Abstract:
A multi-band receiver is disclosed. The multi-band receiver includes a low-noise amplifier (LNA) and a mixer. The LNA includes a switched receiving circuit, a loading circuit, and a switching circuit. The switched receiving circuit has a first receiving circuit for receiving a first signal corresponding to a first frequency, and a second receiving circuit for receiving a second signal corresponding to a second frequency. The loading circuit is utilized for providing a specific load to the switched receiving circuit. The switching circuit is used for controlling whether the first signal or the second signal is transferred to the loading circuit. The mixer is coupled to the low-noise amplifier for receiving an output signal generated from the LNA and for down-converting the output signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates to a wireless communication system, and more particularly, to a multi-band receiver utilized in a wireless communication system.  
         [0003]     2. Description of the Prior Art  
         [0004]     WLAN is a fast-developing and fast-changing technique both in its standard and its applications. For example, 802.11a utilizes a 5 GHz transmission band, but 802.11b and 802.11g utilize a 2.4 GHz transmission band. Furthermore, 802.11a has the disadvantage of short transmission distance and is inconvenient, because the 5 GHz transmission band is not available in some regions. Therefore, few products are designed which will only support 802.11a. The next generation of WLAN techniques and WLAN products will simultaneously support 802.11a and 802.11g so that the transmission efficiency can be raised and the number of users can be increased. Furthermore, the transmission quality demands can be met.  
         [0005]     Nowadays, a dual-band/multi-band receiver comes in two categories. The first category establishes multiple receivers in a chip. In “A triple-band 900/1800/1900 MHz low-power image-reject front-end for GSM” ISSCC of Tech. Papers, pp. 408-409, Feb. 2001, a multi-band receiver is disclosed. Please refer to  FIG. 1 , which is a block diagram of the multi-band receiver  100  according to the prior art. As shown in  FIG. 1 , the multi-band receiver  100  comprises three single-band receivers  110 ,  120 , and  130 . Each single-band receiver  110 ,  120 ,  130  comprises a low-noise amplifier (LNA)  112 ,  122 ,  132  for receiving an RF signal RF 1 , RF 2 , RF 3  (for example, 900 MHz, 1800 MNz, and 1900 MHz RF signals, respectively), a band-pass filter  114 ,  124 ,  134 , and a mixer  116 ,  126 ,  136 . Because three independent single-band receivers  110 ,  120 ,  130  are set up in a chip, the chip area is substantially occupied.  
         [0006]     The second category utilizes a single circuit to achieve a multi-band receiver. For example, in “A SiGe low noise amplifier for 2.4/5.2/5.7 GHz WLAN applications”, IEEE international solid-state circuits conference, pp 364-365, San Francisco, USA February 2003, a multiple-band receiver is disclosed. The multi-band receiver is produced through an HBT producing procedure of SiGe. Please refer to  FIG. 2 , which is a diagram of an LNA of another receiver according to the prior art. In “Concurrent dual-band CMOS low noise amplifiers and receiver architectures, Symp.” on VLSI Circ. Dig., pp. 247-250, Jun. 2001, another receiver is disclosed. The LNA  200  successfully utilizes a CMOS producing procedure to achieve the purpose of “dual-band”. But as shown in  FIG. 2 , this circuit structure needs a lot of inductors to generate at least two central frequencies corresponding to dual-band, furthermore, because of the frequency response of the LC tank  220 , the LNA  220  amplifiers produce unwanted noise when only receiving a signal with a specific frequency.  
         [0007]     In U.S. Pat. Nos. 6,072,996 and 6,658,237, further multi-band receivers are disclosed. In these two patents, the multi-band receiver is achieved through establishing multiple receivers in a chip, and thus the details are omitted here.  
       SUMMARY OF THE INVENTION  
       [0008]     It is therefore one of the primary objectives of the claimed invention to provide a multi-band receiver, to solve the above-mentioned problem.  
         [0009]     According to an exemplary embodiment of the claimed invention, a multi-band receiver is disclosed. The multi-band receiver comprises: a low-noise amplifier (LNA) receiving a received signal and thereby outputting an amplified signal, comprising: a receiving module comprising: a first receiving circuit for receiving a first signal of the received signal corresponding to a first frequency; and a second receiving circuit for receiving a second signal of the received signal corresponding to a second frequency; a loading circuit coupled to the receiving module for providing a load to the receiving module; and a switching circuit for controlling the transfer of the first signal or the second signal to the loading circuit; and a mixer coupled to the low-noise amplifier for converting the amplified signal into an output signal.  
         [0010]     In addition, a method for receiving a received signal and thereby outputting an output signal is disclosed. The method comprises: receiving a first signal of the received signal corresponding to a first frequency by a first receiving circuit; and receiving a second signal of the received signal corresponding to a second frequency by a second receiving circuit; providing a load of a loading circuit; and determining the transfer of the first signal or the second signal to the loading circuit and thereby outputting the output signal; wherein the first and the second receiving circuits share the loading circuit.  
         [0011]     Furthermore, a multi-band receiver is disclosed. The multi-band comprises: an amplifier for amplifying a received signal and thereby outputting an amplified signal, wherein the received signal has a first signal corresponding to a first frequency and a second signal corresponding to a second signal; and a mixer coupled to the amplifier, comprising: a multi-band LC tank for providing a first impedance at the first frequency and a second impendence at the second frequency to remove a common-mode noise of the amplified signal; and a mixing circuit coupled to the multi-band LC tank for converting the amplified signal into an output signal.  
         [0012]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a block diagram of a multi-band receiver according to the prior art.  
         [0014]      FIG. 2  is circuit diagram of an LNA according to the prior art.  
         [0015]      FIG. 3  is a diagram of a multi-band receiver according to the present invention.  
         [0016]      FIG. 4  is a diagram of another embodiment of a tunable LC tank of a mixer shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION  
       [0017]     Please refer to  FIG. 3 , which is a diagram of a multi-band receiver  700  according to the present invention. The multi-band receiver  700  comprises an LNA  300  and a mixer  400 . As shown in  FIG. 3 , the LNA  300  comprises a 2.4 G matching network  310 , a 5 G matching network  320 , a gain cell  330  coupled to the 2.4 G matching network  310 , a gain cell  340  coupled to the 5 G matching network  320 , a switching circuit  350  coupled to the gain cells  330 ,  340 , and a loading circuit  370  coupled to the switching circuit  350 . Here, the matching circuits  310 ,  320  are respectively utilized to receive input signals S 1 , S 2  with specific frequencies (2.4 GHz and 5 GHz), where the functions and structures of the matching circuits  310 ,  320  are well known, and thus omitted here. The gain cells  330 ,  340  cooperate with the loading circuit  370  to amplify the received input signal S 1 , S 2 . Please note that in this embodiment, the loading circuit  370  is utilized to fixedly provide a specific load to the gain cells  330 ,  340 . For example, as shown in  FIG. 1 , the capacitance Cd and the inductance Ld of the loading circuit  370 , the transistors M 1 , M 2  and the inductances L s1 , L s2  of the gain cells  330 ,  340  can be selected. Therefore, the LNA  300  for supporting different frequency bands (2.4 GHz and 5 GHz) can be normalized by adjusting the above-mentioned devices such that the LNA  300  can have the most appropriate characteristic. For example, the LNA  300  can have almost the same small-signal gain at both 2.4 GHz and 5 GHz.  
         [0018]     The operation of the LNA  300  is illustrated as follows. First, the external control circuit (now shown) generates two signals EN 1  and EN 2  to control the switching circuit  350  according to the frequency of a received signal to select the transmission route of the received signal. As shown in  FIG. 3 , the switching circuit  350  comprises two switches (transistors M 3  and M 4 ) for receiving the signals EN 1  and EN 2 . For example, if the frequency of the signal to be received is 2.4 GHz, the control circuit sends the signal EN 1  to turn on the transistor M 3  in order to establish the electrical connection between the loading circuit  370  and the gain cell  330 , and sends another signal EN 2  to turn off the transistor M 3  in order to break the electrical connection between the loading circuit  370  and the gain cell  340 . Therefore, the 2.4 G matching circuit  310  receives a 2.4 GHz input signal S 1  from a previous-stage circuit (such as an antenna or a front-end processing device), the gain cell  330  and the loading circuit  370  amplify the received input signal S 1 , and the loading circuit  370  transfers the amplified input signal S 1  to a next-stage circuit (here, the next-stage circuit is the mixer  400 ). Similarly, another 5 GHz input signal can be processed through a similar operation, and details are thus omitted here.  
         [0019]     In this embodiment, because the capacitance and inductance of the loading circuit are set before the above-mentioned operation (this also means that the capacitance Cd and the inductance Ld both have specific impedances), the loading circuit  370  can be achieved through a tunable LC tank. The tunable LC tank can be dynamically adjusted in the operation. In other words, the frequency of the tunable LC tank can be adjusted to be 2.4 GHz when the 2.4 GHz input signal S 1  is received. This also obeys the spirit of the present invention.  
         [0020]     As shown in  FIG. 3 , the mixer comprises a mixing circuit  410  and a tunable LC tank  420 , where the mixing circuit  410  comprises a plurality of transistors M 5 -M 10 , resistors R 1  and R 2 , and a capacitor C. Please note that the function and the circuit structure of the mixing circuit  410  are already well known, and thus omitted here. In fact, the mixer  400  is quite similar to a prior art mixer. The mixer  400  is utilized to receive the RF signal from the previous stage circuit (here, the previous stage is the LNA  300  shown in  FIG. 3 ), and the mixing circuit  410  is utilized to reduce the frequency of the received RF signal. In this embodiment, the tunable LC tank  420  is electrically connected to the mixing circuit  410 . As shown in  FIG. 3 , the tunable LC tank  420  comprises an inductance L′ and a tunable capacitance Cv′, which can be dynamically adjusted according to the RF signal. Therefore, the tunable LC tank  420  can provide high impedance in a predetermined frequency to eliminate the common-mode noise. For example, if the LNA  300  receives a 2.4 GHz RF signal, the central frequency of the tunable LC tank  420  can be set as 2.4 GHz, and if the LNA  300  receives a 5 GHz RF signal, the central frequency of the tunable LC tank  420  can be set as 5 GHz.  
         [0021]     In another embodiment, the tunable LC tank  420  can be achieved through a capacitor and a tunable inductor. The tunable LC tank  420  can also be a tunable LC tank having two central frequencies. For example, the tunable LC tank  420  can comprise two central frequencies, which are 2.4 GHz and 5 GHz. Please refer to  FIG. 4 , which is a diagram of another embodiment of a tunable LC tank  520  of a mixer  400  shown in  FIG. 3 . As shown in  FIG. 4 , the tunable LC tank  520  comprises a tunable capacitor  550 , a capacitor  540 , and a first inductor  530 . In a preferred embodiment, the LC tank  520  further comprises a second inductor (not shown) coupled in parallel with the tunable capacitor  55 . Therefore, the tunable LC tank  520  can comprise two different central frequencies, and can dynamically adjust the two central frequencies. Please note that the tunable LC tank  520  can also be achieved through a tunable inductor, a capacitor, and an inductor. This also obeys the spirit of the present invention. Furthermore, as known by those skilled in the art, the capacitor  540  can be a tunable capacitor. In other words, all tunable LC tanks having two different central frequencies can be embodied. The above-mentioned changes all obey the spirit of the present invention.  
         [0022]     Moreover, please note that in this embodiment, the LNA  300  is utilized to receive signals with two different frequencies, however, only one other corresponding circuit has to be added. For example, if another 6 GHz RF signal has to be received, only a 6 GHz matching circuit and a corresponding gain cell have to be added so that the new LNA  300  can receive signals with three different frequencies. Therefore, the number of frequencies of the received signals is only utilized as an illustration of the present invention, not a limitation.  
         [0023]     In this embodiment, the LNA  300  utilizes a single-end device (that is, the device is a single-input and single-output device). Therefore, the mixer  400  is also selected to be a single-end device to coordinate with the single-end LNA  300 . This can save the pin number when the chip is packaged. However, in fact, a differential device can also be selected to have both an input signal and output signal. This also obeys the spirit of the present invention.  
         [0024]     Furthermore, in this embodiment, the inductors L s1 , L s2  shown in  FIG. 3  can be achieved through the metal wire when the chip is packaged, and the inductor Ld can be achieved through an inductor formed because of the semiconductor procedure. Therefore, the chip area is saved. Furthermore, the present invention can be achieved through the CMOS producing procedure. In addition, because the tunable LC tank can be utilized to remove the common-mode noise, the signal receiving quality is raised by a great amount.  
         [0025]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.