Abstract:
A method of adjusting transmit power in a CDMA portable phone to maintain adjacent channel power rejection (ACPR) passing margin includes the steps of amplifying a first radio frequency (RF) signal according to a first gain to produce a second RF signal, and amplifying the second RF signal according to a second gain to produce a third RF signal. A desired power level of the third RF signal is determined and a new gain value is computed from the desired power level. The first gain value is adjusted to the new value. A system for adjusting transmit power in a CDMA portable phone to maintain adjacent channel power rejection (ACPR) passing margin includes an automatic gain control (AGC) amplifier having an AGC input terminal, an AGC output terminal, and a control signal input terminal. The system also includes a power amplifier (PA) having a PA input terminal and a PA output terminal, wherein the PA input terminal is connected to the AGC output terminal. In addition, the system includes an output power controller having a controller input terminal and a controller output terminal, wherein the controller input terminal is connected to the PA output terminal and the controller output terminal is connected to the AGC input terminal.

Description:
This application claims benefit of provisional application No. 60/139,691, filed Jun. 14, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention generally relates to wireless telephones. More specifically, the present invention relates to techniques involving the automatic adjustment of RF amplification circuitry. 
     I. Description of the Related Art 
     Signals transmitted by wireless telephones are required to satisfy various requirements. For instance, Code Division Multiple Access (CDMA) cellular phones are mandated by the FCC to limit out of channel distortion when transmitting in the radio frequency (RF) spectrum. Adjacent Channel Power Rejection (ACPR) is a metric frequently used to measure out of channel distortion. ACPR is represented as a curve across the spectrum that is centered at a transmitted RF signal&#39;s center frequency. At this center frequency, an ACPR curve is at its maximum. However, an ACPR curve symmetrically attenuates as frequencies depart from this center frequency. ACPR curves are compared against the spectral power characteristics of transmitted RF signals. Current CDMA standards, such as IS-98, require the spectral power characteristics of transmitted CDMA signals to be below a defined maximum ACPR curve at all frequencies and transmit power levels. When a signal complies with such a requirement, the signal is said to have passing margin. When a signal fails to comply with such a requirement, its out of channel distortion is excessive. 
     A wireless phone contains components that amplify RF signals so that they have sufficient power for transmission. Before amplification, a properly modulated RF signal has negligible out of channel distortion. An amplified signal&#39;s out of channel distortion will also be negligible if the amplification process is linear. However, if a signal has been amplified by a non-linear amplification process, its spectrum will include increased out of channel distortion. This increased out of channel distortion may cause a wireless phone to exceed the maximum allowed ACPR. 
     Electronic amplifiers are generally linear devices. However, under certain conditions, amplifiers will behave in a non-linear fashion. These conditions include low supply voltage and high temperature. Non-linear performance can be reduced by adjusting the output power produced through amplification. This reduction of non-linear performance will also reduce out of channel distortion. What is needed is a way to monitor operating conditions to provide the maximum possible output power without surpassing specified ACPR limits. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and system for maintaining adjacent channel power rejection (ACPR) passing margin. The method and system involves the control of an automatic gain control (AGC) amplifier to achieve a power amplifier (PA) output power that is appropriate for the operating conditions. 
     A method of the present invention includes amplifying a first radio frequency (RF) signal according to a first gain to produce a second RF signal and amplifying the second RF signal according to a second gain to produce a third RF signal. The method also includes determining a desired power level of the third RF signal, computing a new gain value from the desired power level, and adjusting the first gain to the new value. 
     A system of the present invention includes an automatic gain control (AGC) amplifier having an AGC input terminal, an AGC output terminal, and a control signal input terminal. The system also includes a power amplifier (PA) having a PA input terminal and a PA output terminal, wherein the PA input terminal is connected to the AGC output terminal. In addition, the system includes an output power controller having a controller input terminal and a controller output terminal, wherein the controller input terminal is connected to the PA output terminal and the controller output terminal is connected to the AGC input terminal. 
     An advantage of the present invention is the maintenance of ACPR passing margin throughout a range of operating voltages and temperatures without unduly compromising output power. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number. 
     FIG. 1 illustrates an RF amplification circuit according to the invention; 
     FIG. 2 illustrates a typical battery discharge curve; 
     FIGS. 3A,  3 B, and  3 C illustrate the spectral characteristics of amplification circuit output signals; 
     FIG. 4 illustrates an output power controller according to the invention; 
     FIG. 5 illustrates a relationship between a battery voltage signal and a digital battery voltage signal according to the invention; 
     FIG. 6 illustrates a relationship between the power level of a power amplifier output signal and a digital power signal according to the invention; 
     FIG. 7 illustrates a relationship between the ambient temperature of an RF amplification circuit and a digital temperature signal according to the invention; 
     FIG. 8 is a flowchart illustrating a lookup table algorithm performed by a processor according to the invention; 
     FIG. 9 illustrates the relationship between power levels and a digital power signal according to the invention; 
     FIG. 10 illustrates the relationship between a digital power signal and a digital automatic gain control signal according to the invention; 
     FIG. 11 is a curve illustrating the relationship between a digital automatic gain control signal and an analog automatic gain control signal according to the invention; and 
     FIG. 12 is a curve illustrating the relationship between a PA input signal and an analog automatic gain control signal according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an RF amplification circuit  100  in a wireless CDMA telephone according to a preferred embodiment of the present invention. This amplification circuit comprises several components. These components include an automatic gain control (AGC) amplifier  108 , a power amplifier (PA)  112 , and an output power controller  120 . Several signals are associated with this amplification circuit. These signals include a CDMA transmit signal  104 , a PA input signal  110 , a PA output signal  114 , a power source signal  116 , a reverse link power control signal  118 , an automatic gain control signal (AGC_V)  122 , and a PA_ON  124 . 
     AGC amplifier  108  receives CDMA transmit signal  104  and amplifies it according to an adjustable gain. In a preferred embodiment, this signal is at a fixed power level. This enables predictable performance of RF amplification circuit  100 . This amplified signal is output by AGC amplifier  108  as PA input signal  110 . 
     The gain of AGC amplifier  108  is controlled by automatic gain control signal (AGC_V)  122 . In a preferred embodiment, this signal is an electrical voltage or current that can be varied to adjust the gain of AGC amplifier  108 . Increasing the voltage of analog control signal  122  also increases the gain of AGC amplifier  108 . In an alternate embodiment, the gain of AGC amplifier  108  can be controlled by a digital signal. 
     PA  112  is a power amplifier that amplifies PA input signal  110 . This amplified signal is output by PA  112  as PA output signal  114 . In a preferred embodiment, PA output signal  114  is directed to an antenna segment of a CDMA phone for wireless transmission. PA  112  operates according to a fixed gain. However, in alternate embodiments, PA  112  can have an adjustable gain. The performance of PA  112  is typically measured by the power level of PA output signal  110 . 
     In a preferred embodiment, CDMA transmit signal, PA input signal  110 , and PA output signal  114  are all RF signals. In other words, these signals exist in the RF spectrum. However, in alternate embodiments, these signals could exist in other frequency ranges. 
     As illustrated in FIG. 1, PA  112  accepts power source signal  116 . In a preferred embodiment, power source signal  116  is a direct current (DC) voltage. This voltage signal is also known as Vdd. Power source signal can be generated by a battery or other external power source. Typical batteries include lithiumion and nickel-metal hydride batteries. Examples of external power sources include car cigarette lighters, and household alternating current (AC) power converted to a DC voltage. Power source signal  116  can be interrupted by PA_ON  124 . PA_ON  124  is a signal that is triggered when a wireless phone is in standby mode. This interrupt capability reduces the current draw on power sources, thereby conserving energy. 
     Output power controller  120  automatically controls the output power by adjusting AGC_V  122 . Specifically, output power controller  120  controls the magnitude of AGC_V  122 . In a preferred embodiment, output power controller  120  accepts PA output signal  114 , and reverse link power control signal  118  as input signals. These input signals are then manipulated according a process described with respect to FIG. 3 to generate automatic gain control signal (AGC_V)  122 . 
     Output power controller  120  accepts PA output signal  114  to estimate the power level of PA output signal  114 . In a preferred embodiment, output power controller also monitors signals representing ambient temperature and the DC supply voltage. These signals are used by output power controller  120  to determine a maximum allowable power level of PA output signal  114 . 
     Output power controller  120  also accepts reverse link power control signal  118  to perform in accordance with directives received from cellular base stations. Digital reverse link power control signal  118  is a digital signal. In a preferred embodiment, this signal is derived from directives received from a cellular base station via a cellular network channel that is dedicated to overhead traffic. These directives command a wireless phone to adjust the power level of PA output signal  114 . Output power controller  120  considers these directives in conjunction with the determined maximum allowable power level of PA output signal  114 . A desired power level of PA output signal  114  results from this consideration. In an alternate embodiment, output power controller  120  does not consider digital reverse link power control signal  118 . Instead, output power controller  120  equates desired power level to maximum allowable power level. Output power controller  120  then converts this desired power level into AGC_V  122  having the appropriate magnitude. 
     FIG. 2 illustrates a battery discharge curve. This curve depicts the typical decline of a battery&#39;s voltage over time as it supplies electrical current necessary to support wireless phone calls. This discharge curve profiles a time interval when battery voltage declines from 4.1 Volts to 3.2 Volts. As illustrated by this curve, the battery&#39;s voltage is greater than 3.7 Volts for the majority of this interval. A battery&#39;s voltage also fluctuates according to temperature. In general, as temperature increases, so does a battery&#39;s voltage. 
     Wireless telephones are capable of operating across a range of voltages. However, for all wireless phones, there is a minimum operational voltage. If a wireless phone&#39;s power source fails to supply power above this voltage, the phone will not function properly. A typical minimum operational voltage for CDMA wireless phones is 3.0 Volts. As illustrated by FIG. 2, if a wireless phone is powered by a battery, it will operate at voltages above this minimum operational voltage for a significant amount of time. 
     When a wireless telephone is operating at voltages greater than the minimum operational voltage, the particular operating voltage affects the performance characteristics of RF amplification circuit  100 . This principle is evident when RF amplification circuit  100  is calibrated to generate PA output signal  114  at a certain power level. For a given output power calibration, the non-linear characteristics of RF amplification circuit  100  will increase as the operating voltage decreases. As discussed above, increased out of channel distortion is a manifestation of an increase in non-linear amplification characteristics. 
     FIGS. 3A,  3 B, and  3 C illustrate the spectral characteristics of PA output signal  114  as a function of the power level of PA output signal  114  and RF amplification circuit&#39;s  100  operating voltage. Each of these figures contains three curves of solid lines. These curves represent the spectral characteristics of a PA output signal  114  when the operating voltage is either 3.2, 3.7, or 4.2 Volts. Each of these solid line curves has a center lobe and two side lobes. The center lobes exist is the middle of the depicted spectrum and have a larger magnitude than the side lobes that exist to the left and right of each center lobe. The center lobes represent the power of PA output signal  114  inside its designated RF transmission channel. The side lobes represent the power of PA output signal  114  outside of its designated RF transmission channel. This indicates the amount of out of channel distortion. In FIGS. 3A,  3 B, and  3 C, each center lobe is of equal magnitude. In contrast, the side lobe magnitudes vary according to operating voltage. Thus, out of channel distortion varies according to operating voltage. 
     Each of these figures also contains a dotted line curve. This dotted line curve is the ACPR limit. As stated above, the spectral characteristics of PA output signal  114  cannot exceed this limit. In particular, FIG. 3C shows that as operating voltage decreases, the out of channel distortion of PA output signal  114  increases and eventually exceeds the ACPR limit. For example, when the operating voltage is either 4.2 Volts or 3.7 Volts, PA output signal  114  is within the ACPR limit. In other words, there is passing margin. However, when the operating voltage is 3.2 Volts, PA output signal  114  exceeds the ACPR limit. In this situation, no passing margin exists. 
     When considering the characteristics described above in light of the fact that operating voltages fluctuate, RF amplification circuit  100  must employ techniques to ensure that ACPR limits are not exceeded at any operating voltage. 
     A conventional technique for guaranteeing compliance with ACPR requirements involves the static calibration of a wireless phone&#39;s amplification characteristics during production. This calibration technique involves powering a phone with its minimum operational voltage and adjusting AGC_V  122  so that PA output signal  114  yields the maximum possible power without exceeding a specified ACPR limit at this minimum voltage. This technique is termed static calibration because once AGC_V  122  is set, it will not be adjusted. Therefore, according to this technique, output power controller  120  merely provides a constant AGC_V  122 . 
     Static calibration is performed at minimum operational voltage because RF amplification circuit  100  is most susceptible to non-linear performance at this voltage. However, static calibration is a less than optimal solution. Since the voltage of power source signal  116  is typically greater than the minimum operating voltage, RF amplification circuit  100  is often capable of producing a higher power PA output signal  114  without exceeding a specified ACPR limit. Therefore, in a preferred embodiment, output power controller  120  dynamically controls AGC_V  122  in a manner that enables RF amplifier circuit  100  to produce a maximum power with passing margin. 
     FIG. 4 illustrates output power controller  120  according to a preferred embodiment. Output power controller  120  comprises several components. These components include a power detector  404 , an analog multiplexer  406 , an analog to digital (AID) converter  408 , a processor  410 , a power limit register  412 , a linearizer  414 , a digital to analog (D/A) converter  416 , and a temperature sensor  418 . 
     Power detector  404  accepts PA output signal  114  and estimates the power of this signal. In a preferred embodiment, power detector  404  can detect RF power over a 30 dB range having an upper limit of 1 watt and a lower limit of 1 milliwatt. Power detector  404  also generates an analog signal that is proportional to this power estimate. In a preferred embodiment, this analog signal is a DC voltage that is linearly proportional to the power level of PA output signal  114 . Power detector  404  sends this analog signal to an input port on analog multiplexer  406 . Power detector  404  can be implemented with analog circuitry, digital processing algorithms, or any other power detection and estimation means known to persons skilled in the relevant arts. 
     Temperature sensor  418  converts the ambient temperature of RF amplification circuit  100  into a temperature signal  436 . In a preferred embodiment, this temperature signal is a DC voltage that is linearly proportional to the ambient temperature. Temperature sensor  418  sends this analog signal to an input port on analog multiplexer  406 . An exemplary temperature sensor  418  is a thermocouple. 
     Battery voltage signal  420  indicates the operating voltage of RF amplification circuit  100 . In a preferred embodiment, battery voltage signal  420  is simply the battery voltage. This voltage can be obtained by connecting conductors to each battery terminal. 
     Analog multiplexer  406  has input ports to accept analog signals generated by power detector  404  and temperature sensor  418 . Analog multiplexer  406  also has an input port to accept battery voltage signal  420 . In a preferred embodiment, analog multiplexer  406  time division multiplexes these signals into a single output signal that is timed according to an input select signal  424 . Input select signal  424  is received from processor  410 . This single output signal will be referred to as ADCIN_V  426 . ADCIN_V  426  comprises information regarding the power level of PA output signal  114 , the ambient temperature of RF amplification circuit  100 , and battery voltage signal  420 . Analog multiplexer  406  sends ADCIN_V  426  to an input port of A/D converter  408 . 
     A/D converter  408  accepts ADCIN_V  426  via an input port and converts it into a composite data signal  428 . Composite data signal  428  comprises three distinct digital signals: TEMP_N, PO_N, and BATT_N. These three digital signals quantitatively describe the power level of PA output signal  114 , the ambient temperature of RF amplification circuit  100 , and the magnitude of battery voltage signal  420 . A/D converter  408  converts these analog signals into TEMP_N, PO_N, and BATT_N according to defined relationships. These relationships are described below. In a preferred embodiment, A/D converter  408  uses eight bits to encode these digital signals. A/D converter  408  sends these signals to processor  410  according to a standard computer bus architecture. In an alternate embodiment, these signals are sent to processor according to any data interface known to persons skilled in the relevant arts. 
     FIG. 5 is a curve illustrating the relationship between BATT_N and battery voltage signal  420  according to a preferred embodiment. BATT_N is a quantized digital signal represented by eight bits. Battery voltage signal  420  is represented in Volts. As illustrated, the relationship between BATT_N and battery voltage signal  420  is essentially linear. However, in alternate embodiments, this curve can have any shape. 
     FIG. 6 is a curve illustrating the relationship between PO_N and the power level of PA output signal  114  according to a preferred embodiment. PO_N is a quantized digital signal represented by eight bits. The power level of PA output signal  114  is represented in decibels with respect to a milliwatt (dBm). As illustrated, PO_N increases exponentially with the power level of PA output signal  114 . However, this curve can have any shape. 
     FIG. 7 is a curve illustrating the relationship between TEMP_N and the ambient temperature of RF amplification circuit  100  according to a preferred embodiment. TEMP_N is a quantized digital signal represented by eight bits. The ambient temperature of RF amplification circuit  100  is represented by degrees Celsius. As illustrated, TEMP_N decreases monotonically as the ambient temperature of RF amplification circuit  100  increases. However, this curve can have any shape. 
     Processor  410  is any component that can perform algorithms. Processor  410  also contains memory for information access and storage. In a preferred embodiment, processor  410  is a microprocessor. However, in alternate embodiments, processor  410  may comprise processing capability dispersed among one or more application specific integrated circuits (ASICs) or other hardware capable of performing algorithms. Exemplary processors  410  include reduced instruction set computer (RISC) processors, microcontrollers, finite state machines, personal computer processors, and the mobile station modem (MSM) chip. Processor  410  accepts TEMP_N, PO_N, and BATT_N from A/D converter  408  and performs an algorithm that sets the maximum allowable power level of PA output signal  114 . This maximum allowable power level is output by processor  410  as LIMIT_N  430 . LIMIT_N  430  is an eight bit digital signal sent to power limit register  412  according to a standard computer bus architecture. In an alternate embodiment, LIMIT_N  430  is sent to power limit register  412  according to any data interface known to persons skilled in the relevant arts. 
     Processor  410  generates LIMIT_N  430  according to an algorithm. This algorithm can be described at an abstract level with the following equation: 
     
       
         LIMIT_N= f (BATT_N, TEMP_N, PO_N, external power detected signal  422 ) 
       
     
     The above equation states that LIMIT_N  430  is determined according to a mathematical function that is dependent on four signals: BATT_N, TEMP_N, PO_N, and external power detected signal  422 . Processor  410  can perform this function through mathematical computation. However, in a preferred embodiment, processor  410  performs this function by acccessing a lookup table containing pre-compiled values. 
     FIG. 8 is a flowchart illustrating a lookup table algorithm performed by processor  410  according to a preferred embodiment. The algorithm begins with step  804 . In this step, processor  410  converts TEMP_N, PO_N, and BATT_N into a lookup table address. Next, in step  806 , processor  410  accesses the contents of this lookup table address. The contents of this address specify the maximum achievable power level of PA output signal  114  that will satisfy specified ACPR requirements. Step  808  is performed next. In step  808 , processor  410  converts the accessed table entry into LIMIT_N  430 . As described above, LIMIT_N  430  is a digital signal that can be represented by any number of bits. 
     The lookup table described above contains maximum power levels of PA output signal  114  that satisfy a specified ACPR requirement. In a preferred embodiment, each of these powers is based on a combination of temperature, operating voltage, and the existing power level of PA output signal  114 . The contents of maximum power lookup table can be determined by empirical methods. An exemplary empirical method comprises operating RF power amplification circuit  100  at various combinations of temperature, operating voltage, and PA output signal  114  power level to determine the maximum achievable power level within ACPR limits for each combination. Once this maximum power level is determined for a given combination, it is placed in the lookup table described above. In a preferred embodiment, this lookup table is stored in memory that is contained in processor  410 . 
     In alternate embodiments, maximum power lookup table can store a function that is based on a theoretical formula. An exemplary formula is provided below: 
     
       
         LIMIT_N=max(min(( a ·BATT_N+ b ·TEMP_N+C·PO_N),  d ), e ), 
       
     
     Where: 
     a, b, and c=a function or curve fit based on measured data from RF amplification circuit  100 ; 
     d=a value equal to a minimum allowed power level of PA output signal  114  to meet specified CDMA performance standards; and 
     e=a value equal to a maximum allowed power level of PA output signal  114  to meet FCC requirements. 
     FIG. 9 illustrates the relationship between power levels in decibels with respect to a milliwatt (dBm) and LIMIT_N  430 . As stated above, LIMIT_N  430  is a digital signal that quantitatively represents the maximum allowable power level of PA output signal  114 . In this figure LIMIT_N is a digital signal represented by eight bits. In a preferred embodiment, the correspondence or relationship between LIMIT_N units and the power level of PA output signal  114  in dBm is linear. 
     If RF amplification circuit  100  is powered by an external power source such as a car cigarette lighter, External power detected signal  422  is enabled. Processor  410  monitors external power detected signal  422 . If this signal is enabled, processor  410  does not perform the algorithms described above. Rather, processor  410  sets LIMIT_N  430  to a predetermined value. In a preferred embodiment, this predetermined LIMIT_N  430  value is 255. When using the relationship defined in FIG. 9, this value corresponds to a PA output signal  114  power level of 29 dBm. 
     As described above, power limit register  412  receives LIMIT_N  430 , from processor  410 . Power limit register  412  also receives reverse link power control signal  118 . Power limit register generates a dBm_N  432  signal and sends it to linearizer  414 . dBm_N is a digital signal that quantitatively represents the desired power level of PA output signal  114 . In a preferred embodiment, dBm_N  432  is a digital signal represented by eight bits. 
     Power limit register  412  compares the values of LIMIT_N  430  and reverse link power control signal  118 . Based on this comparison, power limit register  412  generates dBm_N  432  according to the following equation: 
      dBm_N=min(LIMIT_N, reverse link power signal  118 ) 
     Effectively, the output of power limit register  412  is the minimum of LIMIT_N  430  and reverse link power control signal  118 . 
     Linearizer  414  translates the desired dBm_N  432  signal into an AGC_N  434  signal. AGC_N  434  is an initial representation of AGC_V  122 . In a preferred embodiment, AGC_N  434  is a digital signal represented by eight bits. After being generated, AGC_N  434  is sent to D/A converter  416 . 
     FIG. 10 illustrates the relationship between dBm_N  432  and AGC_N  434 . In a preferred embodiment, this relationship is substantially linear. However, at higher dBm_N  432  levels, this relationship becomes non-linear. This nonlinearity is purposefully added to correct for non-linear characteristics of AGC amplifier  108 . AGC amplifiers  108  often have unique non-linear characteristics. Therefore, the relationship between dBm_N  432  and AGC_N  434  must be calibrated in each linearizer  414 . 
     D/A converter  416  translates AGC_N  434  into AGC_V  122 . AGC_V  122  is a DC voltage that controls the gain of AGC amplifier  108 . In a preferred embodiment, CDMA transmit signal  104  has a fixed power level. Therefore, the gain of AGC amplifier  108  is the only variable that controls the power level of PA output signal  114 . 
     FIG. 11 is a curve illustrating the relationship between AGC_V  122  and AGC_N  434 . In a preferred embodiment, this curve is linear. However, in alternate embodiments, this curve can have any shape. 
     FIG. 12 is a curve illustrating the relationship between PA input signal  110  and AGC_V  122 . The curve is essentially linear. However, as AGC_V  122  increases, this relationship becomes non-linear. As discussed above with respect to FIG. 10, these non linear characteristics are corrected by linearizer  414 . 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.