Abstract:
An automatic gain control circuit that maximizes front-end signal attenuation is disclosed. The automatic gain control circuit comprises a keyed automatic gain control circuit and an intermodulation detector. The intermodulation detector detects signal interference and generates an intermodulation detection flag. The keyed automatic gain control circuit uses the intermodulation detection flag to control front-end signal attenuation. A method for maximizing front-end signal attenuation for the automatic gain control circuit is also disclosed.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to automatic gain control circuits. More specifically, the present invention relates to an automatic gain control circuit that maximizes front-end signal attenuation.  
         BACKGROUND OF THE INVENTION  
         [0002]    A majority of receiver designs employ some form of front-end automatic gain control (AGC) to limit the amount of signal power present at the mixer input. This limits the signal being presented to the mixer and maintains a higher dynamic range. Three often-used terms in receiver front-end AGC circuits are “wideband AGC” (WBAGC), “narrowband AGC” (NBAGC), and “keyed AGC” (KAGC). WBAGC refers to the wide bandwidth signal strength indication of the total FM band. NBAGC refers to the on-channel bandwidth signal strength indication of a desired signal as defined by the bandwidth of the intermediate frequency (IF) strip. KAGC refers to a design that utilizes a control algorithm that limits the front-end attenuation based on the level of the desired signal.  
           [0003]    Referring to the block diagram in FIG. 4, the conventional implementation of AGC has been to use a WBAGC circuit  40  as a control mechanism for front-end signal attenuation. Modification to the WBAGC circuit  40  has been to use the NBAGC to limit the amount of WBAGC that can be applied to the front-end for signal attenuation. Referring to the block diagram in FIG. 5, this modification is most commonly called the KAGC circuit  50 .  
           [0004]    In such AGC circuits  40 ,  50 , a situation is often present where there is a desired signal  60  that is weak and an undesired signal  62  (i.e. an undesired interferer) that is strong (FIGS. 6, 7). In the conventional WBAGC circuit  40 , the amount of the front-end attenuation (i.e. attenuation magnitude A, B) is dictated entirely by the RF strength of the undesired signal  62 . The attenuation magnitude, A, of the desired signal  60  is typically approximately equivalent to the attenuation magnitude, B, of the undesired signal  62 . Referring to FIG. 6, the WBAGC circuit  40  can essentially attenuate the desired signal  60  below any listenable level (i.e. a noise floor) by the attenuation magnitude, A, after the AGC is applied. When the attenuation of the desired signal  60  is below any listenable level, the situation is commonly referred to as desensitization or “flushing.” Thus, without KAGC, the desired signal  60  is flushed.  
           [0005]    The KAGC circuit  50  works satisfactorily for conditions in which the undesired signals do not produce intermodulation (IM) products that fall on the desired signal. Referring to FIG. 7, the KAGC circuit  50  prevents the desired signal  70  from being flushed for such conditions and is above the noise floor. Hence, the KAGC circuit  50  reduces the amount of desensitization of the desired signal  70 . Similar to the WBAGC circuit  40  for FIG. 6, the attenuation magnitude, A, of the desired signal  70  is typically approximately equivalent to the attenuation magnitude, B, of the undesired signal  72  in the KAGC circuit  50 .  
           [0006]    The amount of attenuation magnitude A, B in the KAGC circuit  50  is limited by the strength of the weak signal station. The limit to which this attenuation is applied is set with an internal reference in the front end IC (i.e. RFIC). This reference is compared with the narrowband level voltage or received signal strength indicator (RSSI). Once this narrowband level voltage reaches the threshold value of the comparator, no further attenuation is applied. The amount of the front-end AGC is limited with the help of the narrowband IF signal.  
           [0007]    Three different signal condition situations, which occur without producing any intermodulation (IM) products at the desired signal frequency, are covered with the present conventional systems that employ a combination of both the conventional WBAGC circuit  40  and the KAGC circuit  50 . In a first situation (not shown), when the desired and undesired signals are both weak, no attenuation is applied in an AGC action for the WBAGC circuit  40 . In a second situation for the WBAGC circuit  40  as seen in FIG. 8, when the desired signal  80  and the undesired signal  82  are strong, the desired AGC action is to apply attenuation until the undesired signal  82  reaches the threshold level. Thus, the desired signal  80  is attenuated down in magnitude that is approximately equivalent to A, and the undesired signal is attenuated down in magnitude that is approximately equivalent to B, where A is equal to B. In a third situation for the KAGC circuit  50  as seen in FIG. 9, when the desired signal  90  is weak and more than one strong undesired signal  92  produces an out-of-band IM product  94 , the desired AGC action is to apply AGC until the desired signal  90  is desensitized to the KAGC level.  
           [0008]    However, the deficiency as seen in FIG. 10, when two strong undesired signals  102  produce an inband IM product  104 , the deficiency of the third situation is the KAGC&#39;s inability to decipher between the desired signal  100  and the IM product  104  that occupies the same bandwidth as the desired signal  100 . These types of IM products  104  are one subset of generalized FM undesired spurious responses. These responses are generated by non-linear mixing operations that include harmonics of an IF signal, the local oscillator signal, and signals at the receiver input.  
           [0009]    It is contemplated by the applicants that conventional AGC circuits  40 ,  50  may be enhanced to detect a spurious response at the desired frequency. Therefore, it is an objective of the applicants to overcome the fallbacks of conventional AGC circuits  40 ,  50  to allow the front-end to exert full attenuation of the incoming signals without being limited by conventional AGC circuits  40 ,  50 .  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly one embodiment of the present invention is directed to an automatic gain control circuit that maximizes front-end signal attenuation. The automatic gain control circuit comprises an intermodulation detector and a keyed automatic gain control circuit. The intermodulation detector detects front-end signal interference and generates an intermodulation detection flag. The keyed automatic gain control circuit uses the intermodulation detection flag to control the front-end signal attenuation.  
           [0011]    Another embodiment of the invention comprises means for detecting signal interference, means for generating an intermodulation detection flag, and means for controlling the keyed automatic gain control circuit.  
           [0012]    Another embodiment of the invention is directed to a method for maximizing front-end signal attenuation for an automatic gain control circuit. The automatic gain control circuit comprises a keyed automatic gain control circuit and an intermodulation detector. The method comprises the steps of receiving a desired signal and an undesired signal, producing signal interference, detecting the signal interference, generating a detection flag, deactivating the keyed automatic gain control circuit and flushing the undesired signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of an enhanced automatic gain control (AGC) system according to the present invention;  
         [0014]    [0014]FIG. 2 is a representative view of a signal condition when an inband intermodulation (IM) product is produced by two signals;  
         [0015]    [0015]FIG. 3 is a representative view of a signal condition including a weak desired signal and a strong undesired signal when no inband IM products are generated;  
         [0016]    [0016]FIG. 4 is a block diagram of a conventional wideband AGC (WBAGC) circuit;  
         [0017]    [0017]FIG. 5 is a block diagram of a conventional keyed AGC (KAGC) circuit;  
         [0018]    [0018]FIG. 6 is a representative view when a desired signal is flushed in the WBAGC circuit of FIG. 4;  
         [0019]    [0019]FIG. 7 is a representative view when the KAGC circuit of FIG. 5 prevents the desired signal from being flushed;  
         [0020]    [0020]FIG. 8 is a representative view of a signal condition showing an AGC application for the WBAGC circuit of FIG. 4;  
         [0021]    [0021]FIG. 9 is a representative view of a signal condition showing an AGC application for the KAGC circuit of FIG. 5 that generates an out-of-band intermodulation product; and  
         [0022]    [0022]FIG. 10 is a representative view of a signal condition showing an AGC application for the KAGC circuit of FIG. 5 that generates an inband intermodulation product. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The AGC system, which is shown generally at  10  in FIG. 1, enhances the capabilities of the conventional KAGC circuit  50  by detecting a spurious response at a desired frequency. Once this is accomplished, it will allow the front-end to exert full attenuation on the incoming signals by essentially turning the KAGC function off without being limited by the KAGC function. In the following description of the preferred embodiment, it is assumed that the WBAGC and the KAGC are fully turned on.  
         [0024]    The detection of signal interference can be accomplished as follows: a typical FM detector (i.e. an FM demodulator) is a circuit whose output voltage is proportional to the difference between a reference frequency and the frequency of an input signal. Hence, large frequency excursions or deviations of the input signal produces large voltage swings at the output. One source of large frequency variations beyond the standard FM deviations is the direct result of IM products being present on the desired signal (FIG. 10). Fast voltage swings at the output generate broad frequency spectrums that are used to generate ultra sonic noise  14  (USN). In the AGC system  10 , means for detecting front-end signal interference, such as an IM detector  12 , detects the USN  14 . Means for generating, such as the IM detector  12 , generates an intermodulation (IM) detection flag  19 . Means for controlling the KAGC circuit  50 , such as the IM detection flag  19 , is used as a control signal for controlling the KAGC action (i.e. turning off the KAGC function) at the front-end of the receiver.  
         [0025]    Because there are several other conditions that can result in USN activity, this particular IM detection flag  19  alone that is generated by the IM detector  12  in the presence of USN  14  is not sufficient to reliably predict the IM product presence. It should be noted however, that the USN activity that is generated as a result of the IM situation is appreciably higher than any other scenario that may result in USN activity. This is readily observed from the fact that IM products are typically generated with higher order harmonics. A higher order harmonic will imply that the frequency deviations of the FM signals are also being amplified with the order of the harmonics involved. Hence, this will typically give rise to a higher quantitative amount of USN  14 .  
         [0026]    To further limit the probability of a false trigger of the KAGC system, a level signal or field strength signal indicator  16  can also be used as an input for the IM detector  12  in order to generate the IM detection flag  19 . The field strength indicator  16  is used to set the KAGC threshold and is located at the output of the long amplifier in the KAGC circuit  50 . With this vital information available, it can be readily determined when the desired signal has reached a low RF level at the point of the AGC set threshold. This information, coupled with the knowledge that the WBAGC is active, can provide one of the triggers for turning off the KAGC function.  
         [0027]    Another source of signal interference that can also be used in order to generate the IM detection flag  19  is the AM wideband (AMWB) signal. As the name would indicate, AMWB is the measure of AM that is created on a FM signal due to the presence of multipath interference. The field strength indicator  16  may be sent to an AMWB detector (not shown) when the desired signal is rapidly changing. Hence, the field strength indicator  16  attempts to track the AMWB signal, which results in a full-wave rectified AM signal that is proportional to the amount of amplitude of the desired signal. The AMWB detector generates a DC voltage that is projected off of the AMWB signal from the field strength indicator  16 . The AMWB detector essentially detects the DC average of the field strength indicator  16 , which in turn provides an amount of variation in the desired signal. Although the AMWB detector is not shown, it may be similarly located where the IM detector  12  is shown in FIG. 1.  
         [0028]    AMWB is commonly used in the receiver design to detect the presence of multipath interference in the FM signal transmission. It would appear that in the presence of an IM signal, there would be less multipath interference generated. In an IM situation, it is already established that the desired signal is very weak. Thus, the amount of AM on this signal is also less when compared to a relatively high desired signal. Hence, a lesser amount of AMWB indication can also be used as an IM detection flag  19  in controlling the KAGC function.  
         [0029]    Another source of signal interference that can also be used in order to generate the IM detection flag  19  in the AGC system  10  is the IF frequency  18  itself. Over-modulations of the IF that effect the IM signals can also be detected at the IF.  
         [0030]    For the AGC system  10  described above, there are two situations that produce IM products that are at the frequency of the desired channel. In a first situation as seen in FIG. 2, when the desired signal  20  (shown at 98.1 MHz) is weak and the undesired signals  22  are strong (shown at 98.9 MHz and 99.7 MHz), an inband IM product  24  is generated and the IM detector  12  is triggered (i.e. FM(IM)=2F 1 −F 2 ; FM(IM)=2*98.9−99.7=98.1). When the IM product  24  is generated, the audio level of the IM product  24  will be twice of what it&#39;s being broadcast. Thus, the KAGC function does not turn on, and the AGC system  10  applies attenuation to eliminate the undesirable signal  22  by applying enough AGC to bring the undesired signal to the start of AGC because the IM product  24  is competing with the desired signal  20 .  
         [0031]    In a second situation as seen in FIG. 3, when the desired signal  30  (shown at 98.1 MHz) is very weak (i.e. the S/N is below a listenable level) and the undesired signal  32  (shown at 98.5 MHz) is strong, no inband IM products are generated. Thus, the KAGC does not turn on, and the AGC system  10  applies attenuation to flush the undesirable signal  32 .  
         [0032]    This approach may desensitize the desired signal  30 . However, the desensitization of the desired signal  30  does not have a major importance in the AGC system  10  if it is below a listenable level. If this did happen, the output of the receiver would be static (i.e. no signal present). From a user&#39;s standpoint, it would be preferable to listen to static than the IM product.  
         [0033]    As shown above, the AGC system  10  uses the detection flags  14 ,  16 , and  18  to help determine the presence of IM products, and when present, allow the KAGC function to switch off with controlling means, such as a control signal  19 , so that the undesirable signal may become flushed. While maintaining the implementation of the KAGG circuit  50  when no inband IM products are present, the AGC system  10  employs the advantage of turning the KAGC function off when inband IM products are generated. Thus, the front-end of the receiver exerts maximum attenuation in order to minimize the effects of the undesired signal. The KAGC function also turns off when a desired signal that is below the KAGC threshold level is very weak and when a strong undesired signal that turns the WBAGC on is present. Thus, the result is a limited amount of front-end attenuation because there is little or no KAGC signal present to control the amount of the attenuation. If the KAGC is turned off completely, the front-end will fully attenuate the undesired signal. Thus, when the KAGC is completely turned off, it does not matter if the desired signal is attenuated with the undesired signal because it had poor listening quality to begin with.  
         [0034]    It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.