PATENT DOCUMENT

Publication Number: US-8798198-B2
Application Number: US-201113222553-A
Country: US
Kind Code: B2

Title: Calibration systems for wireless electronic devices

Abstract:
A calibration system may be provided for calibrating wireless communications circuitry in an electronic device during manufacturing. The calibration system may include data acquisition equipment for receiving an amplitude-modulated calibration signal from the electronic device. The calibration system may include calibration computing equipment for extracting pre-distortion coefficients from the amplitude-modulated calibration signal. The calibration computing equipment may be configured to detect a bulk phase drift in the amplitude-modulated calibration signal. The calibration computing equipment may be configured to remove the bulk phase drift from the amplitude-modulated calibration signal. The wireless communications circuitry may include a power amplifier that distorts a signal generated by the wireless communications circuitry. The wireless communications circuitry may include a pre-distortion compensator for countering the distortion. The pre-distortion coefficients determined from the phase drift corrected amplitude-modulated calibration signal may be used by the pre-distortion compensator when countering the distortion.

Claims:
What is claimed is: 
     
       1. A method of calibrating an electronic device having wireless communications circuitry with a calibration system, wherein the calibration system includes data acquisition equipment and calibration computing equipment and wherein the wireless communications circuitry comprises a calibration pattern generator and local oscillator, the method comprising:
 with the wireless communications circuitry, transmitting an amplitude-modulated calibration signal to the data acquisition equipment; 
 with the data acquisition equipment, transferring the amplitude-modulated calibration signal to the calibration computing equipment; 
 with the calibration computing equipment, extracting a phase drift correction from the amplitude-modulated calibration signal; 
 with the calibration computing equipment, extracting pre-distortion coefficients from the amplitude-modulated calibration signal; 
 with the calibration pattern generator, generating a known amplitude-modulated calibration pattern; 
 with the local oscillator, generating a carrier signal; and 
 with the wireless communications circuitry, before transmitting the amplitude-modulated calibration signal to the data acquisition equipment, modulating the carrier signal with the known amplitude-modulated calibration pattern to create the amplitude-modulated calibration signal. 
 
     
     
       2. The method defined in  claim 1  further comprising:
 with the calibration computing equipment, applying the phase drift correction to the amplitude-modulated calibration signal. 
 
     
     
       3. The method defined in  claim 2  wherein extracting the pre-distortion coefficients from the amplitude-modulated calibration signal comprises extracting the pre-distortion coefficients from the phase drift corrected amplitude-modulated calibration signal. 
     
     
       4. The method defined in  claim 1  wherein the wireless communications circuitry comprises a power amplifier that amplifies the amplitude-modulated calibration signal prior to transmitting the amplitude-modulated calibration signal to the data acquisition equipment. 
     
     
       5. The method defined in  claim 4  wherein the amplitude-modulated calibration signal is distorted by the power amplifier and wherein extracting the pre-distortion coefficients from the amplitude-modulated calibration signal comprises comparing the distorted amplitude-modulated calibration signal with the known amplitude-modulated calibration pattern. 
     
     
       6. The method defined in  claim 5  wherein the wireless communications circuitry further comprises a pre-distortion compensator, and wherein the pre-distortion compensator is configured to alter a communications signal generated by the wireless communications circuitry using the pre-distortion coefficients. 
     
     
       7. The method defined in  claim 1  further comprising, transmitting the pre-distortion coefficients to the electronic device. 
     
     
       8. The method defined in  claim 7  further comprising storing the pre-distortion coefficients using the wireless communications circuitry. 
     
     
       9. The method defined in  claim 1  wherein extracting the phase drift correction from the amplitude-modulated calibration signal comprises using a linear regression procedure to extract the phase drift correction. 
     
     
       10. A calibration system for wireless communications calibration of an electronic device, comprising:
 data acquisition equipment having at least one local oscillator having a first frequency for receiving an amplifier-distorted amplitude-modulated calibration signal from the electronic device, wherein the electronic device comprises at least one local oscillator having a second frequency and wherein the first frequency has a drift with respect to the second frequency that causes a phase drift in the amplifier-distorted amplitude-modulated calibration signal; and 
 calibration computing equipment for extracting a phase drift correction from the amplifier-distorted amplitude-modulated calibration signal. 
 
     
     
       11. The calibration system defined in  claim 10  wherein the electronic device further comprises a power amplifier, wherein the power amplifier induces amplifier distortion in the amplifier-distorted amplitude-modulated calibration signal, and wherein the calibration computing equipment is configured to apply the phase drift correction to the amplifier-distorted amplitude-modulated calibration signal. 
     
     
       12. The calibration system defined in  claim 11  wherein the electronic device further comprises a pre-distortion compensator for pre-distorting an input signal to the power amplifier and wherein the calibration computing equipment is configured to extract pre-distortion coefficients from the phase drift corrected amplifier-distorted amplitude-modulated calibration signal. 
     
     
       13. The calibration system defined in  claim 12  wherein the electronic device comprises a calibration pattern generator that generates a known amplitude-modulated calibration pattern, wherein extracting the pre-distortion coefficients from the phase drift corrected amplifier-distorted amplitude-modulated calibration signal comprises comparing the known amplitude-modulated calibration pattern with the phase drift corrected amplifier-distorted amplitude-modulated calibration signal. 
     
     
       14. The calibration system defined in  claim 13  wherein the data acquisition equipment is coupled to the calibration computing equipment using a General Purpose Interface Bus cable and wherein the data acquisition equipment is configured to transfer amplifier-distorted amplitude-modulated calibration data to the calibration computing equipment using the General Purpose Interface Bus cable. 
     
     
       15. A method for obtaining pre-distortion coefficients for an electronic device having a power amplifier and a first oscillator that operates at a first frequency, the method comprising:
 with the power amplifier, amplifying an amplitude-modulated calibration signal; 
 transmitting the amplified amplitude-modulated calibration signal to data acquisition equipment having a second oscillator that operates at a second frequency, wherein drift between the first and second frequencies causes bulk phase drift in the amplified amplitude-modulated calibration signal; 
 with calibration computing equipment, removing the bulk phase drift from the amplified amplitude-modulated calibration signal; and 
 with the calibration computing equipment, measuring an amplifier distortion in the amplified amplitude-modulated calibration signal. 
 
     
     
       16. The method defined in  claim 15  further comprising:
 with the data acquisition equipment, converting the amplified amplitude-modulated calibration signal into in-phase/quadrature-phase data samples; and 
 with the data acquisition equipment, transferring the in-phase/quadrature-phase data samples to the calibration computing equipment. 
 
     
     
       17. The method defined in  claim 16  further comprising:
 with the calibration computing equipment, using the measured amplifier distortion to obtain the pre-distortion coefficients. 
 
     
     
       18. The method defined in  claim 17  wherein measuring the amplifier distortion comprises:
 with the calibration computing equipment, comparing a received amplitude associated with the in-phase/quadrature-phase data samples to a known input amplitude; and 
 with the calibration computing equipment, comparing a received phase associated with the in-phase/quadrature-phase data samples to the known input amplitude.

Description:
BACKGROUND 
     This related to calibration, and more particularly, to calibration of wireless communications circuitry in electronic devices. 
     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 at 700 MHz, 850 MHz, 900 MHz, 1800 MHz, 1900 MHz, and 2100 MHz. Electronic devices may use short-range wireless communications circuitry to handle communications with nearby equipment. For example, electronic devices may communicate using the WiFi® (IEEE 802.11) bands at 2.4 GHz and 5 GHz and the Bluetooth® band at 2.4 GHz. 
     Wireless communications circuitry often includes a power amplifier that amplifies an electronic input signal to produce an amplified electronic output signal having more power than the input signal. The ratio of power of the output signal to the power of the input signal is commonly referred to as the gain of an amplifier. Power amplifiers commonly suffer from a non-linear gain in which a high power input signal is not amplified as much as a low power input signal (i.e., the gain is reduced for high power signals), and in which the phase of the output signal changes as a function of the input power. To compensate for this and other distortions of a signal by a power amplifier, wireless communications circuitry often includes a pre-distortion compensator circuit that alters an input signal to the power amplifier such that the output of the power amplifier is linear (i.e., the signal gain is the same) for a broader range of input signal powers. Pre-distortion compensator circuits take, as inputs, pre-distortion coefficients based on measured performance of the power amplifier. 
     In order to provide electronic devices that perform uniformly across all devices, each electronic device may be calibrated during manufacturing before delivery to end users. Calibration operations include determination of pre-distortion coefficients by measuring the performance of wireless communications power amplifiers in each device. Measuring the performance of power amplifiers in each electronic device can be time consuming and can therefore slow the pace of production of the devices and can increase the cost of productions. 
     It would therefore be desirable to provide improved calibration systems for electronic devices with wireless communications capabilities. 
     SUMMARY 
     A calibration system may be provided for calibrating wireless communications circuitry in an electronic device during manufacturing. 
     An electronic device may have wireless communications circuitry for handling wireless communications. The wireless communications circuitry may include transceiver circuitry and a power amplifier. The wireless communications circuitry may be configured to communicate with external equipment such as cellular network equipment and wireless local area network equipment. The transceiver may include a pre-distortion compensator for pre-processing a signal to the power amplifier and a calibration pattern generator to be used during calibration of the power amplifier. The calibration system may be configured to optimize pre-distortion coefficients that allow the pre-distortion compensator to preprocess a signal from the transceiver to the power amplifier such that the output signal from the power amplifier is a linear function of the input signal to the pre-distortion compensator. 
     During calibration, the calibration pattern generator may be used to generate a known, amplitude-modulated (AM) input signal that is transmitted using the wireless communications circuitry to data acquisition equipment in the calibration system. The data acquisition equipment may capture and digitize the transmitted AM test signal. The digitized AM signal may then be transferred to calibration computing equipment in the calibration system. The calibration computing equipment may then be used to extract bulk phase drift information from the digitized AM signal. The calibration computing equipment may then be used to correct the digitized AM signal using the extracted bulk phase drift information. The calibration computing equipment may then be used to compare the phase drift corrected AM signal with the known AM input signal and to extract pre-distortion coefficients from the compared input and phase drift corrected AM signals. The calibration computing equipment may then be used to transmit the determined pre-distortion coefficients back to the electronic device. The pre-distortion coefficients may be stored in the electronic device and used when altering signals with the pre-distortion compensator. 
     Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an illustrative electronic device having wireless communications circuitry in accordance with an embodiment of the present invention. 
         FIG. 2A  is a graph of power amplifier performance for a typical power amplifier. 
         FIG. 2B  is a graph showing how the required pre-distortion phase compensation of a typical power amplifier depends on the amplitude of an input signal. 
         FIG. 3  is a diagram of illustrative wireless communications circuitry having a power amplifier and a transceiver having a calibration pattern generator and a pre-distortion compensator in accordance with an embodiment of the present invention. 
         FIG. 4  is a diagram of an illustrative calibration system for determining pre-distortion coefficients including timing control equipment, data acquisition equipment, and calibration computing equipment in accordance with an embodiment of the present invention. 
         FIG. 5  is a graph showing a conventional calibration pattern for calibrating a pre-distortion compensator. 
         FIG. 6  is a graph showing an illustrative calibration pattern for improved determination of pre-distortion coefficients during manufacturing in accordance with an embodiment of the present invention. 
         FIG. 7  is a graph showing an illustrative in-phase/quadrature-phase representation of a calibration pattern of the type shown in  FIG. 6  that is affected by relative local oscillator frequency drift in accordance with an embodiment of the present invention. 
         FIG. 8  is an illustrative graph showing a phase drift correction function determined from amplitude-modulated calibration data of the type shown in  FIG. 7  in accordance with an embodiment of the present invention. 
         FIG. 9  is an illustrative graph showing phase drift corrected calibration data in accordance with an embodiment of the present invention. 
         FIG. 10  is an illustrative graph showing an in-phase/quadrature-phase representation of phase drift corrected calibration data of the type shown in  FIG. 9  in accordance with an embodiment of the present invention. 
         FIG. 11  is a pair of graphs showing an illustrative comparison of corrected calibration data of the type shown in  FIG. 9  in accordance with an embodiment of the present invention. 
         FIG. 12  is a flow chart of illustrative steps involved in performing wireless communications calibration of electronic devices having wireless communications circuitry in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Electronic devices such as cellular telephones, media players, computers, set-top boxes, wireless access points, and other electronic equipment having wireless communications circuitry may be calibrated during manufacturing. Wireless communications circuitry is often able to support multiple radio access technologies. For example, a device may include wireless communications circuitry for handling communications associated with “2G”, “3G” and “4G” communications protocols. Wireless communications circuitry may include capabilities for handling communications using cellular telephone protocols, WiFi® (IEEE 802.11) communications protocols, Bluetooth® communications protocols, etc. Wireless communications using these communications protocols may be implemented using by modulating a carrier signal using a phase-shift keying (PSK) modulation scheme. A calibration system including calibration computing equipment and data acquisition equipment capable of receiving and processing PSK modulated signals may be used during calibration of devices. Following calibration, a device may be shipped to a customer. 
     An illustrative electronic device of the type that may be calibrated during manufacturing is shown in  FIG. 1 . As shown in  FIG. 1 , electronic device  10  may include processing circuitry  12 , input-output devices  14 , and wireless communications circuitry  16 . Processing circuitry  12  may include microprocessors, microcontrollers, digital signal processor integrated circuits, application-specific integrated circuits, and other processing circuitry. Volatile and non-volatile memory circuits such as random-access memory, read-only memory, hard disk drive storage, solid state drives, and other storage circuitry may also be included in processing circuitry  12 . 
     Processing circuitry  12  may use input-output devices  14  to obtain user input and to provide output to a user. Input-output devices  14  may include speakers, microphones, sensors, buttons, keyboards, displays, touch sensors, wireless circuitry such as wireless local area network transceiver circuitry and cellular telephone network transceiver circuitry, and other components for receiving input and supplying output. 
     Wireless communications circuitry  16  may include one or more power amplifiers such as power amplifier  18 . Wireless communications circuitry  16  may include transceiver circuitry for handling communications at the GPS frequency of 1575 MHz, cellular telephone communications (e.g., communications in cellular bands at 700, 800, 900, 1800 1900, and 2100 MHz) or wireless local area network communications (e.g., in bands at 2.4 GHz or 5 GHz). Transceiver circuitry associated with wireless circuitry  16  may include or be associated with circuits such as low-noise amplifiers (LNAs) that are used to amplify incoming signals and power amplifiers such as power amplifier  18 . Power amplifier  18  may be used to amplify outgoing signals. Transceiver circuitry associated with wireless circuitry  16  may include storage and processing circuitry and may communicate with other storage and processing circuitry in device  10 . Storage may be used to store software code or calibration coefficients such as pre-distortion coefficients obtained during calibration operations for device  10 . 
     Wireless communications circuitry  16  may include one or more antennas such as antenna  19 . Antenna  19  may be formed using any suitable type of antenna. For example, antenna  19  may include antennas with resonating elements that are formed from loop antenna structures, patch antenna structures, inverted-F antenna structures, closed and open slot antenna structures, planar inverted-F antenna structures, helical antenna structures, strip antennas, monopoles, dipoles, 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 (e.g., for handling WiFi® traffic or other wireless local area network traffic) and another type of antenna may be used in forming a remote wireless link antenna (e.g., for handling cellular network traffic such as voice calls and data sessions). There may be multiple antennas in device  10 . These antennas may be fixed or may be tunable. 
     Power amplifiers such as power amplifier  18  often exhibit non-linear amplification of high power input signals as shown in  FIG. 2A . Graph  20  of  FIG. 2A  shows how the amplified output power of a signal that is output from a common power amplifier changes with respect to the input power of a signal that is input to the power amplifier. A linear amplification (or constant gain) at all input powers is indicated by dashed line  22  (i.e., the slope of dashed line  22  is constant thereby indicating a constant gain at any input power). Solid line  24  shows that the actual output power of a common amplifier follows linear response curve  22  for relatively small input power. However, for input signals with relatively larger input power, the corresponding output power, as shown by real amplification curve  24 , deviates from linear curve  22 . Power amplifiers may exhibit other distorting behavior. For example, in addition to distorting the amplitude of an input signal, a power amplifier may include capacitive or inductive components that affect output of portions of an input signal and thereby generate distortions in the phase of an input signal. Power amplifiers often distort the frequency of an input signal in addition to distortions of the amplitude and phase of the input signal. In order to compensate for these signal distorting effects (e.g., amplitude distortion, phase distortion, frequency distortion, etc.), an electronic device such as device  10  of  FIG. 1  may be provided with wireless communications circuitry that includes pre-distortion circuitry that applies an inverse (e.g., compensating) distortion to the input signal to a power amplifier that cancels or reduces known (e.g., previously measured) power amplifier distortions to the signal. 
       FIG. 2B  shows how the pre-distortion compensation required to correct a phase distortion of a typical power amplifier varies with respect to the amplitude of the input signal to the power amplifier. As shown by phase compensation curve  107  of graph  105  of  FIG. 2B , for relatively small input amplitudes, a typical power amplifier generates a large phase distortion and therefore requires a large phase compensation. For relatively larger input amplitudes, the required phase compensation is smaller and asymptotes to a minimum phase compensation for very large input amplitudes. 
     Wireless communications circuitry of the type that may be provided with pre-distortion compensation circuitry is shown in  FIG. 3 . As shown in  FIG. 3 , an electronic device of the type shown in  FIG. 1  may include wireless communications circuitry  16  having a transceiver such as transceiver  30  for use in transmitting and receiving radio-frequency (RF) signals. Transceiver  30  may include receiver circuitry such as receiver circuitry  33  that is coupled to one or more antennas such as antenna  19 . Receiver circuitry  33  of transceiver  30  may be configured to process incoming signals such as incoming signal  37 . Transceiver  30  may include one or more local oscillators such as local oscillator  32 . Local oscillator  32  may be configured to generate a carrier signal that is mixed with a modulating data signal using mixing circuitry  35  prior to transmission of data using wireless communications circuitry  16 . During normal operations of device  10 , data signals may be generated using baseband circuitry  31 . Data signals generated with baseband circuitry  31  may be mixed with carrier signals generated by local oscillator  32 . Mixing carrier signals generated by local oscillator  32  with data signals generated using baseband circuitry  31  may include modulating the carrier signal with the data signal. Modulating the carrier signal with the data signal may include any suitable type of carrier signal modulation (e.g., amplitude modulation, frequency modulation, phase modulation, etc.). Modulating carrier signals with data signals using digital in-phase/quadrature-phase (I/Q) phase-shift-keying (PSK) is sometimes described herein as an example. 
     As shown in  FIG. 3 , transceiver  30  may be coupled to power amplifier  18 . Power amplifier  18  may be configured to amplify a signal generated by transceiver  30  prior to transmission of the signal using a radio-frequency front end such as RF front end  38  and one or more antennas such as antenna  19 . Signals such as transmitted signal  39  may be transmitted using antenna  19 . Antenna  19  may be formed separately from RF front end  38  or may be formed as a portion of RF front end  38 . RF front end  38  may include one or more integrated circuits for performing filtering, low-noise amplification, frequency conversion, etc. of received RF signal  37  and transmitted RF signal  39 . If desired, RF front end  38  may be associated with a power detector such as power detector  21 . Power detector  21  and power amplifier  18  may form a portion of a power feedback loop (e.g., a power feedback loop internal to device  10 , a power feedback loop that uses communications with an external network, etc.) for determining a desired amount of amplification by power amplifier  18 . For example, RF front end  38  and power detector  21  may form a portion of a power control feedback loop (e.g., open loop power control, closed loop power control, etc.) in which the detected power of received RF signal  37  is used to determine a desired level of power amplification by power amplifier  18 . Power detector  21  may be formed separately from RF front end  38  or may be formed as an integrated portion of RF front end  38 . 
     As shown in  FIG. 3 , transceiver  30  may include a pre-distortion compensator such as pre-distortion compensator  36  (also sometimes referred to herein as pre-distortion compensation circuitry, pre-distortion circuitry, etc.). Pre-distortion compensator  36  may be configured to compensate for distortions of a signal by power amplifier  18  (e.g., to alter a signal generated by transceiver  30  such that, following amplification using power amplifier  18 , the signal generated by transceiver  30  is substantially undistorted). As an illustrative example of pre-distortion compensation for power amplifier distortion of a signal, transceiver  30  may generate a signal having a low power (small amplitude) component PL (power low) and a high power (large amplitude) component PH (power high). Power amplifier  18  may amplify low power component PL by a gain factor g such that the output signal from power amplifier  18  corresponding to low power component PL has an output power PO=g*PL. Power amplifier  18  may have a non-linear (distorting) amplification for high power input signals. Therefore, power amplifier  18  may amplify high power component PH by a gain factor (for example) 0.5*g such that the output signal from power amplifier  18  corresponding to high power component PH is only PO=0.5*g*PH. In this example, pre-distortion compensator  36  may be configured to boost (e.g., increase the amplitude) high power component PH by a factor of 2 prior to amplification by power amplifier  18  so that the output signal from power amplifier  18  corresponding to pre-distorted high power component PH_pdist=2.0*PH is PO=0.5*g*PH_pdist=g*PH. In this way, a linear gain may be applied to input signals PL and PH. This is merely illustrative. 
     In practice, power amplifier  18  may have more complicated distorting effects on an input signal (e.g., frequency distortion, phase distortion, etc.). Pre-distortion compensator  36  may be configured to pre-distort an input signal to power amplifier  18  to compensate for these more complicated effects. Pre-distortion compensator  36  may be configured to pre-distort the input signal in a way that is inverse to the distortions of power amplifier  18  so that the output signal from power amplifier  18  suffers from reduced distortions in amplitude, frequency, and/or phase. In order to provide pre-distortion compensators such as pre-distortion compensator  36  that compensate for signal distortion by associated power amplifiers, the signal distortion exhibited by the each associated power amplifier may first be measured (e.g., the power amplifier output may be calibrated). 
     In order to measure signal distortion caused by a power amplifier such as power amplifier  18 , a transceiver  30  may be provided with a calibration pattern generator such as calibration pattern generator  34 . Calibration pattern generator  34  may be configured to generate a predetermined signal having a known amplitude-modulated calibration pattern. During calibration operations, the known amplitude-modulated calibration pattern may be mixed with a carrier signal from local oscillator  32  to form a known pre-amplified calibration signal. The known pre-amplified calibration signal may include in-phase and quadrature phase (I/Q) components. The known pre-amplified calibration signal may be passed through pre-distortion compensator  36  without modification. If desired, the known pre-amplified calibration signal may be passed directly to power amplifier  18  without passing through pre-distortion compensator  38 . The known pre-amplified calibration signal may then be amplified by power amplifier  18  to form an amplified amplitude-modulated calibration signal. The amplified amplitude-modulated calibration signal may be transmitted to a calibration system using RF front end  38  and antenna  19 . The amplitude-modulated calibration signal may include distortions in amplitude, phase, and/or frequency generated during amplification using power amplifier  18 . The calibration system may be configured to use the amplifier-distorted amplitude-modulated calibration signal to measure the power amplifier distortion and to extract pre-distortion coefficients based on the measured power amplifier distortion. 
     Once the signal distortion of a power amplifier such as power amplifier  18  has been measured, pre-distortion information for pre-distortion compensator  36  may be represented by one or more pre-distortion coefficients. Pre-distortion coefficients measured during calibration of wireless communications circuitry  16  may be stored by device  10  in memory associated with transceiver  30 , or other memory associated with wireless circuitry  16 . During normal operation of a device such as device  10 , pre-distortion compensator  36  may use pre-distortion coefficients determined during calibration operations to pre-distort signals generated by mixing circuitry  35  prior to signal amplification by power amplifier  18 . During normal operation of device  10 , power detector  21  may be used to detect the power of a signal (e.g., an amplified signal from power amplifier  18 , an incoming signal such as incoming signal  37 , etc.). Power detector  21  may be used as a portion of an open loop or closed loop power control system that adjusts the amount of amplification by power amplifier  18  depending on the detected power the signal. 
       FIG. 4  is a diagram of an illustrative calibration system that may be used in calibrating devices such as device  10 . As shown in  FIG. 4 , calibration system  50  may include timing control equipment  41  and signal receiving and processing equipment such as data acquisition equipment  40  coupled to computing equipment such as calibration computing equipment  42 . Timing control equipment  41  may include external computing equipment configured to initiate a calibration sequence. Timing control equipment  41  may be configured to precisely control the time at which a known amplifier-distorted amplitude-modulated calibration signal is transmitted from device  10  to data acquisition equipment  40 . Data acquisition equipment  40  may include one or more antennas such as antenna  52 . Data acquisition equipment  40  may be configured to use antenna  52  to receive the amplifier-distorted amplitude-modulated calibration signal transmitted wirelessly by device  10  along wireless path  44  during calibration operations. Data acquisition equipment  40  may include a local oscillator such as local oscillator  54  that produces a signal at a frequency that is substantially similar to the frequency of the carrier signal produced by local oscillator  32  of device  10 . The amplifier-distorted calibration signal that is received by data acquisition equipment  40  may include the local oscillator carrier signal generated by local oscillator  32  of device  10  modulated with the known AM calibration pattern. Local oscillator  54  of data acquisition equipment  40  may be used to generate a signal to be mixed with the received amplifier-distorted AM calibration signal carrying the known AM calibration pattern so that amplifier-distorted calibration data may be extracted from the received signal. During calibration operations, the frequency of the signal produced by local oscillator  54  of data acquisition equipment  40  may drift with respect to the frequency of the carrier signal produced by local oscillator  32  of device  10 . A frequency drift of this type may appear in calibration data as a phase distortion (e.g., a bulk phase drift, also sometime referred to herein as phase drift) in addition to distortions produced by power amplifier  18  of device  10 . 
     Communications systems that use local oscillators commonly overcome relative local oscillator frequency drifts by including an un-modulated frequency locking component in communicated data. This frequency calibration component is additional data used to “lock” one local oscillator to the frequency of the other. 
     Radio-frequency calibration signals may be captured by data acquisition equipment  40  and converted to in-phase/quadrature-phase (I/Q) data samples by data acquisition equipment  40 . I/Q data samples may be transferred to calibration computing equipment  42  along path  46 . Path  46  may be a wired or wireless path. In one preferred embodiment that is sometimes described herein as an example, data acquisition equipment  40  and calibration computing equipment  42  may be connected by a path  46  that includes a General Purpose Interface Bus (GPIB) cable. 
     Transfer of digitized calibration data from data acquisition equipment  40  to calibration computing equipment  42  may take an amount of time that is proportional to the amount of calibration data that is transferred. Common calibration systems typically suffer from a data “bottleneck” during transfer of data from RF receiver components to calibration computers. Including a frequency locking component in calibration data may therefore be inefficient because the additional frequency locking component of the calibration data must be transferred (along with the amplitude-modulated pre-distortion calibration data) from data acquisition equipment  40  to calibration computing equipment  42 . While calibration pattern generators are commonly preprogrammed to produce calibration patterns that include frequency locking components, calibration systems such as calibration system  50  may be provided that more efficiently obtain calibrated pre-distortion coefficients for devices such as device  10  by extracting frequency locking information from amplitude-modulated pre-distortion calibration data that does not contain a dedicated un-modulated frequency locking component. 
     Calibration system  50  may be configured to calibrate pre-distortion settings for tens, hundreds, thousands, tens of thousands, hundreds of thousands, millions, tens of millions, or more of devices such as device  10 . Providing calibration systems  50  that are capable of determining frequency locking information from amplitude-modulated pre-distortion calibration data (i.e., without requiring transfer of dedicated un-modulated frequency locking data) may therefore significantly reduce the time required to move devices such as device  10  from manufacturing to delivery to end users. 
     Amplitude-modulated pre-distortion calibration data (e.g., I/Q data samples) that is transferred from data acquisition equipment  40  to calibration computing equipment  42  may be processed using calibration computing equipment  42 . Calibration computing equipment  42  may extract a local oscillator frequency drift correction (e.g., a bulk phase drift correction or phase drift correction) from the amplitude-modulated pre-distortion calibration data. Calibration computing equipment  42  may apply the determined bulk phase drift correction to the amplitude-modulated pre-distortion calibration data to generate phase drift corrected calibration data. Amplitude-modulated pre-distortion calibration may include amplifier distortions generated by power amplifier  18  ( FIG. 3 ). Calibration computing equipment  42  may measure the amplifier distortion of the amplifier-distorted amplitude-modulated pre-distortion calibration data by comparing the phase drift corrected calibration data to the known AM calibration pattern that was generated by calibration pattern generator  34  ( FIG. 3 ). Calibration computing equipment  42  may then extract pre-distortion coefficients from the results of the comparison of the phase drift corrected calibration data and the known AM input signal. Following extraction of the pre-distortion coefficients from the amplitude-modulated pre-distortion calibration data, calibration computing equipment  42  may transmit the pre-distortion coefficients along path  48  to transceiver  30  of device  10 . Path  48  may be a wired or wireless communications path. 
     Pre-distortion coefficients measured during calibration of wireless communications circuitry  16  may be stored by device  10  in memory associated with transceiver  30 , or other memory associated with wireless circuitry  16 . During normal operation of a device such as device  10 , pre-distortion compensator  36  may use pre-distortion coefficients determined during calibration operations to pre-distort signals generated by transceiver  30  prior to signal amplification by power amplifier  18  so that the signal that has been amplified by power amplifier  18  has the same signal quality as the signal that is input to the pre-distorter. 
       FIG. 5  is a graph of a typical calibration pattern containing both amplitude-modulated calibration data and an un-modulated frequency locking component. As shown in  FIG. 5 , graph  49  contains calibration pattern  51 . Calibration pattern  51  includes frequency locking portion  53  and amplitude-modulated portion  56 . Frequency locking portion  53  is commonly used to lock the frequency of a local oscillator associated with a transmitter with the frequency of a local oscillator associated with a receiver. During typical calibration operations, frequency locking portion  53  of calibration pattern  52  is transmitted starting at a time CT 0  and continuing until a time CT 1 . Following transmission of frequency locking portion  53 , at time CT 1 , amplitude-modulated portion  56  is transmitted until a later time CT 2 . The quantity of data contained in frequency locking portion  53  is often comparable to the quantity of data contained in amplitude-modulated portion  56 . Transmitting calibration patterns containing both un-modulated frequency locking and amplitude-modulated calibration portions is therefore inefficient for calibration of large numbers of devices. 
       FIG. 6  is a graph of a calibration pattern that may be used to calibrate a device such as device  10  using calibration system  50  of  FIG. 4 . As shown in  FIG. 6 , graph  60  contains only amplitude-modulated calibration pattern  62 . 
     Amplitude-modulated calibration pattern  62  may be transmitted beginning at a time T 0  and ending at a time T 1 . The data contained in known amplitude-modulated calibration pattern  62  may be substantially less than typical calibration patterns that contain an un-modulated frequency locking component. Amplitude-modulated calibration pattern  62  may be represented in polar space as in-phase (I) and quadrature phase (Q) components. 
     The amplitude of a signal may be expressed in I/Q space components as shown by the following equation:
 
AMPLITUDE= SQRT ( I   2   +Q   2 ),  (1)
 
where SQRT( ) indicates the square root function. The phase of a signal may be expressed in I/Q space components as shown by the following equation:
 
PHASE=arctan( Q/I ),  (2)
 
where arctan( ) represents the arctangent function. Equations 1 and 2 may be used to convert from amplitude/phase space to I/Q space (i.e., from an amplitude/phase representation of data to an I/Q representation of the data). A calibration pattern of the type shown in  FIG. 6  may be mixed with a carrier signal, amplified (and distorted by the amplifier) and transmitted to data acquisition equipment  40  as described above in connection with  FIGS. 3 and 4 .
 
     An amplitude-modulated calibration signal that has been distorted by a power amplifier and captured using a local oscillator that is drifting with respect to the transmitting local oscillator is shown in  FIG. 7 . Graph  70  shows an I/Q polar representation  62 ′ of modulated calibration curve  62  of  FIG. 6 . I/Q polar representation  62 ′ includes an additional phase distortion (e.g., a bulk phase drift) due to relative frequency drift of local oscillator  54  of data acquisition equipment  40  with respect to local oscillator  32  of device  10 . In graph  70 , the amplitude of signal  62 ′ is represented by the distance of a point from the zero crossing of the I and Q axes. In graph  70 , the phase of signal  62 ′ is represented by the angular distance of a point counter-clockwise from the I axis. Amplitude-modulated calibration pattern  62  is not phase modulated. Therefore, in the absence of relative local oscillator drift, no bulk angular change in curve  62 ′ would be observed. Therefore, information about the relative local oscillator frequency drift is contained in the overall phase drift of amplitude-modulated calibration signal  62 ′ in direction  72  between the point transmitted at time T 0  and the point transmitted at time T 1 . Calibration computing equipment  42  of calibration system  50  may be configured to extract the information about the relative local oscillator frequency drift (e.g., the phase drift) from amplitude-modulated calibration signal  62 ′. 
     Given I and Q and the relative arrival time of each data point in amplitude-modulated calibration curve  62 ′, computing equipment  42  may be used to convert I/Q polar representation  70  of amplitude-modulated calibration curve  62 ′ into phase vs. time representation  80  of amplitude-modulated calibration curve  62 ′, as shown in  FIG. 8 . Calibration computing equipment  42  may use equation 2 to determine the phase of each point from the I and Q of each point. Calibration computing equipment  42  may use any suitable curve fitting algorithm (e.g., linear regression, least squares fitting, least absolute deviation fitting, etc.) to extract phase drift information such as phase drift information curve  82  from amplitude-modulated calibration curve  62 ′. Phase drift information curve  82  may be used by computing equipment  42  to generate a local oscillator frequency drift correction (also sometimes referred to herein as a phase drift correction of bulk phase drift correction). The local oscillator frequency drift correction may be used by calibration computing equipment  42  to remove the phase distortion of amplitude-modulated calibration curve  62 ′ due to the relative frequency drift of local oscillator  54  of data acquisition equipment  40  and local oscillator  32  of wireless communications circuitry  16 . Calibration computing equipment  42  may remove the phase distortion of amplitude-modulated calibration curve  62 ′ by subtracting from each data point on amplitude-modulated calibration curve  62 ′ a corresponding point on phase drift information curve  82 . 
     The effect of subtracting from each data point on amplitude-modulated calibration curve  62 ′ a corresponding point on phase drift information curve  82  is shown in graph  90  of  FIG. 9 . As shown in  FIG. 9 , subtracting a corresponding point on phase drift information curve  82  from each data point on amplitude-modulated calibration curve  62 ′ results in phase drift corrected calibration data curve  62 ″. Phase drift corrected calibration data curve  62 ″ may have small variations in phase (e.g., due to phase distortions generated by power amplifier  18 ) and may have bulk phase drifts substantially removed. 
     Phase drift corrected AM calibration data curve  62 ″ may be represented in I/Q space as shown in  FIG. 10 . As shown in  FIG. 10 , phase drift corrected AM calibration data curve  62 ″ of graph  100  may contain data points at substantially one phase (i.e., most of the data points lie at a single angular distance from the I axis). Data points on frequency-corrected calibration data curve  62 ″ may have a wide range of amplitudes (i.e., distances from the zero crossing of the I and Q axes) corresponding to the amplitude modulations in the amplitude-modulated calibration pattern generated by calibration pattern generator  34  ( FIG. 3 ). Data points on phase drift corrected calibration data curve  62 ″ may show excursions such as deviations  102  from a single phase. Because amplitude-modulated calibration pattern  62  generated by calibration pattern generator  34  does not include phase modulations, deviations  102  may represent deviations caused by distortions induced by power amplifier  18 . Calibration computing equipment  42  may be configured to extract information about distortions caused by power amplifier  18  from phase drift corrected calibration data curve  62 ″. Extracting information about distortions caused by power amplifier  18  from frequency-corrected calibration data curve  62 ″ may include comparing data points on phase drift corrected calibration data curve  62 ″ to corresponding data points in known amplitude-modulated calibration pattern  62 . Each data point on phase drift corrected calibration data curve  62 ″ may have an associated phase drift corrected phase and amplitude. 
     Phase drift corrected phase and phase drift corrected amplitude of data points on phase drift corrected calibration data curve  62 ″ may be compared with a corresponding amplitude in known amplitude-modulated calibration pattern  62  using calibration computing equipment ( FIG. 3 ). Graphs  110  and  120  of  FIG. 11  show how phase drift corrected (or simply drift-corrected) phase and phase drift corrected (or simply drift-corrected) amplitude respectively of data points on phase drift corrected calibration data curve  62 ″ may compare with a corresponding input amplitude in known amplitude-modulated calibration pattern  62 . As shown in  FIG. 11 , PHASE-AMPLITUDE curve  112  may show deviations from a single phase such as single phase  116 . AMPLITUDE-AMPLITUDE curve  114  may show deviations from linear amplitude ratio (gain) curve  118 . Because amplitude-modulated calibration pattern  62  generated by calibration pattern generator  34  does not include phase modulations, deviations of PHASE-AMPLITUDE curve  112  from single phase  116  may represent deviations caused by distortions induced by power amplifier  18 . Because a perfect power amplifier would amplify each input amplitude by a common factor, deviations of AMPLITUDE-AMPLITUDE curve  114  from linear amplitude ratio  118  may represent deviations caused by distortions induced by power amplifier  18 . Calibration computing equipment  42  may be configured to extract pre-distortion coefficients from the comparison the PHASE-AMPLITUDE and AMPLITUDE-AMPLITUDE comparisons of frequency-corrected calibration data curve  62 ″ and known amplitude-modulated calibration pattern  62 . 
     To conduct wireless communications calibration of a device using a calibration system of the type shown in  FIG. 4 , the steps of the illustrative flowchart of  FIG. 12  may be performed. 
     At step  200 , device  10  may be used to generate and transmit an amplifier-distorted amplitude-modulated calibration signal to data acquisition equipment such as data acquisition equipment  40  of  FIG. 3 . Generating an amplifier-distorted amplitude-modulated calibration signal may include generating a known amplitude-modulated calibration pattern using calibration pattern generator  34 , generating a carrier signal using local oscillator  32 , modulating the carrier signal with the known amplitude-modulated calibration pattern and amplifying the modulated carrier signal using power amplifier  18 . Transmitting the amplifier-distorted amplitude-modulated calibration signal may include using an RF front end such as RF front end  38  and one or more antennas such as antenna  19  to transmit the data. 
     At step  202 , the amplifier-distorted amplitude-modulated calibration signal may be digitized using data acquisition equipment  40  of  FIG. 4 . The amplifier-distorted amplitude-modulated calibration signal may be transformed by data acquisition equipment  40  into an I/Q pair representation. 
     At step  204 , the I/Q pair representation of the amplifier-distorted amplitude-modulated calibration signal may be transferred from data acquisition equipment  40  to calibration computing equipment  42 . 
     At step  206 , calibration computing equipment  42  may be used to extract a relative local oscillator frequency drift correction from the amplifier-distorted amplitude-modulated calibration signal. Extracting the relative local oscillator frequency drift correction from the amplifier-distorted amplitude-modulated calibration signal may include using a suitable curve fitting operation that determines a bulk phase drift from the amplifier-distorted amplitude-modulated calibration signal. 
     At step  208 , calibration computing equipment  42  may be used to phase drift correct (i.e., apply a phase drift correction) the amplifier-distorted amplitude-modulated calibration signal (i.e., remove the bulk phase drift induced by the relative frequency drift of local oscillators associated with device  10  and data acquisition equipment  40 ). 
     At step  210 , pre-distortion coefficients may be extracted from the phase drift corrected amplifier-distorted amplitude-modulated calibration signal. Extracting pre-distortion coefficients from the phase drift corrected amplifier-distorted amplitude-modulated calibration signal may include comparing the phase drift corrected amplifier-distorted amplitude-modulated calibration signal to the known input amplitude-modulated calibration pattern generated by calibration pattern generator  34  of device  10 . Comparing the phase drift corrected amplifier-distorted amplitude-modulated calibration signal to the known input amplitude-modulated calibration pattern may include comparing received phase information to known input amplitude information and comparing received amplitude information to known input amplitude information. 
     At step  212 , the extracted pre-distortion coefficients may be transferred from calibration computing equipment to device  10  to be stored on device  10  and used by device  10  when generating signals during normal operation of device  10 . 
     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.

Metadata:
Filing Date: 20110831
Publication Date: 20140805
Grant Date: 20140805
Priority Date: 20110831
Inventors: DO GARY LANG
Assignee: APPLE INC
CPC Classifications: [{"code": "H03F1/3288", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F1/3258", "inventive": true, "first": true, "tree": "[]"}, {"code": "H03F1/3288", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F3/245", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F3/189", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F3/189", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F1/3258", "inventive": true, "first": true, "tree": "[]"}, {"code": "H03F3/245", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F1/3247", "inventive": true, "first": false, "tree": "[]"}, {"code": "H03F2201/3233", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 47744402