Patent Publication Number: US-2007105515-A1

Title: Apparatus and method for providing automatic gain control

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to automatic gain control (AGC) for apparatuses such as television signal receivers, and more particularly, to an apparatus and method for providing AGC that avoids excessive tuner gain reduction and compensates for interference from both analog and digital signals.  
      2. Background Information  
      Apparatuses such as television signal receivers use AGC to control the gain of a tuner in order to maintain the amplitude of the tuner&#39;s output signal at a relatively constant level. One problem associated with current AGC techniques may occur when a relatively weak desired signal is being received in the presence of much stronger undesired adjacent signals that overload the tuner and interfere with the reception of the desired signal.  
      The aforementioned problem is particularly applicable to television signal receivers capable of receiving both analog and digital signals. Prior to the introduction of digital television, adjacent channel frequencies were never assigned in the same geographical area. This practice, in the vast majority of cases, prevented interference from adjacent channel frequencies. With the introduction of digital television, however, it was required that adjacent channels be used such that both analog and digital signals could be transmitted during a transition period until virtually all television signal receivers are replaced with new units capable of digital reception. As a result, a relatively weak desired analog or digital television signal may suffer interference from stronger undesired adjacent analog or digital signals.  
      Known AGC techniques detect the presence of stronger undesired adjacent signals and compensate for them by reducing the gain of the tuner. In certain cases, however, the tuner gain may be reduced to a very low level such that the desired signal is below a critical level for proper demodulation. For example, in cases where the undesired adjacent signals are 20 to 40 dB stronger than the desired signal, known AGC techniques often reduce the tuner gain to a level that prevents proper demodulation. Known AGC techniques are also deficient in that they fail to make adequate provision for interference from both digital and analog signals.  
      Accordingly, there is a need for an apparatus and method for providing AGC that addresses the foregoing problems, and thereby avoids excessive tuner gain reduction and compensates for interference from both analog and digital signals. The present invention addresses these and/or other issues.  
     SUMMARY OF THE INVENTION  
      In accordance with an aspect of the present invention, signal processing apparatus is disclosed. According to an exemplary embodiment, the signal processing apparatus comprises tuning means for tuning an RF signal to generate an IF signal. First filtering means filter the IF signal to generate a filtered IF signal. AGC detecting means enables generation of an AGC signal for the tuning means responsive to the filtered IF signal. The AGC detecting means includes second filtering means for attenuating a predetermined carrier frequency.  
      In accordance with another aspect of the present invention, a method for providing AGC is disclosed. According to an exemplary embodiment, the method comprises steps of using a tuner to tune an RF signal to generate an IF signal, filtering the IF signal to generate a filtered IF signal, generating an AGC signal responsive to the filtered IF signal, wherein the generating step includes attenuating a predetermined carrier frequency, and providing the AGC signal to the tuner.  
      In accordance with still another aspect of the present invention, a television signal receiver is disclosed. According to an exemplary embodiment, the television signal receiver comprises a tuner operative to tune an RF signal to generate an IF signal. A first filter is operative to filter the IF signal to generate a filtered IF signal. An AGC detector is operative to enable generation of an AGC signal for the tuner responsive to the filtered IF signal. The AGC detector includes a second filter operative to attenuate a predetermined carrier frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:  
       FIG. 1  is a block diagram of signal processing apparatus according to an exemplary embodiment of the present invention;  
       FIG. 2  is a schematic circuit diagram of the AGC detector of  FIG. 1  according to an exemplary embodiment of the present invention;  
       FIG. 3  is a frequency response graph illustrating relationships between output voltage and input frequency according to an exemplary embodiment of the present invention; and  
       FIG. 4  is a flowchart illustrating steps according to an exemplary embodiment of the present invention. 
    
    
      The exemplifications set out herein illustrate preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.  
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring now to the drawings, and more particularly to  FIG. 1 , signal processing apparatus  100  according to an exemplary embodiment of the present invention is shown. Signal processing apparatus  100  may for example represent the front-end processing circuitry of a receiving device such as television signal receiver and/or other device.  
      As shown in  FIG. 1 , signal processing apparatus  100  comprises tuning means such as tuner  10 , first filtering means such as surface acoustic wave (SAW) filter  20 , AGC detecting means such as AGC detector  30 , AGC processing means such as AGC processing block  40 , amplifying means such as amplifier  50 , another filtering means such as SAW filter  60 , demodulation and processing means such as demodulation and processing block  70 , audio processing and output means such as audio processing and speakers block  80 , and display means such as video display  90 . Some of the aforementioned elements of  FIG. 1  may be embodied using integrated circuits (ICs), and some elements may for example be included on one or more ICs. For clarity of description, certain elements associated with signal processing apparatus  100  such as certain control signals (e.g., channel selection signals), power signals and/or other elements may not be shown in  FIG. 1 .  
      Tuner  10  is operative to perform a signal tuning function. According to an exemplary embodiment, tuner  10  receives an RF input signal from a signal source such as a terrestrial, cable, satellite, internet and/or other signal source, and performs the signal tuning function by filtering and frequency downconverting (i.e., single or multiple stage downconversion) the RF input signal to thereby generate an IF signal between 41 and 47 MHz. This IF signal is represented at Point A in  FIG. 1 . The RF input signal and IF signal may include audio, video and/or data content, and may be of an analog modulation scheme (e.g., NTSC, PAL, SECAM, etc.) and/or a digital modulation scheme (e.g., ATSC, QAM, etc.). Also according to an exemplary embodiment, tuner  10  receives an RF AGC signal from AGC processing circuitry  40  which enables an AGC function.  
      SAW filter  20  is operative to filter the IF signal provided from tuner  10  to thereby generate differential, filtered IF signals. These filtered IF signals are represented at Point B in  FIG. 1 . According to an exemplary embodiment, SAW filter  20  includes one or more individual SAW filters which remove a substantial portion of the undesired, adjacent channel energy from the IF signal provided from tuner  10  to generate the differential, filtered IF signals. Differential, or balanced, operation with respect to a circuit ground minimizes interference from stray coupling, including capacitive coupling from the input of SAW filter  20  that can degrade out of band rejection of SAW filter  20 . According to this exemplary embodiment, the frequency response of SAW filter  20  slightly exceeds the frequency range from 41 to 47 MHz. In this manner, digital adjacent channel interference can be well controlled given that digital television signals are characterized by having a very uniform distribution of power over their bandwidth. As indicated in  FIG. 1 , one of the differential, filtered IF signals output from SAW filter  20  is provided to AGC detector  40  to enable the AGC function of tuner  10 .  
      AGC detector  30  is operative to sample a predetermined signal and enable generation of an RF AGC signal for tuner  10 . According to the exemplary embodiment of  FIG. 1 , AGC detector  30  samples one of the differential, filtered IF signals output from SAW filter  20  and generates an output signal which enables generation of the RF AGC signal. Since only a portion of the undesired, adjacent channel energy is present in the differential, filtered IF signal provided from SAW filter  20 , the RF AGC signal may be generated having an optimum balance of digital adjacent channel interference and the desired signal. As will be described later herein, AGC detector  30  also includes filtering means for attenuating a predetermined carrier frequency, namely an analog sound carrier, to thereby minimize any analog adjacent channel interference. AGC detector  30  may be used in multiple tuner environments. As such, AGC detector  30  may receive control signals (not shown) that vary its operating characteristics as a function of the selected tuner. Further details regarding AGC detector  30  will be provided later herein.  
      AGC processing block  40  is operative to perform processing functions associated with generating the RF AGC signal for tuner  10 . According to an exemplary embodiment, AGC processing block  40  performs functions including, but not limited to, monitoring thresholds above which gain reduction begins, and adjusting AGC speed. AGC processing block  40  may for example be implemented using an IC such as a NXT2004 manufactured by ATI. However, with respect to the inventive concepts of the present invention, AGC processing block  40  is not required. Accordingly, the output signal of AGC detector  30  could be directly applied to tuner  10  as the RF AGC signal.  
      Amplifier  50  is operative to amplify the filtered IF signals provided from SAW filter  20  to thereby generate an amplified IF signal. SAW filter  60  is operative to filter the amplified IF signal provided from amplifier  50  to thereby generate another set of differential, filtered IF signals for demodulation and further processing.  
      Demodulation and processing block  70  is operative to demodulate and further process (e.g., decode, etc.) the differential, filtered IF signals provided from SAW filter  60  to thereby generate demodulated audio and/or video signals for output. According to an exemplary embodiment, demodulation and processing block  70  is operative to perform various different types of signal demodulation including analog demodulation (e.g., NTSC, PAL, SECAM, etc.) and digital demodulation (e.g., ATSC, QAM, etc.), as well as various types of signal decoding including analog decoding (e.g., NTSC, PAL, SECAM, etc.) and digital decoding (e.g., MPEG, etc.).  
      Audio processing and speakers block  80  is operative to process the demodulated audio signals provided from demodulation and processing block  70  and provide an audio output. Video display  90  is operative to provide a video display corresponding to the demodulated video signals provided from demodulation and processing block  70 .  
      Referring to  FIG. 2 , a schematic circuit diagram of AGC detector  30  of  FIG. 1  according to an exemplary embodiment of the present invention is shown. As shown in  FIG. 2 , AGC detector  30  comprises resistors R 1  to R 13 , capacitors C 1  to C 10 , inductors L 1  to L 3 , transistors Q 1  and Q 2 , diodes D 1  and D 2 , and a ceramic resonator X 1 . Exemplary values for resistors R 1  to R 13 , capacitors C 1  to C 10 , and inductors L 1  to L 3  are shown in  FIG. 2 . Other values could also be used. Transistors Q 1  and Q 2  of  FIG. 2  are each embodied as a dual gate metal oxide semiconductor field effect transistor (MOSFET), such as a model BF1005 transistor manufactured by Infineon. Diodes D 1  and D 2  of  FIG. 2  are each embodied as a Schottky diode, such as a model 1PS76SB17 diode manufactured by Phillips. Ceramic resonator X 1  of  FIG. 2  may for example be embodied as a model MKTGA47M2CAHP00B05 ceramic filter manufactured by Murata.  
      As shown in  FIG. 2 , resistor R 9 , capacitor C 9 , inductor L 3 , and ceramic resonator X 1  represent a “trap” filter  35 . According to an exemplary embodiment, trap filter  35  is operative to attenuate a predetermined carrier frequency, namely a 47.25 MHz analog sound carrier. In analog television, such as NTSC, signal power is concentrated near the carriers, specifically the picture and sound carriers. In the presence of analog channel interference, the adjacent sound carrier of 47.25 MHz is very close to the band edge of the desired signal. The presence of this sound carrier can produce too much power and thereby cause the gain of tuner  10  to be adversely reduced more than desired. With trap filter  35  of  FIG. 2 , this problem is addressed in that ceramic resonator X 1  is tuned to shunt 47.25 MHz frequencies. Inductor L 3  and capacitor C 9  are provided to optimize impedances and resistor R 9  is provided to control the amount of attenuation of the 47.25 MHz sound carrier.  
      By controlling the frequency response of SAW filter  20  and providing trap filter  35  as described above, the resulting RF AGC signal applied to tuner  10  is not only optimized to prevent overload of a much greater variation of interfering signal levels, but is also optimized for both analog and digital interfering signals. The benefits of the present invention are evident from the frequency response graph of  FIG. 3  described below.  
      Referring to  FIG. 3 , a frequency response graph  300  illustrating relationships between output voltage and input frequency according to an exemplary embodiment of the present invention is shown. In particular,  FIG. 3  shows a plot of output voltage versus input frequency for a signal applied to SAW filter  20  at Point A of  FIG. 1  and an output voltage measured at Point C of  FIG. 1 . Two frequency responses are shown in graph  300  of  FIG. 3 . Curve X is taken without the addition of trap filter  35  of  FIG. 2 . Curve Y is taken with the addition of trap filter  35  and shows the adjustment in frequency response made to optimize operation for analog interfering signals. In graph  300  of  FIG. 3 , the frequency response between 47 and 48 MHz is the adjacent channel bandwidth that is processed to effect the gain control of tuner  10  in the presence of adjacent analog channel interference.  
      Referring to  FIG. 4 , a flowchart  400  illustrating steps according to an exemplary embodiment of the present invention is shown. For purposes of example and explanation, the steps of  FIG. 4  will be described with reference to elements of signal processing apparatus  100  of  FIG. 1 . The steps of  FIG. 4  are merely exemplary, and are not intended to limit the present invention in any manner.  
      At step  410 , signal processing apparatus  100  tunes an RF signal to generate a corresponding IF signal. According to an exemplary embodiment, tuner  10  receives an RF input signal from a signal source such as a terrestrial, cable, satellite, internet and/or other signal source, and performs the signal tuning function by filtering and frequency downconverting (i.e., single or multiple stage downconversion) the RF input signal to thereby generate an IF signal between 41 and 47 MHz, at step  410 . This IF signal is represented at Point A in  FIG. 1 . As previously indited herein, the RF input signal and IF signal may include audio, video and/or data content, and may be of an analog modulation scheme (e.g., NTSC, PAL, SECAM, etc.) and/or a digital modulation scheme (e.g., ATSC, QAM, etc.).  
      At step  420 , signal processing apparatus  100  filters the IF signal to generate filtered IF signals. According to an exemplary embodiment, SAW filter  20  filters the IF signal generated by tuner  10  at step  410  to thereby generate differential, filtered IF signals at step  420 . These filtered IF signals are represented at Point B in  FIG. 1 . As previously indicated herein, the frequency response of SAW filter  20  slightly exceeds the frequency range from 41 to 47 MHz and thereby contains some digital adjacent channel interference.  
      At step  430 , signal processing apparatus  100  generates an AGC signal responsive to one of the filtered IF signals by attenuating a predetermined carrier frequency. According to an exemplary embodiment, AGC detector  30  samples one of the differential, filtered IF signals generated by SAW filter  20  at step  420  and generates an output signal which enables generation of the RF AGC signal at step  430 . As previously indicated herein, AGC detector  30  includes trap filter  35  which attenuates the 47.25 MHz analog sound carrier and thereby controls analog adjacent channel interference. The AGC signal generated at step  430  may be the direct output of AGC detector  30 , or may be the output of AGC processing block  40  as previously described herein.  
      At step  440 , signal processing apparatus  100  provides the AGC signal to its tuner  10 . According to an exemplary embodiment, the AGC signal generated at step  430  is provided to tuner  10  from either AGC detector  30  or AGC processing block  40  depending on the particular embodiment. The AGC signal in turn controls the gain of tuner  10  and thereby facilitates the RF AGC function of signal processing apparatus  100 .  
      As described herein, the present invention provides an apparatus and method for providing AGC that avoids excessive tuner gain reduction and compensates for interference from both analog and digital signals. The present invention may be applicable to various apparatuses, either with or without a display device. Accordingly, the phrases “signal processing apparatus” and “television signal receiver” as used herein may refer to systems or apparatuses including, but not limited to, television sets, computers or monitors that include a display device, and systems or apparatuses such as set-top boxes, video cassette recorders (VCRs), digital versatile disk (DVD) players, video game boxes, personal video recorders (PVRs), radios, computers or other apparatuses that may not include a display device.  
      While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.