Abstract:
There are various mobile communication standards such as GSM, EDGE, and W-CDMA. For a GSM or EDGE system, a receiver must be configured to work with an IF signal with a center frequency and bandwidth of 200 KHz. For WCDMA system, the same receiver must be configured to work with an IF signal with a center frequency of 600 KHz to 1000 KHz and band width of 2000 KHz. Accordingly, a configurable frequency IF filter with the capability to operate with multiple standards is provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/813,375 filed Jun. 14, 2006, which is incorporated herein by reference in its entirety. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a reconfigurable/programmable intermediate frequency (IF) filter. 
       BACKGROUND OF THE INVENTION 
       [0003]    Today many radio frequency (RF) receivers are super heterodyne receivers.  FIG. 1  illustrates a RF receiver  100  that employs the heterodyne principle to down-convert and demodulate data from a RF signal. Generally, data are transmitted on a high frequency signal because of the intrinsic relationship between the RF&#39;s wavelength and the size of an antenna. The high frequency signal used to piggy back an information signal of lower frequency is called a carrier signal. 
         [0004]    In an heterodyne system, a carrier signal is removed from a transmitted RF signal by mixing the received signal with another locally generated signal. The mixing process yields several signals at various frequency bands. The frequency band of interests is the intermediated frequency (IF) of the system, which contains data signals in modulated form. As illustrated in  FIG. 1 , RF signals received by an antenna are amplified and outputted to a mixer  110 . The output of mixer  110  is inputted into an IF filter  120 . IF filter  120  performs several important functions such as image rejection, amplification, and bandpass filtration. Depending upon the application, IF filter  120  may be a Bessel filter or more commonly a Butterworth filter. The latter is designed to provide a maximum frequency plateau of minimum ripple across the bandpass frequency of the filter. The former is designed to perform in the substantially the same way but with a time delay. 
         [0005]    Currently there are several co-existing communication standards such as: global system for mobile communication (GSM), a second generation (2G) technology; universal mobile telecommunications system (UMTS), a third generation technology (3G) (UMTS is also known as wideband code division multiple access (W-CDMA)); enhanced data GSM environment (EDGE); and CDMA2000. Each standard typically operates at a different IF frequency and has a different bandwidth. Thus, each standard requires a different IF filter configuration. 
         [0006]    One class of filters with a high frequency response is the transconductor capacitor (G m C) filter.  FIG. 2  illustrates a conventional transconductor circuit  200  used to implement a G m C filter. Circuit  200  includes a pair of transistors  202  and  204 , a pair of resistors  206  and  208 , and a pair of current sources  210  and  212 . The differential input voltages are received by the gates of transistors  202  and  204 . In operation, transistor  202  outputs a current (I out ) when it is biased by a differential voltage (V in+ ). The ratio of the output current and input voltage defines the transconductance (G m ) of transistor  202 . Thus, the G m  of circuit  200  is: 
         [0000]    
       
         
           
             
               G 
               m 
             
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                 ∂ 
                 
                   I 
                   out 
                 
               
               
                 ∂ 
                 
                   V 
                   
                     i 
                      
                     
                         
                     
                      
                     n 
                   
                 
               
             
           
         
       
     
         [0007]    To increase the linearity of circuit  200 , degenerative resistors  206  and  208  are coupled between the sources of transistors  202  and  204 . Further, each source of transistors  202  and  204  is independently biased by current source  210  and  212 . In this configuration, DC current flow through resistors  206  and  208  is not present and only AC current flow is allowed. This yields a transconductor with a better performance due to the elimination of voltage drop across the degenerative resistors. 
         [0008]    As mentioned, each communication standard operates at a different IF frequency and bandwidth. Hence a receiver is typically designed to work optimally with a certain communication standard. For example, a GSM or EDGE compatible receiver must be configured to work with an IF signal with a center frequency of 200 KHz. For WCDMA, the same receiver must be configured to work with an IF signal with a center frequency of 600 KHz to 1000 KHz. Hence, in current receiver systems, a specific set of filters is designed and manufactured for each communication standard. 
         [0009]    Accordingly, what is needed is a filter stage that can be implemented across various communication standards. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         [0010]    The present invention is described with reference to the accompanying drawings. 
           [0011]      FIG. 1  illustrates a block circuit diagram of a conventional receiver. 
           [0012]      FIG. 2  illustrates a circuit diagram of a conventional transconductor. 
           [0013]      FIG. 3  illustrates a block circuit diagram of a transceiver according to an embodiment of the present invention. 
           [0014]      FIG. 4  illustrates a circuit diagram of a G m -C bandpass filter. 
           [0015]      FIG. 5  illustrates a circuit diagram of a G m -C bandpass filter according to an embodiment of the present invention. 
           [0016]      FIG. 6A  illustrates a circuit diagram of a switchable Gm cell implemented in the circuit of  FIG. 5 . 
           [0017]      FIG. 6B  illustrates a circuit diagram of two parallel Gm cells implemented in the circuit of  FIG. 5 . 
           [0018]      FIG. 7  illustrates a circuit diagram of an adjustable resistance G m  stage. 
           [0019]      FIG. 8  illustrates a Gain (dB) vs. frequency chart for the filter of  FIG. 5  under various operating modes. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    This specification discloses one or more embodiments that incorporate the features of this invention. The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. An embodiment of the present invention is now described. While specific methods and configurations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the art will recognize that other configurations and procedures may be used without departing from the spirit and scope of the invention. 
         [0021]      FIG. 3  illustrates a wireless receiver  300 , according to an embodiment of the present invention, that includes an antenna  305 , a low noise amplifier (LNA)  310 , mixers  315  and  320 , a local oscillator  325 , filter stages  340  and  345 , amplifiers  350  and  355 , and a pair of analog to digital converters (ADC)  360  and  365 . 
         [0022]    RF signals received by antenna  305  are forwarded to LNA  310 . The received RF signals are single-ended RF signals. Depending upon the communication standard used, the received RF signals typically range from 800 MHz to 2.1 GHz. For example, a GSM network may be implemented at 800 MHz or 1.9 GHz. W-CDMA is typically implemented at 2.1 GHz. 
         [0023]    In receiver  300 , LNA  310  amplifies RF signals and provide a low noise amplification. The amplified signals are then converted into a in-phase (I) signal portion and a quadrature (Q) signal portion by mixers  315  and  320 , respectively. Mixers  315  and  320  also down converts each of the I and Q signals to a lower frequency signal. Both mixers  315  and  320  operate in the substantial same way, as such only the operation of mixer  315  will be described. Mixer  315  mixes the I signal (or Q) with a local signal  327  that is generated by a voltage control oscillator (VCO)  325 . The frequency of signal  327  is generally selected to match with the frequency of the carrier signal of the received RF signal. In this way, the input I signal or (Q signal) is down converted to an intermediate frequency signal by mixer  315 . For a GSM or EDGE system, the IF signal has a center frequency of 200 KHz and a frequency bandwidth of 200 KHz. For a W-CDMA system, the IF signal has a center frequency of 1 MHz and a frequency bandwidth of 1.8 MHz. 
         [0024]    Although not necessarily required, the IF signal may be amplified by an IF amplifier (not shown). After amplification, the IF signal is feed through filter stage  340 . Filter stage  340  comprises several stages of filters and variable gain amplifiers. Filter stage  340  also performs DC offset rejection, signal amplification, and bandpass filtration. In this way, the IF signal may be processed to obtain the proper gain and frequency bandwidth. For example, filter stage  340  comprises multiple stages of Butterworth filters. Alternatively, filter stage  340  comprises multiple stages of Chebyshev or Bessel filters. A combination of Butterworth, Chebyshev or Bessel filters may also be used. Filter stage  345  is implemented in the same way as filter stage  340 . 
         [0025]    As shown in  FIG. 3 , the output of filter stage  340  or  345  may further be amplified using amplifier  350  and  355 . This amplification stage is optional and generally depends on the application. After the final amplification stage, the amplified signal is routed to an analog to digital converter (ADC)  360 / 365 . 
         [0026]    Conventionally, receiver  300  is implemented using a conventional transconductor capacitor (G m C) bandpass (BP) filter  400 , such as the one shown in  FIG. 4 . Filter  400  will be further discussed below. In an embodiment, receiver  300  is implemented using a reconfigurable G m C bandpass filter  500 , such as the one shown in  FIG. 5 . G m C bandpass filter  500  will be further discussed below. 
         [0027]    As shown in  FIG. 4 , G m C bandpass filter  400  includes three G m  stages  410 ,  420 , and  430 , a resistor  440 , a capacitor  450 , a second resistor  460 , and a second capacitor  470 . Resistor  440  and capacitor  450  provide a first resistor-capacitor pair that generally determines the bandwidth frequency of filter  400 . Resistor  460  and capacitor  470  provide a second resistor-capacitor pair that determines the center frequency of filter  400 . For example, to adjust the bandwidth of filter  400 , the RC constant of the resistor  440 -capacitor  450  pair may be adjusted. To adjust the center frequency, the RC constant of the resistor  460 -capacitor  470  pair may be adjusted. Further, G m  stage  410  is used to primarily control the overall signal gain of filter  400 . G m  stages  420  and  430  are used to primarily control the overall frequency response of filter  400 . 
         [0028]    As shown in  FIG. 5 , reconfigurable G m C bandpass filter  500  includes three programmable G m  stages  510 ,  520 , and  530 , a resistor  540 , a capacitor  550 , a second resistor  560 , and a second capacitor  570 . In an embodiment, capacitors  550  and  570  are adjustable capacitors. These adjustable capacitors serve to compensate for the system processing corners or variations. Adjustable capacitors are well known in the art. Further, resistors  540  and  560  may also be adjustable and may be implemented as a variable G m  stage as shown in  FIG. 7 , which will be further discussed below. 
         [0029]    In an embodiment, G m  stages  510 ,  520 , and  530  are programmable or reconfigurable in order to increase the overall G m  of the system. In this manner, the frequency response of the filter may be manipulated. As mentioned, the frequency response of a filter may be affected by changing the time constant of the filter, the RC value. The general relationship between RC, G m  and frequency is: 
         [0000]      frequency∞ 1/ RC=G   m   /C    
         [0030]    Thus, to affect the frequency response of a filter, one may manipulate the capacitance or the G m  value of the circuit. Filter  500  is a multi-standard filter because its G m  may be adjusted such that the frequency response of filter  500  is dramatically changed. Again, the IF center frequency of a GSM system is approximately 200 KHz and approximately 1000 KHz for a W-CDMA system. Filter  500  can operate in either environment by increasing or lowering the G m  value of G m  stages  510 ,  520 , and  530 . For W-CDMA application, the G m  of the G m  stages  510 ,  520 , and  530  has to be increased relative to where the G m  value is set at for GSM application. 
         [0031]    Each of the G m  modules or stages includes two G m  circuits connected in parallel. G m  stage  510  includes G m  circuits  512  and  514 . G m  stage  520  includes G m  circuits  522  and  524 . G m  stage  530  includes G m  circuits  532  and  534 . Each of the G m  circuits  514 ,  524 , and  534  has a larger G m  value than its respective parallel G m  stage. In an embodiment, the G m  of circuit  514  is 10 times greater than G m  of circuit  512 . Similarly, the G m  of circuits  524  and  534  is 10 times greater than the G m  of circuits  522  and  532 , respectively. Even though the each of the G m  of circuits  514 ,  524 , and  534  is 10 times greater than the Gm of the respective parallel circuit, other multiples could also be employed such as 15×, 20×, etc. 
         [0032]    In GSM mode, G m  circuits  512 ,  522 , and  532  are enabled and G m  circuits  514 ,  524 , and  532  are disabled to provide the necessary gain and IF characteristics for GSM operation. In this manner, only one of the G m  circuit of the parallel circuit pair (e.g. G m  stage  510  or  520 ) is enabled at any time. In W-CDMA mode, G m  circuits  512 ,  522 , and  532  are disabled and G m  circuits  514 ,  524 , and  532  are enabled to provide the necessary gain and IF characteristics for W-CDMA operation. In this manner, filter  500  exhibits a larger overall G m  and yields a larger IF frequency (compared to GSM) as required W-CDMA. Alternatively, both circuits of the parallel circuit pair could be enabled at the same time. However, the G m  ratio of the circuit pair would have to be manipulated such that the parallel G m  circuit pair would yield a desired G m  value. For example, G m  stage  510  may be configured such that both G m  circuits  512  and  514  are enabled in GSM mode, and 1 of the G m  circuits  512  and  514  is disabled in W-CDMA mode. 
         [0033]    The G m  of a G m  circuits may be controlled with many methods. One of the methods is to manipulate the transistor&#39;s channel width and length ratio. For a transistor, the relationship between G m , channel length, and width is: 
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         [0000]    In filter  500 , the surface area of G m  circuits  512  and  514  are generally the same. The major difference is in the W/L ratio of transistors in each of the circuits. For example, the W/L ratio for circuit  512  may be 2μ/6.3μ≈0.317. For circuit  514 , the W/L ratio is reversed, 6.3μ/2μ≈3.150. 
         [0034]      FIG. 6A  illustrates a G m  cell  600 , which is one example embodiment that can be used to implement the G m  circuit of G m  stages  510 ,  520 , and  530 . G m  cell  600  includes two positive channel metal oxide semiconductors (PMOS)  610  and  620  and two negative channel MOS (NMOS)  630  and  640 . The gates of PMOS  620  and NMOS  630  are coupled to a voltage input. The drains of PMOS  620  and NMOS  630  are coupled together and comprise the output node in which I out  is obtained. The source of PMOS  610  is coupled to a voltage controller. The drain of PMOS  610  is coupled to the source of PMOS  620 . The source of NMOS  640  is coupled to ground and the drain is coupled to the source of NMOS  630 . Both the gates of PMOS  610  and NMOS  640  are coupled together and to a gain controller. Further an inverter is coupled to the gate of PMOS  610 . 
         [0035]      FIG. 6B  illustrates an example implementation of G m  stage  510 . As shown, G m  stage  510  includes G m  circuits  512  and  514  connected in parallel. The input nodes of circuits  512  and  514  are commonly coupled to an input source (not shown). Similarly, the output nodes of circuits  512  and  514  are commonly coupled to an output node. Transistors  605 A-B,  615 A-B,  625 A-B, and  635 A-B are similar to transistors  610 ,  620 ,  630 , and  640  of G m  cell  600 . In the preferred embodiment, while in GSM mode, the cell with a smaller G m  value is enabled and the cell with the larger G m  value is disabled. In W-CDMA mode, the cell with the larger G m  value is enabled and the cell with the smaller G m  value is disabled. To disable cell  600 , PMOS  610  and NMOS  640  are both disabled. In this way, the physical connection is maintained but G m  contribution of the cell is eliminated. As mentioned, G m  stage  510  may be configured such that both cells are enabled for GSM mode and only 1 of the cell is enabled for W-CDMA mode or vice versa. Such configuration may be readily performed by one skilled in the art. 
         [0036]      FIG. 7  illustrates a G m  stage  700 , which is one example embodiment that can be used to implement adjustable resistors  540  and  560 . G m  stage  700  includes 4 GM cells arranged as shown such that the total resistance is: 
         [0000]    
       
         
           
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         [0000]    When G m  stage  700  is being implemented as resistor  540 , the change in G m  affects the bandwidth of G m C filter  500 . When G m  stage  700  is being implemented as resistor  560 , the change in G m  affects the center frequency of G m C filter  500 . As such, G m  stage  700  may be regulated to obtain a desired bandwidth and center frequency. Gm can be regulated the same way as illustrated in  FIG. 6A  and  FIG. 6B . 
         [0037]      FIG. 8  illustrates a frequency vs. dB chart for GmC filter  500 . As shown, the frequency bandwidth of W-CDMA mode is approximately 10 times the bandwidth of GSM mode. 
       CONCLUSION 
       [0038]    While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.