Abstract:
A tuning method and a tuning apparatus for tuning a filter are disclosed. The tuning method includes: configuring the filter as a VCO; utilizing the VCO to generate an oscillation signal according to a driving signal; comparing a frequency of the oscillation signal with a reference frequency to generate a comparison result; converting the comparison result into the driving signal in order to establish a feedback mechanism. Therefore, the inner components such as the gm and capacitance inside the VCO are completely tuned when the VCO generates an oscillation signal having a wanted frequency. Since the VCO is inside the filter and the components of the filter and the VCO are similar, the driving signal can be utilized to make the filter operate in a desired center frequency under a well-designed relationship between the frequency of the oscillation signal and the center frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/597333, which was filed on Nov, 23, 2005 and entitled “APPARATUS AND METHOD FOR FILTER FREQUENCY TUNING”. 
     
    
     BACKGROUND  
       [0002]     The invention relates to a tuning apparatus and method, and more particularly, to an apparatus for tuning a center frequency of a filter and related method thereof.  
         [0003]     Filters have been widely used in many applications. In general, the implementations of the filters can be roughly divided into two types of structures, which are discrete-time switch-capacitor filters and continuous-time filters (including gm-C filter, MOSFET-C filter, etc.). Because of the limitation of clock frequency, a high-frequency filter is often implemented by the continuous-time structure. Furthermore, in different types of continuous-time filters, the gm-C filter is the most general.  
         [0004]     However, the variation degree of some characteristics, such as cut-off frequency (center frequency), of the continuous-time filter is often larger than 30% due to the influences of variations in the manufacturing process and temperature variations. Therefore, a tuning mechanism should be added into the filter to overcome the process and temperature variations such that the frequency response of the filter is not affected by the influences of the process and temperature variations.  
         [0005]     In the gm-C filter structure, the cut-off frequency (center frequency) can be represented by the following equation w c =K*g m,u /C, where g m,u  represents a gm(trans-conductance) value of a trans-conductor per unit, C represents a total capacitance value corresponding to a node, and K is a scaling factor larger than 0. From the above equation, it can be seen that when the center frequency w c  deviates from a target value, the gm value g m,u  or the capacitance value C can be adjusted to tune the center frequency w c  back to the target value. In order to achieve the purpose of tuning the center frequency w c , either the gm value g m,u  or the capacitance value C can be adjusted. Please note that adjusting the gm value g m,u  or the capacitance value C is equivalent. The spirit of the two adjusting methods is the same.  
         [0006]     Taking the method of adjusting the gm value g m,u  as an example, please refer to  FIG. 1 , which shows a conventional tuning structure  100  of adjusting the gm value g m,u  of a main filter  110 . Please note, the main filter  110  is a target filter to be tuned. Furthermore, in  FIG. 1 , the tuning operation is performed by a PLL (including FD  120  the charge pump  121  and the loop filter  122 ) cooperating with a VCO  130 . Please note, under the tuning structure  110  shown in  FIG. 1 , the VCO  130  is better composed of the same trans-conductor cells as those of the main filter  110 , the VCO  130  has the same environment (e.g., loading, etc.) as the main filter  110 , and the gm value of the trans-conductor circuits of the VCO  130  and the main filter  110  are controlled by the same control signal Vc. Therefore, if the tracking relationship between the VCO  130  and the main filter  110  is better, the tuning operation of the tuning structure can be more accurate. In other words, when the VCO  130  is tuned, the main filter  110  is also tuned because they have similar environment. Assume that the center frequency w c  of the main filter  110  is ideally equal to the K*g m,u /C, and the oscillation frequency w o  of the VCO  130  is equal to N*g m,u /C. When the PD  120  of the PLL is locked to a certain frequency, the control signal Vc is adjusted to change the gm value g m,u  such that the f o =(½π) (N*g m,u /C)=f ref . As mentioned previously, the main filter  11   0  and the VCO  130  has a good tracking relationship (e.g., they have the same gm value g m,u ). Therefore, f c =(½π) (K*g m,u /C)=(K/N) f o =(K/N) f ref . Obviously, if the values K, N, and f ref  can be selected properly, the center frequency of the main filter  110  can be tuned to a target frequency.  
         [0007]     Please refer to  FIG. 2 , which is a diagram of another conventional tuning structure  200 . Please note, the tuning structure  200  shown in  FIG. 2  utilizes a similar concept. The tuning structure  200  utilizes similar trans-conductor cells to form the master filter  230  (in general, the master filter shown in  FIG. 2  has lower levels), and utilizes the characteristic of the master filter  230  to perform the tuning tasks. For example, a two-level Biquad LPF has a 90 degree phase delay at the point where w o =N*g m,u /C. Therefore, when the signal having the frequency f ref  is inputted into the master filter  230 , the entire tuning structure  200  utilizes the phase detector (PD)  220  to determine whether the phase difference is 90 degrees. Additionally, recall as mentioned previously, the feedback mechanism is implemented by a PLL including a charge pump  221  and a loop filter  222 , the negative feedback mechanism adjusts the control signal Vc to make the f o =(½π) (N*g m,u /C)=f ref . From the above-mentioned structure, it can be easily seen that f c =(½π) (K*g m,u /C)=(K/N) f o =(K/N) f ref . Therefore, the tuning structure  200  shown in  FIG. 2  can also achieve the same tuning goal.  
         [0008]     The above-mentioned structures both needs a PLL including a PD, a charge pump, and a loop filter. It is known that the PLL occupies a larger area and as one result, this increases the cost. Please refer to  FIG. 3 , which is a diagram of another conventional tuning mechanism  300 . The tuning mechanism  300  utilizes a digital circuit  320  to perform a negative feedback control. The entire tuning method shown in  FIG. 3  is more similar to the tuning structure shown in  FIG. 1 . The difference between the tuning structures shown in  FIG. 3  and  FIG. 1  is that the tuning structure shown in  FIG. 3  utilizes a digital circuit  320  (i.e., a digital FD) to compare the frequency f ref  with the frequency f c  generated by the VCO  330  instead of utilizing a PLL. Thereby, the comparison result is transformed into a control signal through a DAC  340  in order to adjust the frequency f c . Similarly, because of the tracking relationship between the VCO  330  and the main filter, when the oscillation signal of the VCO  330  is tuned, the cut-off frequency of the main filter  310  can be tuned successfully.  
         [0009]     The influences caused by the process and temperature variations upon the frequency f c , can be alleviated through the above-mentioned tuning mechanisms. Obviously, the above-mentioned tuning mechanisms need either a VCO or a master filter. Furthermore, either the VCO or the master filter is often a two-level system. In addition, in order to make the environment similar to the main filter, all dummy devices, dummy loading, and other circuits, which have originally been set up in the main filter, also have to be copied and implemented inside the tuning structure (e.g., the above-mentioned VCO or master filter) to make the environment similar. In most of the applications, the tuning structure often occupies a huge area larger than 20% of the entire circuit. Therefore, the above-mentioned tuning mechanism consumes a large area and high cost resulting in an uneconomical solution.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore one of the primary objectives of the claimed disclosure to provide a tuning apparatus, to solve the above-mentioned problem.  
         [0011]     According to an exemplary embodiment of the claimed disclosure, a tuning apparatus for tuning a filter is disclosed. The filter comprises a voltage-controlled oscillator (VCO) circuit. The tuning apparatus comprises: an enabling circuit, electrically connected to the filter, for controlling the filter to enter a tuning mode by disabling the entire filter except for the VCO to thereby allow the VCO to generate an oscillation signal according to a driving signal; and for controlling the filter to enter a normal mode from the tuning mode by enabling the entire filter to operate according to a driving signal when a frequency of the oscillation signal is equal to a reference frequency; a frequency detector, electrically connected to the enabling circuit and the VCO, for comparing a frequency of the oscillation signal outputted by the VCO with the reference frequency to generate a comparison result; and a controlling circuit, electrically connected to the frequency detector and the filter, for adjusting the driving signal according to the comparison result in the tuning mode, obtaining the driving signal in the tuning mode when the frequency of the oscillation signal is equal to the reference frequency, and outputting the driving signal to the filter in the normal mode.  
         [0012]     According to an exemplary embodiment of the claimed disclosure, a tuning method for tuning a filter is disclosed. The filter comprises a voltage-controlled oscillator (VCO) circuit, the tuning method comprises: controlling the filter to enter a tuning mode by disabling the entire filter except for the VCO allowing the VCO to generate an oscillation signal according to a driving signal; comparing a frequency of the oscillation signal outputted by the VCO with a reference frequency to generate a comparison result; for adjusting the driving signal according to the comparison result in the tuning mode, obtaining a driving signal in the tuning mode when the frequency of the oscillation signal is equal to the reference frequency, and outputting the driving signal to the filter in a normal mode; and controlling the filter to enter the normal mode from the tuning mode by enabling the entire filter to operate according to the driving signal when the frequency of the oscillation signal is equal to the reference frequency.  
         [0013]     Furthermore, a gm replica circuit is disclosed. The gm replica circuit comprises: a pair of input transistors, each of a first input transistor and a second input transistor of the pair of the input transistors receiving a reference voltage, the first input transistor coupled to a reference current; a current mirror coupled to the pair of input transistors; and a gm setting device, the gm setting device having three ends, a control end of the three ends directly connected to the second current mirror and the second input transistor, the other two ends respectively connected to the first input transistor and the second input transistor.  
         [0014]     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  
       [0015]      FIG. 1  is a diagram of a conventional tuning structure.  
         [0016]      FIG. 2  is a diagram of another conventional tuning structure.  
         [0017]      FIG. 3  is a diagram of another conventional tuning structure.  
         [0018]      FIG. 4  is a diagram of a tuning apparatus of a first embodiment according to the present disclosure.  
         [0019]      FIG. 5  shows a general two-level biquad filter.  
         [0020]      FIG. 6  shows a general two-level biquad filter.  
         [0021]      FIG. 7  shows an LC ladder filter.  
         [0022]      FIG. 8  is a diagram of the entire circuit when the main filter is in the normal mode.  
         [0023]      FIG. 9  is a diagram of the gm replica circuit shown in  FIG. 4   
         [0024]      FIG. 10  is a diagram of a tuning device of a second embodiment according to the present disclosure. 
     
    
     DETAILED DESCRIPTION  
       [0025]     In the following filter frequency tuning device and related method thereof, the tuning procedure is basically divided into two steps. The first step is to use a VCO inside a main filter to adjust the cut-off frequency of the main filter in order to remove the influence of the process variation of the center frequency f c . The second step is to utilize a gm replica circuit on-line control to remove the influence of the temperature variation of the center frequency f c .  
         [0026]     The entire implementation of the present disclosure is illustrated as follows. Please refer to  FIG. 4 , which is a diagram of a tuning apparatus  400  of a first embodiment according to the present disclosure. As shown in  FIG. 4 , the tuning apparatus  400  comprises an enabling circuit  420 , a frequency detector (FD)  430 , and a controlling circuit  440 . The controlling circuit  440  comprises a digital-to-analog converter  441 , and a Gm replica circuit  442 . First, when the entire main filter  410  is in a tuning mode, a part of the main filter  410  is configured as the VCO structure  411  by the enabling circuit  420 . The present disclosure does not limit the options for configuration. For example, a set of trans-conductor cells, which forms a two gm-C integrator structure, can be set active, and other trans-conductor cells are temporarily disabled such that the VCO structure  411  can be formed.  
         [0027]     A part of a filter can be configured as a VCO circuit. For example, a portion can be selected form a biquad filter or a LC ladder structure to serve as a VCO circuit. Other filter structure (not limited to biquad or LC ladder filter) can become a VCO circuit when a portion of the filter structure is enabled and other portion is disabled. Please refer to  FIG. 5  and  FIG. 6 , which respectively show a general two-level biquad filter. Please also refer to  FIG. 7 .  FIG. 7  shows an LC ladder filter. Basically, a simple VCO circuit can be formed by two gm cells and two capacitors, where a first capacitor is coupled to the input end of a first gm cell and the output end of a second gm cell, and a second capacitor is coupled to the input end of the second gm cell and the output end of the first gm cell. Therefore, as shown in  FIG. 5 ,  FIG. 6 , and  FIG. 7 , the highlighted portion can be regarded as the above-mentioned VCO circuit  411 . In addition, the gm-cells, which are utilized to form the VCO, are not limited to be the active gm-cells inside the filter. Because in a filter, some dummy gm-cells are often added at each node to compensate for the capacitor loading, the dummy gm-cells can also be utilized to form the VCO. This change also obeys the spirit of the present disclosure.  
         [0028]     When the VCO structure  411  is formed, the VCO  411  starts to oscillate an oscillation signal having a frequency w o =N*g m,u /C according to a driving signal. At this time, any of the above-mentioned three conventional mechanisms could be used to compare the frequency f o  with the frequency f ref . In this embodiment, similar to the digital circuit shown in  FIG. 3 , the digital frequency detector (FD)  430  compares the frequency the frequency f o  with the frequency f ref . The comparing result outputted from the FD  430  is then inputted into the controlling circuit  440  to generate the driving signal. The FD  430  becomes stable when the frequency f o  and the frequency f ref  are equal. Please note, because the VCO  411  is inside the main filter  410 , when the VCO  411  is completely tuned, theoretically the main filter  410  is tuned.  
         [0029]     Here, the controlling circuit  440  comprises a DAC  441  and a gm replica circuit  442 . The DAC  441 , as mentioned previously, is utilized to convert the comparing result into a driving signal. In the prior art, the driving signal can be utilized for the main filter  410 , however, in the present disclosure, the gm replica circuit  442  is further utilized to adjust the driving signal such that the gm value of the gm cell in the main filter  440  is not influenced by temperature variation. Basically, the gm replica circuit  442  can maintain the gm value by adjusting the driving signal. The operation and function of the gm replica circuit  442  will be described in the following disclosure.  
         [0030]     Therefore, when the VCO  411  is completely tuned, the main filter  410  is configured as the original circuit instead of the VCO  411 . That is, when the frequency of the oscillation signal is equal to the reference frequency, the enabling circuit  420  switches the entire main filter  410  from the tuning mode to a normal mode. At this time, the main filter  410  can execute the original function and the main filter  410  is thereafter not utilized as a VCO.  
         [0031]     Please refer to  FIG. 8 , which is a diagram of the entire circuit when the main filter  410  is in the normal mode. As shown in  FIG. 8 , since there is no VCO circuit in the normal mode, when the main filter  410  is in the normal mode the FD  430  has no input frequency source to compare with the reference frequency. In the normal mode, the DAC  441  outputs a constant analog signal (e.g., a current signal) to the gm replica circuit  442 . The gm replica circuit  442  generates a driving signal to replicate a gm value to the gm cell in the main filter  410 . The gm value is invariant when environment temperature varies. In other words, the tuned center frequency is temperature insensitive because of the gm replica circuit  442 . The operation and structure of the gm replica circuit  442  will be discussed in the following disclosure.  
         [0032]     Please refer to  FIG. 9 , which is an exemplary detailed gm replica circuit. The gm replica circuit  442  comprises a current mirror, an NMOS m 1 , and a pair of input transistors m 2  and m 3 . A reference voltage difference ΔV ref  is inputted to the gates of the input transistors m 2  and m 3 . In addition, the node A and the node B are output nodes of the current mirror. The node A is further connected to the drain of the transistor m 2  and a reference current I ref . Please note, in this embodiment, the reference current I ref  is outputted from the DAC  441 . The node B is directly connected to the gate of the NMOS m 1  and connected to the drain of the transistor m 3 . Therefore, the driving signal of the node B can be utilized to adjust the resistance of the NMOS m 1  such that the gm value of the entire gm replica circuit  442  is tuned back to a desired value.  
         [0033]     The basic concept of the gm replica circuit  442  is to utilize the inner negative feedback loop to fix the gm value of the gm replica circuit  442  as gm=I ref /ΔV ref , where I ref  is a temperature-insensitive reference current, and ΔV ref  is a temperature-insensitive reference voltage, which can be generated from a bandgap voltage generator.  
         [0034]     The following is offered as proof. When the voltage difference ΔV ref  is applied to the gates of the transistor m 3  and m 2 , an additional current Δi is induced to flow through the transistor m 3 . An additional current Δi also appears and flows through the transistor m 2  toward the node A. Therefore, the current flowing from the current mirror to the node A would be (I ref −Δi), where (I ref −Δ i ) would be equal to Δi because of the current mirror. Therefore, I ref  is equal to 2*Δi . Since gm=2*Δi/ΔV ref =I ref /ΔV ref , the resulting gm is I ref /ΔV ref , which is invariant when temperature varies. In other words, if the Iref and the Vref are both temperature-independent, the Gm value of the gm replica circuit is stable and the control voltage used to control the Gm value can also be utilized in the main filter. The reference voltage Vref can be generated from a bandgap circuit to ensure that the reference voltage ΔV ref  is not influenced by temperature variances. The reference current Iref is received from the DAC.  
         [0035]     Moreover, the entire circuit (i.e., the negative feedback) can automatically adjust the voltage V c  to make the gm value always remain fixed at I ref /ΔV ref . For example, if the ambient temperature increases, the gm value increases accordingly. The current Ai increase because 2*Δi=Gm*ΔV ref . Therefore, for the current mirror, the current (I ref −Δi) flowing toward the node A decreases because of the increase of the current Δi. And in the node B, the current above the node B is equal to (Iref−Δi), but the current below the node B is equal to Δi. Therefore, the driving voltage of the node B is pulled down by the current Δi. In other words, the gate voltage of the NMOS ml decreases such that the gm value decreases. Obviously, the entire circuit is a negative feedback mechanism such that the gm value of the entire gm replica circuit is fixed.  
         [0036]     Therefore, as long as the gm cell of the gm replica circuit  442  is equal to the gm cell of the main filter  410 , the gm value of the main filter  410  can be maintained as that of the gm replica circuit  442  because they share the same driving voltage.  
         [0037]     In addition, the gm replica circuit  442  is an optional device, and it is not a limitation of the present disclosure. For example, the implementation of the gm replica circuit is not limited. For example, if the filter can further comprise a negative feedback loop according to its gm cell, the same effect can be achieved. On the other hand, if the entire filter tuning structure includes the gm replica circuit, the output of the DAC can be directly utilized to change the I ref  or ΔV ref  in order to further adjust the gm value such that the oscillation frequency of the VCO can be equal to the target value.  
         [0038]     Moreover, in some applications, if the temperature variation is insignificant, or the gm cell is insensitive to the temperature variation, the tuning device  400  requires only the DAC  441 . This means that the gm replica circuit  442  is no longer needed. Please refer to  FIG. 10 , which is a diagram of a tuning device  1000  of a second embodiment according to the present disclosure. As shown in  FIG. 10 , the tuning device  1000  does not comprise a gm replica circuit. This also obeys the spirit of the present disclosure.  
         [0039]     In addition, in the above disclosure, a digital FD and a DAC are used for comparing the oscillation frequency and a reference frequency. Please note, the above-mentioned mechanism is only an embodiment, not a limitation of the present disclosure. For example, the present disclosure can also utilize a PLL including an FD, charge pump, and a low pass filter. That is, the FD is utilized to compare the frequencies, and the charge pump and the low pass filter can convert the comparison result of the FD into a driving signal for the main filter. This can also achieve the goal of tuning the center frequency of the main filter.  
         [0040]     Please note, in the above disclosure, only the gm value is tuned. But in the actual implementation, either the gm value or the capacitance can be tuned to change the center frequency of the main filter. This change also obeys the spirit of the present disclosure.  
         [0041]     In contrast to the prior art, the present disclosure can utilize the components of the filter to build a VCO and use the VCO to tune the filter. Therefore, the present disclosure does not need another VCO to perform the tuning operation. This saves the cost and the area that is otherwise required by the VCO. Furthermore, because the present disclosure directly utilizes the VCO circuit inside the filter, the environment of the VCO and the filter are guaranteed to be identical . Therefore, the mismatch problem between the VCO and the filter, which may introduce poor tuning performance, is no longer a concerned. In other words, the present disclosure has improved tuning performance.  
         [0042]     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.