PRIORITY 
   This application claims priority to an application entitled “Apparatus for Improving Intermodulation Distortion Characteristics in a CDMA Mobile Terminal” filed in the Korean Industrial Property Office on Mar. 22, 2000 and assigned Serial No. 2000-14520, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a received signal processing technique in a CDMA mobile terminal, and in particular, to an apparatus for reducing intermodulation distortion generated while processing a received signal. 
   2. Description of the Related Art 
   An existing digital CDMA (Code Division Multiple Access) mobile terminal has a transmission frequency band of 824-849 MHz and a reception frequency band of 869-894 MHz. An analog AMPS (Advanced Mobile Phone System) mobile terminal also has the same transmission and reception frequency bands. Since the CDMA mobile terminal and the AMPS mobile terminal have the same frequency bands, the CDMA mobile terminal is so designed to reduce and/or eliminate interference with the AMPS signals. That is, the CDMA mobile terminal is so designed as to have the constant reception performance, even though there exist AMPS signals around the CDMA channel. 
   In this connection, there has been proposed an IS-95 intermodulation distortion characteristic specification. The CDMA mobile terminal should be so manufactured as to satisfy this specification. For reference, the IS-95 intermodulation distortion characteristic specification is shown in Table 1 below. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
               1 st  Jammer Signal 
               2 nd  Jammer Signal 
             
             
                 
               Traffic Signal 
               (±900 KHz Offset) 
               (±1700 KHz Offset) 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               1 
               −101 dBm 
               −43 dBm 
               −43 dBm 
             
             
               2 
                −90 dBm 
               −32 dBm 
               −32 dBm 
             
             
               3 
                −79 dBm 
               −21 dBm 
               −21 dBm 
             
             
                 
             
           
        
       
     
   
   In the prior art, there have been proposed several methods for satisfying the above conditions. However, the proposed methods are all performed by software. That is, in the existing methods, the mobile terminal operates according to software processing to satisfy the above-stated specification. 
     FIG. 1  shows a typical structure of the conventional CDMA mobile terminal. Specifically,  FIG. 1  shows a structure of a signal processor for processing transmission and reception signals. 
   Referring to  FIG. 1 , a receiver of the CDMA mobile terminal includes a duplexer  100 , a low-noise amplifier (LNA)  112 , an RF (Radio Frequency) filter  114 , a mixer  116 , and an IF (Intermediate Frequency) filter  118 . Further, the receiver includes an attenuator  110  provided in front of the low-noise amplifier  112  to satisfy the intermodulation distortion (IMD) characteristics. The attenuator  110  is provided to solve a reception performance degradation problem due to the intermodulation noise components which are generated when the two CDMA and AMPS continuous waves having the same frequency band are input to the low-noise amplifier  112 . The attenuator  110  solves the reception performance degradation problem by properly controlling a gain of the signals applied to the low-noise amplifier  112 . The IF filter  123  inputs the modulated IF signals and filters the predetermined transmitting IF frequency signals. The mixer  121  mixes the output of the IF filter  123  with the predetermined local oscillating signals and thus outputs the RF signals. The RF filter  119  filters the mixed signals and detects the RF signals. The drive amplifier  117  amplifies the level of the RF signals so as to obtain the power for driving the power amplifier  113  as stated below. The duplexer  100  switches the connection of the antennas and the transmitting and receiving portion. The RF filter  115  filters the output of the driving amplifier  117 , once again. The power amplifier  113  amplifies the filtered RF signals to the predetermined level. The isolator  111  is connected between the output end of the power amplifier  113  and the input end of the duplexer  100  so as to avoid the transmitting distortion of the reflection wave, thereby transmitting the output of the power amplifier  113  to the duplexer  100  without any attenuating. The duplexer  100  switches antenna  130  between a transmitting and receiving mode. 
   Now, an operation of the CDMA mobile terminal will be described in detail. In accordance with the unique characteristics of the CDMA system, a level of an IF signal output from the mixer  116  has a constant value regardless of the strength of a signal received at an antenna ANT. The mixer  116  performs this operation under the control of a controller (now shown). 
   For information, a voltage level for controlling the mixer  116  is adjusted depending on the strength of the signals received at the receiver. Such a power control method is generally called “open-loop power control”. There is a case where the received signal strength becomes higher than a predetermined level. In this case, the controller attenuates the received signal depending on the received signal strength by outputting an intermodulation distortion (IMD) characteristic control signal. To this end, the controller generates a control signal for properly controlling a gain of the attenuator  110 . This control signal is generally referred to as an “IMD characteristic control signal” or “IMD control voltage”. By doing so, it is possible to decrease the intermodulation distortion, thereby solving the reception performance degradation problem. 
   In this case, however, the conventional CDMA mobile terminal of  FIG. 1  has the following disadvantages. That is, the attenuator  110  is controlled through a software process which is performed by the controller. Hence, this method inevitably includes an additional software process, making it difficult to control the attenuator  110  in real time. Further, the additional software process may raise an overload problem of the controller. In particular, since the attenuator  110  is controlled by software, it is very difficult to match the correct control timing. 
   In other words, the controller calculates the control signal for the attenuator  110  by performing the software process on the received jammer signals, so that the control signal will be generated after certain time delay. In addition, since the hardware interacts with the software, it is necessary to control an accurate operating point. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a hardware apparatus for controlling an attenuation of a received signal in order to solve a reception performance degradation problem due to intermodulation distortion in a CDMA mobile terminal. 
   To achieve the above and other objects, there is provided an apparatus for improving an intermodulation distortion characteristic by controlling a gain of an attenuator in a CDMA mobile terminal. The apparatus includes a converter for passing input signal components except an IF signal component, amplifying the passed input signal to a specific level, converting the amplified signal to a DC signal, comparing the DC signal with first and second thresholds, and generating a control signal for controlling an operating point of the attenuator by synthesizing the compared result signals. 
   Preferably, the converter comprises a filter for passing the input signal components except the IF signal component; an IF amplifier for amplifying a signal output from the filter to the specific level; an integrator for converting a signal output from the IF amplifier to a DC signal; first and second comparators for comparing a signal output from the integrator with the first and second thresholds, respectively; and a synthesizer for synthesizing signals output from the first and second comparators. 
   Preferably, the filter is comprised of a notch filter for passing the input signal components except the IF signal component. Alternatively, the filter is comprised of a high-pass filter for filtering a high-frequency band signal and a low-pass filter for passing a low-frequency band signal. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a block diagram illustrating a structure of a CDMA mobile terminal with an IMD characteristic improving device according to the prior art; 
       FIG. 2  is a block diagram illustrating a structure of a CDMA mobile terminal with an IMD characteristic improving device according to an embodiment of the present invention; and 
       FIG. 3  is a block diagram illustrating a detailed structure of the converter in the IMD characteristic improving device according to an embodiment of the present invention. 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
     FIG. 2  shows a structure of a CDMA mobile terminal with an IMD characteristic improving device according to an embodiment of the present invention. Referring to  FIG. 2 , the IMD characteristic improving device according to an embodiment of the present invention additionally includes a converter  220  for controlling an attenuation of an attenuator  210  interposed between a duplexer  200  and a low-noise amplifier (LNA)  212 . The converter  220  is provided to adjust an operating point of the attenuator  210  so as to satisfy the IMD characteristics. The IF filter  233  inputs the modulated IF signals and filters the predetermined transmitting IF frequency signals. The mixer  231  mixes the output of the IF filter  233  with the predetermined local oscillating signals and thus outputs the RF signals. The RF filter  229  filters the mixed signals and detects the RF signals. The drive amplifier  227  amplifies the level of the RF signals so as to obtain the power for driving the power amplifier  223  as stated below. The duplexer  200  switches the connection of the antennas and the transmitting and receiving portion. The RF filter  225  filters the output of the driving amplifier  227 , once again. The power amplifier  223  amplifies the filtered RF signals to the predetermined level. The isolator  221  is connected between the output end of the power amplifier  223  and the input end of the duplexer  200  so as to avoid the transmitting distortion of the reflection wave, thereby transmitting the output of the power amplifier  223  to the duplexer  200  without any attenuating. The duplexer  200  switches antenna  230  between a transmitting and receiving mode. 
     FIG. 3  shows the detailed structure of the converter  220  in the IMD characteristic improving device according to an embodiment of the present invention. Referring to  FIG. 3 , unlike the conventional IMD improving device in which the controller controls an attenuation of the attenuator by software, the IMD improving device according to an embodiment of the present invention includes the converter  220  in order to control an attenuation of the attenuator by hardware. The converter  220  includes a filter  321  for passing signal components except an IF signal component, an IF amplifier  322  for amplifying the signal output from the filter  321 , an integrator  323  for converting the signal output from IF amplifier  322  to a DC (Direct Current) signal, first and second comparators  324  and  325  for comparing the DC signal output from the integrator  323  with their predetermined thresholds, respectively, and a synthesizer  326  for synthesizing the signals output from the first and second comparators  324  and  325 . 
   In operation, the filter  321  receives a signal output from a mixer  216  and passes the signal components except an IF signal component. The filter  321  can be implemented by a notch filter, and can also be comprised of a high-pass filter and a low-pass filter. The filter  321  is provided to prevent the IF signal component from being applied to the IF amplifier  322 . The IF amplifier  322  amplifies the signal output from the filter  321  to a specific level which is high enough so that the first and second comparators  324  and  325  can compare the level with their thresholds. The integrator  323  converts the signal output from the IF amplifier  322  to a DC signal. The first and second comparators  324  and  325  compare the signal output from the integrator  323  with their thresholds. 
   In the embodiment of the present invention, the two comparators  324  and  325  are used to satisfy the received-signal processing capability required in the IS-95 IMD characteristic specification as shown in Table 1. The IS-95 IMD characteristic specification specifies the IMD characteristics for the received signal strengths of −90 dBm and −79 dBm. That is, the two comparators  324  and  325  are used to control the attenuator  210  for the received signal strengths of −90 dBm and −79 dBm in order to satisfy the IS-95 IMD characteristic specification. However, according to the specification, the comparator is not required for the IMD characteristic at the received signal strength of −101 dBm. 
   The thresholds of the comparators  324  and  325  are previously determined according to operating circumstances. For example, an operating point where the comparator operates is previously set by hardware, in order to satisfy the IMD characteristic at −90 dBm. When the converted DC signal for an erroneous input signal is higher than the predetermined threshold, the comparator outputs a HIGH signal to increase the attenuation. The same operation is performed even for the IMD characteristic at −79 dBm. 
   The synthesizer  326  synthesizes the signals output from the first and second comparators  324  and  325 , and provides the attenuator  210  with its output signal as an IMD control signal. 
   When the received signal strength is −90 dBm, the jammer signal level should be −32 dBm according to the IMD characteristic specification. In this case, only the first comparator  324  generates an output signal, while the second comparator  325  generates no output signal. 
   However, when the received signal strength is −79 dBm, the jammer signal level should be −21 dBm according to the IMD characteristic specification. In this case, both the first and second comparators  324  and  325  generate their output signals, and the synthesizer  326  synthesizes the signals output from the first and second comparators  324  and  325 , and provides the attenuator  210  with its output signal as an IMD control signal. 
   As described above, the CDMA mobile terminal according to an embodiment of the present invention can solve the reception performance degradation problem due to intermodulation distortion of the received signal. In addition, it is possible to realize real-time control by controlling the attenuator by hardware rather than by software to satisfy the IMD characteristics. 
   While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Disclosed is an apparatus for improving an intermodulation distortion characteristic by controlling a gain of an attenuator in a CDMA mobile terminal. The apparatus includes a converter for passing input signal components except an IF signal component, amplifying the passed input signal to a specific level, converting the amplified signal to a DC signal, comparing the DC signal with first and second thresholds, and generating a control signal for controlling an operating point of the attenuator by synthesizing the compared result signals.