Abstract:
A system for providing automatic gain control (AGC) comprises a signal path with an RF input, a plurality of power detectors in communication with the signal path, each of the power detectors operable to measure a total broadband power level of a signal in the signal path, each of the power detectors positioned to monitor a point in the signal path corresponding to a change in signal bandwidth, a control system operable to receive from each of the power detectors information associated with the power level and to adjust attenuation in the signal path in response to the information to achieve desired gain control.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to co-pending and commonly-assigned U.S. patent application Ser. No. 11/442,643, filed concurrently herewith, entitled, “DIGITAL ATTENUATOR CIRCUITS AND METHODS FOR USE THEREOF,” the disclosure of which is hereby incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates, in general, to Automatic Gain Control (AGC) in a system with an RF signal input and, more particularly, to AGC for broadband tuners. 
     BACKGROUND OF THE INVENTION 
     Currently, most television (TV) tuners employ narrow tracking filters near the their signal inputs which are used to diminish, or at least significantly limit, undesired portions of signals. Such filters generally are approximately 20 MHz bandpass filters and adjustable to pass a signal of interest. Placement of the filters near a signal input allows for a significant number of unwanted bands to be substantially removed before the signal reaches a demodulator as a narrow band signal. A narrow-band power measurement device at the baseband input to the demodulator is then used to provide Automatic Gain Control (AGC) by measuring the narrow band signal power and adjusting gain (usually at the signal input). The demodulator is usually located near the output of the signal path. While it might seem at first that detecting signal power near the output would allow signal power levels closer to the input to fluctuate without adequate control, the use of the narrow tracking filters can help to reduce power in a significant portion of the spectrum before the signal reaches amplifiers and other circuitry that are prone to distortion. 
     A disadvantage of narrow tracking filters is that they are not easily accommodated on semiconductor chips. In fact, with current technology, prior art systems have yet to implement narrow tracking filters on silicon because of the large inductors required in narrow tracking filter designs. Prior art systems that perform AGC by measuring narrow-band power near the output suffer from input power problems when narrow tracking filters are not present. For example, an RF signal input may include information on many channels, and the demodulator focuses on only one of those channels at a time. Thus, the narrow-band power measurement device adjusts the gain according to the measured power of whichever signal the demodulator is locked on to. However, the narrow-band power measurement device will not detect the power in the other channels. In other words, the AGC loop bases its determination on output power in the narrow band of interest but does not account for the full power spectrum of the input signal. As a result, high-power signals in those undesired channels are not controlled and can cause distortion in circuits that are close to the input, such as amplifiers. 
     One solution has been to implement narrow tracking filters separate from other parts of a tuner that are on a silicon chip. However, implementing large parts of a chip tuner off-chip can be expensive and defeats the purpose of integration. Further, implementing the narrow filters off-chip generally fails to provide an acceptable broadband input return loss, making such designs undesirable for cable applications. Currently, the prior art offers no substitute for narrow tracking filters. As a result, integration of tuner systems on chips has been impeded. 
     The AGC systems described above generally use analog attenuators to control the gain. Analog attenuators usually provide a continuous range of attenuation, and, therefore, a system can usually achieve a continuous range of signal power levels. A control system can then settle the signal power level at or very near a single reference power level. Engineers have been reluctant to try digital attenuators due to concern that discrete levels of attenuation offered by such attenuators would not allow a control system to settle, since there would be a small amount of error in a detected signal power level versus a reference signal power level. Accordingly, prior art AGC systems lack digitally-controlled attenuation. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to systems and methods which provide AGC with respect to tuners having broadband input. In one example system, a plurality of broadband power detectors are placed in the signal path at points that correspond to changes in bandwidth of the signal, for example, after filters. The broadband power detectors are capable of detecting the total power for the signal at their respective points from desired and undesired signals. Further, an embodiment of the system includes a plurality of attenuators that are distributed throughout the signal path. The measurements from the broadband power detectors are sent to a central control system that determines if attenuation should be adjusted. When the control system adjusts attenuation, it increases or decreases attenuation at one or more of the distributed attenuators. 
     In one example embodiment, the attenuators are digital attenuators that provide discrete levels of attenuation in the signal path. The central control system then compares signal power at each power detector to one or more ranges (“windows”) of power levels and attempts to bring the signal power within the windows. Thus, the control system can settle in a more stable manner than if signal power was compared to a single reference power level. 
     One example embodiment includes a state machine that measures signal power at one broadband power detector at a time and attempts to settle the measured power. After the power measured by the first broadband power detector is settled in a defined power window, the state machine moves to the next broadband power detector and attempts to settle the power detected thereby. 
     Various embodiments use one or more techniques to distribute attenuation in the signal path. In addition to placing attenuators throughout the signal path, some embodiments increase or decrease attenuation to a specified level in one attenuator and then move to the next attenuator, where such embodiments continue to increase or decrease attenuation to a specified level before moving once again to another attenuator. Such embodiments then cycle back to the first attenuator, if desired, to further increase or decrease attenuation. Such embodiments distribute attenuation changes throughout the signal path during gain control operations. Distribution of attenuation may help to ensure that signal power is not left uncontrolled at various parts of the signal path. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an illustration of an exemplary system adapted according to one embodiment of the invention; 
         FIG. 2  is an illustration of an exemplary system adapted according to one embodiment of the invention for providing AGC in a RF tuner; 
         FIG. 3  is an illustration of an exemplary peak detector adapted according to one embodiment of the invention; 
         FIG. 4  is an illustration of an exemplary digital attenuator; 
         FIGS. 5A-5B  are an illustration of an exemplary method for providing AGC in the system of  FIG. 2 ; and 
         FIGS. 6A and 6B  are illustrations of an exemplary method adapted according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is an illustration of exemplary system  199  adapted according to one embodiment of the invention. System  199  is a Radio Frequency (RF) tuner with Automatic Gain Control (AGC). In some embodiments, some or all of the components of system  199  may be disposed on a same semiconductor chip. However, in other embodiments, various components, including filter  140 , may be off-chip. Single-chip systems, multi-chip systems, and non-chip systems are within the scope of the invention. 
     The signal path includes front-end attenuator  100  that receives an RF input signal, low-noise amplifier (LNA)  110 , filter  120 , up conversion mixer  130 , filter  140 , down conversion mixer  150 , and digital attenuator  155 . Although not shown in these figures, the signal path continues on to other components, such as amplifiers, filters, and one or more demodulators or other baseband processing circuits. The invention is not limited to the particular components shown in  FIG. 1 , as a signal path with AGC can be implemented in a variety of configurations. 
     A plurality of broadband power detectors  160 ,  170 ,  180  are in communication with the signal path at various points. As used herein, “broadband” refers to a signal that includes one or more channels in addition to any channels that are processed by a demodulator or other baseband processing circuit at a given time, without regard to residual power from channels that have been substantially filtered out. For example, in a TV tuner that has a single-channel demodulator, a broadband signal includes two or more TV channels. Power detector  160  is placed after LNA  110  (LNA  110  includes filter circuitry, not shown), power detector  170  is placed after filter  120 , and power detector  3  is placed after mixer  150  and filter  140 . The placement of each of power detectors  160 ,  170 ,  180  of the illustrated embodiment is at a point that corresponds to a bandwidth change of the signal, and each of power detectors  160 ,  170 ,  180  measures the total power in the signal at each respective placement point. Thus, while LNA  110  will change the power and frequency make-up of the RF signal, power detector  160  measures the total power of the RF signal at its respective point in the signal path. The same is true for detectors  170  and  180 . 
     System  199  also includes control block  190 . Control block  190  receives measured power levels from detectors  160 ,  170 ,  180  and controls the attenuation by adjusting digital attenuators  100  and/or  150 . Control block  190  can be analog or digital. Control block  190 , in this example, includes user inputs, allowing a user to specify, for example, optimal power levels at points in the signal path or takeover points for attenuators  100  and  150  (explained in mode detail below). 
       FIG. 2  is an illustration of exemplary system  299  adapted according to on embodiment of the invention for providing AGC in a RF tuner.  FIG. 2  adds detail to the embodiment shown in  FIG. 1 . In this example, the illustrated parts of system  299  are all disposed in a single semiconductor chip, except for filter  220 . However, as mentioned above, various embodiments of the invention are not limited to being single-chip, multi-chip, partly-chip, or non-chip. In alternative embodiment, filter  220  may also be placed the same chip as the other components. 
     At the top of system  299  is shown the signal path that includes, beginning at RF input, digital attenuator  200 . Attenuator  200  provides a maximum of 36 dB attenuation in approximately 0.05 dB steps. The signal path also includes LNA  205 , which has bandpass filtering, thereby amplifying the signal and changing its bandwidth. Digital attenuator  210  follows LNA  205 , and it provides 6.4 dB of attenuation at 0.05 dB steps. Up-conversion mixer  215  receives the signal from attenuator  210 , mixes the signal, and sends the signal off-chip to Intermediate Frequency (IF) filter  220 . The signal then returns to the chip where it is fed to image-reject down-conversion mixer  225 . Digital attenuator  230  provides a maximum of 12.8 dB attenuation at 0.05 dB steps, which is followed by fixed gain amplifier  235  to the output. 
     System  299  includes two broadband peak detectors (i.e., a type of power detector—it should be understood that various embodiments can employ other kinds of power detectors, such as Root Mean Square power detectors and the like alternatively to or in addition to peak detectors)  240  and  245 . Each peak detector  240  and  245  is in communication with the signal path in a point that corresponds to a change in signal bandwidth. For instance, peak detector  240  is placed after LNA  205 , which includes bandpass filtering; thus the bandwidth of the signal is changed by LNA  205  before it is detected by peak detector  240 . Also, peak detector  245  is placed after filter  200 . The output of each peak detector  240  and  245  is a voltage signal that is proportional to the power detected. 
     The output of peak detector  240  is fed to comparators  250 , which compare the output of peak detector  240  to a plurality of levels, here five separate voltage levels. Similarly, the output of peak detector  245  is fed to comparators  260 , which compare the output of peak detector  245  to a plurality of levels, here five voltages, which may be the same or different from the voltages compared to the output of detector  240 . 
     System  299  also includes control logic block  270 , which controls the attenuation in the signal path through use of counter circuits  275 ,  280 , and  285 , as explained in more detail below. Digital words output from counter circuit  275  control attenuator  200 . Words output from counter  280  control attenuator  210 , and words from counter  285  control attenuator  230 . Each of counter circuits  275 ,  280 , and  285  are driven by programmable clock generator  290 . 
     Since attenuators  200 ,  210 , and  230  are digital, they each provide attenuation in discrete steps rather than in a continuous range. Accordingly, a control system that simply compares signal power to a single reference value may have trouble settling, since discrete attenuation will rarely (if ever) cause the signal power to be the same as the reference value. By contrast, system  299  uses each comparators  250  and  260  to determine if the voltage signals output by detectors  240  and  245  fall into one or more respective windows defined by VL, VML, VM, VMH, and/or VH. In one example, attenuation is adjusted so that the output from each detector  240  and  245  falls within the window defined by VML and VMH. In another example, attenuation is adjusted so that the output from each detector  240  and  245  falls within the window defined by VL and VH. Wider power windows tend to reduce the amount the adjusting that occurs; however, they also tend to provide less precise gain control. In each such example, once the power level from one or both of detectors  240  and  245  is within an acceptable defined range, attenuation changes may be discontinued until the power level either decreases or increases to the extent that it leaves the window. 
       FIG. 3  is an illustration of exemplary peak detector  300  adapted according to one embodiment of the invention. In this example, signal power is sensed at V inn  and V inp . V out  is sent to comparators, and it is proportional to the total broadband signal power that it senses at its associated point in the signal path. The range of power that is detected by peak detector  300  can be based, in part, on how large the power windows are. For instance, if a power window of interest is 8 dB, peak detector  300  may be designed to accurately measure in a 10 dB range. In one example, the optimum power level for a signal at a point sensed by detector  300  is known, or at least approximately known based upon based upon some considerations. For example, optimum power level for a signal at a given point may be based upon a power level that is within a linear operating range of a subsequent circuit element The voltage output by peak detector at the optimum power is then defined in the system as VM. The corresponding comparators may then be programmed accordingly to set VM and applicable windows. 
       FIG. 4  is an illustration of an exemplary digital attenuator  400 . The attenuation provided by attenuator  400  is a function of its resistance. The resistance is a function of the number of discrete, resistive legs  401 - 403  that are switched “on” at a given time. The switching of resistive legs  401 - 403  is controlled by digital control lines. In this example, the digital control lines are in communication with a counter (e.g., counter  275  of  FIG. 2 ), and each digital control line corresponds to one bit of the word output from the counter. This is an example technique that employs counters to control an attenuation circuit. Other examples of appropriate digital attenuator circuits may be found in U.S. patent application Ser. No. 11/442,643, filed concurrently herewith, entitled, “DIGITAL ATTENUATOR CIRCUITS AND METHODS FOR USE THEREOF.” 
     The embodiments illustrated in  FIGS. 1 ,  2 ,  3 , and  4  are merely examples, and other ways of arranging a tuner according to the invention are possible. For instance, different ranges for attenuators, different types of attenuators, different signal path configurations, different power windows, different broadband power detectors, and the like are all possible. In fact, principles of the invention are applicable in general to broadband tuners and are not limited to any specific design. 
       FIGS. 5A-5B  are an illustration of exemplary method  500  for providing AGC in system  299  ( FIG. 2 ). Method  500  is merely exemplary, as other system designs may use different methods for operation. Additionally, other methods for operation may be used in system  299 . 
     The basic flow of method  500  is one that first settles the power measured by peak detector  240  in a power window, then settles peak detector  245 . Once the power levels measured by both of peak detectors  240  and  245  are settled, the method moves to a rest state where signal power is detected and compared to power windows, but no adjustments are made to attenuation. The method may also advance to the rest state if all of the attenuators are at a maximum or a minimum and no change to attenuation is possible at the given signal level. The method moves from the rest state if the signal power level is outside of a power window for a given peak detector  204  or  245  and it is possible to change the attenuation. 
     Steps  501 - 508  operate generally to cause the power detected by peak detector  240  to settle in a power window defined by VML and VMH. In step  501 , total signal power is measured at peak detector  240 . Peak detector  240  outputs a voltage signal that is indicative of the power level. In step  502 , the signal power level is compared to one or more power windows at comparators  250 . In this example, the output voltage signal is compared to a voltage window defined by VML and VMH. A step  503 , a determination is made if the signal power is within the window. If it is, method  500  then advances to an operation that settles the power detected by peak detector  245 . If the signal power level is not within the power window, then it is determined if the power is below the power window (i.e., below VML) at step  504 . If the power level is below the power window, then it is determined whether attenuators  200  and  210  are at the lowest attenuation possible at step  505 . If attenuators  200  and  210  are at the lowest attenuation possible, then method  500  advances to an operation that settles the power detected by peak detector  245 . If it is still possible to adjust attenuation down, then attenuation is incremented down in step  506 . 
     System  299 , in this example, increments attenuation in a specific way. Each attenuator  200 ,  210 , and  230  is controlled by AGC control logic  270  that causes counters  275 ,  280 ,  285  to output digital words to respective attenuators  200 ,  210 , and  230 , thereby adjusting the attenuation of each. AGC control logic takes action based on the signal power level comparison (step  502 ) at time periods based on the frequency of clock  290 . The time periods allow for the signal to “settle out” before subsequent adjustments are performed. 
     Further, as a result of the comparing, AGC control logic  270  causes one of counters  275  or  280  to count up a single step or down a single binary step, and only one counter is adjusted at a time. In this example, a counter value of ten zeroes at counter  275  causes the lowest level of attenuation for attenuator  200 , and a value of ten ones represents the highest level of attenuation. When it is determined by AGC control logic  270  that the detected power is below the power window, then AGC control logic  270  outputs a mode that specifies that the next counter adjustment should be down. The appropriate counter  275  or  280  is then adjusted at the next clock cycle. 
     Attenuation is distributed in system  299  in order to even out signal levels in the signal path so as to avoid large power level differences between the front end and back end of the signal path. In this example, when attenuation is incremented down (step  506 ), attenuator  200  is incremented first. This may be repeated until attenuator  200  reaches its bottom rollover point (i.e., counter  275  is at all zeros), when AGC control logic  270  begins incrementing attenuator  210  down. Method  500  returns to step  501  after each incrementation. 
     Returning to step  504 , if it determined that the signal power is not below the power window (i.e., it is above VMH), method  500  progresses to step  507 , where it checks whether both of attenuators  200  and  210  are at highest attenuation (i.e., counters  275  and  280  are at high rollover—all ones). If both attenuators  200  and  210  are at high rollover, then method  500  advances to an operation that settles the signal power detected by detector  245  in a window. If attenuators  200  and  210  are not at high rollover, then attenuation is incremented up in step  508 . 
     Incrementing attenuation up is performed, in this example, in a specific way. Similar to the operation described above, attenuation is adjusted by incrementing counters  275  and  280  one at a time and at clock cycles. In step  508 , when incrementing attenuation up, AGC control logic  270  increases the attenuation of attenuator  210  first. Once attenuator  210  reaches a takeover point, then AGC control logic  270  begins increasing the attenuation of attenuator  200 . Takeover points can be defined by values of counters  275 ,  280 , and  285 . In this example, a takeover point of attenuator  210  can be set to be a value between all zeros of counter  280  and all ones, for instance, the base-ten value sixty-three. AGC control logic  270  increments attenuator  210  until a takeover point is reached. Then, AGC control logic  270  begins to increase the attenuation of attenuator  200 . In this example, attenuator  200  has no takeover point for simplicity. Once the high rollover point of attenuator  200  is reached, then AGC control logic  270  begins increasing the attenuation of attenuator  210  once more. In this manner AGC control logic  270  increments attenuator  210  up to at least a portion of its maximum value, adjusts attenuator  200  up until it reaches the maximum attenuation of attenuator  200 , then once again increase the attenuation of attenuator  210  (if need be) until its maximum attenuation value is reached. Method  500  returns to step  501  after each incrementation. The use of takeover points helps to increase the distribution of attenuation throughout the signal path. 
     Once the signal power level measured by peak detector  240  is settled in a power window (or if both of attenuators  200  and  210  are at rollover points and no more adjustment is possible), method  500  moves to an operation wherein the signal power level detected by peak detector  245  is settled. At step  510 , the signal power level is measured at peak detector  245 . The power level is compared to a power window by comparators  260 . 
     If it is determined in step  512  that the signal power level is within the window, then method  500  moves to a rest state to be described further below. The window may be the same or different than that described above with regard to peak detector  240 . In this example, for simplicity, the power window is defined by VML and VMH. If it is determined that the signal power level is outside of the power window, then method  500  moves to step  513  where it is determined if the signal power level is below the power window. 
     If the power level detected by peak detector  245  is below the power window, then it is determined whether all of attenuators  200 ,  210 , and  230  are at their lowest attenuations. If all of attenuators  200 ,  210 , and  230  are at low rollover, then method  500  moves to the rest state. If one or more of attenuators  200 ,  210 , and  230  can be adjusted down, then attenuation is incremented down in step  515 . 
     Incrementing down in step  515  is performed in a specific way, in this example. As described above, only one attenuator  200 ,  210 , and  230  is adjusted at a time at single binary steps, and adjustments are performed at clock cycles of clock  290 . In response to power levels detected at peak detector  245 , AGC control logic  270  adjusts attenuator  200  down first, then adjusts attenuator  210  down if and when attenuator  200  reaches its low rollover point. AGC control logic  270  then adjusts attenuator  230  once attenuator  210  reaches its minimum. Method  500  returns to step  510  after each binary incrementation. 
     Returning to step  513 , if it is determined that the signal power level is not below the power window (i.e., it is above VMH), then it is determined whether all of attenuators  200 ,  210 , and  230  are at their maximum values in step  516 . If each attenuator  200 ,  210  and  230  is at its high rollover point, then method  500  moves to a rest state. If the attenuation of one or more of attenuators  200 ,  210 , and  230  can be increased, then attenuation is incremented up in step  517 . 
     Incrementing attenuation up is performed in a specific way in this example. As described above, only one attenuator  200 ,  210 , and  230  is adjusted at a time at single binary steps, and adjustments are performed at clock cycles of clock  290 . Both of attenuators  210  and  230  have takeover points. In response to power levels detected at peak detector  245 , AGC control logic  270  adjusts attenuator  230  up first, then adjusts attenuator  210  up if and when attenuator  230  reaches its low rollover point or a takeover point. AGC control logic  270  adjusts attenuator  200  up if and when attenuator  210  reaches its high rollover point or a takeover point. AGC control logic  270  then increases the attenuation of attenuator  230  once again if attenuator  200  reaches its maximum. Accordingly, AGC control logic  270  increases the attenuation of attenuator  210  once again if and when attenuator  230  reaches its maximum or another takeover point. In this manner, incrementing attenuation up is performed at least partly at attenuator  230 , then at attenuator  210 , then at attenuator  200 , and it returns to adjusting attenuator  230  up. As mentioned above, using takeover points helps to distribute attenuation throughout the signal path. Method  500  returns to step  510  after each binary incrementation. 
     In the example above, the use of takeover points was described only with regard to increasing attenuation; however, various embodiments of the invention are not so limited. For instance, when adjusting attenuation down, AGC control logic  270  can switch to decreasing the attenuation of attenuator  230  when the takeover point of attenuator  210  has been reached through incrementally decreasing attenuation. Further, each of attenuators  200 ,  210 , and  230  may have one or more takeover points, so that a given attenuator is not necessarily limited to having one or zero takeover points. Takeover points may be set in a number of ways. For instance, takeover points may be hardwired in logic of AGC control logic  270 , calculated by AGC control logic  270  according to an algorithm, determined by user input into AGC control logic  270 , or the like. 
     Once the signal power level measured by peak detector  245  is settled in a power window (or if all of attenuators  200 ,  210 , and  230  are at rollover points and no more adjustment is possible), method  500  moves to a rest state. At step  520 , clock  290  is stopped, though measuring of the signal power levels at peak detectors  240  and  245  continues. Step  521  defines when method  500  leaves the rest state. If method  500  is in the rest state as a result of power levels at both peak detectors  240  and  245  being settled in power windows, then method  500  will turn clock  290  on and return to step  504  if the power level detected at peak detector  240  leaves the power window. Similarly, method  500  will turn clock  290  on and return to step  513  if the power level detected at peak detector  245  leaves the power window. 
     If method  500  is in the rest state because all of attenuators  200 ,  210 , and  230  are at rollover points, then method  500  returns to step  504  if the power level detected at peak detector  240  is outside of the power window and adjusting the attenuation accordingly would not cause rollover in one of attenuators  200  or  210 . Similarly, method  500  returns to step  514  if the power level detected at peak detector  245  is outside of the power window and adjusting the attenuation accordingly would not cause rollover in one of attenuators  200 ,  210  or  230 . 
     An alternative way to describe the AGC operation of system  299  is to illustrate it as an operation of a state machine executed by AGC control logic  270 .  FIGS. 6A and 6B  are illustrations of exemplary method  600  adapted according to one embodiment of the invention. Equations in  FIG. 6B  explain the relations and variables of the states in  FIG. 6A . 
     Method  600  starts out at state 00. In state 00, AGC control logic  270  operates to settle the power detected by peak detector  240  in a power window defined by VML and VMH. Method  600  changes from state 00 if the signal power is settled in the window or if both of attenuators  200  and  210  are at rollover point so that adjusting attenuation further is not possible. 
     Method  600  moves to state 01, which is not used. Accordingly, method  600  advances to state 11. In state 11, AGC control logic  270  operates to settle the power detected by peak detector  245  in a power window defined by VML and VMH. Method  600  changes from state 11 if the signal power detected by peak detector  245  is settled in the window or if all of attenuators  200 ,  210 , and  230  are at rollover point so that adjusting attenuation further is not possible. 
     Another condition that will cause method  600  to advance from state 11 is when the power level detected by one of detectors  240  or  245  is settled in its respective window and the power detected by the other of detectors  240  or  245  is below its respective window and it is not possible to decrease attenuation further. The state machine does not advance from state 11 when the power level detected by one of detectors  240  or  245  is settled in its respective window and the power detected by the other of detectors  240  or  245  is above its respective window. 
     Method  600  advances to state 10 from state 11. State 10 is the idle state or rest state, and AGC control logic  270  turns off clock  290 . Peak detectors  240  and  245  and comparators  250  and  260  are operating so that AGC control logic  270  monitors signal power level. If the signal power detected by either of peak detectors  240  or  245  falls outside of its respective window, then method  600  advances to either state 00 or state 11 (unless, of course, adjusting attenuation is not possible because attenuators  200 ,  210 , and  230  are at rollover points). It is possible to define a different widow for monitoring at each of detectors  240  and  245  in state 10 than the windows used in states 00 and 11. For instance, it is possible to perform method  600  so that it advances to either of states 00 or 11 when the signal power detected by either of peak detectors  240  or  245  falls outside of VH and VL. Widening the window may increase the time it takes to advance from state 10. 
     The embodiments illustrated in  FIGS. 5 and 6  are merely examples, and other methods of performing AGC according to the invention are possible. For instance, different takeover points for attenuators, different orders for measuring power from peak detectors, different numbers and placements of peak detectors, and the like are all possible. In fact, principles of the invention are applicable in general to broadband tuners and are not limited to any specific technique. 
     Various embodiments of the invention may include one or more advantages over prior art solutions. For instance, some prior art systems reduce the power from unwanted or unmonitored bands through the use of narrow band filters. In some embodiments of the present invention, though, it is possible to reduce or eliminate the need for narrow band filters in the signal path, since the broadband power detectors (e.g., detector  245  of  FIG. 2 ) allow a control loop to react to total broadband signal power. 
     Trading narrow band filters for broadband power detectors may provide for smaller, faster, and cheaper systems. For instance, since it can be difficult to create narrow band filters on semiconductor substrates, embodiments that reduce or eliminate the need for such filters can be used in systems that have a higher degree of integration. 
     Further, designers in the past have been reluctant to use digital attenuators out of a concern that it is difficult to cause an AGC system to settle since attenuation from such filters is in discrete steps, thereby causing some amount of error at each measurement. Various embodiments of the present invention solve the problem by comparing signal power to a reference window of levels rather than to a single reference value. Accordingly, a control system can settle. Further, various embodiments allow the reference window to be set by user input or by an algorithm, so that wider and narrower windows can be tailored for a given application. For instance, in a system wherein it is important to have slower state changes, the window can be widened. 
     Distributing attenuation throughout a signal path can provide a higher degree of control over that provided by prior art systems. For instance, various prior art systems detect signal power near the end of the signal path and adjust attenuation only near the input. However, the power detected in the signal near the output does not always represent the power near the input to an acceptable degree of accuracy. As a result, prior art systems may experience distortion caused by excessive signal power near the input, especially from bands that are not being demodulated at a given time. However, various embodiments of the present invention detect total broadband signal power at a plurality of points in the signal path and distribute attenuation throughout the signal path, thereby lessening the occurrence of distortion near the input. 
     Various embodiments of the invention perform attenuation adjustments based on a clock cycle designed to allow for some amount of delay between possible adjustments. The amount of delay can be increased or decreased to allow for the signal and the various circuits in the signal path to reach steady state, while still providing quick response times. Further, various embodiments adjust attenuation in small steps (e.g., 0.05 dB), by, for example, adjusting only one attenuator at a time. Small adjustments at a controlled rate can help a demodulator to remain locked on a signal as attenuation is adjusted. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.