Abstract:
An automatic gain control (AGC) circuit that includes an RF amplifier with first and second distinct active gain control regions, wherein a gain of the RF amplifier varies during operation in the active gain control regions

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/601,026, filed Aug. 12, 2004, and entitled “Advanced Digital Receiver.” 
     
    
     REFERENCE REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable 
       SEQUENTIAL LISTING 
       [0003]    Not applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present invention relates generally to digital communication techniques, and more particularly, to an apparatus for and method of adjusting the automatic gain control unit of a receiver. 
         [0006]    2. Description of the Background of the Invention 
         [0007]    Signal communications systems transmit a data stream from a transmitter to a receiver through a communication channel. Specifically, a transmitter modulates a carrier wave in response to the data stream to generate a radio frequency (RF) signal and transmits the RF signal through the communication channel. An analog front-end of a receiver detects the RF signal from the communication channel and down-mixes the RF signal to develop a near-baseband intermediate frequency (IF) signal. The IF signal is thereafter demodulated and decoded to develop estimates of the data stream. 
         [0008]    The analog front-end is designed with automatic gain control (AGC) that presents an IF signal with constant power to the demodulation circuitry even as the power level of the RF signal detected from the channel varies. To achieve this, the front-end incorporates an RF amplifier that amplifies the RF signal, a mixer to generate an IF signal from the amplified RF signal, and an IF amplifier to amplify the generated IF signal to develop an amplified IF signal that is presented to the demodulation circuitry. Control circuitry in the front-end monitors the power level of the signal received from the channel and adjusts the gains of the RF and IF amplifiers accordingly so that the power level at the output of the front-end is maintained at a constant level. 
         [0009]    Typical front-ends use a two-mode AGC, which operates in a first operating mode if the power level of the received signal is low and in a second operating mode if the power level of the received signal is high. The AGC that is operating in the first operating mode sets the gain of the RF amplifier to a maximum level and adjusts the gain of the IF amplifier as necessary to produce an output signal of constant power. If the power level of the received signal is high, then the AGC operates in the second operating mode whereby the front-end sets the gain of the IF amplifier to a constant gain and adjusts the gain of the RF amplifier as needed to maintain an output signal of constant power. 
         [0010]    Having two operating modes in the AGC prevents saturation of the RF amplifier when the receiver receives a signal with a high power level. However, saturation of the RF amplifier can still occur, especially in situations when signals in adjacent channels interfere with the signal in the desired channel. This is because the control circuitry of a typical front-end selects the operating mode of the AGC by averaging the received signal power in a desired channel without considering the power levels of signals in adjacent channels. If the power level of the signal received in the desired channel is low but the power level of a signal in an adjacent channel is high, the AGC will operate in the first operating mode (i.e., maximum RF gain) and the strength of the signal in the adjacent channel will cause the RF amplifier to become saturated and cause distortion of the signal in the desired channel. In addition, the two-mode AGC does not allow the front end to compensate for fast changes in signal power that, for example, could be caused by reflections from large moving objects (e.g., trucks, planes, etc.) because the gain of the RF amplifier cannot be adjusted quickly without causing an instability in the gain control loop due to excessive delays in the control path. 
       SUMMARY OF THE INVENTION 
       [0011]    According to one aspect of the invention, an automatic gain control (AGC) circuit comprises an RF amplifier that has first and second distinct active gain control regions, wherein a gain of the RF amplifier varies during operation in the active gain control regions. 
         [0012]    According to another aspect of the invention, a circuit for amplifying a signal includes a first amplifier that develops a first amplified signal from the signal, wherein a first gain is associated with the first amplifier. The circuit further includes a second amplifier that generates a second amplified signal from a signal derived from the first amplified signal. In addition, the circuit includes a controller that is responsive to the power level of the signal for selecting an operating mode for the circuit from at least three operating modes and for controlling the first gain and the second gain in accordance with the operating mode. 
         [0013]    Other aspects and advantages of the present invention will become apparent upon consideration of the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a receiver in a communications system; 
           [0015]      FIG. 2  depicts an embodiment of an automatic gain control unit (AGC) of an analog front-end of the receiver of  FIG. 1 ; 
           [0016]      FIG. 3A  depicts an IF gain control curve of the AGC unit of  FIG. 2 ; 
           [0017]      FIG. 3B  depicts an RF gain control curve of the AGC unit of  FIG. 2 ; 
           [0018]      FIG. 4  comprises a state diagram illustrating operation of a control system of the AGC unit of  FIG. 2 ; 
           [0019]      FIG. 5  comprises a block diagram of a control system of the AGC unit of  FIG. 2  that operates in a manner similar to the operation illustrated by  FIG. 4 ; 
           [0020]      FIG. 6A  depicts a gain characteristic curve of an IF amplifier in the AGC unit of  FIG. 2 ; 
           [0021]      FIG. 6B  depicts a flow chart of a selector of the controller of  FIG. 4 ; and 
           [0022]      FIGS. 7A-7C  are a series of diagrams on a synchronized time scale illustrating one aspect of the operation of the controller of  FIG. 2  in response to received power level. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]      FIG. 1  illustrates a receiver  100  suitable for receipt and decoding of a signal transmitted through a channel. The receiver  100  comprises an analog front-end  102 , a demodulator  104 , and a decoder  106 . In addition, a control system  108  monitors and controls the operation of the various components of the receiver  100 . The analog front-end receives an RF signal at an input  110  and develops an IF signal at an output  112 . The control system  108  operates the front-end  102  such that the power level of the IF signal developed at the output  112  is maintained at a desired level even as the power level of the RF signal at the input  110  fluctuates. 
         [0024]      FIG. 2  depicts an automatic gain control (AGC) unit  200  of the analog front-end  102  of the receiver  100 . The AGC unit  200  comprises an RF amplifier  202 , a mixer  204 , an IF amplifier  206 , and a down-converter oscillator  208 . The AGC unit  200  further provides additional control signals to the control system  108  including an RF GAIN CONTROL signal on a line  210  to control the gain (RF GAIN ) of the RF amplifier  202  and an IF GAIN CONTROL signal on a line  212  to control the gain (IF GAIN ) of the IF amplifier  206 . Only signals relevant to an understanding of the present embodiment are shown herein. 
         [0025]    The RF amplifier  202  of the analog front-end  102  receives a signal RF INPUT  from the channel at the input  110 . The RF amplifier  202  amplifies the signal RF INPUT  to develop a signal RF OUT  on a line  214  that is provided to the mixer  204 . The mixer  204  uses a stable local oscillator output signal received on a line  216  from the down-converter oscillator  208  to down-convert the RF OUT  signal to an intermediate frequency signal IF IN  on a line  218 . The IF amplifier  206  receives the IF IN  signal from the mixer  204  and amplifies the IF IN  signal to generate a signal IF OUT  on a line  220 . Some embodiments may use components such as a Bandpass Filter between the mixer  204  and the IF amplifier  206  in order to remove out of band interference from the signal IF IN . Referring back to  FIG. 1 , it can be appreciated that the IF OUT  signal on the line  220  is identical to the analog near-baseband IF signal on the output line  112  provided by the analog front-end receiver  102  to the demodulator  104 . 
         [0026]    The control system  108  provides the RF GAIN CONTROL signal on the line  210  that determines the RF GAIN  applied by the RF amplifier  202  in accordance with a predetermined gain characteristic curve of the RF amplifier  202 . Similarly, the control system  108  provides the IF GAIN CONTROL signal on line  212  that determines the IF GAIN  applied by the IF amplifier  206  in accordance with a predetermined gain characteristic curve of the IF amplifier  206 . The control system  108  selectively controls the RF GAIN  and the IF GAIN  using the RF GAIN CONTROL and IF GAIN CONTROL signals on lines  210  and  212 , respectively, to optimize the signal-to-noise and distortion performance of the analog front end  102 , even in the presence of interference from adjacent channels. 
         [0027]    In one embodiment, the control system  108  estimates the power level RF PL  of the received RF INPUT  signal from the RF GAIN , the IF GAIN , and the power level of the IF OUT  signal as follows: 
         [0000]      RF PL =IF OUT −(RF GAIN +IF GAIN   +K ) 
         [0028]    where K is a predetermined constant and measurements are in dB or dBm. It should be apparent that the value of the RF GAIN  in the above equation can be estimated from the value of the RF GAIN CONTROL signal on the line  210  and the gain characteristic curve of the RF amplifier  202 . Similarly, the value of the IF GAIN  can be estimated using the value of the IF GAIN CONTROL signal on the line  212  and the gain characteristic curve of the IF amplifier  206 . 
         [0029]    The control system  108  operates the AGC unit  200  in one of four operating modes MODE 0 , MODE 1 , MODE 2 , and MODE 3  in accordance with the calculated value of RF PL .  FIG. 3A  depicts an IF gain control curve  300  that shows the IF GAIN  applied by the IF amplifier  206  during the operating modes MODE 0 , MODE 1 , MODE 2 , and MODE 3 . The IF gain control curve  300  has a first active region  302 , a first static region  304 , a second active region  306 , and a second static region  308  in which the IF amplifier  206  is operable during operation in MODE 0 , MODE 1 , MODE 2 , and MODE 3 , respectively. Similarly, as shown in  FIG. 3B , the RF GAIN  applied by the RF amplifier  202  is controlled in accordance with the RF gain control curve  310 . The RF gain control curve  310  has a first static region  312 , a first active region  314 , a second static region  316 , and a second active region  318  that are in effect during MODE 0 , MODE 1 , MODE 2 , and MODE 3 , respectively. 
         [0030]    The control system  108  operates the AGC unit  200  in MODE 0  when the power level RF PL  of the RF INPUT  signal is less than a first threshold level S MIN . The control system  108  operates the AGC unit  200  in MODE 1  when the RF power level RF PL  is greater than S MIN  but less than a second threshold level S NOM . Similarly, the AGC unit  200  operates in MODE 2  when the RF power level RF PL  is greater than S NOM  but less than a third threshold level S MAX . Finally, the control system  108  operates the AGC unit  200  in MODE 3  when the RF power level RF PL  is greater than the level S MAX . Although not shown in  FIG. 3A  and  FIG. 3B , some embodiments of the control system  108  incorporate a degree of hysteresis between the certain ones or all of the different modes of operation of the AGC unit  200 . Those of skill in the art would recognize that fewer or more operating modes can be used without departing from the spirit of the invention. 
         [0031]      FIG. 4  illustrates a state diagram  400  of the control system that may be used to control the AGC unit  200 . At block  402 , “Initialize,” control system  108  initializes the various elements of the AGC unit  200 . Block  402  then calculates the power level RF PL  of the received signal, RF INPUT  and, in some embodiments, causes the AGC unit  200  to proceed to block  404 . In other embodiments, shown as a dashed line in  FIG. 6 , the control system  108  compares the calculated RF PL  to the threshold values S MIN , S NOM , and S MAX  and selects an appropriate operating mode for the AGC unit  200  as described below. For example, the AGC unit  200  directly transitions from block  402 , “Initialize,” to block  406 , “MODE 1 -Adjust RF Gain,” when S NOM &gt;RF PL &gt;S MIN  without first transitioning into MODE 0 . 
         [0032]    At block  404 , “MODE 0 -SET RF GAIN,” the IF amplifier  206  operates in the first active region  302  and the control system  108  adjusts the signal controlling IF GAIN  to control the gain of the IF amplifier  206  while the RF amplifier  202  operates in the first static region  312  with the RF GAIN  set to RF GAIN MAX . In this mode, the control system  108  adjusts the signal controlling IF GAIN  linearly with respect to the power level RF PL  so that IF GAIN  is in a range between IF GAIN MAX  and IF GAIN NOM . It can be appreciated that setting the RF amplifier gain to RF GAIN MAX , when RF PL  is less than S MIN  provides the greatest signal amplification at the output of the IF amplifier  206  while overcoming noise present at the RF amplifier input coupled to the line  110 . The AGC unit  200  then transitions to block  406  when RF PL  is greater than S MIN . 
         [0033]    At block  406 , “MODE 1 -Adjust RF Gain,” the control system  108  operates the RF amplifier  202  in the first active region  314  of the RF gain control curve  310 . Depending upon the power level of the RF INPUT  signal, the signal controlling the RF GAIN  is slewed so that the RF GAIN  is between RF GAIN MAX  and RF GAIN NOM . The RF GAIN  is adjusted in accordance with the power level RF PL  so that the IF GAIN  is maintained at a constant gain of IF GAIN NOM . Changes in the RF PL  while the AGC is operating in this mode may cause the IF GAIN  to deviate from IF GAIN NOM . However, the control system  108  adjusts the RF GAIN  so that the IF GAIN  signal returns to IF GAIN NOM  Preferably, the signal controlling the RF GAIN  is adjusted linearly with respect to the power level RF PL . Adjusting the RF GAIN  while maintaining the IF GAIN  constant allows the AGC unit  200  to compensate for strong adjacent channel interference without significantly degrading the receiver performance. If RF PL ≧S NOM , the control system  108  transitions the AGC unit  200  to block  408 . However, if the power level RF PL  becomes less than S MIN , the control system  108  transitions the AGC unit  200  to block  404 . 
         [0034]    At block  408 , “MODE 2 -SET RF GAIN,” the control system  108  operates the RF amplifier  202  in the static region  316  by setting the RF GAIN  to RF GAIN NOM . The control system  108  operates the IF amplifier  206  in the second active region  306  and adjusts the signal controlling IF GAIN  so that the IF GAIN  is in a range between IF GAIN NOM  and IF GAIN MIN . Preferably, the IF GAIN  is adjusted linearly with respect to RF PL . This allows the AGC unit  200  to adjust for strong adjacent channel interference without further degrading the signal-to-noise performance at the output of the IF amplifier  206 . If RF PL &lt;S NOM , the control system  108  transitions the AGC unit  200  to block  406 . Otherwise, if RF PL ≧S MAX , the control system  108  transitions the AGC unit  200  to block  410 . 
         [0035]    At block  410 , “MODE 3 -Adjust RF Gain,” the control system  108  operates the RF amplifier  202  in the second active region  318  by adjusting the signal that controls the RF GAIN  so that RF GAIN  is in a range between RF GAIN NOM  and RF GAIN MIN  while maintaining the IF GAIN  at a constant gain of IF GAIN MIN . As described above with respect to “MODE 1 -Adjust RF GAIN,” the IF GAIN  may deviate from IF GAIN MIN  in response to a change in RF PL . However, the control system adjusts the RF GAIN  such that the IF GAIN  returns to IF GAIN MIN . The RF GAIN  is generally adjusted linearly with respect to the power level RF PL . This allows the AGC unit  200  to adjust for a received RF INPUT  signal with high power. If RF PL &lt;S MAX , the control system  108  transitions the AGC unit  200  back to block  408 . Although not indicated in  FIG. 4 , it can be understood that in some embodiments of the state diagram  400  include techniques to provide hysteresis when transitioning between the various modes. Illustratively, some embodiments of the state diagram  400  transition the AGC unit  200  from block  410  to block  408  when RF PL &lt;S MAX −Δ, where Δ signifies the desired degree of hysteresis. It can be understood that transitions of the AGC unit  200  between other blocks of the state diagram  400  may also include a similar offset. 
         [0036]    Estimating the RF PL  from the RF GAIN  and the IF GAIN  of the RF amplifier  202  and IF amplifier  206 , respectively, may difficult to implement. To overcome this, some implementations of the control system  108  may use the IF OUT  signal developed at the line  220  of  FIG. 2  to select the operating mode of the AGC  200 .  FIG. 5  shows a block diagram of a control system  500  that can be used in such an implementation. An analog to digital converter  501  receives the signal IF OUT  on the line  502  and provides a digital value corresponding to the signal to a squarer  503  that develops a signal on a line  504  that represents the power level of the signal IF OUT . A comparator block  505  receives the signal on the line  504  and a reference signal IF REF  on a line  506 . The signal IF REF  represents the power level of the signal desired at the output line  220  of the AGC  200 . A subtractor  508  calculates a difference between the IF OUT  and IF REF  signals and provides the result to an integrator  510 , which averages the difference between the IF OUT  and IF REF  signals over time and develops a signal IF GC  on a line  512 . The actual gain IF GAIN  applied by the IF amplifier  206  is determined by the IF GC  signal in accordance with the gain characteristic curve of the IF amplifier  206 . 
         [0037]    A comparator  518  receives the IF GC  signal on a line  520  and a signal IF HIGH  on a line  522 . The signal IF HIGH  is the IF GAIN CONTROL signal that must provided to the IF amplifier  206  on a line  212  to set the gain thereof to IF GAIN NOM . A subtractor  524  in the comparator calculates a difference between the IF GC  and IF HIGH  signals and provides the resulting signal to an integrator  526 . The integrator  526  averages the difference over time and develops a signal RF GC     —     MODE     —     1  on a line  528 . The signal RF GC     —     MODE     —     1  corresponds to the RF GAIN CONTROL signal that must provided to the RF amplifier  202  on the line  210  when the gain of the IF amplifier  206  is set to IF GAIN NOM  to cause the AGC unit  200  to produce an output signal on the output line  220  having a power level IF REF . 
         [0038]    A comparator  530  receives the IF GC  signal on a line  532  and a signal IF LOW  on a line  534 . The signal IF LOW  is the IF GAIN CONTROL signal that must be provided to the IF amplifier  206  on a line  212  to sets the gain thereof to IF GAIN MIN . A subtractor  536  in the comparator calculates a difference between the IF GC  and IF LOW  signals and provides the resulting signal to an integrator  538 . The integrator  538  averages the difference between the two signals over time and develops a signal RF GC     —     MODE      —     3  on a line  540 . The signal RF GC     —     MODE     —     3  corresponds to the RF GAIN CONTROL signal that must be provided the RF amplifier  202  on the line  210  when the gain of the IF amplifier  206  is set to IF GAIN MIN  so that the AGC unit  200  produces an output signal on the line  220  having a power level IF REF . 
         [0039]    A selector  542  receives the signals RF GC     —     MODE     —     1 , RF GC     —     MODE     —     3 , and IF GC  on the lines  528 ,  540 , and  544 , respectively. In addition, the selector  542  receives signals RF GC     —     MODE     —     0  and RF GC     —     MODE     —     2  on the lines  546  and  548 , respectively. The signals RF GC     —     MODE     —     0  and RF GC     —     MODE     —     2  are signals that if provided to RF amplifier  202  on the line  210  set the gain of the RF amplifier  202  to RF GAIN MAX  and RF GAIN NOM , respectively. The selector  542  compares the signal IF GC  to threshold values that correspond to the operating modes of the AGC unit  200 , selects a desired operating mode for the AGC  200 , and generates a signal RF GC  on a line  550  in accordance with the desired operating mode. The selector  542  selects one of the signals RF GC     —     MODE     —     0 , RF GC     —     MODE      —     1 , RF GC     —     MODE     —     2 , or RF GC     —     MODE     —     3  in accordance with the operating modes MODE 0 , MODE 1 , MODE 2 , and MODE 3 , respectively, to generate the signal RF GC . 
         [0040]      FIG. 6A  depicts an example of a gain characteristic curve that approximates the actual gain characteristic curve of the IF amplifier  206 . The gain characteristic curve of  FIG. 6A  is used by the selector  542  to determine the desired operating mode. Typically, the gain characteristic curve of the IF amplifier  206  maps the voltage of the signal IF GC  to the gain of the IF amplifier  206 . However, it should be apparent that one or more parameters of the signal IF GC  and/or one or more other parameter(s), e.g., ambient temperature, could be used to map to the gain of the IF amplifier  206 . 
         [0041]      FIG. 6B  depicts a flow chart of a control loop that illustrates operation of one embodiment of the selector  442  of the control system  108  of the AGC unit  200 . A block  602  compares IF GC &lt;IF GC     —     1 , and if the result of the comparison is true, a block  604  selects MODE 0  as the desired operating mode and sets RF GC  to RF GC     —     MODE      —     0 . Otherwise, a block  606  compares IF GC     —     1 ≦IF GC &lt;IF GC     —     2 , and if the result is true, a block  608  sets the desired operating mode to MODE 1  and RF GC  to RF GC     —     MODE     —     1 . If the comparison of the block  606  is false, then a block  610  compares IF GC     —     2 ≦IF GC &lt;IF GC     —     3 , and if the result is true, a block  612  sets the desired operating mode to MODE 2  and RF GC  to RF GC     —     MODE     —     2 . If none of the comparisons of the blocks  602 ,  606 , and  610  generates a positive result (i.e., IF GC &gt;IF GC     —     3 ), a block  614  sets the desired operating mode to MODE 3  and RF GC  to RF GC     —     MODE     —     3 . After selecting the desired operating mode and the value of the signal RF GC , control from the blocks  604 ,  608 ,  612 , and  614  returns to the block  602 . 
         [0042]    Referring once again to  FIG. 2 , some embodiments of the AGC unit  200 , incorporate an IF amplifier  206  having a wider bandwidth than the RF amplifier  202  wherein the IF GAIN  can be adjusted faster than the RF GAIN . During operation, the AGC unit  200  may be required to quickly transition between operating modes in response to sudden changes in the input signal power level. In response, the IF GAIN  can be immediately adjusted to compensate for the sudden change in the input signal and for the slower response of the RF amplifier  202 . The RF GAIN CONTROL and IF GAIN CONTROL signals on the lines  210  and  212 , respectively, are thereafter adjusted simultaneously until the RF GAIN  and IF GAIN  gains reach levels that are in accordance with the new operating mode of the AGC unit  200 . As an example, consider the behavior over time of a received signal depicted in  FIG. 7A , where the power level of the received signal at time T 0  is at a level RF MODE-2  less than S MAX  and greater than S NOM . At time T 1 , the power level of the signal drops to a power level RF MODE-1  that is less than S NOM  and greater than S MIN . In accordance with  FIGS. 3A and 3B  the AGC unit  200  is operated at  316  in MODE 2  during the time period between times T 0  and T 1  and is operated at  314  in MODE 1  after time T 1 .  FIGS. 7B and 7C  show how the RF GAIN  and the IF GAIN  are adjusted in response to the signal power level behavior depicted in  FIG. 7A . During the period of time when the AGC unit  200  is operating in MODE 2  (i.e., between times T 0  and T 1 ), the IF GAIN  is set to IF GAIN-MODE-2  and the RF GAIN  is set to RF GAIN-MODE-2 . At time T 1  the AGC unit  200  begins a transition from MODE 2  to MODE 1  in response to the change in the power level of the input signal depicted in  FIG. 7A . The AGC unit  200  enters a transition period by immediately increasing the IF GAIN  to IF GAIN-TRANS  and slewing the RF GAIN  from RF GAIN-MODE-2  to RF GAIN-MODE-1 . The value of IF GAIN-TRANS  is selected to compensate for the new power level of the received signal. In the example depicted by  FIGS. 7A-7C , the transition period occupies the period of time between times T 1  and T 2  during which the RF GAIN  is increased and the IF GAIN  is decreased. The transition period ends when the RF GAIN  and the IF GAIN  reach levels dictated by the new mode of operation of the AGC unit  200 . The control system operates the AGC unit  200  to compensate for fast changes in signal power while minimizing distortion. It should be apparent to those of skill in the art that similar variations in the gains of the amplifiers would be appropriate during other transition periods. 
         [0043]    Some embodiments integrate the control system  108  with the circuitry of the demodulator  104  of the receiver  100 . Other embodiments implement the entire analog front end  102  as part of the demodulator  104  circuitry of the receiver. Yet other embodiments implement the AGC  200  as part of the demodulator  108 . Other combinations should be apparent to those of skill in the art. 
         [0044]    Variations in the implementation of the invention will occur to those of skill in the art. Illustratively, some or all of the generation and calculation of signals can be performed by application-specific or general-purpose integrated circuits, by discrete components, or in software. While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.