellular telephone) bands that may be handled by device  10  include the 800 MHz band, the 850 MHz band, the 900 MHz band, the 1800 MHz band, the 1900 MHz band, the 2100 MHz band, the 700 MHz band, and other bands. The long-range bands used by device  10  may include the so-called LTE (Long Term Evolution) bands. The LTE bands are numbered (e.g.,  1 ,  2 ,  3 , etc.) and are sometimes referred to as E-UTRA operating bands. Long-range signals such as signals associated with satellite navigation bands may be received by the wireless communications circuitry of device  10 . For example, device  10  may use wireless circuitry to receive signals in the 1575 MHz band associated with Global Positioning System (GPS) communications. Short-range wireless communications may also be supported by the wireless circuitry of device  10 . For example, device  10  may include wireless circuitry for handling local area network links such as WiFi® links at 2.4 GHz and 5 GHz, Bluetooth® links at 2.4 GHz, etc. 
     As shown in  FIG. 1 , device  10  may include storage and processing circuitry  28 . Storage and processing circuitry  28  may include storage such as 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. Processing circuitry in storage and processing circuitry  28  may be used to control the operation of device  10 . This processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, application specific integrated circuits, etc. 
     Storage and processing circuitry  28  may be used to run software on device  10 , such as internet browsing applications, voice-over-internet-protocol (VOIP) telephone call applications, email applications, media playback applications, operating system functions, functions related to communications band selection during radio-frequency transmission and reception operations, etc. To support interactions with external equipment such as base station  21 , storage and processing circuitry  28  may be used in implementing communications protocols. Communications protocols that may be implemented using storage and processing circuitry  28  include internet protocols, wireless local area network protocols (e.g., IEEE 802.11 protocols—sometimes referred to as WiFi®), protocols for other short-range wireless communications links such as the Bluetooth® protocol, IEEE 802.16 (WiMax) protocols, cellular telephone protocols such as the “2G” Global System for Mobile Communications (GSM) protocol, the “2G” Code Division Multiple Access (CDMA) protocol, the “3G” Universal Mobile Telecommunications System (UMTS) protocol, and the “4G” Long Term Evolution (LTE) protocol, MIMO (multiple input multiple output) protocols, antenna diversity protocols, etc. Wireless communications operations such as communications band selection operations may be controlled using software stored and running on device  10  (i.e., stored and running on storage and processing circuitry  28  and/or input-output circuitry  30 ). 
     Input-output circuitry  30  may include input-output devices  32 . Input-output devices  32  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  32  may include user interface devices, data port devices, and other input-output components. For example, input-output devices may include touch screens, displays without touch sensor capabilities, buttons, joysticks, click wheels, scrolling wheels, touch pads, key pads, keyboards, microphones, cameras, buttons, speakers, status indicators, light sources, audio jacks and other audio port components, digital data port devices, light sensors, motion sensors (accelerometers), capacitance sensors, proximity sensors, etc. 
     Input-output circuitry  30  may include wireless communications circuitry  34  for communicating wirelessly with external equipment. Wireless communications circuitry  34  may include radio-frequency (RF) transceiver circuitry formed from one or more integrated circuits, power amplifier circuitry, low-noise input amplifiers, passive RF components, one or more antennas, transmission lines, and other circuitry for handling RF wireless signals. Wireless signals can also be sent using light (e.g., using infrared communications). 
     Wireless communications circuitry  34  may include radio-frequency transceiver circuitry  90  for handling various radio-frequency communications bands. For example, circuitry  90  may include transceiver circuitry  36 ,  38 , and  42 . Transceiver circuitry  36  may handle 2.4 GHz and 5 GHz bands for WiFi® (IEEE 802.11) communications and may handle the 2.4 GHz Bluetooth® communications band. Circuitry  34  may use cellular telephone transceiver circuitry  38  for handling wireless communications in cellular telephone bands such as at 850 MHz, 900 MHz, 1800 MHz, 1900 MHz, and 2100 MHz and/or the LTE bands and other bands (as examples). Circuitry  38  may handle voice data and non-voice data traffic. 
     Transceiver circuitry  90  may include global positioning system (GPS) receiver equipment such as GPS receiver circuitry  42  for receiving GPS signals at 1575 MHz or for handling other satellite positioning data. In WiFi® and Bluetooth® links and other short-range wireless links, wireless signals are typically used to convey data over tens or hundreds of feet. In cellular telephone links and other long-range links, wireless signals are typically used to convey data over thousands of feet or miles. 
     Wireless communications circuitry  34  may include one or more antennas  40 . Antennas  40  may be formed using any suitable antenna types. For example, antennas  40  may include antennas with resonating elements that are formed from loop antenna structure, patch antenna structures, inverted-F antenna structures, slot antenna structures, planar inverted-F antenna structures, helical antenna structures, hybrids of these designs, etc. Different types of antennas may be used for different bands and combinations of bands. For example, one type of antenna may be used in forming a local wireless link antenna and another type of antenna may be used in forming a remote wireless link antenna. 
     As shown in  FIG. 1 , wireless communications circuitry  34  may also include baseband processor  88 . Baseband processor may include memory and processing circuits and may also be considered to form part of storage and processing circuitry  28  of device  10 . 
     Baseband processor  88  may be used to provide data to storage and processing circuitry  28 . Data that is conveyed to circuitry  28  from baseband processor  88  may include raw and processed data associated with wireless (antenna) performance metrics for received signals such as received power, transmitted power, frame error rate, bit error rate, channel quality measurements based on received signal strength indicator (RSSI) information, channel quality measurements based on received signal code power (RSCP) information, channel quality measurements based on reference symbol received power (RSRP) information, channel quality measurements based on signal-to-interference ratio (SINR) and signal-to-noise ratio (SNR) information, channel quality measurements based on signal quality data such as Ec/Io or Ec/No data, information on whether responses (acknowledgements) are being received from a cellular telephone tower corresponding to requests from the electronic device, information on whether a network access procedure has succeeded, information on how many re-transmissions are being requested over a cellular link between the electronic device and a cellular tower, information on whether a loss of signaling message has been received, information on whether paging signals have been successfully received, and other information that is reflective of the performance of wireless circuitry  34 . This information may be analyzed by storage and processing circuitry  28  and/or processor  88  and, in response, storage and processing circuitry  28  (or, if desired, baseband processor  58 ) may issue control commands for controlling wireless circuitry  34 . For example, baseband processor  88  may issue commands that direct transceiver circuitry  90  to switch into use desired transmitters/receivers and antennas. 
     Antenna diversity schemes may be implemented in which multiple redundant antennas are used in handling communications for a particular band or bands of interest. In an antenna diversity scheme, storage and processing circuitry  28  may select which antenna to use in real time based on signal strength measurements or other data. In multiple-input-multiple-output (MIMO) schemes, multiple antennas may be used in transmitting and receiving multiple data streams, thereby enhancing data throughput. 
     Illustrative locations in which antennas  40  may be formed in device  10  are shown in  FIG. 2 . As shown in  FIG. 2 , electronic device  10  may have a housing such as housing  12 . Housing  12  may include plastic walls, metal housing structures, structures formed from carbon-fiber materials or other composites, glass, ceramics, or other suitable materials. Housing  12  may be formed using a single piece of material (e.g., using a unibody configuration) or may be formed from a frame, housing walls, and other individual parts that are assembled to form a completed housing structure. The components of device  10  that are shown in  FIG. 1  may be mounted within housing  12 . Antenna structures  40  may be mounted within housing  12  and may, if desired, be formed using parts of housing  12 . For example, housing  12  may include metal housing sidewalls, peripheral conductive members such as band-shaped members (with or without dielectric gaps), conductive bezels, and other conductive structures that may be used in forming antenna structures  40 . 
     As shown in  FIG. 2 , antenna structures  40  may be coupled to transceiver circuitry  90  by paths such as paths  45 . Paths  45  may include transmission line structures such as coaxial cables, microstrip transmission lines, stripline transmission lines, etc. Impedance matching circuitry, filter circuitry, and switching circuitry may be interposed in paths  45  (as examples). Impedance matching circuitry may be used to ensure that antennas  40  are efficiently coupled to transceiver circuitry  90  in desired frequency bands of interest. Filter circuitry may be used to implement frequency-based multiplexing circuits such as diplexers, duplexers, and triplexers. Switching circuitry may be used to selectively couple antennas  40  to desired ports of transceiver circuitry  90 . For example, a switch may be configured to route one of paths  45  to a given antenna in one operating mode. In another operating mode, the switch may be configured to route a different one of paths  45  to the given antenna. The use of switching circuitry between transceiver circuitry  90  and antennas  40  allows device  10  to switch particular antennas  40  in and out of use depending on the current performance associated with each of the antennas. 
     In a device such as a cellular telephone that has an elongated rectangular outline, it may be desirable to place antennas  40  at one or both ends of the device. As shown in  FIG. 2 , for example, some of antennas  40  may be placed in upper end region  42  of housing  12  and some of antennas  40  may be placed in lower end region  44  of housing  12 . The antenna structures in device  10  may include a single antenna in region  42 , a single antenna in region  44 , multiple antennas in region  42 , multiple antennas in region  44 , or may include one or more antennas located elsewhere in housing  12 . 
     Antenna structures  40  may be formed within some or all of regions such as regions  42  and  44 . For example, an antenna such as antenna  40 T- 1  may be located within region  42 - 1  or an antenna such as antenna  40 T- 2  may be formed that fills some or all of region  42 - 2 . Similarly, an antenna such as antenna  40 B- 1  may fill some or all of region  44 - 2  or an antenna such as antenna  40 B- 2  may be formed in region  44 - 1 . These types of arrangements need not be mutually exclusive. For example, region  44  may contain a first antenna such as antenna  40 B- 1  and a second antenna such as antenna  40 B- 2 . 
     Transceiver circuitry  90  may contain transmitters such as radio-frequency transmitters  48  and receivers such as radio-frequency receivers  50 . Transmitters  48  and receivers  50  may be implemented using one or more integrated circuits (e.g., cellular telephone communications circuits, wireless local area network communications circuits, circuits for Bluetooth® communications, circuits for receiving satellite navigation system signals, power amplifier circuits for increasing transmitted signal power, low noise amplifier circuits for increasing signal power in received signals, other suitable wireless communications circuits, and combinations of these circuits). 
       FIG. 3  is a diagram showing how radio-frequency path  45  may be used to convey radio-frequency signals between an antenna  40  and radio-frequency transceiver  91 . Antenna  40  may be one of the antennas of  FIG. 2  (e.g., antenna,  40 T- 1 ,  40 T- 2 ,  40 B- 1 ,  40 B- 2 , or other antennas). Radio-frequency transceiver  91  may include receivers and/or transmitters in transceiver circuitry  90 , wireless local area network transceiver  36  (e.g., a transceiver operating at 2.4 GHz, 5 GHz, 60 GHz, or other suitable frequency), cellular telephone transceiver  38 , or other radio-frequency transceiver circuitry for receiving and/or transmitting radio-frequency signals. 
     Conductive path  45  may include one or more transmission lines such as one or more segments of coaxial cable, one or more segments of microstrip transmission line, one or more segments of stripline transmission line, or other transmission line structures. Path  45  may include a first conductor such as signal line  45 A and may include a second conductor such as ground line  45 B. Antenna  40  may have an antenna feed with a positive antenna feed terminal  58  (+) that is coupled to signal path  45 A and a ground antenna feed terminal  54  (−) that is coupled to ground path  45 B. If desired, circuitry such as filters, impedance matching circuits, switches, amplifiers, and other radio-frequency circuits may be interposed within path  45 . 
     As shown in  FIG. 3 , antenna  40  may include a resonating element  41  and antenna tuning circuitry. Resonating element  41  may be formed from a loop antenna structure, patch antenna structure, inverted-F antenna structure, slot antenna structures, planar inverted-F antenna structures, helical antenna structures, hybrids of these designs, etc. The use of antenna tuning circuitry may help device  10  cover a wider range of communications frequencies than would otherwise be possible. 
     In general, it is desirable for device  10  to be able to exhibit wide band coverage (e.g., for device  10  to be able to support operation in multiple frequency bands corresponding to different radio access technologies). For example, it may be desirable for antenna  40  to be capable of operating in a higher frequency band that covers the GSM sub-bands at 1800 MHz and 1900 MHz and the data sub-band at 2100 MHz, a first lower frequency band that covers the GSM sub-bands at 850 MHz and 900 MHz, and a second lower frequency band that covers the LTE band at 700 MHz, the GSM sub-bands at 710 MHz and 750 MHz, the UMTS sub-band at 700 MHz, and other desired wireless communications bands. 
     The band coverage of antenna  40  may be limited by its volume (i.e., the amount of space that is occupied by antenna  40  within housing  12 ). For an antenna having a given volume, a higher band coverage (or bandwidth) results in a decrease in gain (e.g., the product of maximum gain and bandwidth is constant). As a result, increasing the volume of antenna  40  will generally increase its band coverage. Increasing the volume of antennas, however, may not always be feasible if a small form factor is desired. 
     To satisfy consumer demand for small form factor wireless devices, one or more of antennas  40  may be provided with antenna tuning circuitry. The antenna tuning circuitry may include a radio-frequency tunable component such as tunable component (sometimes referred to as an adjustable antenna tuning element)  100  and an associated control circuitry such as control circuit  102  (see, e.g.,  FIG. 3 ). Tunable element  100  and/or control circuit  102  may sometimes be formed as an integral part of antenna resonating element  41  or as a separate discrete surface-mount component that is attached to antenna resonating element  41 . 
     For example, antenna tuning element  100  may include switching circuitry based on one or more switches or continuously tunable load components. Control circuit  102  may be used to place tunable element  100  in the desired state by sending appropriate control signals Vc via path  104 . The switching circuitry may, for example, include a switch that can be placed in an open or closed position. When the switch is placed in its open position (e.g., when control signal Vc has a first value), antenna  40  may exhibit a first frequency response. When the switch is placed in its closed position (e.g., when control signal Vc has a second value that is different than the first value), antenna  40  may exhibit a second frequency response. By using an antenna tuning scheme of this type, a relatively narrow bandwidth (and potentially compact) design can be used for antenna  40 , if desired. 
     In one suitable embodiment of the present invention, antenna  40  may be an inverted-F antenna.  FIG. 4A  is a schematic diagram of an inverted-F antenna that may be used in device  10 . As shown in  FIG. 4A , inverted-F antenna  40  may have an antenna resonating element such as antenna resonating element  41  and a ground structure such as ground G. Antenna resonating element  41  may have a main resonating element arm such as arm  96 . Short circuit branch such as shorting path  94  may couple arm  96  to ground G. An antenna feed may contain positive antenna feed terminal  58  (+) and ground antenna feed terminal  54  (−). Positive antenna feed terminal  58  may be coupled to arm  96 , whereas ground antenna feed terminal  54  may be coupled to ground G. Arm  96  in the  FIG. 4A  example is shown as being a single straight segment. This is merely illustrative. Arm  96  may have multiple bends with curved and/or straight segments, if desired. 
     In the example of  FIG. 4A , inverted-F antenna  40  may include an antenna tuning element  100  interposed in shorting path  94 . Antenna tuning element  100  may, for example, be a switchable impedance matching network, a switchable inductive network, a continuously tunable capacitive circuit, etc. 
     In another suitable arrangement of the present invention, resonating element  41  of inverted-F antenna  40  may include an antenna tuning element  100  coupled between the extended portion of resonating arm  96  and ground G (see, e.g.,  FIG. 4B ). In such an arrangement, a capacitive structure such as capacitor  101  may be interposed in shorting path  94  so that antenna tuning circuit  100  is not shorted to ground at low frequencies. In the example of  FIG. 4B , antenna tuning element  100  may be a switchable inductor, a continuously tunable capacitive/resistive circuit, etc. 
     In general, antenna  40  may include any number of antenna tuning elements  100 . As shown in  FIG. 4C , short circuit branch  94  may include at least one tunable element  100 - 1  that couples arm  96  to ground. Tunable element  100 - 1  may be a switchable inductive path, as an example (e.g., element  100 - 1  may be activated to short arm  96  to ground). If desired, antenna tuning element  100 - 3  may be coupled in parallel with the antenna feed between positive antenna feed terminal  58  and ground feed terminal  54 . Tunable element  100 - 3  may be an adjustable impedance matching network circuit, as an example. 
     As another example, antenna tuning element  100 - 4  may be interposed in antenna resonating arm  96 . Antenna tuning element  100 - 4  may be a continuously adjustable variable capacitor (as an example). If desired, additional tuning elements such tuning element  100 - 2  (e.g., continuously tunable or semi-continuously tunable capacitors, switchable inductors, etc.) may be coupled between the extended portion of arm  96  to ground G. 
     The placement of these tuning circuits  100  in  FIGS. 4A ,  4 B, and  4 C is merely illustrative and do not serve to limit the scope of the present invention. Additional capacitors and/or inductors may be added to ensure that each antenna tuning circuit  100  is not shorted circuited to ground at low frequencies (e.g., frequencies below 100 MHz). In general, antennas  40  in device  10  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, hybrids of these designs, etc. At least a portion of antennas  40  in device  10  may contain at least one antenna tuning element  100  (formed at any suitable location on the antenna) that can be adjusted so that wireless circuitry  34  may be able to cover the desired range of communications frequencies. 
     By dynamically controlling antenna tuning elements  100 , antenna  40  may be able to cover a wider range of radio-frequency communications frequencies than would otherwise be possible. A standing-wave-ratio (SWR) versus frequency plot such as SWR plot of  FIG. 5A  illustrates the band tuning capability for antenna  40 . As shown in  FIG. 5A , solid SWR frequency characteristic curve  124  corresponds to a first antenna tuning mode in which antenna  40  exhibits satisfactory resonant peaks at low-band frequency f A  (e.g., to cover the 850 MHz band) and high-band frequency f B  (e.g., to cover the 1900 MHz band). In the first antenna tuning mode, the antenna tuning elements  100  of antenna  40  may be placed in a first configuration (e.g., antenna tuning elements  100  may be provided with a first set of control signals). 
     Dotted SWR frequency characteristic curve  126  corresponds to a second antenna tuning mode in which the antennas of device  10  exhibits satisfactory resonant peaks at low-band frequency f A ′ (e.g., to cover the 750 MHz band) and high-band frequency f B ′ (e.g., to cover the 2100 MHz band). In the second antenna tuning mode, the antenna tuning elements  100  may be placed in a second configuration that is different than the first configuration (e.g., antenna tuning circuits  100  may be provided with a second set of control signals that is different than the first set of control signals). 
     If desired, antenna  40  may be placed in a third antenna tuning mode in which antenna  40  exhibits satisfactory resonant peaks at both low-band frequencies f A ′ and f A  (e.g., to cover both the 750 and 850 MHz bands) and at high-band frequencies f B  and f B ′ (e.g., to cover both the 1900 and 2100 MHz bands), as shown by SWR characteristic curve  128 . In the third antenna tuning mode, the antenna tuning elements  100  may be placed in a third configuration that is different than the first and second configurations (e.g., antenna tuning elements  100  may be provided with a third set of control signals that is different than the first and second sets of control signals). A combination of tuning methods may be used so that the resonance curve  128  exhibits broader frequency ranges than curves  124  and  126 . 
     In another suitable arrangement, antenna  40  may be placed in a fourth antenna tuning mode in which antenna  40  exhibits satisfactory resonant peaks at mid-band frequencies f C  and f D  (e.g., to cover frequencies between the low and high bands), as shown by SWR characteristic curve  130  of  FIG. 5B . In the fourth antenna tuning mode, the antenna tuning circuits  100  may yet be placed in another different configuration. The SWR curves of  FIGS. 5A and 5B  are merely illustrative and do not serve to limit the scope of the present invention. In general, antenna(s)  40  may include antenna tuning circuits  100  that enable device  10  to transmit and receive wireless signals in any suitable number of radio-frequency communications bands. 
     Antenna tuning element  100  may be any switchable or tunable electrical component that can be adjusted in real time. Antenna tuning element  100  may have a first terminal A and a second terminal B that may be coupled to desired locations on antenna resonating element  41  and a third terminal operable to receive control signal Vc from control circuit  102 .  FIG. 6A  shows one suitable circuit implementation of tunable element  100 . As shown in  FIG. 6A , element  100  may include a radio-frequency switch  150  and a load circuit  152  coupled in series between terminals A and B. Switch  152  may be implemented using a p-i-n diode, a gallium arsenide field-effect transistor (FET), a microelectromechanical systems (MEMs) switch, a metal-oxide-semiconductor field-effect transistor (MOSFET), a high-electron mobility transistor (HEMT), a pseudomorphic HEMT (PHEMT), a transistor formed on a silicon-on-insulator (SOI) substrate, etc. The state of the switch can be controlled using signal Vc generated from control circuit  102  (see, e.g.,  FIG. 3 ). For example, a high Vc will turn on or close switch  402  whereas a low Vc will turn off or open switch  402 . 
     Load circuit  152  may be formed from one or more electrical components. Components that may be used as all or part of circuit  152  include resistors, inductors, and capacitors. Desired resistances, inductances, and capacitances for circuit  152  may be formed using integrated circuits, using discrete components (e.g., a surface mount technology inductor) and/or using dielectric and conductive structures that are not part of a discrete component or an integrated circuit. For example, a resistance can be formed using thin lines of a resistive metal alloy, capacitance can be formed by spacing two conductive pads close to each other that are separated by a dielectric, and an inductance can be formed by creating a conductive path (e.g., a transmission line) on a printed circuit board. 
     In another suitable arrangement, tunable element  100  may include a switch  154  (e.g., a single-pole triple-throw radio-frequency switch) and multiple load circuits  150 - 1 ,  150 - 2 , and  150 - 3 . As shown in  FIG. 6B , switch  154  may have ports P 1 , P 2 , P 3 , and P 4 . Terminal B of tunable element  100  may be coupled to port P 1  while terminal A of tunable element  100  may be coupled to port P 2  via circuit  150 - 1 , to port P 3  via circuit  150 - 2 , and to port P 4  via circuit  150 - 3 . As described previously, load circuits  150 - 1 ,  150 - 2 , and  150 - 3  may include any desired combination of resistive components, inductive components, and capacitive components formed using integrated circuits, discrete components, or other suitable conductive structures. Switch  154  may be controlled using signal Vc generated by control circuit  102 . For example, switch  154  may be configured to couple port P 1  to P 2  when Vc is at a first value, to couple port P 1  to P 3  when Vc is at a second value that is different than the first value, and to couple port P 1  to P 4  when Vc is at a third value that is different than the first and second values. 
     The example of  FIG. 6B  in which tunable element  100  includes three impedance loading circuits is merely illustrative and does not serve to limit the scope of the present invention. If desired, tunable element  100  may include a radio-frequency switch having any number of ports configured to support switching among any desired number of loading circuits. If desired, switch  154  may be configured such that more than one of the multiple loading circuits  150  may be coupled to port P 1  in parallel. 
     In another suitable arrangement, tunable element  100  may include a variable capacitor circuit  156  (sometimes referred to as a varactor). As shown in  FIG. 6C , varactor may have first terminal A, second terminal B, and a control terminal operable to receive signal Vc from control circuit  300 . Control circuit  102  may be adjusted so that Vc adjusts the capacitance of varactor  156  to the desired amount. Varactor  156  may be formed using integrated circuits, one or more discrete components (e.g., SMT components), etc. In general, varactor  156  may be continuously variable capacitors or semi-continuously adjustable capacitors. 
     It may be desirable to have a way of characterizing the performance of antenna tuning element  100  to determine its behavior when assembled within device  10 . One way of testing the performance of antenna tuning element  100  involves mounting antenna tuning element  100  within an actual device  10  so that antenna tuning element  100  is placed in its true application environment (e.g., antenna tuning element  100  is placed in its intended assembled state, enabling element  100  to be presented with actual loading and operating conditions). 
       FIG. 7  shows a test system such as test system (sometimes referred to as a test station)  200  that can be used to test device  10  that includes at least one antenna tuning element  100 . Electronic device  10  that is being tested may be referred to as a device under test (DUT). As shown in  FIG. 7 , test system  200  may include a test host such as test host  202 , a test chamber such as test chamber  240 , a signal generator such as signal generator  204 , testers such as testers  220  and  234  (e.g., power meters  220 - 1  and  220 - 2  and spectrum analyzers  234 - 1 ,  234 - 2 , and  234 - 3 ), and other radio-frequency test equipment. Test host  202  may, for example, be a host personal computer or other types of computing equipment that can be used to control the operation of signal generator  204 , power meters  220 , spectrum analyzers  234 , and other test accessories in test system  200 . 
     DUT  10  may be placed within test chamber  240  during test operations. Test chamber  240  may have a cubic structure (six square walls) or a rectangular prism-like structure (six rectangular walls), if desired. Test chamber  240  may be internally lined with absorbent material. The absorbent material may be formed from pyramid-shaped foams (see, e.g., liner  242 ), rubberized material, or other suitably lossy material. Test chamber  240  may sometimes be referred to as an anechoic chamber. If desired, reverberation chambers (e.g., chambers with one or more tuners that can be moved to different orientations to obtain varying spatial distribution of electrical and magnetic field strength) may also be used. 
     DUT  10  may be attached to a positioner such as positioner  244  when DUT  10  is placed within test chamber  240 . Positioner  244  is a computer-controlled or manually-controlled positioning device that can be used to change the position/orientation of DUT  10  within test chamber  240  during testing. For example, positioner  244  may include actuators for controlling lateral and/or rotational movement of DUT  10  and may therefore sometimes be referred to as a DUT rotator. DUT rotator  244  may be controlled using control signals generated by test host  100  routed over path  243 . 
     Signal generator  204  may be used for generating radio-frequency test signals at desired fundamental frequencies (e.g., frequencies at which device  10  may operate during normal wireless transmission). These test signals may be provided to DUT  100  via a coaxial cable, radio-frequency transmission line path, and/or other suitable conductive paths. If desired, signal generator  204  may be a radio communications tester of the type that is sometimes referred to as a call box or a base station emulator. 
     Signal generator  204  may be operated directly or via computer control (e.g., when signal generator  204  receives commands from test host  202 ). When operated directly, a user may control signal generator  204  by supplying commands directly to the signal generator using a user input interface of signal generator  204 . For example, a user may press buttons in a control panel on the signal generator while viewing information that is displayed on a display in generator  204 . In computer controlled configurations, test host  202  (e.g., software running autonomously or semi-autonomously on the computer) may communicate with signal generator  204  (e.g., by sending and receiving data over a wired path or a wireless path between the computer and the signal generator). 
     Signal generator my provide radio-frequency test signals to an amplifying circuit such as radio-frequency power amplifier  206 . Power amplifier  206  may be used to amplify the radio-frequency test signals so that test signals are delivered to DUT  10  at the desired power level. The gain provided by power amplifier  206  may be controlled using test host  202 . For example, consider a scenario in which the desired power to be delivered to DUT  10  is equal to 20 dBm during a first test iteration and is equal to 45 dBm during a second test iteration. Power amplifier  206  may be configured to provide a first signal gain during the first test iteration and may be configured to provide a second signal gain that is greater than the first signal gain during the second test iteration (e.g., radio-frequency power amplifier may provide more gain when the requested power to be delivered to DUT  10  is higher). 
     The amplified radio-frequency test signals at the output of power amplifier  206  may be output to an input port P 0  of radio-frequency switch  208 - 1  (e.g., a first single-pole quadruple-throw radio-frequency switch). Signals received at port P 0  may be routed to a selected one of output ports P 1 , P 2 , P 3 , and P 4 . Test host  202  may generate a control signal Vc that can be used to place switch  208 - 1  in a desired state. In the example of  FIG. 7  in which switch  208 - 1  is operable in four different states, signal Vc may be a two-bit digital signal. When test host  202  sets Vc to be equal to “00,” port P 0  may be coupled to P 1  (e.g., switch  208 - 1  may be placed in a first state). When test host  202  sets Vc to be equal to “01,” port P 0  may be coupled to P 2  (e.g., switch  208 - 1  may be placed in a second state). When test host  202  sets Vc to be equal to “10,” port P 0  may be coupled to P 3  (e.g., switch  208 - 1  may be placed in a third state). When test host  202  sets Vc to be equal to “11,” port P 0  may be coupled to P 4  (e.g., switch  208 - 1  may be placed in a fourth state). 
     Output ports P 1 -P 4  of switch  208 - 1  may be coupled to corresponding ports P 11 , P 12 , P 13 , and P 14  of radio-frequency switch  208 - 2  (e.g., a second single-pole quadruple-throw radio-frequency switch) via respective radio-frequency isolator circuits. Port P 1  of switch  208 - 1  may be coupled to port P 11  of switch  208 - 2  via a first radio-frequency isolator  210 - 1 ; port P 2  of switch  208 - 1  may be coupled to port P 12  of switch  208 - 2  via a second radio-frequency isolator  210 - 2 ; port P 3  of switch  208 - 1  may be coupled to port P 13  of switch  208 - 2  via a third radio-frequency isolator  210 - 3 ; and port P 4  of switch  208 - 1  may be coupled to port P 14  of switch  208 - 2  via a fourth radio-frequency isolator  210 - 4 . 
     First isolator circuit  210 - 1  may be configured to selectively pass radio-frequency signals at frequency fa while preventing any signals reflected back via port P 11  from leaking back to power amplifier  206  (e.g., signals reflected from DUT  10  may be drained to a ground terminal of the isolator). Second isolator circuit  210 - 2  may be configured to selectively pass radio-frequency signals at frequency fb while preventing any reflected signals from leaking back to power amplifier  206  via port P 12 . Third isolator circuit  210 - 3  may be configured to selectively pass radio-frequency signals at frequency fc while preventing any reflected signals from leaking back to power amplifier  206  via port P 13 . Fourth isolator circuit  210 - 4  may be configured to selectively pass radio-frequency signals at frequency fd while preventing any reflected signals from leaking back to power amplifier  206  via port P 14 . If desired, radio-frequency band-pass filters may be used in place of the isolators to provide desired frequency selectivity. 
     Radio-frequency switch  208 - 2  may be configured to selectively couple one of ports P 11 -P 14  to its output port P 10 . For example, switch  208 - 2  may receive a two-bit digital signal Vc from test host  202 . When test host  202  sets Vc to be equal to “00,” port P 10  may be coupled to P 11  (e.g., switch  208 - 2  may be placed in a first state). When test host  202  sets Vc to be equal to “01,” port P 10  may be coupled to P 12  (e.g., switch  208 - 2  may be placed in a second state). When test host  202  sets Vc to be equal to “10,” port P 10  may be coupled to P 13  (e.g., switch  208 - 2  may be placed in a third state). When test host  202  sets Vc to be equal to “11,” port P 10  may be coupled to P 14  (e.g., switch  208 - 2  may be placed in a fourth state). 
     Frequencies fa, fb, fc, and fb represent desired fundamental operating frequencies to be tested and may be equal to 750 MHz, 900 MHz, 1950 MHz, and 2100 MHz, respectively (as an example). Frequency fa may be less than fb, fb may be less than fc, and fc may be less than fd. The example of  FIG. 7  in which switches  208 - 1  and  208 - 2  are configurable in four different states is merely illustrative and does not serve to limit the scope of the present invention. If desired, switches  208 - 1  and  208 - 2  may include any number of ports for performing switching among any suitable number of radio-frequency channels and bands. 
     Port P 10  of switch  208 - 2  may be coupled to a first terminal of radio-frequency switch  212 - 1  (e.g., a first single-pole double-throw radio-frequency switch). The first terminal of radio-frequency switch  212 - 1  may be switchably coupled to a selected one of a second terminal and a third terminal of switch  212 - 1 . The state of switch  212 - 1  (i.e., whether the first terminal is connected to the second terminal or whether the first terminal is connected to the third terminal) may be controlled using control signal Vc generated using test host  202 . 
     Switch  212 - 1  may be coupled to switch  212 - 2  (e.g., a second single-pole double-throw radio-frequency switch) via respective low pass filters. In particular, switch  212 - 2  may have a first terminal that is coupled to DUT  10  via path  218 , a second terminal that is coupled to the second terminal of switch  212 - 1  via first low pass filter circuit  214 , and a third terminal that is coupled to the third terminal of switch  212 - 1  via second low pass filter  216 . The state of switch  212 - 2  (i.e., whether the first terminal is connected to the second terminal or whether the first terminal is connected to the third terminal) may be controlled using control signal Vc generated from test host  202 . 
     Filter  214  may be switched into use when radio-frequency switches  208  (i.e., switches  208 - 1  and  208 - 2 ) are placed in the first and second states while filter  216  is switched out of use. Filter  216  may be switched into use when radio-frequency switches  208  are placed in the third and fourth states while filter  214  is switched out of use. Filter  214  may have a cutoff frequency of fb′ (i.e., radio-frequency signals greater than fb′ will be attenuated), where frequency fb′ is greater than fb but less than fc. Filter  214  may therefore be used to attenuate spurious signals at harmonic frequencies (e.g., frequencies that are integer multiples of fundamental frequency fa or fb) that may have been generated due to the non-linear behavior of the test equipment interposed between switch  212 - 1  and signal generator  204 . Filter  216  may have a cutoff frequency of fd′ (i.e., radio-frequency signals greater than fd′ will be attenuate), where frequency fd′ is greater than fd. Filter  216  may therefore be used to attenuate spurious emissions at harmonic frequencies (e.g., frequencies that are integer multiples of fundamental frequency fc or fd). 
     Power meter  220 - 1  may be coupled to path  218  via broadband radio-frequency coupler  222 . Coupler  222  may be used to divert a small fraction of the transmitted radio-frequency test signals that are being conveyed from switch  212 - 2  to DUT  10 . As an example, coupler  222  may be a −20 dB coupler and may be used to extract one percent of the delivered powered with which the signals are being transmitted to DUT  10 . A power reading of −5 dBm may therefore translate to an actual delivered power level of 15 dBm (−5 plus 20), whereas a power reading of 13 dBm may translate to an actual delivered power level of 33 dBm (13 plus 20). The term “actual delivered power level” refers to the output power level at which the radio-frequency signals are being delivered to DUT  10 . 
     Power meter  220 - 2  may be coupled to path  218  via broadband radio-frequency coupler  224 . Coupler  224  may be used to divert a small fraction of the signals that have been reflected back from DUT  10 . Coupler  224  may also be a −20 dB coupler (as an example). Power meters  220  (i.e., power meters  220 - 1  and  220 - 2 ) may include radio-frequency receiver circuitry that is able to gather information on the magnitude and phase of signals transmitted to and reflected from DUT  10  (i.e., radio-frequency signals that are reflected from DUT  10  or radio-frequency signals that have passed through at least a portion of DUT  10 ). 
     By analyzing transmitted signals using power meter  220 - 1  and reflected signals using power meter  220 - 2 , the magnitude and phase of the complex impedance (sometimes referred to as a reflection coefficient) of DUT  10  may be determined. For example, by analyzing the transmitted and reflected signals, power meters  220 - 1  and  220 - 2  may collectively be used to obtain linear measurements such as S-parameter measurements that reveal information about whether DUT  100  exhibits satisfactory radio-frequency performance. For example, S 11  (complex impedance) measurements and/or an S 21  (complex forward transfer coefficient) measurements may be obtained and computed using test host  202  based on data gathered using power meters  220 . The values of S 11  and S 21  may be measured at desired fundamental frequencies. 
     Spectrum analyzer  234 - 1  may also be coupled to path  218  via broadband radio-frequency coupler  226 . Coupler  226  may be used to divert a small fraction of the signals that have been reflected back from DUT  10  to spectrum analyzer  234 - 1 . Coupler  224  may be a −20 dB coupler (as an example). As shown in  FIG. 7 , switches  228 - 1  and  228 - 2  and high pass filters  230  and  232  may be interposed between coupler  226  and spectrum analyzer  234 - 1 . Spectrum analyzers such as spectrum analyzers  234 - 1 ,  234 - 2 , and  234 - 3  may be a radio-frequency tester suitable for performing radio-frequency measurements on signals received from DUT  10 . Results gathered using these testers  234  may be retrieved by test host  202  for further processing. 
     Switches  228 - 1  and  228 - 2  may be single-pole double-throw radio-frequency switches. Switch  228 - 1  may have a first terminal that is coupled to coupler  226 , whereas switch  228 - 2  may have a first terminal that is coupled to spectrum analyzer  234 - 1 . Switch  228 - 1  may have a second terminal that is coupled to a second terminal of switch  228 - 2  via filter  230  and may have a third terminal that is coupled to a third terminal of switch  228 - 2  via filter  232 . Filter  230  may be used for selectively passing through signals having frequencies greater than fb″ (e.g., signals wither frequencies fb″ will be attenuated), whereas filter  232  may be used for selectively passing through signals having frequencies greater than fd″ (e.g., signals wither frequencies fd″ will be attenuated). 
     Filter  230  may be switched into use when radio-frequency switches  208  are placed in the first and second states while filter  232  is switched out of use (e.g., switches  228 - 1  and  228 - 2  may be configured to couple tester  234 - 1  to coupler  226  via filter  230 ). Configuring filter  230  in this way serves to attenuate signals at fundamental frequencies fa or fb while passing through signals at the harmonic frequencies (e.g., 2*fa, 3*fa, . . . , n*fa, 2*fb, 3*fb, . . . , n*fb). Frequency fb″ may greater than fb and less than 2*fa (as an example). 
     Filter  232  may be switched into use when radio-frequency switches  208  are placed in the third and fourth states while filter  230  is switched out of use (e.g., switches  228 - 1  and  228 - 2  may be configured to couple tester  234 - 1  to coupler  226  via filter  232 ). Configuring filter  232  in this way serves to attenuate signals at fundamental frequencies fc or fd while passing through signals at the harmonic frequencies (e.g., 2*fc, 3*fc, . . . , n*fc, 2*fd, 3*fd, . . . , n*fd). Frequency fd″ may greater than fd and less than 2*fc (as an example). 
     Test system  200  may be used to characterize how the presence of antenna tuning element  100  affects the antenna performance of DUT  10  (e.g., to determine whether antenna tuning element  100  contains a manufacturing defect). DUT  10  may or may not be powered on. In the scenario in which DUT is powered on, DUT  10  need not be placed in normal user mode (e.g., the radio-frequency transceiver circuitry is not actively transmitting wireless traffic). This type of testing in which DUT  10  is powered off or in idle mode may be referred to as passive antenna testing. During passive antenna testing, the antenna may be energized using a radio-frequency input test signal that is generated by signal generator  204  and fed to DUT  10 . In particular, the input radio-frequency signal may be fed directly to antenna tuning element  100  via direct physical contact with a radio-frequency test probe that is connected to path  218  (as an example). 
     As described previously, power meters  220  and tester  234 - 1  may be used to measure signals conveyed along path  218  via direct conduction (e.g., signals are conveyed to the power meters and tester via conducted means). Simply obtaining conductive test measurements may not be sufficient in characterizing the antenna because no radiated signal from DUT  10  is measured. Certain defects in element  100  may cause a drop in antenna efficiency without a corresponding or measurable change to antenna input impedance (i.e., certain defects cannot be detected by simply monitoring S 11 ). In these cases, only a radiated test is capable of detecting such variations. This requires a test antenna that samples signals radiated from DUT  10  within test chamber  240 . 
     A test antenna such test antenna  248  may be placed within test chamber  240 . Test antenna  248  may be used for receiving radio-frequency signals emanated from DUT  10  upon excitation from the input radio-frequency test signals that are conveyed to DUT  10  over path  218 . Test antenna  248  may be a horn antenna, a patch antenna, or other types of radio-frequency wave guides. Radio-frequency signals received using test antenna  248  may be grouped into separate paths based on the polarization orientation. Horizontally polarized (HP) received signals may be fed to port P 0  of radio-frequency switch  252 - 1  via path  250 - 1 , whereas vertically polarized (VP) received signals may be fed to port P 0  of radio-frequency switch  252 - 2  via path  250 - 2  (as an example). 
     Switch  252 - 1  may be coupled to a second spectrum analyzer  234 - 2  via radio-frequency switch  256 - 1 , whereas switch  252 - 2  may be coupled to a third spectrum analyzer  234 - 3  via radio-frequency switch  256 - 2 . Switches  252 - 1  and  252 - 2  may be single-pole triple-throw radio-frequency switches, whereas switches  256 - 1  and  256 - 2  may be single-pole double-throw radio-frequency switches. 
     Switch  256 - 1  may have a first terminal that is coupled to tester  234 - 2 , a second terminal that is coupled to port P 1  of switch  252 - 1 , and a third terminal that is selectively coupled to one of ports P 2  and P 3  in switch  252 - 1 . High pass filter circuits such as filters  258  and  260 , radio-frequency switch  254 - 1  (e.g., a single-pole double-throw RF switch), and radio-frequency power amplifier  264  may be coupled between switch  252 - 1  and the third terminal of switch  256 - 1 . In particular, switch  254 - 1  may have a first terminal that is coupled to the third terminal of switch  256 - 1  via power amplifier  264 , a second terminal that is coupled to port P 2  of switch  252 - 1  via filter  258 , and a third terminal that is coupled to port P 3  of switch  252 - 1  via filter  260 . Only one of filters  258  and  260  may be switched into use at any given point in time. When switched into use, filter  258  may serve to attenuate signals having frequencies less than fb″ and to pass through signals having frequencies greater than fb″ (e.g., for passing through signals at harmonic frequencies associated with fundamental frequency fa or fb). When switched into use, filter  260  may serve to attenuate signals having frequencies less than fd″ and to pass through signals having frequencies greater than fd″ (e.g., for passing through signals at harmonic frequencies associated with fundamental frequency fc or fd). Power amplifier  264  may be used to amplify signals with sufficient gain such that signals arriving at tester  234 - 2  can be properly measured. 
     Test host  202  may configured switch  252 - 1  to coupled port P 0  to P 1  and may configured switch  256 - 1  to coupled its first terminal to its second terminal when performing linear radio-frequency measurements at the fundamental frequency currently being tested (e.g., fa, fb, fc, or fd). Switch  256 - 1  may be configured to couple its first terminal to its third terminal when performing non-linear radio-frequency measurements at the harmonic frequencies (i.e., when characterizing harmonic distortion levels). In particular, test host  202  may configure switch  252 - 1  to couple port P 0  to P 2  and may configure switch  254 - 1  to couple its first terminal to its second terminal when switches  208  are placed in the first or second state. Alternatively, test host  202  may configure switch  252 - 1  to couple port P 0  to P 3  and may configure switch  254 - 1  to couple its first terminal to its third terminal when switches  208  are placed in the third or fourth state. Radio-frequency measurements obtained using spectrum analyzer  234 - 2  may be reflective of the wireless behavior of horizontally polarized signals received via antenna  248 . 
     The vertically polarized signals may be fed to spectrum analyzer  234 - 3  via path  250 - 2  and switches  252 - 2  and  256 - 2 . Switch  256 - 2  may have a first terminal that is coupled to tester  234 - 3 , a second terminal that is coupled to port P 1  of switch  252 - 2 , and a third terminal that is selectively coupled to one of ports P 2  and P 3  in switch  252 - 2 . High pass filter circuits such as filters  266  and  268 , radio-frequency switch  254 - 2  (e.g., a single-pole double-throw RF switch), and radio-frequency power amplifier  272  may be coupled between switch  252 - 2  and the third terminal of switch  256 - 2 . In particular, switch  254 - 2  may have a first terminal that is coupled to the third terminal of switch  256 - 2  via power amplifier  272 , a second terminal that is coupled to port P 2  of switch  252 - 2  via filter  266 , and a third terminal that is coupled to port P 3  of switch  252 - 2  via filter  268 . Only one of filters  266  and  268  may be switched into use at any given point in time. When switched into use, filter  266  may serve to attenuate signals having frequencies less than fb″ and to pass through signals having frequencies greater than fb″ (e.g., for passing through signals at harmonic frequencies associates with fundamental frequency fa or fb). When switched into use, filter  268  may serve to attenuate signals having frequencies less than fd″ and to pass through signals having frequencies greater than fd″ (e.g., for passing through signals at harmonic frequencies associated with fundamental frequency fc or fd). Power amplifier  272  may be used to amplify signals with sufficient gain such that signals arriving at tester  234 - 3  can be properly measured. 
     Test host  202  may configured switch  252 - 2  to coupled port P 0  to P 1  and may configured switch  256 - 2  to coupled its first terminal to its second terminal when performing linear radio-frequency measurements at the fundamental frequency currently being tested (e.g., fa, fb, fc, or fd). Switch  256 - 2  may be configured to couple its first terminal to its third terminal when performing non-linear radio-frequency measurements at the harmonic frequencies (i.e., when characterizing harmonic distortion levels). In particular, test host  202  may configure switch  252 - 2  to couple port P 0  to P 2  and may configure switch  254 - 2  to couple its first terminal to its second terminal when switches  208  are placed in the first or second state. Alternatively, test host  202  may configure switch  252 - 2  to couple port P 0  to P 3  and may configure switch  254 - 2  to couple its first terminal to its third terminal when switches  208  are placed in the third or fourth state. Radio-frequency measurements obtained using spectrum analyzer  234 - 3  may be reflective of the wireless behavior of vertically polarized signals received via antenna  248 . 
     Test results obtained using both testers  234 - 2  and  234 - 3  may be retrieved and analyzed using test host  202  when determining whether antenna tuning element  100  satisfies design criteria. All the radio-frequency switches in test system  200  (e.g., switches  208 ,  212 - 1 ,  212 - 2 ,  228 - 1 ,  228 - 2 ,  252 - 1 ,  252 - 2 ,  254 - 1 ,  254 - 2 ,  256 - 1 , and  256 - 2 ) may be controlled by test host  202  via adjustable signal Vc. 
     If desired, a physical object such as object  246  may be placed in the vicinity of DUT  10  during testing. The presence of object  246  may serve to simulate an actual user scenario in which a users hand or other body part(s) may impact the antenna performance of DUT  10 . Object  246  may therefore sometimes be referred to as a phantom object. Positioner  244  may be used to vary the position and orientation of DUT  10  relative to object  246  during device characterization. If desired, object  246  may be formed using dielectric material, metal, ceramic, plastic, rubber, foam, or other suitable material. If desired, the position/orientation of object  246  may also be adjusted manually or automatically during testing. 
       FIG. 8  is a flow chart of illustrative steps for using the test system of  FIG. 7  to characterize DUT  10 . At step  400 , test chamber  240  may be calibrated to determine an associated path loss. Path loss may be defined as the amount of power attenuation experienced by signals propagating down a transmission line path. The path loss associated with test chamber  240  may therefore be defined as the amount of power attenuation experienced by signals traveling from the output of signal generator  204  to the corresponding test input contact point with DUT  10 . This calibrated path loss value may be factored into the measured values to take into account path loss variation from test station to test station. 
     At step  402 , test host  202  may select a fundamental frequency for testing (e.g., fundamental frequency f 0  may be set to fa, fb, fc, or fd). At step  404 , test host  202  may configure signal generator  204  to output radio-frequency signals at a selected output power level. At step  406 , positioner  244  may be used to rotate/move DUT  10  in different orientations and positions relative to phantom object  246 . 
     While radio-frequency test signals are being fed to DUT  10  for energizing antenna tuning element  100  and while DUT  10  being placed in various orientations, power meters  220 - 1  and  220 - 2  may be used to measured delivered power and reflected power levels, respectively. Spectrum analyzer  234 - 1  may be used to measure different harmonic distortion levels in the reflected signals corresponding to different DUT orientations. Spectrum analyzers  234 - 2  and  234 - 3  may be used to measure different harmonic distortion levels in the radiated signals corresponding to the different DUT orientations. For example, the power meters and the spectrum analyzers may be used to collectively measure fundamental and harmonic emission patterns and to measure soft breakdown levels to determine whether antenna tuning element  100  satisfies design criteria. 
     If there are additional output power levels to be tested, processing may loop back to step  404  (as indicated by path  412 ). Processing may loop back to step  402  if there are additional fundamental frequencies to be tested, as indicated by path  414 . The steps of  FIG. 8  are merely illustrative and do not serve to limit the scope of the present invention. If desired, test system  200  may include any number of power meters, spectrum analyzers, filters, radio-frequency switches, and other radio-frequency test equipment for obtaining linear and nonlinear radio-frequency measurements at any number of desired frequencies. 
       FIG. 9  is a plot showing measured power level Pmeas versus input power level Pin (i.e., power that is delivered to DUT  100 ) illustrating harmonic distortion measurements on a log scale (in units of dBm) for a given fundamental frequency. As shown in  FIG. 9 , line  350  plots the measured signal level at fundamental frequency f 0 , line  352  plots the measured signal level at second harmonic frequency  2   f   0 , and line  354  plots the measured signal level at third harmonic frequency  3   f   0 . Generally, for a given input power, the output power level measured at  2   f   0  is less than the output power level measured at f 0 . Similarly, the output power level measured at  3   f   0  is less than the output power level measured at  2   f   0  at the given input power level. 
     As illustrated in  FIG. 9 , a  2   nd  harmonic distortion value HD 2  may be defined as the ratio of the measured output power level at  2   f   0  corresponding to a given input power Pi to the measured output power level at f 0  corresponding to input power level Pi. A 3 rd  harmonic distortion value HD 3  may be defined as the ratio of the measured output power level at  3   f   0  corresponding to given input power Pi to the measured output power level at f 0  corresponding to power level Pi. In general, an n th  harmonic distortion value HDn is defined as the ratio of output power level measured at the n th  harmonic frequency n*f 0  to the output power level measured at the fundamental frequency f 0 . Harmonic distortion values may be represented in units of dBc. Test host  202  may be used to compute these different order harmonic distortion values for each desired input power Pin. 
     Harmonic distortion values also tend to decrease as input power Pin is lowered. At a certain input power level, antenna tuning element  100  may experience reliability issues such as oxide breakdown or soft breakdown (e.g., a condition in which a semiconductor device in element  100  can no longer be controlled in a predictable manner). In the example, of  FIG. 9 , curves  352  and  354  may rise dramatically when Pin is raised beyond oxide break threshold power level Pob (as indicated by dotted lines  356  and  358 , respectively). Oxide break threshold power level Pob may vary as a function of frequency, temperature, power supply level, and other factors that impact the operation of antenna tuning element  100 . The computed oxide break threshold level Pob may be compared to a predetermined threshold to determine whether antenna tuning element satisfies device reliability requirements. 
     The foregoing is merely illustrative of the principles of this invention and various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20120402
Publication Date: 20150630
Grant Date: 20150630
Priority Date: 20120402
Inventors: MOW MATTHEW A.
DRAGONE, JR. ROCCO V.
BIEDKA THOMAS E.
SCHLUB ROBERT W.
CABALLERO RUBEN
Assignee: APPLE INC
CPC Classifications: [{"code": "H04B17/103", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R29/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01Q9/0421", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01Q5/328", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01Q5/328", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B17/103", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01Q9/0421", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R29/10", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 49234068