Abstract:
The present invention is directed to systems and methods for providing an AGC circuit for maintaining a constant output power level from an amplifier. More specifically, the AGC circuit includes a circuitry for determining whether an input signal is a QAM or a CW signal. A QAM/CW gain switch is then controlled depending upon the input signal. Depending upon the mode of the QAM/CW gain switch, the AGC circuit either attenuates the power level of the signal or bypasses the signal. The bypassed or attenuated signal is then compared to a reference signal so that the AGC circuit produces an adjusting voltage accordingly. The amplifier finally receives the adjusting voltage and attenuates the output power level of the signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to amplifiers, and more specifically to automatic gain control circuits. 
       BACKGROUND OF THE INVENTION 
       [0002]    Amplifiers include automatic gain control (AGC) circuits to ensure that the power level output signal remains constant. Typically, a quadrature amplitude modulation (QAM) modulated signal is transmitted from a headend facility and transmitted downstream to amplifiers for amplification and subsequently to subscriber equipment for final processing. An amplifier having a single pilot QAM AGC detector typically includes a bandpass filter (BPF) that has a 3 dB (decibel) bandwidth narrower than the QAM signal bandwidth, which is transmitted in a channel, to ensure that the BPF has a sufficient out-of-band rejection to reject the signal in the adjacent channels. 
         [0003]    At times, however, the QAM modulation is turned off resulting in a continuous wave (CW) signal, which has, in concept, a 3 dB bandwidth of 0 Hz (Hertz). So when the QAM modulation is turned off, the signal power level filtered by the BPF and sensed by the AGC circuit is somewhat higher than when the QAM modulation is on. The exact amount depends on the system requirement and the design of the filter, but it is typically between 1 and 6 dB. Consequently, since the AGC circuit determines the output power level within the entire channel, the amplifier then incorrectly adjusts the power level lower by the same amount, e.g., 1-6 dB. Therefore, what is needed is a system and method for detecting the on and off condition of the QAM modulation as well as maintain the desired output power level of the amplifier regardless whether the QAM modulation is on or off. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. In the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0005]      FIG. 1  is a block diagram of a communications system that typically includes amplifiers for boosting the power level of signals during transmission to subscriber equipment, such as set-top boxes, cable modems, etc. 
           [0006]      FIG. 2  is an electrical block diagram of a conventional amplifier that includes an AGC circuit. 
           [0007]      FIG. 3  is an electrical block diagram of the conventional QAM AGC circuit of  FIG. 2 . 
           [0008]      FIG. 4  is an electrical block diagram of an AGC circuit for adjusting the power level of an amplifier in accordance with the present invention. 
           [0009]      FIG. 5  illustrates the QAM/CW gain switch in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Preferred embodiments of the invention can be understood in the context of a broadband communications system. Note, however, that the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. All examples given herein, therefore, are intended to be non-limiting and are provided in order to help clarify the description of the invention. 
         [0011]    The present invention is directed towards an AGC circuit that detects a power level of a signal at the output of an amplifier. The AGC circuit then adjusts the power level according to a reference power level so that the output power level of the signal remains constant. An advantage of the present invention is that the AGC circuit is capable of dynamically discriminating between both a QAM modulated signal and a CW signal. In this manner, the AGC circuit of the present invention correctly adjusts the output power level regardless of the input signal. 
         [0012]      FIG. 1  is a block diagram of a communications system  100  that typically includes amplifiers  110  for boosting the power level of signals during transmission to subscriber equipment  115 , such as set-top boxes, cable modems, etc. The signals are generally QAM modulated by headend equipment  120  prior to transmission to optical nodes  125 , which convert the optical signals into electrical signals. The amplifiers  110  then amplify the QAM modulated signals received from the optical nodes  125  and further transmit the amplified signals downstream. 
         [0013]      FIG. 2  is an electrical block diagram of a conventional amplifier  110  that includes a QAM AGC circuit. The AGC circuit  205  is capable of monitoring a QAM modulated pilot carrier signal in the forward signals. The DC voltage values of the pilot carrier signals include multi-level data having peak values at varying times and different rates. Initially, the forward signals are transmitted through one or more gain stages  210  for amplifying the forward signal. A Bode circuit  215  varies the signal level by attenuation in accordance with the amplifier design. A final output gain stage  220  subsequently processes the forward signal, if necessary, which is then transmitted to an output port  225  for further transmission downstream. The output of the final gain stage  220  is also coupled to the AGC circuit  205  that is used to further control the attenuation of the Bode circuit  215  in response to the power level of the amplified forward signal. 
         [0014]      FIG. 3  is an electrical block diagram of the conventional QAM AGC circuit  205  of  FIG. 2 . The AGC circuit  205  includes a bandpass filter (BPF)  305  and one or more gain stages  310  for filtering and amplifying, respectively, the pilot signal. The filtered pilot signal is then transmitted to a radio frequency (RF) detector  315  that demodulates the signal to recover a baseband modulating signal. The demodulated baseband signal is then amplified through a video amplifier  320 . A buffer  325  and a resistor-capacitor (RC) filter  330  then average the baseband signal in order to establish a DC voltage that represents the average power level voltage of the QAM carrier. An error integrator  345  then compares the average voltage with a reference voltage  340  and provides an adjusting voltage to a loop filter  350  for filtering. The filtered adjusting voltage is provided to the Bode circuit  215  for attenuation as necessary so that the amplifier  110  continues to provide a stable signal having a desired power level. 
         [0015]    As previously mentioned, due to the constraints on adjacent channel rejection and the limitation of filter sharpness, a practical BPF  305  would have the passband bandwidth narrower than the symbol rate of the QAM signal. Therefore, the detected power is less than the full channel power, for example, 3 to 6 dB (decibel) less. Accordingly, this loss in detected power is designed into the AGC  205  so that an incorrect adjustment is not made. The AGC circuit  205  is then useful in adjusting a QAM modulated signal, but at times, it is advantageous to turn the QAM modulation off for amplifier balancing or trouble shooting. When the QAM modulation is turned off, the signal is a CW signal where the peak power equals the average power. When a CW signal is received, the QAM AGC  205  detects 3-6 dB more power as designed by the amplifier than compared to when the QAM modulation is on and incorrectly adjusts the output power level 3-6 dB lower. 
         [0016]      FIG. 4  is an electrical block diagram of an AGC circuit for adjusting the power level of an amplifier regardless of input signal in accordance with the present invention. An input signal, which may be either a QAM modulated signal or a CW signal, is received at the BPF  305 . The BPF  305  filters the input signal and allows a predetermined pilot carrier signal to pass through to a first gain stage  310 . The gain stage  310  amplifies the signal to some power level depending upon many factors, such as the location of the amplifier in the system  100 . A QAM/CW gain switch  405 , which is discussed further below, receives the amplified signal and forwards it to the second gain stage  310  for further amplifying, if necessary. The RF detector  315  demodulates the signal to recover a baseband modulated signal. The demodulated baseband signal is then amplified through the video amplifier  320 . 
         [0017]    In accordance with the present invention, a peak detector  410  samples pulses of the demodulated signal to establish a peak voltage, which is then provided to a comparator  415 . Concurrently, a buffer  420  and an RC filter  425  also receive the demodulated signal. The buffer and RC filter combination averages the demodulated signal and provides the comparator  415  an averaged voltage. The comparator  415  then determines from the two received signals whether a QAM signal or a CW signal is present. More specifically, if a QAM signal is present, the peak voltage detected by the peak detector  410  will be greater than the average voltage provided by the buffer and RC filter combination. Furthermore, if a CW signal is present, the peak voltage is equal to the average voltage. It will be appreciated that the comparator  415  can be implemented using either hardware, software, or a combination of both. 
         [0018]      FIG. 5  illustrates the QAM/CW gain switch  405  in accordance with the present invention. When the comparator  415  determines that the input signal is a QAM modulated signal, the comparator  415  positions the QAM/CW gain switch  405  in a first mode  505 . When the comparator  415  determines that the input signal is a CW signal, the comparator  415  positions the QAM/CW gain switch  405  in a second mode  510 . Depending upon the position of an internal switch  515 , there is a difference in the adjusted power value of the input signal. The QAM/CW gain switch  405  accommodates the differences in measured power levels of the channel regardless of the input signal. 
         [0019]    More specifically, when the input signal is a QAM modulated signal, the internal switch  515  is closed ensuring a additional 0 dB of gain. The error integrator  345  receives the averaged signal having a 0 dB of additional gain from the output of the RC filter  425  and compares the power level of the averaged signal with the reference output power level  340 . The error integrator  345  then provides an adjusting voltage accordingly to a loop filter  350  for filtering. The filtered adjusting voltage is provided to the Bode circuit  215  so that the amplifier continues to provide a stable signal having a desired power level. 
         [0020]    On the other hand, when the input signal is a CW signal, the internal switch  515  is opened forcing the input signal through a resistor  520  having some attenuation value ensuring that there is, for example, a 3 dB loss. It will be appreciated that the dB attenuation value can be adjusted, for example, some value between 3 and 6 dB, depending upon the design of the amplifier. The error integrator  345  receives the averaged signal having a 3 dB loss from the output of the RC filter  425  and compares the power level of the averaged signal with the reference output power level  340 . The error integrator  345  then provides an adjusting voltage accordingly to a loop filter  350  for filtering. The filtered adjusting voltage is subsequently provided to the Bode circuit  215  so that the amplifier continues to provide a stable signal having a desired power level. 
         [0021]    Therefore, an AGC circuit has been described that can receive both a QAM modulated and CW input signals while providing accurate output power levels. While this invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention, as set forth herein, are intended to be illustrative and not limiting. Various changes may be made without departing from the truth and the full scope of the invention as defined by the following claims.