Abstract:
Disclosed herein are a terrestrial digital multimedia broadcasting (DMB) tuner of a low intermediate frequency (IF) structure which is applied to a mobile communication terminal, such as a mobile phone, and an image rejection mixer applied thereto. In order to improve inter-channel attenuation characteristics, the image rejection mixer locates an oscillation frequency above or beneath the frequency of a target signal to include an image signal of the target signal in a terrestrial DMB band, mixes a radio frequency (RF) signal with the oscillation frequency, and outputs the resulting IF signals to a polyphase filter in a selected arrangement. Because the image signal of the target signal is included in the terrestrial DMB band, the image rejection mixer can satisfy the inter-channel attenuation characteristics without a troublesome or complex design.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is based on, and claims priority from, Korean Application Number 2005-002478, filed Jan. 11, 2005, the disclosure of which is incorporated by reference herein in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a terrestrial digital multimedia broadcasting (DMB) tuner of a low intermediate frequency (IF) structure which is applied to a mobile communication terminal, such as a mobile phone, and an image rejection mixer applied thereto, and more particularly to an image rejection mixer which is capable of being implemented with a single integrated circuit (IC) by excluding an external device, so that it can be made with small size and at low cost and operated at low power, and a terrestrial DMB tuner of a low IF structure using the same.  
         [0004]     2. Description of the Related Art  
         [0005]     In general, digital multimedia broadcasting (DMB) refers to broadcasting capable of sending text, graphics and moving images, as well as high-quality sound of the compact disc (CD) level, over simple audio services such as existing amplitude modulation (AM) broadcasting or frequency modulation (FM) broadcasting. This DMB typically means terrestrial broadcasting that locally provides a free broadcasting service, but, roughly, also includes satellite DMB that provides a pay multimedia broadcasting service using both a satellite and terrestrial network.  
         [0006]     The DMB adopts, for audio broadcasting, a digital audio processing or modulation system, which is very resistant to deterioration or noise, not an existing analog audio processing or modulation system. The digital audio processing system adopts an audio compression scheme of moving picture experts group (MPEG)  1  layer  2  that compresses high-volume data appropriately to transmission and storage thereof, and the digital audio modulation system adopts an orthogonal frequency division multiplexing (OFDM) scheme that provides an excellent mobile reception capability. Allocated as digital audio broadcasting (DAB) bands in Europe are a band-III (174˜240 MHz), which is a very high frequency (VHF) band, an L-band (1452˜1492 MHz), and a satellite DMB band (2630˜2655 MHz).  
         [0007]     In contrast, a part of the band-III (174˜240 MHz) is allocated as a terrestrial DMB band in Korea. At present, TV channels belonging to the band-III, for example, a channel  10  (193˜199 MHz) and channel  12  (204˜210 MHz), are allocated as terrestrial DMB frequencies in Korea. Here, each of the channel  10  and channel  12  includes three DMB channels.  
         [0008]     Research and development have been ceaselessly done for making a DMB tuner in the form of one application specific integrated circuit (ASIC) chip through a complementary metal-oxide semiconductor (CMOS) process using a silicon process according to a recent tendency to provide lightness, thinness, compactness and smallness. For this one-chip implementation, there is a need to make a circuit configuration of the DMB tuner as simple as possible by excluding a device difficult to make in IC form.  
         [0009]     The configuration of a conventional terrestrial DMB tuner is shown in  FIG. 1 .  
         [0010]      FIG. 1  is a circuit diagram showing the configuration of a conventional terrestrial DMB tuner.  
         [0011]     As shown in  FIG. 1 , the conventional terrestrial DMB tuner comprises a band pass filter  11  for passing a radio frequency (RF) signal of a terrestrial DMB band at a predetermined band, an RF amplifier  12  for amplifying an output signal from the band pass filter  11  at a predetermined gain, and an automatic gain control (AGC) amplifier  13  having a gain which is automatically controlled according to a received signal strength. The AGC amplifier  13  acts to amplify an output signal from the RF amplifier  12  at the controlled gain. The conventional terrestrial DMB tuner further comprises a voltage controlled oscillator (VCO)  14  for generating an oscillation frequency for channel selection, a phase locked loop (PLL)  15  for controlling the oscillation frequency of the VCO  24 , a mixer  16  for mixing an output signal from the AGC amplifier  13  with the oscillation frequency to generate an IF signal, an IF surface acoustic wave (SAW) filter  17  for passing the IF signal from the mixer  16  at a predetermined band, and an IF amplifier  18  for amplifying an output signal from the SAW filter  17  at a predetermined gain.  
         [0012]      FIG. 2  is a frequency spectrum of a target signal, image signal and IF signal in the terrestrial DMB tuner of  FIG. 1 . With reference to  FIG. 2 , the conventional terrestrial DMB tuner converts an RF signal of the band-III (174˜240 MHz) into an IF signal of 38.912 MHz, which contains an image signal RFim as well as a target signal RFw. That is, the target signal RFw and the image signal RFim are located at both sides of the IF signal such that they are symmetrically spaced apart from each other by an IF about an oscillation frequency Fo.  
         [0013]     The image signal can be removed by the band pass filter  11  because the frequency of the IF signal is high and the image signal is thus far away from the target signal at a frequency domain. Moreover, the IF signal can be selected at higher selectivity using the IF SAW filter  17 .  
         [0014]     However, the above-mentioned conventional terrestrial DMB tuner has to use the IF SAW filter having excellent selectivity even at a high frequency, since the frequency of the IF signal is high and an active filter implementable with an IC has poor selectivity at the high frequency. The use of the IF SAW filter makes it difficult to make the terrestrial DMB tuner in the form of one IC, resulting in limitations in reducing the size, cost and power consumption of the tuner.  
         [0015]     Approaches to the exclusion of the SAW filter may be a tuner of a zero IF structure and a tuner of a low IF structure. However, the tuner of the zero IF structure has a disadvantage in that reception sensitivity is significantly degraded due to a direct current (DC) offset in the process of OFDM modulation.  
       SUMMARY OF THE INVENTION  
       [0016]     Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an image rejection mixer which is capable of being implemented with a single IC by excluding an external device, so that it can be made with small size and at low cost and operated at low power, and a terrestrial DMB tuner of a low IF structure using the same, which is applied to a mobile communication terminal such as a mobile phone.  
         [0017]     In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of an image rejection mixer which is applicable to a terrestrial digital multimedia broadcasting (DMB) tuner and mixes a radio frequency (RF) signal with first and second oscillation signals to generate an intermediate frequency (IF) signal, the first and second oscillation signals having a phase difference of 90 degrees therebetween, the image rejection mixer comprising: a first multiplier for multiplying the RF signal by the first oscillation signal to generate first and second IF-I signals which are 180 degrees out of phase with each other; a second multiplier for multiplying the RF signal by the second oscillation signal to generate first and second IF-Q signals which are 180 degrees out of phase with each other; a signal selection unit including first and second input terminals for receiving the first and second IF-I signals, respectively, third and fourth input terminals for receiving the first and second IF-Q signals, respectively, first to fourth output terminals, a first signal selector for outputting the first and second IF-I signals to the first and third output terminals and the first and second IF-Q signals to the second and fourth output terminals, respectively, in response to a first switching signal, and a second signal selector for outputting the first and second IF-I signals to the first and third output terminals and the first and second IF-Q signals to the fourth and second output terminals, respectively, in response to a second switching signal; and a polyphase filter including first to fourth input terminals connected respectively to the first to fourth output terminals of the signal selection unit, and first to fourth output terminals, the polyphase filter removing image components contained in signals inputted through the first to fourth input terminals thereof to generate first to fourth IF signals and output them through the first to fourth output terminals thereof, respectively, whereby an image frequency lower than an oscillation frequency is removed when the first signal selector is turned on, and an image frequency higher than the oscillation frequency is removed when the second signal selector is turned on.  
         [0018]     In accordance with another aspect of the present invention, there is provided a terrestrial DMB tuner comprising: a band pass filter for passing an RF signal of a terrestrial DMB band at a predetermined band; an RF amplification circuit for amplifying an output RF signal from the band pass filter; a phase locked loop (PLL) for controlling oscillation in response to a channel selection signal; a two-phase oscillator for generating first and second oscillation signals having a phase difference of 90 degrees therebetween under the oscillation control of the PLL; an image rejection mixer for mixing an output RF signal from the RF amplification circuit with the first and second oscillation signals to generate an IF signal and removing image components contained in the generated IF signal in response to first and second switching signals; an IF filter for passing the resulting IF signal from the image rejection mixer at a predetermined band; and an IF amplification circuit for amplifying an output IF signal from the IF filter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0020]      FIG. 1  is a circuit diagram showing the configuration of a conventional terrestrial DMB tuner;  
         [0021]      FIG. 2  is a frequency spectrum of a target signal, image signal and IF signal in the terrestrial DMB tuner of  FIG. 1 ;  
         [0022]      FIG. 3  is a block diagram showing the configuration of a terrestrial DMB tuner of a low IF structure according to the present invention;  
         [0023]      FIG. 4  is a circuit diagram showing the configuration of an image rejection mixer according to the present invention;  
         [0024]      FIG. 5  is a view illustrating band-III channel allocation and inter-channel attenuation characteristics of the terrestrial DMB tuner of the low IF structure according to the present invention; and  
         [0025]      FIGS. 6   a  to  6   c  are views illustrating a channel selection operation of the terrestrial DMB tuner of the low IF structure according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings.  
         [0027]      FIG. 3  is a block diagram showing the configuration of a terrestrial DMB tuner of a low IF structure according to the present invention.  
         [0028]     With reference to  FIG. 3 , the terrestrial DMB tuner of the low IF structure according to the present invention comprises a band pass filter  100  for passing an RF signal of a terrestrial DMB band at a predetermined band, an RF amplification circuit  200  for amplifying an output RF signal from the band pass filter  100 , an interface unit  300  for converting serial data containing a channel selection signal and first and second switching signals SS 1  and SS 2  into parallel data and outputting the channel selection signal and first and second switching signals SS 1  and SS 2  contained in the converted parallel data, a PLL  400  for controlling oscillation in response to the channel selection signal from the interface unit  300 , and a two-phase oscillator  500  for generating first and second oscillation signals LO 1  and LO 2  having a phase difference of 90 degrees therebetween under the oscillation control of the PLL  400 . The terrestrial DMB tuner of the low IF structure according to the present invention further comprises an image rejection mixer  600  for mixing an output RF signal from the RF amplification circuit  200  with the first and second oscillation signals LO 1  and LO 2  to generate an IF signal and removing image components contained in the generated IF signal in response to the switching signals SS 1  and SS 2 , an IF filter  700  for passing the resulting IF signal from the image rejection mixer  600  at a predetermined band, and an IF amplification circuit  800  for amplifying an output IF signal from the IF filter  700 .  
         [0029]     Preferably, each of the RF amplification circuit  200  and IF amplification circuit  800  includes a fixed-gain amplifier with a fixed gain and/or an AGC amplifier with a gain which is automatically controlled according to a received signal strength.  
         [0030]     The IF signal has a low IF of about 850 to 900 KHz.  
         [0031]     The RF amplification circuit  200 , interface unit  300 , PLL  400 , two-phase oscillator  500 , image rejection mixer  600 , IF filter  700  and IF amplification circuit  800 , among the above-stated components, can be provided in one IC.  
         [0032]      FIG. 4  is a circuit diagram showing the configuration of the image rejection mixer  600  according to the present invention.  
         [0033]     With reference to  FIG. 4 , the image rejection mixer  600  according to the present invention includes a first multiplier  620 , a second multiplier  630 , a signal selection unit  640 , and a polyphase filter  650 . The image rejection mixer  600  further includes a distributor  610  for distributing the output RF signal from the RF amplification circuit  200  to the first multiplier  620  and the second multiplier  630 .  
         [0034]     In  FIG. 4 , the first multiplier  620  multiplies the RF signal by the first oscillation signal LO 1  to generate first and second IF-I signals XI(t) and X{overscore (I)}(t) which are 180 degrees out of phase with each other.  
         [0035]     The second multiplier  630  multiplies the RF signal by the second oscillation signal LO 2  to generate first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) which are 180 degrees out of phase with each other.  
         [0036]     The signal selection unit  640  includes first and second input terminals IN 1  and IN 2  for receiving the first and second IF-I signals XI(t) and X{overscore (I)}(t), respectively, third and fourth input terminals IN 3  and IN 4  for receiving the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t), respectively, first to fourth output terminals OUT 1  to OUT 4 , a first signal selector  641  for outputting the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3  and the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the second and fourth output terminals OUT 2  and OUT 4 , respectively, in response to the first switching signal SS 1 , and a second signal selector  642  for outputting the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3  and the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the fourth and second output terminals OUT 4  and OUT 2 , respectively, in response to the second switching signal SS 2 .  
         [0037]     The second signal selector  642  in the signal selection unit  640  is turned on by the second switching signal SS 2  which is provided when a first DMB channel is selected, and the first signal selector  641  in the signal selection unit  640  is turned on by the first switching signal SS 1  which is provided when a second or third DMB channel is selected. Alternatively, the first signal selector  641  in the signal selection unit  640  may be turned on by the first switching signal SS 1  which is provided when the third DMB channel is selected, and the second signal selector  642  in the signal selection unit  640  may be turned on by the second switching signal SS 2  which is provided when the first or second DMB channel is selected.  
         [0038]     Here, one of the first switching signal SS 1  and second switching signal SS 2  is selectively provided as an ON signal to selectively turn on one of the first signal selector  641  and second signal selector  642 .  
         [0039]     In detail, the first signal selector  641  in the signal selection unit  640  includes a first switch SW 1  which is turned on in response to the first switching signal SS 1  to output the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3 , respectively, and a second switch SW 2  which is turned on in response to the first switching signal SS 1  to output the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the second and fourth output terminals OUT 2  and OUT 4 , respectively. Preferably, each of the first and second switches SW 1  and SW 2  is implemented with an amplifier.  
         [0040]     The second signal selector  642  in the signal selection unit  640  includes a third switch SW 3  which is turned on in response to the second switching signal SS 2  to output the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3 , respectively, and a fourth switch SW 4  which is turned on in response to the second switching signal SS 2  to output the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the fourth and second output terminals OUT 4  and OUT 2 , respectively. Preferably, each of the third and fourth switches SW 3  and SW 4  is implemented with an amplifier.  
         [0041]     The polyphase filter  650  includes first to fourth input terminals X 1 -X 4  connected respectively to the first to fourth output terminals OUT 1  to OUT 4  of the signal selection unit  640 , and first to fourth output terminals Y 1  to Y 4 . The polyphase filter  650  acts to remove image components contained in signals inputted through the first to fourth input terminals X 1 -X 4  to generate first to fourth IF signals IF 1  to IF 4  and output them through the first to fourth output terminals Y 1  to Y 4 , respectively.  
         [0042]      FIG. 5  illustrates band-III channel allocation and inter-channel attenuation characteristics of the terrestrial DMB tuner of the low IF structure according to the present invention. As shown in this drawing, a TV channel  10  or  12  is used for the terrestrial DMB tuner, although other TV channels may of course be used. In this case, the TV channel  10  or  12  must have attenuation characteristics of about 40 dB with other neighboring TV channels, and terrestrial DMB channels of the TV channel  10  or  12  must have attenuation characteristics of about 20 dB thereamong.  
         [0043]     Hence, in order to satisfy the inter-channel attenuation characteristics without a troublesome or complex design, the terrestrial DMB tuner of the low IF structure according to the present invention sets an oscillation frequency to locate an image signal of a target signal in a terrestrial DMB band, as shown in  FIG. 6 .  
         [0044]      FIGS. 6   a  to  6   c  illustrate a channel selection operation of the terrestrial DMB tuner of the low IF structure according to the present invention.  
         [0045]     In the image rejection mixer of the present invention, when a first DMB channel DMB-CH 1  is selected, an oscillation frequency for selection of the first DMB channel DMB-CH 1  is located above the DMB channel DMB-CH 1 , as shown in  FIG. 6   a . When a second DMB channel DMB-CH 2  is selected, an oscillation frequency for selection of the second DMB channel DMB-CH 2  is located above or beneath the DMB channel DMB-CH 2 , as shown in  FIG. 6   c . When a third DMB channel DMB-CH 3  is selected, an oscillation frequency for selection of the third DMB channel DMB-CH 3  is located beneath the DMB channel DMB-CH 3 , as shown in  FIG. 6   b.    
         [0046]     Next, the function and effect of the present invention will be described in detail in conjunction with the annexed drawings.  
         [0047]     With reference to FIGS.  3  to  6 , in the terrestrial DMB tuner of the present invention, a terrestrial DMB signal inputted through an antenna ANT is passed at a predetermined band by the band pass filter  100  and then amplified by the RF amplification circuit  200 .  
         [0048]     Meanwhile, in the terrestrial DMB tuner of the present invention, a serial/parallel converter SP of the interface unit  300  converts serial data SD containing a channel selection signal and first and second switching signals SS 1  and SS 2  into parallel data. A first register Re 1  of the interface unit  300  outputs the switching signals SS 1  and SS 2  contained in the parallel data converted by the serial/parallel converter SP to the image rejection mixer  600 . A second register Re 2  of the interface unit  300  outputs the channel selection signal contained in the parallel data converted by the serial/parallel converter SP to the PLL  400 . Here, the serial data SD is data that is provided according to channel selection in a terminal or device to which the terrestrial DMB tuner of the present invention is applied.  
         [0049]     Thereafter, the PLL  400  controls oscillation of the two-phase oscillator  500  in response to the channel selection signal from the interface unit  300 , and the two-phase oscillator  500  generates first and second oscillation signals LO 1  and LO 2  having a phase difference of 90 degrees therebetween under the oscillation control of the PLL  400 . The two-phase oscillator  500  then outputs the generated first and second oscillation signals LO 1  and LO 2  to the image rejection mixer  600 .  
         [0050]     For example, as shown in  FIG. 6   a , when the first DMB channel DMB-CH 1  contained in the channel  12  is selected, the two-phase oscillator  500  generates “206.136 MHz” as the first oscillation frequency LO 1  to select a center frequency “205.264 MHz” of the first DMB channel DMB-CH 1 , so the image rejection mixer  600  outputs an IF signal of about 872 KHz.  
         [0051]     As shown in  FIG. 6   b , when the third DMB channel DMB-CH 3  contained in the channel  12  is selected, the two-phase oscillator  500  generates “207.88 MHz” as the second oscillation frequency LO 2  to select a center frequency “208.736 MHz” of the third DMB channel DMB-CH 3 , so the image rejection mixer  600  outputs an IF signal of about 856 KHz.  
         [0052]     Also, as shown in  FIG. 6   c , when the second DMB channel DMB-CH 2  contained in the channel  12  is selected, the two-phase oscillator  500  selectively generates “206.136 MHz” as the first oscillation frequency LO 1  or “207.88 MHz” as the second oscillation frequency LO 2  to select a center frequency “207.008 MHz” of the second DMB channel DMB-CH 2 . As a result, the image rejection mixer  600  outputs an IF signal of about 872 KHz when the first oscillation frequency LO 1  of 206.136 MHz is generated, and an IF signal of about 856 KHz when the second oscillation frequency LO 2  of 207.88 MHz is generated.  
         [0053]     Thereafter, the image rejection mixer  600  mixes an output RF signal from the RF amplification circuit  200  with the first and second oscillation signals LO 1  and LO 2  to generate an IF signal, removes image components contained in the generated IF signal in response to the first and second switching signals SS 1  and SS 2  and outputs the resulting IF signal to the IF filter  700 , which will be described later in detail with reference to  FIG. 4 .  
         [0054]     The IF filter  700  passes the IF signal from the image rejection mixer  600  at a predetermined band, and the IF amplification circuit  800  amplifies and outputs an output IF signal from the IF filter  700 .  
         [0055]     A detailed description will hereinafter be given of the operation of the image rejection mixer  600  with reference to  FIGS. 3 and 4 .  
         [0056]     With reference to  FIGS. 3 and 4 , in the image rejection mixer  600 , the distributor  610  distributes the output RF signal from the RF amplification circuit  200  to the first multiplier  620  and the second multiplier  630 . At this time, the first multiplier  620  multiplies the RF signal by the first oscillation signal LO 1  to generate first and second IF-I signals XI(t) and X{overscore (I)}(t) which are 180 degrees out of phase with each other. Also, the second multiplier  630  multiplies the RF signal by the second oscillation signal LO 2  to generate first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) which are 180 degrees out of phase with each other.  
         [0057]     Thereafter, the signal selection unit  640  receives the first and second IF-I signals XI(t) and X{overscore (I)}(t) and the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) through the first to fourth input terminals IN 1  to IN 4 , respectively, and outputs them through the first to fourth output terminals OUT 1  to OUT 4  in different arrangements based on the first and second switching signals SS 1  and SS 2  from the interface unit  300 .  
         [0058]     In more detail, the signal selection unit  640  of the present invention includes the first signal selector  641  and the second signal selector  642 . The first signal selector  641  or second signal selector  642  is selectively operated in response to the first switching signal SS 1  or second switching signal SS 2 . For example, when only the first signal selector  641  is turned on by the first switching signal SS 1 , the first and second IF-I signals XI(t) and X{overscore (I)}(t) and the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) are connected to the first to fourth output terminals OUT 1  to OUT 4  as in Table 1 below.  
         [0059]     Alternatively, in the case where only the second signal selector  642  is turned on by the second switching signal SS 2 , the first and second IF-I signals XI(t) and X{overscore (I)}(t) and the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) are connected to the first to fourth output terminals OUT 1  to OUT 4  as in the Table 1 below.  
         [0060]     That is, when the first signal selector  641  is turned on, an oscillation frequency is set lower than the frequency of a target signal, thereby making it possible to remove an image signal of a frequency lower than the oscillation frequency. Alternatively, when the second signal selector  642  is turned on, an oscillation frequency is set higher than the frequency of a target signal, thereby making it possible to remove an image signal of a frequency higher than the oscillation frequency.  
                                             TABLE 1                                       ARRANGEMENTS OF SIGNALS           OUTPUTTED THROUGH           OUTPUT TERMINALS OF           SIGNAL SELECTION UNIT            OUTPUT TERMINALS   OUT1   OUT2   OUT3   OUT4               FIRST SIGNAL SELECTOR:   XI(t)   XQ(t)   X{overscore (I)}(t)   X{overscore (Q)}(t)       ON =&gt; LOWER OSCILLATION       FREQUENCY       SECOND SIGNAL SELECTOR:   XI(t)   X{overscore (Q)}(t)   X{overscore (I)}(t)   XQ(t)       ON =&gt; HIGHER OSCILLATION       FREQUENCY                  
 
         [0061]     For example, the first signal selector  641  in the signal selection unit  640  may be turned on by the first switching signal SS 1  which is provided when the second or third DMB channel is selected, and the second signal selector  642  in the signal selection unit  640  may be turned on by the second switching signal SS 2  which is provided when the first DMB channel is selected.  
         [0062]     When the second or third DMB channel is selected, the first signal selector  641  is turned on in response to the first switching signal SS 1 , so as to output the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3  and the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the second and fourth output terminals OUT 2  and OUT 4 , respectively, as shown in the above Table 1.  
         [0063]     In more detail, the first switch SW 1  of the first signal selector  641  is turned on in response to the first switching signal SS 1  to output the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3 , respectively, and the second switch SW 2  of the first signal selector  641  is turned on in response to the first switching signal SS 1  to output the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the second and fourth output terminals OUT 2  and OUT 4 , respectively.  
         [0064]     For another example, the first signal selector  641  in the signal selection unit  640  may be turned on by the first switching signal SS 1  which is provided when the third DMB channel is selected, and the second signal selector  642  in the signal selection unit  640  may be turned on by the second switching signal SS 2  which is provided when the first or second DMB channel is selected.  
         [0065]     When the first or second DMB channel is selected, the second signal selector  642  is turned on in response to the second switching signal SS 2 , so as to output the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3  and the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the fourth and second output terminals OUT 4  and OUT 2 , respectively, as shown in the above Table 1.  
         [0066]     In more detail, the third switch SW 3  of the second signal selector  642  is turned on in response to the second switching signal SS 2  to output the first and second IF-I signals XI(t) and X{overscore (I)}(t) to the first and third output terminals OUT 1  and OUT 3 , respectively, and the fourth switch SW 4  of the second signal selector  642  is turned on in response to the second switching signal SS 2  to output the first and second IF-Q signals XQ(t) and X{overscore (Q)}(t) to the fourth and second output terminals OUT 4  and OUT 2 , respectively.  
         [0067]     The polyphase filter  650  includes the first to fourth input terminals X 1 -X 4  connected respectively to the first to fourth output terminals OUT 1  to OUT 4  of the signal selection unit  640 , and the first to fourth output terminals Y 1  to Y 4 . The polyphase filter  650  removes image components contained in signals inputted through the first to fourth input terminals X 1 -X 4  to generate first to fourth IF signals IF 1  to IF 4  and output them through the first to fourth output terminals Y 1  to Y 4 , respectively.  
         [0068]     Where the polyphase filter  650  is implemented with a four-phase filter consisting of R and C as shown in  FIG. 4 , it can be operated as will be described below.  
         [0069]     First, when the third DMB channel DMB-CH 3  is selected as shown in  FIG. 6   b , among  FIGS. 6   a  to  6   c , signals are selected as shown in the above Table 1 and then inputted to the first to fourth input terminals X 1  to X 4  of the polyphase filter  650  as in Table 2 below.  
                                                     TABLE 2                                       ARRANGEMENT OF               SIGNALS INPUTTED           THROUGH INPUT           TERMINALS OF           POLYPHASE FILTER                INPUT TERMINALS   X1   X2   X3   X4                       SIGNALS   XI(t)   XQ(t)   X{overscore (I)}(t)   X{overscore (Q)}(t)                      
 
         [0070]     For example, if an image signal “X A (t)” and a target signal “X B (t)” are contained in the input RF signal and X{overscore (I)}(t) and XQ(t) are defined as in the following equation 1, the signals at the first to fourth input terminals X 1  to X 4  of the polyphase filter  650  can be expressed as in the following equation 2.  
                 XI   ⁡     (   t   )       =         A   2     ⁢   ∠     -     90   ⁢   °     +       B   2     ⁢   ∠90°         ⁢     
     ⁢       XQ   ⁡     (   t   )       =         A   2     ⁢   ∠0°     +       B   2     ⁢   ∠0°                 [     Equation   ⁢           ⁢   1     ]                   X   ⁢           ⁢   1   ⁢     :     ⁢     XI   ⁡     (   t   )         =         A   2     ⁢   ∠     -     90   ⁢   °     +       B   2     ⁢   ∠   ⁢           ⁢   90   ⁢   °         ⁢     
     ⁢       X   ⁢           ⁢   2   ⁢     :     ⁢     XQ   ⁡     (   t   )         =         A   2     ⁢   ∠0°     +       B   2     ⁢   ∠   ⁢           ⁢   0   ⁢   °         ⁢     
     ⁢       X   ⁢           ⁢   3   ⁢     :     ⁢   X   ⁢     I   _     ⁢     (   t   )       =         A   2     ⁢   ∠90°     +       B   2     ⁢   ∠     ⁢           -     90   ⁢   °         ⁢     
     ⁢       X   ⁢           ⁢   4   ⁢     :     ⁢   X   ⁢       Q   _     ⁡     (   t   )         =         A   2     ⁢   ∠180°     +       B   2     ⁢   ∠   ⁢           ⁢   180   ⁢   °                 [     Equation   ⁢           ⁢   2     ]             
 
         [0071]     In the above equations 1 and 2, “A” represents image data and “B” represents target data.  
         [0072]     Outputted at the first output terminal Y 1  of the polyphase filter  650  in  FIG. 4  is a signal as in the following equation 3, which is the sum of a signal obtained by legging the signal at the first input terminal X 1  by a resistor R 1  and a signal obtained by leading the signal at the second input terminal X 2  by a capacitor C 1 .  
         [0073]     Outputted at the second output terminal Y 2  is a signal as in the following equation 4, which is the sum of a signal obtained by legging the signal at the second input terminal X 2  by a resistor R 2  and a signal obtained by leading the signal at the third input terminal X 3  by a capacitor C 2 .  
         [0074]     Outputted at the third output terminal Y 3  is a signal as in the following equation 5, which is the sum of a signal obtained by legging the signal at the third input terminal X 3  by a resistor R 3  and a signal obtained by leading the signal at the fourth input terminal X 4  by a capacitor C 3 .  
         [0075]     Outputted at the fourth output terminal Y 4  is a signal as in the following equation 6, which is the sum of a signal obtained by legging the signal at the fourth input terminal X 4  by a resistor R 4  and a signal obtained by leading the signal at the first input terminal X 1  by a capacitor C 4 .  
                     Y   ⁢           ⁢   1     =       ⁢       [         A   2     ⁢   ∠     -     90   ⁢   °     -     45   ⁢   °     +       B   2     ⁢   ∠90°     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠0°     +     45   ⁢   °     +       B   2     ⁢   ∠0°     +     45   ⁢   °       ]                 =       ⁢     B   ⁢           ⁢   ∠45°                   [     Equation   ⁢           ⁢   3     ]                       Y   ⁢           ⁢   2     =       ⁢       [         A   2     ⁢   ∠0°     -     45   ⁢   °     +       B   2     ⁢   ∠0°     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠90°     +     45   ⁢   °     +       B   2     ⁢   ∠     -     90   ⁢   °     +     45   ⁢   °       ]                 =       ⁢     B   ⁢           ⁢   ∠135°                   [     Equation   ⁢           ⁢   4     ]                       Y   ⁢           ⁢   3     =       ⁢       [         A   2     ⁢   ∠90°     -     45   ⁢   °     +       B   2     ⁢   ∠     -     90   ⁢   °     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠180°     +     45   ⁢   °     +       B   2     ⁢   ∠180°     +     45   ⁢   °       ]                 =       ⁢       B   ⁢           ⁢   ∠     -     135   ⁢   °                   =       ⁢     B   ⁢           ⁢   ∠225°                   [     Equation   ⁢           ⁢   5     ]                       Y   ⁢           ⁢   4     =       ⁢       [         A   2     ⁢   ∠180°     -     45   ⁢   °     +       B   2     ⁢   ∠180°     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠     -     90   ⁢   °     +     45   ⁢   °     +       B   2     ⁢   ∠90°     +     45   ⁢   °       ]                 =       ⁢     B   ⁢           ⁢   ∠135°                   [     Equation   ⁢           ⁢   6     ]             
 
         [0076]     As can be seen from the above equations 3 to 6, only the target data B of the third channel is outputted under the condition that the image data A is removed.  
         [0077]     Next, when the first DMB channel DMB-CH 1  is selected as shown in  FIG. 6   a , signals are selected as shown in the above Table 1 and then inputted to the first to fourth input terminals X 1  to X 4  of the polyphase filter  650  as in Table 3 below.  
                                                     TABLE 3                                       ARRANGEMENT               OF SIGNALS           INPUTTED THROUGH           INPUT TERMINALS           OF POLYPHASE FILTER                INPUT TERMINALS   X1   X2   X3   X4                       SIGNALS   XI(t)   X{overscore (Q)}(t)   X{overscore (I)}(t)   XQ(t)                      
 
         [0078]     For example, if a target signal “X A (t) ” and an image signal “X B (t)” are contained in the input RF signal, the signals at the first to fourth input terminals X 1  to X 4  of the polyphase filter  650  can be expressed as in the following equation 7.  
                 X   ⁢           ⁢   1   ⁢     :     ⁢     XI   ⁡     (   t   )         =         A   2     ⁢   ∠     -     90   ⁢   °     +       B   2     ⁢   ∠   ⁢           ⁢   90   ⁢   °         ⁢     
     ⁢       X   ⁢           ⁢   2   ⁢     :     ⁢   X   ⁢       Q   _     ⁡     (   t   )         =         A   2     ⁢   ∠180°     +       B   2     ⁢   ∠   ⁢           ⁢   180   ⁢   °         ⁢     
     ⁢       X   ⁢           ⁢   3   ⁢     :     ⁢   X   ⁢       I   _     ⁡     (   t   )         =         A   2     ⁢   ∠90°     +       B   2     ⁢   ∠     -           ⁢     90   ⁢   °         ⁢     
     ⁢       X   ⁢           ⁢   4   ⁢     :     ⁢     XQ   ⁡     (   t   )         =         A   2     ⁢   ∠0°     +       B   2     ⁢   ∠   ⁢           ⁢   0   ⁢   °                 [     Equation   ⁢           ⁢   7     ]             
 
         [0079]     In the above equation 7, “A” represents target data and “B” represents image data.  
         [0080]     Outputted at the first output terminal Y 1  of the polyphase filter  650  in  FIG. 4  is a signal as in the following equation 8, which is the sum of a signal obtained by legging the signal at the first input terminal X 1  by the resistor R 1  and a signal obtained by leading the signal at the second input terminal X 2  by the capacitor C 1 .  
         [0081]     Outputted at the second output terminal Y 2  is a signal as in the following equation 9, which is the sum of a signal obtained by legging the signal at the second input terminal X 2  by the resistor R 2  and a signal obtained by leading the signal at the third input terminal X 3  by the capacitor C 2 .  
         [0082]     Outputted at the third output terminal Y 3  is a signal as in the following equation 10, which is the sum of a signal obtained by legging the&#39;signal at the third input terminal X 3  by the resistor R 3  and a signal obtained by leading the signal at the fourth input terminal X 4  by the capacitor C 3 .  
         [0083]     Outputted at the fourth output terminal Y 4  is a signal as in the following equation 11, which is the sum of a signal obtained by legging the signal at the fourth input terminal X 4  by the-resistor R 4  and a signal obtained by leading the signal at the first input terminal X 1  by the capacitor C 4 .  
                     Y   ⁢           ⁢   1     =       ⁢       [         A   2     ⁢   ∠     -     90   ⁢   °     -     45   ⁢   °     +       B   2     ⁢   ∠90°     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠180°     +     45   ⁢   °     +       B   2     ⁢   ∠180°     +     45   ⁢   °       ]                 =       ⁢     A   ⁢           ⁢   ∠225°                   [     Equation   ⁢           ⁢   8     ]                       Y   ⁢           ⁢   2     =       ⁢       [         A   2     ⁢   ∠180°     -     45   ⁢   °     +       B   2     ⁢   ∠180°     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠90°     +     45   ⁢   °     +       B   2     ⁢   ∠     -     90   ⁢   °     +     45   ⁢   °       ]                 =       ⁢     A   ⁢           ⁢   ∠315°                   [     Equation   ⁢           ⁢   9     ]                       Y   ⁢           ⁢   3     =       ⁢       [         A   2     ⁢   ∠90°     -     45   ⁢   °     +       B   2     ⁢   ∠     -     90   ⁢   °     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠0°     +     45   ⁢   °     +       B   2     ⁢   ∠0°     +     45   ⁢   °       ]                 =       ⁢     A   ⁢           ⁢   ∠45°                   [     Equation   ⁢           ⁢   10     ]                       Y   ⁢           ⁢   4     =       ⁢       [         A   2     ⁢   ∠0°     -     45   ⁢   °     +       B   2     ⁢   ∠0°     -     45   ⁢   °       ]     +                     ⁢     [         A   2     ⁢   ∠     -     90   ⁢   °     +     45   ⁢   °     +       B   2     ⁢   ∠90°     +     45   ⁢   °       ]                 =       ⁢     A   ⁢           ⁢   ∠315°                   [     Equation   ⁢           ⁢   11     ]             
 
         [0084]     As can be seen from the above equations 8 to 11, only the target data A of the first channel is outputted under the condition that the image data B is removed.  
         [0085]     On the other hand, when the second DMB channel DMB-CH 2  is selected as shown in  FIG. 6   c , the polyphase filter  650  performs the same operation as that for the third DMB channel if an oscillation frequency is set lower than the frequency of the second DMB channel, and the same operation as that for the first DMB channel if the oscillation frequency is set higher than the frequency of the second DMB channel.  
         [0086]     As described above, the present invention proposes an image rejection mixer which allows an image of a selected one of DMB channels of a TV channel for terrestrial DMB to be present in the TV channel, such that it is appropriate to be applied to a terrestrial DMB tuner of a low IF structure. Therefore, the proposed image rejection mixer can be made with small size and at low cost and operated at low power. This invention also proposes a terrestrial DMB tuner with such an image rejection mixer.  
         [0087]      FIGS. 6   a  to  6   c  illustrate the channel selection operation of the terrestrial DMB tuner of the low IF structure according to the present invention.  
         [0088]     With reference to  FIG. 6   a , in the image rejection mixer of the present invention, when the first DMB channel DMB-CH 1  is selected, the first signal selector  641  is turned on by the first switching signal SS 1  to locate the oscillation frequency for selection of the first DMB channel DMB-CH 1  above the DMB channel DMB-CH 1 .  
         [0089]     With reference to  FIG. 6   b , in the image rejection mixer of the present invention, when the third DMB channel DMB-CH 3  is selected, the second signal selector  642  is turned on by the second switching signal SS 2  to locate the oscillation frequency for selection of the third DMB channel DMB-CH 3  beneath the DMB channel DMB-CH 3 .  
         [0090]     With reference to  FIG. 6   c , in the image rejection mixer of the present invention, when the second DMB channel DMB-CH 2  is selected, the first signal selector  641  is turned on by the first switching signal SS 1  or the second signal selector  642  is turned on by the second switching signal SS 2 , to locate the oscillation frequency for selection of the second DMB channel DMB-CH 2  above or beneath the DMB channel DMB-CH 2 .  
         [0091]     As apparent from the above description, the present invention provides a terrestrial DMB tuner which is applied to a mobile communication terminal, such as a mobile phone, and an image rejection mixer applied thereto. The image rejection mixer can be implemented with a single IC by excluding an external device, so that it can be made with small size and at low cost and operated at low power.  
         [0092]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.