Abstract:
A loopback module includes a plurality of signal paths and is designed to mix an incoming phone signal with a signal from a signal generator to produce a loopback signal at the receive frequency of the phone. The loopback signal is applied to the phone&#39;s antenna. The phone then evaluates the loopback signal to determine the appropriate offset for the transmitter chain at that frequency and power level. This process is iterated until the desired number of frequencies and power levels are tested for proper calibration. The offsets are stored in memory for later use by the phone.

Description:
FIELD OF THE INVENTION 
     This invention pertains to a device and method for calibrating the power output of a mobile communications device. 
     BACKGROUND OF THE INVENTION 
     In a mobile communication system, the transmit power of the mobile station is controlled to meet two sometimes competing objectives. The first objective is to maintain minimum signal quality standards. If the signal is fading, the mobile station will increase its transmit power so that the received signal at the base station meets the minimum signal quality standard. The second objective is to reduce adjacent channel and co-channel interference so that other devices also using that particular base station may communicate clearly. If the transmit power of a particular device is too high, some of the power may spill into neighboring channels causing interference with transmission from other mobile stations. Therefore, the mobile station will, whenever possible, reduce its transmit power to avoid interference provided that the minimum signal quality standard can be maintained at the 1000 level. 
     To effectively control the power level of the mobile station, it is desirable that the power amplifier of the mobile terminal have a linear performance over both frequency and the dynamic range of the power levels required. Unfortunately, mobile devices are the sum of several electronic components, none of which necessarily behaves linearly. Therefore, a typical mobile device will have a non-linear curve when comparing an expected power output to actual power output as seen in FIG.  1 . This curve changes at each of the operating frequencies of the mobile device. To compensate for this nonlinearity, the mobile device incorporates a set of offsets (see FIG. 1) and stores them in non-volatile memory. These offsets are designed to bring the actual power output into a linear relation with the expected output. For example, where the actual power output exceeds the expected power output, a negative offset is stored to reduce the actual power output (the circled portion of FIG.  1 ). 
     In order to calculate these offsets, manufacturers typically measure the output power level at many points across both the frequency band and the dynamic power range of the transmitter. The higher the number of points, the better the accuracy (and linearity) of the resulting output signal. Where Time Division Multiplexing Access (TDMA) is used, the number of power levels is restricted, and thus the total number of points is relatively reasonable. However, where Channel Division Multiplexing Access (CDMA) is used, an infinite number of power levels may be used theoretically, resulting in effectively infinite number of points to be tested. 
     Complicating the problem, while the circuits used in different devices of the same product line are theoretically the same, individual variation within the parts used to create the circuits in the different devices have individual variations, which results in the offsets being unique to each device. Thus, each device must be tested individually to ensure proper calibration of the device. 
     Conventionally, this calibration is done with an expensive rack of equipment including an antenna connected to a receiver and transmitter, several power supply sources, and a processor (typically in a personal computer) to control the rack and communicate with the processor in the mobile device. Initially, the receiver of the mobile device is calibrated by generating a signal at a set frequency and power level and applying it to the mobile device&#39;s antenna. The rack processor evaluates the readings within the mobile device processor and calculates an offset, which is then stored by the mobile device. This process is repeated for a number of points at different frequencies and power levels. This is not a fast process because the test equipment must “settle” at each frequency. 
     After calibration of the receiver chain, the transmitter chain is calibrated. This involves the mobile device transmitting at a number of frequencies and power levels to the antenna of the test equipment. The device communicates with the rack processor and tells the rack processor that it transmitted on x frequency at y power. The rack processor then compares this information to the frequency and power that was received at the test equipment. Again, the test equipment takes time to settle at each operative frequency and power level tested. From the comparison, the rack processor can calculate an offset, which is sent, typically by a serial communication line to the mobile device, which then stores the offset in its memory. 
     This calibration process can be time consuming and costly by adding test time in the factory and demanding expensive testing equipment. Given the intense competition to produce an economical mobile device, any increase in the production cost is undesirable. Thus, manufacturers try to reduce time by speeding up the measurement capability and/or the communication between the test equipment and the mobile device so that the testing is accomplished faster; or the manufacturers cut corners and test fewer points across the bandwidth and the dynamic range of the transmitter. Alternatively, the parts used to assemble the device may be made to a more exacting standard such that the devices within the product line behave identically or the parts themselves behave more linearly, so that fewer non-linear instances occur. All of these solutions have shortcomings. The first solution typically involves creating more expensive test equipment, the cost of which is then passed on to the cost of the device. The second solution increases the errors that may occur during the use of the device, especially where improper offsets are stored in the memory and the end result is poorer performance of the device. The final solution also results in a more expensive device because the cost of the more precise parts is higher. 
     Accordingly, there remains a need in the field of mobile communications device testing, and particularly in the field of mobile phone testing, to provide an economical method and device which reduces the time necessary to test and calibrate a mobile phone without adding substantially to the cost of the test equipment. 
     SUMMARY OF THE INVENTION 
     The present invention is a loopback module used for calibrating the receiver and transmitter chains of a mobile telephone. The loopback module is controlled by the mobile telephone during the calibration procedure. The phone transmits a signal from the phone antenna to the loopback module. The loopback module changes the frequency of the transmitted signal to create a loopback signal, which is then fed back to the phone through the antenna. Software in the phone evaluates the loopback signal to determine the appropriate offset for the transmitter chain at that frequency and power level. This process is iterated until the desired number of frequencies and power levels are tested for proper calibration. The offsets are stored in memory for later use by the phone. 
     An alternate use of the loopback module is a general integrity check for the phone components. A signal is generated in the phone, sent to the loopback module, and a loopback signal is received by the phone from the loopback module. If the loopback signal fits within a window of acceptable responses, then the phone is considered to be O.K. to calibrate. If the phone is outside the window of acceptable responses, then the phone is slated for further testing to determine the component which is causing the poor response. Upon location and replacement of the defective component(s), the phone is tested again until an acceptable response is acquired, at which time the phone is calibrated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a graph of the desired power levels of the mobile communications device compared to the actual power levels, illustrating the nature of the offsets used to calibrate devices; 
     FIG. 2 is a schematic diagram of a cellular phone of the present invention; 
     FIG. 3 is a block diagram of the calibration system of the present invention; 
     FIG. 4 is a detailed schematic diagram of the loopback module of the present invention; 
     FIG. 5 is a simplified flow diagram illustrating the calibration process of the present invention; 
     FIG. 6 is a detailed flow diagram of the receiver calibration process of the present invention; 
     FIG. 7 is a detailed flow diagram of the transmitter calibration process of the present invention; and 
     FIG. 8 is a schematic diagram of an alternate use of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and particularly to FIG. 2, a mobile communication device, such as a cellular telephone, is shown and indicated generally by the numeral  10 . Mobile telephone  10  is a fully functional radio transceiver capable of transmitting and receiving digital and/or analog signals over an RF channel according to known standards, such as Telecommunications Industry Association (TIA), IS- 54 , IS- 136 , and IS- 95 . The present invention, however, is not limited to cellular telephones, but may also be implemented in other types of mobile communication devices including, without limitation, pagers and personal digital assistants. 
     The mobile telephone  10  includes an operator interface  12  and a transceiver unit  24  contained in a housing. Users can dial and receive status information from the mobile telephone  10  via the operator interface  12 . The operator interface  12  consists of a keypad  16 , display  18 , microphone  20 , and speaker  22 . The keypad  16  allows the user to dial numbers, enter data, respond to prompts, and otherwise control the operation of the mobile telephone  10 . The display  18  allows the operator to see dialed digits, call status information, messages, and other stored information. An interface control  14  interfaces the keypad  16  and display  18  with the telephone&#39;s control logic  26 . The microphone  20  and speaker  22  provide an audio interface that allows users to talk and listen on their mobile telephone  10 . Microphone  20  converts the user&#39;s speech and other sounds into audio signals for subsequent transmission by the mobile telephone  10 . Speaker  22  converts audio signals received by the mobile telephone  10  into audible sounds that can be heard by the user. In general, the microphone  20  and speaker  22  are contained in the housing of the mobile telephone  10 . However, the microphone  20  and speaker  22  can also be located in a headset that can be worn by the user. 
     The transceiver unit  24  comprises a transmitter  30 , receiver  40 , and antenna assembly  50 . The transceiver circuitry or radio communications circuit is typically contained on a printed circuit board (not shown). The transmitter  30  includes a digital signal processor  32 , modulator  34 , and RF amplifier  36 . The digital signal processor  32  converts analog signals from the microphone  20  into digital signals, compresses the digital signal, and inserts error-detection, error-correction, and signaling information. Modulator  34  converts the signal to a form that is suitable for transmission on an RF carrier. The RF amplifier  36  amplifies the signal to a suitable power level for transmission. In general, the transmit power of the telephone  10  can be adjusted up and down in two decibel increments in response to commands it receives from its serving base station. This allows the mobile telephone  10  to only transmit at the necessary power level to be received and reduces interference to nearby units. It is precisely this power level adjustment ability that is calibrated by the first aspect of the present invention. 
     The receiver  40  includes a receiver/amplifier  42 , demnodulator  44 , and digital signal processor  46 . The receiver/amplifier  42  contains a band pass filter, low level RF amplifier, and mixer. Received signals are filtered to eliminate side bands. The remaining signals are passed to a low-level RF amplifier and routed to an RF mixer assembly. The mixer converts the frequency to a lower frequency that is either amplified or directly provided to the demodulator  44 . The demodulator  44  extracts the transmitted bit sequence from the received signal. The digital signal processor  46  decodes the signal, corrects channel-induced distortion, and performs error-detection and correction. The digital signal processor  46  also separates control and signaling data from speech data. The control and signaling data are passed to the control logic  26 . Speech data is processed by a speech decoder and converted into an analog signal which is applied to speaker  22  to generate audible signals that can be heard by the user. 
     The control logic  26  controls the operation of the telephone  10  according to instructions stored in a program memory  28 . Control logic  26  may be implemented by one or more microprocessors. The functions performed by the control logic  26  include power control, channel selection, timing, as well as a host of other functions. The control logic  26  inserts signaling messages into the transmitted signals and extracts signaling messages from the received signals. Control logic  26  responds to any base station commands contained in the signaling messages and implements those commands. When the user enters commands via the keypad  16 , the commands are transferred to the control logic  26  for action. 
     The antenna  50  is operatively connected by a conventional transmission line to the transmitter  30  and receiver  40  for radiating and receiving electromagnetic waves. Electrical signals from the transmitter  30  are applied to the antenna  50  which converts the signal into electromagnetic waves that radiate out from the antenna  50 . Conversely, when the antenna  50  is subjected to electromagnetic waves radiating through space, the electromagnetic waves are converted by the antenna  50  into an electrical signal that is applied to the receiver  40 . 
     Turning now to FIG. 3, a simple block diagram of the calibration system of the present invention is shown The mobile phone  10  is operatively connected to an equipment rack  51  and to a loopback module  60 . The loopback module  60  is operatively connected to the equipment rack  51 . The equipment rack  51  includes a signal generator  52 , such as the HP8924 sold by Hewlett Packard at 11311 Chinden Blvd, Boise Idaho 83714, a power source  54 , and a processor  56  such as a personal computer (PC). 
     The power source  54  supplies power to the loopback module  60  and the phone  10  as well as to the elements of the equipment rack  51 . The signal generator  52  communicates with the loopback module  60 . The processor  56  is operatively connected to the phone  10  through a conventional serial connection. The phone  10  is connected by its antenna  50  to the loopback module  60  and by a serial connection  58  to a series of switches within the loopback module  60  as will be explained in greater detail below. 
     The loopback module  60 , best seen in FIG. 4, includes a housing  62  with a power port  64 , an antenna port  66  operatively connected to the antenna  50  of the mobile phone  10 , a signal generator port  68  operatively connected to the signal generator  52 , and a switch control port  70  operatively connected to the phone  10 . Within the housing  62  is a loopback circuit  72  which mixes transmitted RF signals from the mobile phone  10  and generates a loopback signal receive frequency of the mobile phone  10  for use in calibrating the phone&#39;s transmitter  30 . The loopback circuit  72  also includes a direct signal path  74  for connecting the signal generator  52  and phone  10  to calibrate the phone&#39;s receiver  40 . 
     The loopback circuit  72  includes a first signal path  74  directly connecting antenna port  66  and signal generator port  68 , a second signal path  76  connecting the antenna port  66  to a first input of a mixer  90 , a third signal path  84  connecting the signal generator port  68  to a second input of the mixer  90 , and a fourth signal path  86  connecting the output of the mixer  90  to the antenna port  66 . 
     The first signal path  74 , called the direct signal path herein, is used to transmit signals from port  68  to port  66  for calibrating the receiver  40 . The second signal path  76 , called the transmit signal path, transmits signals applied at port  66  by the mobile phone  10  to the mixer  90 . The transmit signal path  76  includes first and second branches  92  and  94 . First branch  92  provides a direct, non-attenuating path from port  66  to the mixer  90 . Second branch  94  includes a pair of 20 dB attenuators  97  to attenuate the transmitted signal. This is necessary at higher power levels. A pair of switches  96  and  98  select between the first and second branches  92  and  94 . 
     The third signal path  84 , called the mixing signal path, transmits injection signals applied at port  68  by the signal generator  52  to the mixer  90 . These injection signals are mixed with the transmitted signal to generate a loopback signal at the receive frequency of the mobile phone  10 . The mixing signal path  84  includes a pair of amplifiers  88  to amplify the injection signal. 
     The fourth signal path  86  referred to as the loopback signal path, transmits the loopback signal output by the mixer  90  to port  66 . Loopback signal path  86  includes first and second branches,  100  and  102  respectively. Each branch  100 ,  102  of the loopback signal path  86  includes an amplifier  103 ,  103 ′ and a filter  101 ,  101 ′ respectively corresponding to two different frequency bands. Loopback signal path  86  is coupled to antenna port  66  by a coupler  108 . Alternatively, loopback signal path  86  could connect to a separate output port (not shown) instead of antenna port  66 . 
     In use, the loopback module  60  helps calibrate the phone  10  using primarily the processor or control logic  26  of the phone  10  to complete the calibration process. The offsets created by this calibration are stored in the memory  28  of the phone  10  and the phone  10  is ready to ship. In particular, the memory  28  of the phone  10  may have software preprogrammed into the phone  10 , or the PC  56  could upload the program at the start of the calibration process. While the control logic  26  is expected to perform all of the calculations detailed below, it is possible to supplement the control logic  26  with the computing power of the PC  56  if needed or desired. It is believed however, that most communication between the PC  56  and the control logic  26  may be eliminated, thereby reducing the calibration time required. 
     FIG. 5 shows an overview of the preferred method of calibrating the phone  10 . It is to be understood that the aforementioned software within the memory  28  of the phone  10  preferably implements this method. The calibration sequence begins at block  110 . The phone  10 , the equipment rack  51 , and the loopback module  60  are turned on (block  112 ). The PC  56  instructs the phone  10  to begin calibration of the receiver portion  40  of the phone  10  (block  114 ). After the receiver  40  is calibrated, the transmitter  30  of the phone is then calibrated (block  116 ) and the calibration procedure ends (block  118 ). 
     The calibration of the receiver portion  40  is shown in FIG. 6, where block  114  is exploded into its component steps. Specifically, the calibration of the receiver portion  40  begins at block  128 . If not already turned on in block  112 , the phone  10  and the test equipment, including the loopback module  60 , is turned on (block  130 ). The phone  10  sets the loopback module  60  to pass-thru mode (block  132 ) by manipulating switches  78 ,  80  to direct signals along the first signal path  74 . The receiver  40  is tuned to a first channel n (block  134 ). The signal generator  52  is then set to the same channel n (block  136 ). This tuning is accomplished by the phone  10  communicating with the PC  56 , which then instructs the signal generator  52  to tune to the correct channel. 
     The signal generator  52  is then set to a power level y (block  138 ). Again, this is accomplished from the phone  10  to the PC  56  to the generator  52 . The signal from the signal generator  52  enters the loopback module  60  at the signal generator port  68 , passes along the direct signal path  74 , and is applied to the antenna  50  of the phone  10  via the antenna port  66 . The phone  10  measures the power level of the received signal and calculates an offset value based on the actual level of the signal (block  140 ). The offset is then saved in memory  28  (block  142 ). The phone  10  knows the actual level of the signal generated through the link with the PC  56 . 
     The phone  10  then checks to see if this offset is the end of the desired power levels for this channel (block  144 ). If the answer is no, then the signal generator increments the power level (block  146 ) and steps  140  and  142  are repeated. When the power levels at that particular channel have been exhausted, i.e. the answer to block  144  is yes, then the phone  10  checks to see if this is the last channel to be tested (block  148 ). If the answer is no, then the signal generator  52  increments the channel (block  150 ) and repeats steps  138 ,  140 ,  142 ,  144  and  146  as described above until the channels have been exhausted. When the channels have been exhausted, i.e. the answer to block  148  is yes, then the receiver calibration ends (block  152 ). 
     The calibration of the transmitter portion  30  is shown in FIG. 7, where block  116  is exploded into its component steps. Initially, the loopback module  60  is set to loopback mode (block  154 ). This is effectuated by instructing switches  78  and  80  to select the second signal path  76 . This causes the injection signal from the signal generator  52  to enter the signal generator port  68  and pass through the two amplifiers  88  to the mixer  90 . 
     The signal generator  52  is tuned to the difference between the transmit frequency and the receive frequency for a given channel. For example, if the phone were operating in a 800 MHz mode, the transmit frequency range is between 824.040-848.970 MHz and the receive frequency range is between 869.040-893.970 MHz. Thus, the signal generator  52  would be tuned to 45 MHz, representing the difference between the transmit frequency and the receive frequency for a given channel n. Likewise, in the 1900 MHz mode, the signal generator  52  would be tuned to 80 MHz because the transmit frequency range is 1850-1910 MHz and the receive frequency is 1930-1990 MHz. 
     Because more phones are now operating in at least two modes representing different frequency bands, the loopback module  60  should also be equipped to handle these multiple modes. As a result, the loopback signal path  86  includes the first frequency branch  100  and the second frequency branch  102  controlled by switches  104 ,  106 . The phone  10  knows which frequency band is being tested and selects the appropriate frequency branch  100 ,  102  accordingly (block  155 ). In the disclosed loopback module  60 , the amplifier and filter combination  101 ,  103  in the first branch  100  operate at the 800 MHz band and exclude any transients and harmonics which may interfere with the calibration process. Likewise, the amplifier and filter combination  101 ′,  103 ′ in the second branch  102  operate at the 1900 MHz band and similarly exclude transients and harmonics outside the desired frequency range. 
     The receiver  40  of the phone is tuned to channel n (block  156 ) and the transmitter is tuned to channel n as well (block  158 ). The transmitter  30  is set to an output power level y (block  160 ) and transmits. The transmitted signal is applied at the antenna port  66  of the loopback module  60  and directed through either non-attenuated branch  94  or attenuated branch  92 . If the power level y is relatively high, such that the mixer  90  or other electrical components within the circuit  72  might be damaged, the phone  10  actuates switches  96  and  98  to select the attenuated branch  92 , which results in a 40 dB reduction in the strength of the phone signal before it arrives at the mixer  90 . If the power level y is relatively low, then the phone signal is routed through the non-attenuated branch  94  by switches  96  and  98 . The phone controls the operation of all the switches through the control port  70 . 
     The transmitted phone signal is then mixed in the mixer  90  with the signal from the signal generator  52  to create a loopback signal at the receive frequency of the selected channel n. This loopback signal then exits the mixer  90  by loopback signal path  86 , where it travels through the selected frequency path  100  or  102  as determined by the position of the switches  104  and  106 . The loopback signal is amplified and filtered by the appropriate elements and passed to the coupler  108 , which couples the loopback signal to the antenna port  66 , which now serves as both an output port and an input port. It should be understood that the loopback signal need not be coupled back through the first port  66 , rather the loopback signal could also exit through a dedicated output port. 
     The antenna  50  now receives the loopback signal. The phone  10  measures the power of the received signal and calculates an offset from the value observed versus the value the phone thought it transmitted (block  162 ). The phone  10  compensates for any attenuation due to the path taken by the signal. The offset is saved (block  164 ) and the phone checks to see if this is end of the desired input levels (block  166 ). If the answer is no, then the power level is incremented (block  168 ) and steps  162 ,  164 , and  166  are repeated. When the power level increments above a certain “safe” level, the phone  10  switches the loopback module to the attenuated path  92  so that no components are inadvertently damaged during the testing at the higher power levels. If the answer to block  166  is yes, channel n has been tested at all the desired power level test points, then the phone  10  asks if all the channels have been tested (block  170 ). If the answer to block  170  is no, then both the receiver  40  and the transmitter  30  increment to the next channel (block  172 ) and steps  162 ,  164 ,  166 , and  168  are repeated until a yes is returned from block  170 . If a yes is returned from block  170 , then the calibration of the transmitter  30  ends (block  174 ), and the calibration of the phone  10  for that frequency band ends (block  126 , FIG.  5 ). 
     The phone  10  may then be calibrated for a different frequency band if so desired with the appropriate shift of switches  104 ,  106 . While the present invention does require greater care in calibrating the receiver  40  of the phone  10 , great savings are made in the time required calibrating the transmitter  30 . This is due in large part to the fact that the test elements do not have to settle between tuning changes, nor is time wasted communicating back and forth between the phone  10  and the PC  56  during the transmitter calibration. As noted above, it is possible to supplement the computing power of control logic  26  with the PC  56 , but such is not preferred. 
     Another aspect of the present invention is seen in FIG.  8 . The present invention may also be used to simply test phones or to test individual components within the phones. The phone  10  is connected to the loopback module  60 , which in turn is connected to the signal generator  52 . A digital signal processor  200 , which may be part of the control logic  26 , or may be part of either the transmitter  30  or receiver  40 , generates a baseband signal  202 . The signal  202  is preferably transformed into the frequency spectrum in the DSP  200 . In the preferred embodiment, the DSP  200  performs a FFT on the signal  202 . The signal  202  is then modulated by the transmitter  30  and sent through the loopback module  60 . Preferably, the signal is at a relatively low power and need not be attenuated. The baseband signal is mixed to the receiver frequency in the mixer  90  as described above and the resulting loopback signal is sent back to the phone  10  after filtering. The receiver  40  receives the loopback signal and performs the demodulation. 
     Upon arrival at the DSP  200 , the loopback signal is also transformed to generate frequency loopback signal  204 . The loopback signal  204  is compared to the baseband signal  202  as generally seen at  206 . It can be imagined that there is a band around the baseband signal  202 , seen in dotted lines at  208 , which forms an envelope. This envelope represents a “good” phone, i.e. any response falling within this envelope  208  means that the phone tested had an acceptable response. The envelope  208  may be derived from a number of known good phones prior to beginning this type of test. 
     This test uses the full transmit and receive paths, and any gross inherent problems would show up in the received waveform. The test could be used either as a stand alone go/no-go test, or as an initial screen to determine whether the phone should be sent for calibration. 
     Furthermore, by changing the baseband signal  202 , individual elements within either the transmitter chain  30  or the receiver chain  40  may be excited and tested. This provides a great degree of flexibility in testing components without requiring additional expensive testing equipment other than the loopback module  60 . 
     The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the spirit and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.