PATENT DOCUMENT

Publication Number: US-9310422-B2
Application Number: US-201213487149-A
Country: US
Kind Code: B2

Title: Methods and apparatus for testing small form factor antenna tuning elements

Abstract:
A test system for testing a device under test (DUT) is provided. The test system may include a DUT receiving structure configured to receive the DUT during testing and a DUT retention structure that is configured to press the DUT against the DUT receiving structure so that DUT cannot inadvertently shift around during testing. The DUT retention structure may include a pressure sensor operable to detect an amount of pressure that is applied to the DUT. The DUT retention structure may be raised and lowered vertically using a manually-controlled or a computer-controlled positioner. The positioner may be adjusted using a coarse tuning knob and a fine tuning knob. The positioner may be calibrated such that the DUT retention structure applies a sufficient amount of pressure on the DUT during production testing.

Claims:
What is claimed is: 
     
       1. A test system for testing a device under test having device under test contacts, comprising:
 a device receiving structure having first and second opposing sides and device receiving structure contacts configured to receive the device under test contacts when the device under test is mated with the device receiving structure; 
 a device retention structure configured to press the device under test against the device receiving structure with a calibrated amount of pressure; 
 a radio-frequency tester that is coupled to the device receiving structure and that is configured to obtain radio-frequency measurements from the device under test; 
 a first radio-frequency connector that is attached to the first side of the device receiving structure and coupled to the radio-frequency tester, wherein the first radio-frequency connector comprises a first core signal conductor that is coupled to a first device receiving structure contact; and 
 a second radio-frequency connector that is attached to the second side of the device receiving structure and coupled to the radio-frequency tester, wherein the second radio-frequency connector comprises a second core signal conductor that is coupled to a second device receiving structure contact, the first and second device receiving structure contacts contact respective first and second device under test contacts, the first and second device under test contacts are positioned at respective first and second opposing sides of the device under test, the device receiving structure contacts comprise third, fourth, and fifth device receiving structure contacts, and the third device receiving structure contact is coupled to a ground plane. 
 
     
     
       2. The test system defined in  claim 1 , wherein the device under test comprises an antenna tuning element selected from the group consisting of: a radio-frequency switch, a tunable resistive component, a tunable capacitive component, and a tunable inductive component. 
     
     
       3. The test system defined in  claim 1 , further comprising:
 a positioner operable to move the device retention structure so that the device retention structure presses the device under test against the device receiving structure with the calibrated amount of pressure. 
 
     
     
       4. The test system defined in  claim 3 , wherein the positioner comprises a computer-controlled positioner, the test system further comprising:
 a test host configured to store calibration data, wherein the positioner moves the device retention structure towards the device under test by an amount specified by the stored calibration data. 
 
     
     
       5. The test system defined in  claim 1 , wherein the device retention structure includes a pressure sensor operable to detect an amount of force that is applied to the device under test from the device retention structure. 
     
     
       6. The test system defined in  claim 5 , further comprising:
 a positioner operable to move the device retention structure, wherein the positioner is configured to move the device retention structure at a first rate when the device retention structure is not in contact with the device under test, and wherein the positioner is configured to the move the device retention structure at a second rate that is lower than the first rate when the device retention structure is in contact with the device under test. 
 
     
     
       7. The test system defined in  claim 4 , further comprising: a plurality of test pins, wherein the fourth device receiving structure contact is coupled to a first test pin of the plurality of test pins, and wherein the first test pin is coupled to the test host. 
     
     
       8. The test system defined in  claim 7 , wherein the fifth device receiving structure contact is coupled to a second test pin of the plurality of test pins, and wherein the second test pin is coupled to a power supply unit. 
     
     
       9. The test system defined in  claim 8 , wherein the ground plane is coupled to a third test pin of the plurality of test pins, wherein the third test pin is coupled to the power supply unit.

Description:
BACKGROUND 
     This relates generally to wireless communications circuitry, and more particularly, to electronic devices having 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 narrow, 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 testing the antenna tuning elements prior to being assembled within a finished product without damaging the antenna tuning elements. 
     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 device receiving structure (e.g., a test substrate), test host (e.g., a personal computer), a radio-frequency tester (e.g., a vector network analyzer), a power supply unit, cabling (e.g., coaxial cables) for coupling the radio-frequency tester to the device receiving structure, and other test equipment. The antenna tuning element currently being tested by the test system may be referred to as a device under test (DUT), a device component under test, or a circuit under test (CUT). The power supply unit may optionally be used to supply power to the DUT during testing. The test host may send control signals to the DUT that places the DUT in a desired one of multiple possible operating states. The DUT may be mated with the device receiving structure during testing. 
     The test system may also include a device retention structure configured to press against the DUT with a calibrated amount of force (pressure) so that DUT is properly mated with the device receiving structure and so that DUT does not shift around during testing. The amount of force with which the device retention structure presses against the DUT may be determined using device retention structure calibration operations. 
     The device retention structure may include a pressure sensor operable to generate a pressure sensor output indicative of the amount of force that is applied by the device retention structure. The device retention structure may be positioned over the DUT at a height that is controlled by a positioning device (e.g., a computer-controlled positioner or a manually-controlled positioner). The positioning device may, for example, include a first position adjustment mechanism for adjusting the height of the device retention structure with a first degree of accuracy (e.g., a coarse height adjustment mechanism) and a second position adjustment mechanism for adjusting the height of the device retention structure with a second degree of accuracy that is greater than the first degree of accuracy (e.g., a fine height adjustment mechanism). 
     During calibration, the first position adjustment mechanism may be used to move the device retention structure towards the DUT until the device retention structure makes contact with the DUT. The moment at which the device retention structure makes contact with the DUT can be detected by monitoring when the pressure sensor output changes dramatically from a baseline value. Once the device retention structure is in contact with the DUT, the second position adjustment mechanism may be used to move the device retention structure towards the DUT until the pressure sensor output exceeds a predetermined threshold level. 
     Once the pressure sensor output exceeds the predetermined threshold level, a position adjustment setting reflective of the current state of the first and second position adjustment mechanisms may be recorded as test data. Test data may be gathered in this way for different types of DUTs. The test data may then be processed to obtain calibration data. The calibration data may be used to configure the positioner so that the device retention structures presses against the DUT with sufficient force during production testing (e.g., while the radio-frequency tester is being used to obtain radio-frequency measurements from the DUT). 
     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, 4B, and 4C  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 tuning 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 that includes a device under test (DUT) retention structure in accordance with an embodiment of the present invention. 
         FIGS. 8 and 9  are perspective views of a DUT retention structure that is controlled using an associated positioner in accordance with an embodiment of the present invention. 
         FIGS. 10A, 10B, and 10C  are diagrams showing different types of tips that can be used as part of a DUT retention structure in accordance with an embodiment of the present invention. 
         FIGS. 11A, 11B, and 11C  are cross-sectional side views of illustrative coupling structures on a test board on which a DUT can be mounted during test in accordance with an embodiment of the present invention. 
         FIG. 12  is a plot of pressure sensor output value versus DUT retention structure positioner adjustment settings in accordance with an embodiment of the present invention. 
         FIG. 13  is a flow chart of illustrative steps for calibrating a test system to obtain critical DUT retention structure positioner adjustment settings in accordance with an embodiment of the present invention. 
         FIG. 14  is a frequency distribution plot of critical DUT retention structure positioner adjustments settings gathered from two different types of DUTs in accordance with an embodiment of the present invention. 
         FIG. 15  is a flow chart of illustrative steps for operating the test system of  FIG. 7  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 also 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. 
     Device  10  may also receive long-range signals such as signals associated with satellite navigation bands. For example, device  10  may use wireless circuitry to receive signals in the 1575 MHz band associated with the Global Positioning System (GPS) and to receive signals in the 1602 MHz band associated with the Global Navigation Satellite System (GLONASS). 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, 4B, and 4C  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. 
     Having antenna tuning element  100  as part of antenna  40  introduces an additional component that needs to be characterized, because the design of antenna tuning element  100  can substantially impact the antenna performance of device  10 . For example, the position at which element  100  is placed relative to the antenna feed terminals, the orientation of element  100  within device  10 , and other design factors associated with element  100  can affect the wireless operation of device  10 . It may therefore be desirable to have a way of characterizing the performance of antenna tuning element  100  to provide guidance in the antenna design of device  10 . 
     In accordance with an embodiment of the present invention, antenna tuning element  100  may be tested using a test system such as test system  200  of  FIG. 7 . An antenna tuning element  100  that is being characterized using test system  200  may sometimes 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  (e.g., a personal computer), a radio-frequency tester such as radio-frequency tester  204 , a power supply unit such as power supply unit  206 , a test substrate such as test substrate  210 , control circuitry, network circuitry, cabling, and other test equipment. 
     DUT  100  may be placed on top of test substrate  210  during testing. Substrate  210  may be a plastic support structure or other dielectric structure, a rigid printed circuit board substrate such as a fiberglass-filled epoxy substrate (e.g., FR4), a flexible printed circuit (“flex circuit”) formed from a sheet of polyimide or other flexible polymer, or other substrate material. The use of substrate  210  is merely illustrative. In general, test system  200  may include a test fixture having any suitable type of DUT receiving structure (e.g., a printed circuit board, a metal platform, a dielectric support structure, a receptacle having a recess, a test socket, etc.) that is configured to mate with DUT  100  during testing. 
     DUT  100  may, for example, include conductive pads V, W, X, Y, and Z that serve as contact terminals for interfacing with external circuitry and may therefore sometimes be referred to as DUT contacts. In the example of  FIG. 7 , pad V may serve as terminal A for DUT  100 , whereas pad W may serve as terminal B for DUT  100  (see, e.g.,  FIGS. 6A, 6B, and 6C ). Pads X and Y may serve as power supply terminals (e.g., positive and ground power supply voltages may be supplied to DUT  100  via pads X and Y), whereas pad Z may serve as a control terminal (e.g., control signals for configuring the state of DUT  100  may be supplied via pad Z). 
     Test contact members configured to mate with the contact terminals of DUT  100  may be formed on substrate  210 . As shown in  FIG. 7 , conductive pads V′, W′, X′, Y′, and Z′ may be formed on substrate  210 . Substrate  210  may have a first edge portion to which a first radio-frequency connector  220 - 1  is attached and a second edge portion to which a second radio-frequency connector  220 - 2  is attached. Connectors  220 - 1  and  220 - 2  may be coaxial connectors such as SubMiniature version A (SMA) connectors that provide a 50 ohm termination impedance for radio-frequency signals up to 18 GHz (as an example). This is merely illustrative. If desired, other types of connectors such as SubMiniature version B (SMB) connectors, SubMiniature version C (SMC) connectors, Bayonet Neill-Concelman (BNC) connectors, U.FL connectors, and other types of connectors may be used in test system  200 . 
     Connector  220 - 1  may have a core signal conductor  222 - 1  that is coupled to pad V′ via a signal trace  226  that is formed in substrate  210 . Connector  220 - 2  may have a core signal conductor  222 - 2  that is coupled to pad W′ via a signal trace  228  that is formed in substrate  210 . Substrate  210  may include at least one layer such as layer  224  configured to serve as a ground reference for signals propagating through the conductive signal traces formed in other layers of substrate  210 . Connectors  220 - 1  and  220 - 2  may each have an outer conductor that surrounds the core signal conductor and that is coupled to ground  224 . Pad X′ may be coupled to ground plane  224  through a conductive via  240 . 
     When DUT  100  is mated with test substrate  210 , DUT contacts V, W, X, Y and Z may make physical and electrical contact with substrate contacts V′, W′, X′, Y′, and Z′, respectively. As shown in  FIG. 11A , contact pads  330  may represent only one suitable mating mechanism for interfacing with a corresponding DUT. In another suitable arrangement, protruding conductive features such as solder balls  332  may be formed on top of contact pads  330  (see, e.g.,  FIG. 11B ). The presence of solder balls  332  may facilitate proper electrical connection between DUT  100  and the substrate contacts. In yet another suitable arrangement, pins such as pogo pins  334  may be embedded within test substrate  210 . Each pin  334  may have a spring-loaded tip portion  336  configured to make electrical contact with DUT  100  when DUT  100  is mounted on substrate  210 . These examples are merely illustrative. In general, test system  200  may include any type of DUT receiving structure having any suitable type of coupling mechanism for mating with DUT  100  during testing. 
     Power may be supplied to the mated DUT using test pins  252  formed on substrate  210 . Power supply unit  206  may provide a positive power supply voltage and a ground power supply voltage to first and second test pins  252 , respectively, via cable  250 . First test pin  252  may be coupled to pad Y′ via path  244  formed in substrate  210 , whereas second test pin  252  may be coupled to test substrate ground plane  224  (i.e., to short layer  224  to ground). Moreover, power supply unit  206  can be used to monitor an amount of current that is drawn by DUT  100 . Data reflective of the amount of current drawn by DUT  100  over time may be provided from power supply unit  206  to test host  202  via path  232 . Monitoring current using power supply unit  206  in this way ensures that DUT  100  does not consume excessive amounts of power. If desired, test host  202  can also control the amount of power that is supplied to DUT  100  by sending appropriate power adjustment settings to unit  206  via path  232 . 
     The example of  FIG. 7  in which power is supplied to DUT  100  is suitable for an antenna tuning element that contains at least one active component (i.e., a electrical component that needs to be actively powered for proper operation). As an example, radio-frequency switch  152  of  FIG. 6A  may be an active component that needs to be powered during testing. In other arrangements, DUT  100  may not include active components. When DUT  100  only includes passive electrical components (e.g., resistors, capacitors, inductors, and/or other passive load elements), power need not be supplied to DUT  100  during testing. 
     Control signals for configuring the state of DUT  100  may be supplied to the mated DUT using test pin  254 . For example, test host  202  may provide appropriate control signals Vc that place DUT  100  in the desired state via path  256  that is coupled to pin  254 . Consider a scenario in which DUT  100  is a varactor of the type shown in  FIG. 6B . During a first test iteration, test host  202  may send control signals to DUT  100  via path  236  that configure switch  154  to connect ports P 1  and P 2 . During a second test iteration, test host  202  may send control signals to DUT  100  via path  236  that configure switch  154  to connect ports P 1  and P 3 . During a third test iteration, test host  202  may send control signals to DUT  100  via path  236  that configure switch  154  to connect ports P 1  and P 4 . It is generally desirable to characterize DUT  100  in a variety of potential operating states using test system  200 . 
     The example of  FIG. 7  in which only five contacts are shown is merely illustrative. In general, DUT  100  may include at least one contact terminal, at least two contact terminals, at least five contact terminals, at least ten contact terminals, or any suitable number of contact terminals for interfacing with external circuitry (e.g., for interfacing with test equipment during testing and for interfacing with other antenna structures during normal device operation). 
     Conductive traces  226  and  228  and associated ground plane  224  formed as a part of substrate  210  in this way may collectively serve as a microstrip transmission line path through which radio-frequency test signals may be conveyed during testing. In general, substrate  210  may be configured to form any suitable transmission line path such as stripline transmission lines, edge coupled microstrip transmission lines, edge coupled stripline transmission lines, or other suitable transmission line structures through which radio-frequency signals may be conveyed. 
     Tester  204  may be used to generate radio-frequency test signals that are fed to DUT  100  via the transmission lines formed on substrate  210 . Radio-frequency tester  204  may be a vector network analyzer (as an example). Tester  204  may have a first port  216 - 1  to which a first radio-frequency cable  218 - 1  is connected and a second port  216 - 2  to which a second radio-frequency cable  218 - 2  is connected. Radio-frequency cables  218 - 1  and  218 - 2  may, for example, be coaxial cables. In particular, first cable  218 - 1  may have a first end that is connected to tester port  216 - 1  and a second end terminating at a first radio-frequency connector  219 - 1 . Similarly, second cable  218 - 2  may have a first end that is connected to tester port  216 - 2  and a second end terminating at a second radio-frequency connector  219 - 2 . The first port  216 - 1  of tester  204  may be electrically connected to terminal A of a mounted DUT by mating connectors  219 - 1  and  220 - 1 , whereas the second port  216 - 2  of tester  204  may be electrically connected to terminal B of the mounted DUT by mating connectors  219 - 2  and  220 - 2 . Connected using this arrangement, tester  204  may be configured to gather desired radio-frequency measurements such as radio-frequency two-port network parameters from DUT  100 . Radio-frequency tester  204  may receive commands from test host  202  via path  230  that direct tester  204  to gather desired radio-frequency measurement. If desired, test data can be provided from tester  204  to test host  202  via path  230 . 
     Radio-frequency tester  204  may be configured to produce radio-frequency test signals that are applied to DUT  100  via cables  218  (e.g., cables  218 - 1  and  218 - 2 ) and test substrate  210 . Even without being connected to other components to form a completed antenna assembly, DUT  100  may emit radio-frequency signals when being energized by the test signals generated using tester  204 . As electromagnetic test signals are transmitted by tester  204  and applied to DUT  100  through test cable  218 - 1 , corresponding emitted electromagnetic test signals may be received through test cable  218 - 2  (as an example). Tester  204  may also receive reflected signals via cable  218 - 1  (i.e., signals that were reflected from DUT  100  in response to the signals transmitted through test cable  218 - 1 ). 
     The reflected signals gathered in this way may be used to compute a reflection coefficient (sometimes referred to as an S 11  parameter or S 11  scattering parameter). The transmitted signals on cable  218 - 1  and corresponding received signals on cable  218 - 2  may be used to compute a forward transfer coefficient (sometimes referred to as an S 21  parameter or S 21  scattering parameter). The S 11  and S 21  data may include magnitude and phase components. 
     Similarly, tester  204  may also transmit test signals to DUT  100  through test cable  218 - 2 . As test electromagnetic signals are transmitted by tester  204  and applied to DUT  100  through test cable  218 - 2 , corresponding emitted electromagnetic test signals may be received through test cable  218 - 1 . Tester  204  may also receive reflected signals via cable  218 - 2  (i.e., signals that were reflected from DUT  100  in response to the signals transmitted through test cable  218 - 2 ). The emitted and reflected signals gathered in this way may be used to compute reflection coefficient data (sometimes referred to as an S 22  scattering parameter) and forward transfer coefficient data (sometimes referred to as an S 12  scattering parameter). 
     Test host  202  may, for example, analyze the scattering parameter test data to determine whether DUT  100  satisfies design criteria. If the gathered test data deviates from a predetermined level by an unacceptable amount, DUT  100  may be marked as defective. If the gathered test data deviates from the predetermined level by a tolerable amount, DUT  100  may be marked as a passing device. The use of tester  204  for obtaining scattering parameter test data from DUT  100  is merely illustrative and does not serve to limit the scope of the present invention. If desired, tester  204  may be used to gather other types of radio-frequency measurements from DUT  100 . 
     When DUT  100  is mated with substrate  210 , there needs to be a way for securing the position of DUT  100  so that DUT  100  is properly connected to the corresponding test contacts on substrate  210  and so that DUT  100  does not accidentally shift during testing. Conventionally, surface mount components under test are soldered to receiving test fixtures. Soldering DUT  100  to substrate  210  can, however, potentially damage DUT  100  due to excess amounts of heat that is generated from soldering equipment. It may therefore be desirable to have a way of fixing the position of DUT  100  on top of substrate  210  while imparting minimal stress on DUT  100 . 
     In one suitable embodiment of the present invention, test system  200  may include a DUT retention structure such as DUT retention structure  260 . DUT retention structure  260  may include a pressure sensing circuit  262  operable to detect an amount of pressure that is applied at a tip portion  261 . Pressure sensor  262  may produce a pressure sensor output value Dpres that is proportional to the amount of pressure (or force) that is exerted onto tip  261 . In one suitable arrangement, sensor output value Dpres may be output in the form of a digital signal. In other suitable embodiments, the pressure sensor output value may be output in the form of an analog signal. Pressure sensor output value Dpres may be fed to test host  202  via path  266 . 
     During testing, tip portion  261  of DUT retention structure  260  may be positioned over DUT  100  and may be lowered to make physical contact with DUT  100 . Configured in this way, DUT retention structure  260  may be used to apply a sufficient amount of pressure on DUT  100  so that DUT  100  is properly mated with test substrate  210  and so that DUT  100  will not be accidentally knocked out of place during testing. DUT retention structure  260  having tip  261  that is used as such may sometimes be referred to as a “plunger.” Tip  261  may be formed using non-marring material such as acetyl plastic, Delrin® (a polyoxymethylene plastic), other plastics, or other suitable non-marring materials. The use of non-marring materials may help avoid scratches or other damage to DUT  100  when tip  261  is pressed against the surface of DUT  100 . 
       FIG. 10A  shows one suitable shape for tip  261 . As shown in  FIG. 10A , tip  261  may have a flat planar surface  320  configured to make contact with a corresponding surface on DUT  100 . In another suitable arrangement, tip  261  may have a rounded surface  322  configured to make contact with DUT  100  (see, e.g.,  FIG. 10B ). In yet another suitable arrangement, an additional layer of compressible material such as material  324  may be attached to the end of tip  261  (see, e.g.,  FIG. 10C ). Material  324  may be formed using rubberized foam or other suitable elastomeric material. These configurations for tip  261  are merely illustrative. If desired, tip portion  261  of plunger  260  may have any suitable shape for making contact with DUT  100 . 
     The position of plunger  260  may be controlled using a positioner such as positioner  264 . Positioner  264  may be a computer-controlled device (e.g., a positioning device that receives commands from test host  202  via path  266 ) or a manually-controlled positioning device (e.g., a positioning device that is controlled via knob adjustments from test personnel) having actuators for controlling the height of plunger  260  in the direction of arrows  265  (as an example). 
       FIG. 8  shows a perspective view of test system  200 . Test substrate  210  may be secured to a first base member  300  using clamps such as clamps  306 . Test pins  253  may be formed on substrate  210 . Test pins  253  may include power supply signal pins  252 , control signal pins  254 , and other pins that can be used to access DUT  100 . Positioner  264  may be secured on top of a second base member  302  that is adjacent to first base member  300 . The first and second base members may be secured to test platform  304  (sometimes referred to as a test platen). Base members  300  and  302 , clamps  306 , and platform  304  may be constructed using metal, plastic, or other durable materials. 
     As shown in  FIG. 8 , plunger  260  may be attached to a vertical arm  270 . Positioner  264  may include a first linear position adjustment mechanism such as a coarse tuning knob  272  that is used to make coarse height (linear) adjustments for vertical arm  270  in direction  310 . For example, the height of plunger  260  may be adjusted in 1 mm steps using tuning knob  272 . 
     Positioner  264  may also include a second linear position adjustment mechanism such as a fine tuning knob  312  that is used to make fine height adjustments for vertical arm  270  in direction  310  (see, e.g.,  FIG. 9 ). For example, the height of plunger  260  may be adjusted in 0.1 mm steps using tuning knob  310 . In general, the step size associated with fine tuning knob  310  can be any suitable fraction of the step size associated with coarse tuning knob  272 . Positioner  264  may also include a coarse tuning lock  314  and a fine tuning lock  316 . Tuning locks  314  and  316  may be used to prevent damage to DUT  100  by providing a hard stop when plunger  260  is lowered to a minimum allowable distance separating tip  261  from the surface of test substrate  210 . 
     Generally, the different knobs in positioner  264  (e.g., tuning knobs  272 ,  312 ,  314 , and  316 ) may be controlled manually by test personnel or may be controlled pneumatically/automatically based on commands from test host  202 . The example of  FIGS. 7 and 8  in which positioner  264  has two tuning knobs offering two degrees of positioning accuracy is merely illustrative. If desired, positioner  264  may include less than two or more than two positioning adjustment mechanisms for controlling the vertical and/or horizontal position of plunger  260  in any desired step size (e.g., in 1 mm positioning resolution, in 0.1 mm positioning resolution, in 0.05 mm positioning resolution, in 0.01 mm positioning resolution, etc.). 
       FIG. 12  is a plot showing how pressure sensor output Dpres may vary as a function of a plunger height adjustment setting that is reflective of the current state of coarse tuning knob  272  and fine tuning knob  312 . When the height adjustment setting is initialized to S 0 , plunger  260  may be raised to a maximum height within the capabilities of positioner  264 . At this maximum height, tip  261  should not be in contact with DUT  100 , and sensor  262  may provide a baseline sensor output value of P 0 . As the plunger height adjustment setting is increased (e.g., using coarse tuning knob  272 ), plunger  260  may gradually be lowered until DUT  100  makes physical contact with DUT  100 . 
     When plunger  260  makes contact with DUT  100 , output Dpres may increase substantially from the baseline reading of P 0  to an elevated reading P 1  (see, corresponding plunger height adjustment setting S 1 ). At this point, plunger  260  may be further lowered until Dpres exceeds a predetermined pressure threshold level Pt (e.g., by further incrementing the plunger height adjustment setting using fine tuning knob  314 ). The height adjustment setting corresponding to the point at which Dpres exceeds a predetermined threshold Pt may be referred to as a critical adjustment setting S*. It may be desirable to gather critical adjustment settings S* across multiple samples for a given type of DUT and for different types of DUTs. 
       FIG. 13  shows illustrative steps involved in obtaining critical adjustment settings S* when calibrating test station  200 . At step  400 , the plunger height adjustment setting may be reset to S 0  so that positioner  264  raises plunger  260  to its maximum height. At step  401 , DUT  100  may be placed on test board  210  (e.g., so that DUT contacts V, W, X, Y, and Z are aligned to corresponding test contacts V′, W′, X′, Y′, and Z′). 
     At step  402 , coarse tuning knob  272  may be monotonically adjusted to gradually lower plunger  260  until plunger  260  makes contact with DUT  100 . By monitoring when pressure sensor output Dpres spikes upward as plunger  260  is lowered (see,  FIG. 12 ), it is possible to determine when plunger  260  makes physical contact with DUT  100 . Once plunger  260  is in contact with DUT  100 , no additional adjustment to coarse tuning knob  272  should be made. 
     At step  404 , fine tuning knob  312  may be monotonically adjusted to gradually lower plunger  260  until Dpres exceeds a predetermined threshold pressure (i.e., a pressure level which ensures that DUT  100  is securely pressed against test board  210 ). Once Dpres has reached the predetermined threshold pressure, no additional adjustment to fine tuning knob  312  should be made. 
     At step  406 , the current state of coarse tuning knob  272  and fine tuning knob  312  may collectively represent the critical height adjustment setting S* for DUT  100  that is presently being tested. Due of variations in semiconductor packaging processes, it is possible that two samples of an electrical component have different package thicknesses. It may therefore be desirable to calibrate test station  200  by testing more than one sample to ensure that plunger  260  applies sufficient pressure on multiple DUTs  100  in a production line. For example, n samples of the same type of DUT may be tested to obtain a distribution of critical height adjustment settings S* (as indicated by path  408 ). If desired, processing may loop back to step  400  to calibrate test station  200  using different types of DUTs (see, e.g., the different types of antenna tuning elements of  FIGS. 6A, 6B, and 6C ), as indicated by path  410 . 
     The steps of  FIG. 13  are merely illustrative and do not serve to limit the scope of the present invention. The steps of  FIG. 13  may be performed manually by a test operator or may be performed automatically using test software running on test host  202 . In the scenario where the calibration method is automated, a closed loop control system having actuators for moving plunger  260 , stepper motors for adjusting knobs  272  and  312 , a pressure sensor for monitoring the real-time pressure at the tip of plunger  260 , and other electrical test equipment may be used. The actuators and stepper motors may be adjusted in a controlled fashion while the electrical response that is measured using the pressure sensor is being monitored. When this electrical response meets or exceeds a predetermined threshold, a critical pressure setting is recorded. These steps may be repeated on a sample of reference devices to gather numerous critical pressure settings. An average pressure setting may then be computed and used as the operating pressure setting as applied to the DUTs during production testing. 
       FIG. 14  is a frequency distribution plot showing test data that may be gathered using the steps described in connection with  FIG. 13 . Curve  500  may represent a distribution profile for critical adjustment settings S* associated with a first type of DUT, whereas curve  502  may represent a distribution profile for critical adjustment settings S* associated with a second type of DUT that is different than the first type of DUT. 
     Setting Sa may be obtained by sampling the rightmost S* that is part of curve  500  and may represent a setting that, if applied to positioner  264 , will allow all DUTs of the first type to receive at least the predetermined threshold pressure from plunger  260 . Setting Sa′ may be obtained by computing an S* that is three standard deviations (3σ) to the right of the mean of curve  500  and may represent a setting that, if applied to positioner  264 , will allow more than 99% of DUTs of the first type to receive at least the predetermined threshold pressure from plunger  260 . 
     Setting Sb may be obtained by sampling the rightmost S* that is part of curve  502  and may represent a setting that, if applied to positioner  264 , will allow all DUTs of the second type to receive at least the predetermined threshold pressure from plunger  260 . Setting Sb′ may be obtained by computing an S* that is three standard deviations (3σ) to the right of the mean of curve  502  and may represent a setting that, if applied to positioner  264 , will allow more than 99% of DUTs of the second type to receive at least the predetermined threshold pressure from plunger  260 . 
       FIG. 15  is a flow chart of illustrative steps for processing and applying the calibration data. At step  600 , test host  202  may select a desired S* for each type of DUT. As an example, test host  202  may select Sa to be the primary setting for configuring positioner  264  when using test system  200  to test a DUT of the first type. As another example, test host  202  may select Sa′ to be the primary setting for configuring positioner  264  when using test system  200  to test a DUT of the first type. Different types of DUTs may therefore be tested using different primary plunger height adjustment settings determined during step  600 . If desired, test host  202  may alternatively select a single master adjustment setting for configuring positioner  264  regardless of the type of DUT being tested. For example, the master plunger height adjustment setting may be computed by taking the maximum of Sa′ and Sb′ (if only two types of DUTs are being tested). Other ways of selecting the master plunger height adjustment setting may be used. 
     At step  602 , the calibration settings determined during step  600  may be stored on test host  202  and may be applied to positioner  264  so that plunger  260  will be configured to apply sufficient pressure on a corresponding DUT during production testing. At step  604 , calibration of positioner  264  is complete and test system  200  may be used to test any suitable electrical components suitable for mounting on test substrate  210 . 
     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: 20120601
Publication Date: 20160412
Grant Date: 20160412
Priority Date: 20120601
Inventors: NATH JAYESH
HAN LIANG
MOW MATTHEW A.
O'CONNOR HAGAN
NICKEL JOSHUA G.
BEVELACQUA PETER
PASCOLINI MATTIA
SCHLUB ROBERT W.
CABALLERO RUBEN
Assignee: APPLE INC
CPC Classifications: [{"code": "G01R1/0408", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R31/2808", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R31/2837", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R31/2822", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R1/0408", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R31/2808", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R31/2822", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R31/2837", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 49669442