Abstract:
A method and apparatus permit a wireless communication device to be calibrated during manufacturing and subsequently re-calibrated while in use to correct for the effects of temperature changes on transmit power.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/509,080, filed on Oct. 6, 2003, entitled “WLAN RADIO POWER CONTROL USING A POWER DETECTOR,” incorporated herein by reference. 
     
    
     BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present subject matter relates generally to controlling transmit power in a transmitter to account for variations in temperature and frequency.  
         [0004]     2. Background Information  
         [0005]     Wireless local area network (“WLAN”) communication channels typically span a relatively broad range of frequencies in the 2.4 GHz Industry, Scientific and Medical (“ISM”) band and the 5 GHz Unlicensed National Information Infrastructure (“U-NII”) band. Transmit power of WLAN transmitters may vary with ambient temperature due to thermal characteristics of WLAN chipsets and physical design constraints of the associated WLAN transceiver hardware. When transmitting in different WLAN channels in these bands, it is desirable that the transmit power vary only with the baseband transmit signal levels and not with temperature and channel frequency. In practice, however, transmitter signal power can vary significantly across temperature and radio frequency (“RF”) channel frequency. Minimizing transmit power variations to achieve uniform performance and avoid interference to other co-located WLAN networks is desirable.  
       BRIEF SUMMARY  
       [0006]     In accordance with at least some embodiments of the invention, a method and apparatus are disclosed herein that permit a wireless communication device to be calibrated during manufacturing and subsequently re-calibrated while in use to accommodate the effects of temperature changes on transmit power. A preferred embodiment of the manufacturing calibration process comprises determining a first gain setting for a communication device, storing the first gain setting in memory in the communication device, transmitting a calibration signal, changing a second gain setting until transmit power of the communication device is at a predetermined level, obtaining a value indicative of the transmit power, and storing the second gain setting and the value indicative of the transmit power in the communication device&#39;s memory. The calibration signal preferably comprises a tone (e.g., a 5 MHz tone) or other constant envelope signal.  
         [0007]     A periodic gain adjustment calibration process may comprise retrieving a calibrated gain value from memory in a communication device, changing a gain of an amplifier in the communication device to have the retrieved calibrated gain value, transmitting a calibration signal, measuring transmit power of the communication device while transmitting the calibration signal until the transmit power reaches at least a threshold value, computing a new gain value of the amplifier based on the measured transmit power after the transmit power reaches at least the threshold value, and changing the gain of the amplifier to be the new gain value.  
       NOTATION AND NOMENCLATURE  
       [0008]     Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, various companies may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     For a more detailed description of the preferred embodiments of the present invention, reference will now be made to the accompanying drawings, wherein:  
         [0010]      FIG. 1  illustrates a wireless communication device in accordance with the preferred embodiments of the invention;  
         [0011]      FIG. 2  depicts a block diagram of a radio transmitter included in the wireless communication device of  FIG. 1 ;  
         [0012]      FIG. 3  shows a preferred embodiment of a power detector used in the radio of  FIG. 2 ;  
         [0013]      FIG. 4  illustrates a preferred method of calibrating the radio during manufacturing; and  
         [0014]      FIG. 5  illustrates a method of calibrating the radio during normal device operation to adjust for changes in temperature. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims, unless otherwise specified. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary, of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.  
         [0016]     Referring now to  FIG. 1 , a wireless communication device  10  is shown in accordance with a preferred embodiment of the invention. The wireless device  10  may comprise, for example, a wireless transceiver used in a WLAN. As shown, the wireless communication device comprises a host  12 , a baseband processor  14  and a radio  20 . One or two antennas  22  coupled to the radio  20  facilitate one or two-way wireless communications between the device  10  and another device (not shown). The host  12  generally comprises application-specific functionality. In some embodiments, the host may comprise a personal data assistant or a computer such as a wireless-enabled laptop computer. The baseband processor  14  and radio  20  may be fabricated on a circuit card that may be inserted into the computer to permit the computer to wirelessly access a wireless local area network.  
         [0017]     Referring still to  FIG. 1 , the baseband processor  14  comprises a central processing unit (“CPU”)  16  that executes firmware  18  stored in non-volatile memory. At least some of the functionality described herein may be performed by the CPU  16  executing the firmware  18 . The baseband processor  14  also implements a baseband amplifier (not separately shown) which functions to amplify signals being transmitted via radio  20  and antenna  22 .  
         [0018]      FIG. 2  depicts an example of a WLAN transmitter and shows additional detail for the radio transmitter  20 . As shown, the radio comprises two processing paths for the I and Q components of the transmitted signal. The I path comprises low pass filter  30 , amplifier  32 , and mixer  34  while the Q path comprises low pass filter  36 , amplifier  38 , and mixer  40 . The mixers  34  and  40  multiply their respective signals by a signal from a signal generator  44  (the Q path including a 90 degree phase shifter for the signal generator output signal). The output signals from the multipliers are summed together at node  43  to form an intermediate frequency (IF) signal, which is provided to a variable gain amplifier (“VGA”)  46 . The amount of gain implemented in the VGA  46  is controlled by the baseband processor via the transmit gain control signal.  
         [0019]     An RF mixer  50  multiplies the output signal from the VGA  46  by a signal from an RF synthesizer to upshift the transmit signal to a suitable carrier frequency for transmission. The radio also comprises a band pass filter  54  followed by a power amplifier  56 . The antenna  22  couples to the output of the power amplifier through a directional coupler. The power detector  62  is operatively coupled to the directional coupler  58  to determine the power level of transmitted signal. The directional coupler  58  suppresses energy reflected back to the power amplifier from the antenna so that the power detector measures only the transmit power. A power detector  62  comprises a circuit that outputs a signal indicative of the level of transmit power. An exemplary embodiment of the power detector  62  is shown in  FIG. 3  as a squaring circuit  70  followed by a low pass filter  72 . Referring still to  FIG. 2 , the output signal from the power detector is provided to an amplifier  64  and then to the baseband processor  14  for further processing as described herein.  FIG. 2  also shows a non-volatile memory  15  coupled to, or otherwise accessible to, the baseband processor  14 . The non-volatile memory  15  may comprise a re-programmable read only memory such as an electrically erasable read only memory (“EEPROM”) or battery-backed random access memory (“RAM”). The non-volatile memory  15  preferably is used to store various parameters used, as described below, in a closed loop power control (“CLPC”) algorithm implemented at least in part by the CPU  16  of the baseband processor  14  and associated firmware  18 . Those skilled in the art will appreciate various types of transmitter architectures that can implement CLPC based on the teachings contained herein.  
         [0020]     The CLPC algorithm generally comprises two processes. A first process includes a calibration process preferably performed during fabrication of the radio. This calibration process generally is performed once per radio. A second process generally comprises a repeatedly executed calibration process while the radio is being used during normal operation to transmit data.  FIG. 4  illustrates a preferred embodiment of the initial calibration process and  FIG. 5  illustrates a preferred embodiment of the repeatedly executed calibration process.  
         [0021]     Turning now to  FIG. 4 , the initial calibration process comprises blocks  102 ,  104 ,  106 ,  108 , and  110  as shown. In block  102 , the radio is optimized for error vector magnitude (“EVM”) and spectral mask at a first temperature T 1 . The temperature T 1  can be any suitable and controlled temperature. Any suitable mechanism for optimizing the radio for EVM and spectral mask is acceptable such as that described in “Using Error Vector Magnitude measurements to Analyze and Troubleshoot Vector-Modulated Signals” by Agilent Technologies, copyright 1997, 2000, incorporated herein by reference. As a result of the optimization process depicted by block  102 , optimal gain settings are determined for the VGA  46  and the baseband amplifier in the baseband processor. These gain settings are referred to as P VGA (T 1 ) (represented in dB) and P BBG (T 1 ) where the parenthetical reference to “T 1 ” indicates that the VGA and baseband amplifier gain settings are determined at the temperature T 1 . In block  104 , the values of P VGA (T 1 ) and P BBG (T 1 ) are stored in the non-volatile memory  15 .  
         [0022]     At  106 , the baseband processor  14  causes a calibration signal to be transmitted via the radio  20 . The calibration signal preferably comprises a tone (e.g., a 5 MHz tone) or other constant envelope signal. Preferably, the calibration signal is transmitted relatively soon after the baseband processor determines that the transmit power is satisfactory to ensure that the calibration signal captures the temperature environment in which the manufacturing calibration is performed. At  108 , the baseband processor assesses the transmit power level via the power detector  62  and changes the gain of the VGA  46  until a desired transmit power is obtained. In accordance with the preferred embodiment of the invention, the desired transmit power is greater than 0 dBm and generally within a predetermined range of values. In some embodiments, the calibration process of  FIG. 4  is performed for each of a plurality of frequency bands. For example, one frequency band comprises 2.412-2.484 GHz (channels  1 - 14 ). Another band may comprise 5.18-5.32 GHz (channels  36 - 64 ) while other bands may comprise 5.5-5.6 GHz (channels  100 - 120 ) and 5.62-5.805 GHz (channels  124 - 161 ). The choice of the frequency bands on which the calibration is performed is determined based on the nature of the radio  20 . The CLPC algorithm places no restriction on the number of frequency bands except that the algorithm preferably is performed on each of these bands independently. As such, the desired transmit power referred to in block  108  is chosen so that the power detector returns reliable temperature compensated values. This may be about 7 dBm for the channels in the 5 GHz bands and about 4 dBm for the channels in the 2.4 GHz bands. In block  110 , the new gain setting of the VGA and the transmit power associated with the new VGA setting, referred to as C VGA (T 1 ) (represented in dB) and C PD (T 1 ), respectively, are also stored in the non-volatile memory  15 .  
         [0023]     During normal operation, the wireless communication device  10  transmits data according to the requests of the host  12 . During periodic intervals (e.g., every five minutes), the baseband processor  14  causes the device  10  to transition to a calibration mode in which the process of  FIG. 5  is performed. When the periodic calibration mode depicted in  FIG. 5  is performed, the ambient temperature at which the wireless communication device operates may be different from the initial calibration temperature T 1 . The temperature of the periodic calibration mode is referred to as T 2  in  FIG. 5 . The temperature T 2  may be the same or different as the temperature T 1 . The periodic calibration mode of  FIG. 5  recomputes the gain settings for the VGA  46  and the baseband amplifier in accordance with changes in temperature.  
         [0024]     At block  120 , the baseband processor  14  causes the gain setting for the VGA to be changed from its setting usable to transmit data packets (P VGA (T 1 ) to the value of C VGA (T 1 ). AT  122 , the baseband processor  14  causes the calibration signal (in some embodiments the same calibration signals referred in  FIG. 4 ), to be transmitted. The baseband processor  14  then changes at  124  the gain of the VGA until the desired transmit power is obtained in a process similar to that of block  108  in  FIG. 4 .  
         [0025]     A series of calculations are performed by the baseband processor as reflected in blocks  126 ,  128 , and  130 . In block  126 , the baseband processor computes the value of  
           P   ′     ⁢           ⁢   as   ⁢           ⁢     P   ′       =         [     ⁢       C   VGA     ⁡     (     T   2     )         -       C   VGA     ⁡     (     T   1     )       +     10   ⁢       log   ⁡     (         C   PD     ⁡     (     T   2     )           C   PD     ⁡     (     T   1     )         )       .             
 
 The value P′ is a dB representation containing an integer portion and a fractional portion. The integer portion is referred to as D VGA  and the fractional portion is referred to as D BBG . Thus, if P′ is represented as xx.yy, the value of xx is D VGA  and the value of yy is D BBG . In block  128 , the new VGA gain setting for normal data packet transmission preferably is computed as P VGA (T 2 )=P VGA (T 1 )+D VGA . In block  130 , the new baseband amplifier gain setting preferably is computed as  
           P   BBG     ⁡     (     T   2     )       =         P   BBG     ⁡     (     T   1     )       ⁢       antilog   ⁡     (       D   BBG     20     )       .           
 
 The values of D VGA  and D BBG  represent correction factors usable to correct the VGA and baseband gain settings for the effects of changes in temperature. Finally, in block  132 , with the new gain settings computed, normal operation of the wireless communication device  10  resumes. 
 
         [0028]     While the preferred embodiments of the present invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Accordingly, the scope of protection is not limited by the description set out above.