Abstract:
An automatic gain control loop disposed in a receiver is adapted to compensate for varying levels of out of band interference sources by adaptively controlling the gain distribution throughout the receive signal path. One or more intermediate received signal strength indicator (RSSI) detectors are used to determine a corresponding intermediate signal level. The output of each RSSI detector is coupled to an associated comparator that compares the intermediate RSSI value against a corresponding threshold. The take over point (TOP) for gain stages is adjusted based in part on the comparator output values. The TOP for each of a plurality of gain stages may be adjusted in discrete steps or continuously.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims benefit under 35 USC 119(e) of U.S. provisional application No. 60/979,024, filed Oct. 10, 2007, entitled “A Technique For Optimizing Gain Partitioning In A Receiver”, the content of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A receiver system typically consists of a series of stages consisting of pre-selectivity gain and mixing, frequency selectivity (i.e. a filter) and post-selectivity gain and mixing. Conventional receivers either set a total system gain with a predetermined partition between pre- and post selectivity gain, or rely on a separate controller or demodulator to independently adjust pre and post selectivity gains to achieve the linearity/noise tradeoff. 
         [0003]      FIG. 1  is a simplified block diagram of a receiver  100 , as known in the prior art. In receiver  100 , amplifier  110  has a gain G 1  that provides pre-selectivity gain. Frequency converter  120 , which may be a mixer, provides frequency conversion. Filter D 1    130  is typically a bandpass filter adapted to filter out undesired signal. Amplifier  140  has a gain of G 2  and provides post-selectivity gain. A local oscillator (not shown) is often used to provide an oscillating signal to frequency converter  120 . Frequency converter  120 , and filter  130  typically have finite linearity and thus it is desirable to limit the range of signals that are coupled to them. 
         [0004]      FIG. 2A  shows a spectrum of exemplary signals received by filter  130 . The desired signal is shown as having the frequency Fd. The spectrum of the receives signals often includes undesired signal components (also referred to as blockers) shown as having frequencies Fb 1  and Fb 2  that interfere with the desired signal, causing non-linearity, distortion, etc. For example, the spacing and amplitude of the undesired signals Fb 1  and Fb 2  may result in a third order intermodulation distortion product at the output of amplifier  110 . As such, it is not desirable to place too much gain before filter  130  which is adapted to attenuate the blocker signals, as shown in  FIG. 2B . The reduction of the undesired signals enables amplifier  140  to amplify the desired frequencies in without substantially increasing the amplitudes of the undesired signals. 
         [0005]    By reducing the gain G 1  of amplifier  110 , the linearity is improved. Reducing the gain of the first amplifier  110  also reduces the amplitude of signal S 1 . To keep the amplitude of signal S 4  constant, gain G 2  may be increased. The gain redistribution between amplifiers  110  and  140  reduces distortion but also results in degradation of the signal-to-noise (SNR) ratio. Therefore a tradeoff exists between increasing the gain G 1  to improve signal to noise ratio, and degrading linearity performance of the system (increasing the distortion products in the signal) when blockers are present. 
         [0006]    Gains G 1  and G 2  are typically selected such that the total gain G 1 *G 2  is equal to a known value. In accordance with one conventional technique, for a given input signal level S 0 , a predetermined gain partitioning of G 1  and G 2  is used.  FIG. 3  is a block diagram of a conventional receiver  300  configured to achieve a predetermined gain partitioning of G 1  and G 2  using control signal T sys .  FIG. 4  shown plots of gains G 1 , G 2  and G 1 *G 2  (G sys ) for a receiver having predetermined gain partitions. 
         [0007]    In receiver  300 , the gains of the first and second amplifiers  110  and  140 , respectively, are controlled by gain controller  310  that controls the gains G 1  and G 2  in accordance with an algorithm that provides fixed gain partitioning using signal T sys .  FIG. 4  shows examples of the gain G 1  from amplifier  110 , gain G 2  from amplifier  140  as well as the products of these two gains. The attack point (AP) represents the signal level at which total gain G sys  begins to be fall. The take-over point (TOP) represents the signal level at which gain control is passed from signal T 2  to signal T 1 . The TOP and AP values are typically predetermined and fixed. In a typical television system, a demodulator is used to generate control signals T 1  and T 2 . 
         [0008]    In accordance with another conventional technique, the output signal of the second amplification stage is used to determine the gain partitioning.  FIG. 5  is a simplified block diagram of a receiver  500  having gain partitioning controlled by a demodulator  510 . Demodulator  510  is configured to controls the values of G 1  and G 2  depending on the presence and level of blockers. Demodulator  510  operates to control the partitioning of the gain between amplifiers  110  and  140  by sensing the output signal S 4  of second amplifier  140 . Demodulator  510  may be programmed to estimate whether blockers or other undesired signal components are causing distortion in the desired signal. Demodulator  510  then repartitions the gain by adjusting signals T 1  and T 2 . 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    An automatic gain control loop disposed in a receiver is adapted to compensate for varying levels of out of band interference sources by adaptively controlling the gain distribution throughout the receive signal path. One or more intermediate received signal strength indicator (RSSI) detectors are used to determine a corresponding intermediate signal level. The output of each RSSI detector is coupled to an associated comparator that compares the intermediate RSSI value against a corresponding threshold. The take over point (TOP) for gain stages is adjusted based in part on the comparator output values. The TOP for each of a plurality of gain stages may be adjusted in discrete steps or continuously. 
         [0010]    In accordance with the present invention, for a given receiver path gain defined, for example, by the product of the pre and post selectivity gains, the present invention provides a self-contained, compact apparatus and method for adjusting the partitioning between pre and post-selectivity gain to optimize the signal level entering the filter disposed in the receiver. The receiver is thus enabled to continuously trade off linearity against noise depending on the presence or absence of undesired signals (blockers) at other frequencies without relying on the intervention of an external controller or demodulator. 
         [0011]    A receiver, in accordance with one embodiment of the present invention includes, in part, a first amplification stage, a frequency conversion module responsive to the first amplification stage, a filter responsive to the frequency conversion module, a second amplification stage responsive to the filter, and a controller adapted to vary a gain of each of the first and second amplification stages in response to an output signal of the first amplification stage and further in response to an overall gain selected for the receiver. 
         [0012]    A receiver in accordance with another embodiment of the present invention includes, in part, a first amplification stage, a frequency conversion module responsive to the fist amplification stage, a filter responsive to the frequency conversion module, and a second amplification stage responsive to the filter. The receiver is adapted to vary the gains of the first and second amplification stages in response to a first and second feedback signals. 
         [0013]    In one embodiment, the first and second feedback signals are supplied by a controller responsive to signals representative of the output signals of the first and second amplification stages. In one embodiment, the controller is external to the receiver. In one embodiment, the controller is further responsive to the filter. In one embodiment, the receiver includes a third amplification stage. In such embodiments, the controller is further responsive to a third signal representative of the output signal of the third amplification stage. 
         [0014]    A method of controlling the gain of a receiver, in accordance with one embodiment of the present invention, includes, in part, amplifying a received signal to generate a first signal using a first amplification stage, frequency converting the first signal, filtering the frequency converted signal, amplifying the filtered signal to generate a second signal using a second amplification stage, and varying a gain of each of the first and second amplification stage in response to an output signal of the first amplification stage and further in response to an overall gain selected for the receiver. 
         [0015]    A method of controlling the gain of a receiver, in accordance with another embodiment of the present invention, includes, in part, amplifying a received signal to generate a first amplified signal using a first amplification stage, frequency converting the first amplified signal, filtering the frequency converted signal, amplifying the filtered signal to generate a second amplified signal using a second amplification stage, and varying a gain of each of the first and second amplification stage in response to first and second feedback signals. 
         [0016]    In one embodiment, the method further includes, in part, applying signals representative of the first and second amplified signals to a controller, and generating the first and second feedback signals in response to the signals applied to the controller. In one embodiment, the controller is external to the receiver. In one embodiment, the method further includes applying a signal representative of the filtered signal to the controller. In one embodiment, the controller is further responsive to a third amplified signal present in the receiver. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]      FIG. 1  is a simplified block diagram of a receiver, as known in the prior art. 
           [0018]      FIG. 2A  shows a spectrum of exemplary signals received by a filter disposed in a wireless communication receiver. 
           [0019]      FIG. 2B  shows the filtering characteristics of a filter adapted to attenuate the undesired signals shown in  FIG. 2A . 
           [0020]      FIG. 3  is a simplified block diagram of a receiver, as known in the prior art. 
           [0021]      FIG. 4  is a simplified gain diagram of an embodiment of amplifier gains in a system having a predetermined gain partition. 
           [0022]      FIG. 5  is a block diagram of a receiver, as known in the prior art. 
           [0023]      FIG. 6  is a simplified block diagram of a receiver, in accordance with one exemplary embodiment of the present invention. 
           [0024]      FIG. 7  is a simplified block diagram of a receiver, in accordance with another exemplary embodiment of the present invention. 
           [0025]      FIG. 8A ,  8 B and  8 C are examples of gain plots and gain partitioning for the receiver of  FIG. 7 . 
           [0026]      FIG. 9  is a flowchart of steps carried out to perform adaptive gain partitioning, in accordance with one embodiment of the present invention. 
           [0027]      FIG. 10  is a block diagram of a receiver, in accordance with one exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 6  is a block diagram of a receiver  600 , in accordance with one embodiment of the present invention. Receiver  600  is shown as including, in part, amplifiers  110 ,  140 , frequency converter  120 , filter  130  and sensor  610 . A local oscillator (not shown) provides an oscillating signal to frequency converter  120 . Frequency converter  120  may be a mixer, a multiplier, etc. Demodulator  510  may be external or internal to receiver  600 . Sensor  610  sense signal S 1  to determine the strength of the RF signal. Signal S 1  so sensed is supplied to demodulator/controller  510 . Also supplied to demodulator/controller  510  is signal S 4  that is generated by amplifier  140 . In response, demodulator/controller  510  generates signals T 1  and T 2  that are respectively applied to amplifiers  110  and  140  to control their gains. As see from  FIG. 6 , receiver  600  together with demodulator/controller  510  form a pair of control loops L 1  and L 2 , which are independently controlled by the demodulator/controller  510 . Loop L 1  is used to control gain G 1  via signal T 1 , and loop L 2  is used to control gain G 2  via signal T 2 . Demodulator/controller  510  may use any one of a number of different algorithms to vary the gains of amplifiers  110 , and  140  using signals T 1  and T 2 . 
         [0029]      FIG. 7  is a block diagram of a receiver  700 , in accordance with another embodiment of the present invention. Receiver  700  is similar to receiver  600  except that in receiver  700  signal T sys  applied to controller  710  includes information about the overall gain of the two amplification stages. Signal T sys  may be supplied by, e.g., a demodulator. Accordingly in receiver  700 , loop L 1  is used to determine G 1 . Controller  710  knowing the overall gain signal represented by signal T sys  sets the proper gain G 2  using signal T 2 . The gain partitioning of receiver  700  automatically partitions the gains G 1  and G 2  to achieve a desired gain Gsys specified by controller  710  based on input from a single control line Tsys. Because only one control line Tsys is required in receiver  700 , it is easy to implement. Furthermore, receiver  100  may be configured to adapt TOP to trade off linearity with signal to noise ratio depending on the level of blockers. Additionally, controller  710  may be exclusive of the demodulator and thus, controller  710  may be implemented on the same IC as the other elements of the receiver  700 . 
         [0030]      FIGS. 8A ,  8 B and  8 C illustrates an example of gain curves and gain partitioning for the variable gain partitioning receiver of  FIG. 7 .  FIG. 8A  shows the characteristics of the overall gain G sys  of receiver  700 . When signal S 1  exceeds a certain reference level, TOP is reduced until S 1  equals the reference or falls within a certain range of the desired reference, for example, to TOP 1 , as shown in  FIG. 8C . When S 1  falls below the reference, TOP is increased until S 1  once again equals the reference, for example, to TOP 2 , as shown in  FIG. 8B . 
         [0031]    Referring to  FIGS. 6 and 8 , controller  710  operates in the following manner. Assume that the desired channel signal S d  is nearly constant, but blocker levels are fluctuating, causing total signal S 1  to change. When sensor  610  detects that the total signal S 1  has exceeded an optimal reference level, loop L 1  is used to reduce the TOP, effectively reducing G 1  through T 1 . G 2  is increased through T 2  to maintain a constant G sys . Likewise, when sensor  610  detects that S 1  has dropped below the reference level, loop L 1  is used to increase the TOP, effectively increasing G 1  through T 1 . G 2  is decreased through T 2 , again maintaining constant G sys . The optimal reference level varies from application to application and can be programmed dynamically as the application changes. Hysteresis may be used to stabilize the circuit in a digital implementation. 
         [0032]    The receiver  700  of  FIG. 7  does not require an external controller or demodulator to optimize the gain partitioning, making the system very simple to interface with any demodulator, and any communication standard without the need for extensive software development. 
         [0033]    A practical digital implementation is presented in conjunction with the method  900  illustrated below. It provides discrete steps in TOP control and receives a digital S 1  signal. A circuit implementing the method  900 , such as the controller  710  of  FIG. 7 , can compare the input S 1  level to a reference level and increase or decrease a digital word controlling the TOP to compensate. The controller circuit can be clocked at a rate that can depend on the rate that the S 1  signal is being updated. 
         [0034]      FIG. 9  is a flowchart  900  of steps carried out to perform adaptive gain partitioning, in accordance with one embodiment of the present invention. The process begins at step  910  when S 1  (i.e., the output signal of the first amplification stage) value after the first gain stage is updated or upon the next iteration of the control loop if the S 1  value is continuously updated or updated at a rate faster than the rate of the control loop. The controller receives the updated S 1  value. 
         [0035]    At step  920  a determination is made as to whether the S 1  value is substantially the same as the predetermined reference level REF for the application that is presently active. If so, the controller proceeds to step  930  and determines if the S 1  value is less than a predetermined low reference level REFL. If so, the controller proceeds to step  970  and increases the Take-Over-Point, up to a predetermined TOP limit. 
         [0036]    If at step  930  the controller determines that S 1  is not less than the low reference level REFL, the controller instead proceeds to step  940  where the controller determines if S 1  is greater than the high reference level REFH. If not, the controller proceeds back to step  910  to await the next S 1  update without making any changes to the TOP. If, at step  940 , the controller determines that the RSSI is greater than the high reference level REFH, the controller proceeds to step  960  to decrease the TOP down to a predetermined lower limit. 
         [0037]    Referring to step  920 , if the controller determines that S 1  is not substantially equal to the reference level, the controller proceeds to step  950  to determine if S 1  is greater than the reference level. If so, the controller proceeds to step  970  to increase the TOP, but not to exceed the upper limit. If at step  950  the controller determines that S 1  is not greater than the reference level, the controller proceeds to step  960  to decrease the TOP but not smaller than a lower limit. The controller proceeds from either step  960  or step  970 , that is, after adjusting the TOP, back to step  910  to await the next S 1  update. 
         [0038]    It is understood that additional signal strength monitoring loops may be added in the signal path in order to detect which portion of the signal path is experiencing saturation first. Such capability may be useful for allowing the receiver to distinguish between blockers which are far from the desired signal or close to the desired signal. 
         [0039]    A close blocker is referred to as an N+/−1 blocker or adjacent channel blocker (that is, a blocker which is one channel above or below the desired channel N). Blockers further away in frequency are similarly labeled. In many receivers, an N+/−1 blocker may cause a portion of the signal path after mixing or filtering to limit receiver performance before the mixer saturates. A receiver is more susceptible to N+/−1 blockers because the (undesirable) third-order distortion products from these blockers are more severe at frequencies closer to the blockers. To remedy these problems, in accordance with one embodiment of the present invention, an adaptive gain partitioning receiver includes sensors in the signal path to allow the receiver to distinguish between close in blockers, such as N+/1, from N+/−2 and other blockers. 
         [0040]      FIG. 10  is a block diagram of a receiver  1000  that includes a pair of signal strength sensors.  810  and  820 . Receiver  1000  is thus similar to receiver  700  except that receiver  1000  senses strength of signals S 1  and S 3 . The overall gain of the receiver is defined by signal T sys  applied to controller  710 . Receiver  1000  thus detects when the weakest link in the signal path is being strained, and adjusts the gain partition(s) to relieve the strain on that link. In the N+/1 blocker case, S 3  will reach a level where its distortion from filter D 1  and other baseband circuits will begin to affect the signal before the signal S 1  becomes the dominant source of distortion. The controller  710  can decide to reduce the gain G 1  and compensate by increasing gain G 2 , thereby keeping S 3  below a predetermined threshold. Other filters and gain control mechanisms can be introduced in the signal path and controlled in a similar manner. 
         [0041]    The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. The invention is not limited by the number of subbands disposed in the diversity receiver. The invention is not limited by the type of integrated circuit in which the present disclosure may be disposed. Nor is the disclosure limited to any specific type of process technology, e.g., CMOS, Bipolar, or BICMOS that may be used to manufacture the present disclosure. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.