Abstract:
A method to track and control the resonance frequency of a band-pass filter provides a solution for the sensitivity limitations against temperature and process variations. A phase sensing module obtains the phase difference between the input and output and a negative feed-back control architecture can be used to tune the filter&#39;s resonance over the input RF frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application relates to and claims the benefit of priority to PCT Application PCT/IB207/053357 filed Aug. 22, 2007, and of European Patent Application No. 06300886.6 filed Aug. 23, 2006, both of which are hereby incorporated by reference in their entireties for all purposes as if fully set forth herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to filter resonance. In particular, the present invention relates to controlling and tuning the resonance frequency of a band-pass filter. 
       BACKGROUND OF THE INVENTION 
       [0003]    Modern wireless communication increasingly uses more simultaneous receiver (Rx) and transmitter (Tx) communication links such as CDMA networks as well as simultaneous GPS and broadcast features. These Tx and Rx channels operate simultaneously with a large difference of levels. For example, when the Rx signal goes below noise, the Tx signal can be very high with the leaked Tx to Rx signal being also high even when a duplexer for attenuation is used. Furthermore, during communications, a number of interferers may be present at the Rx input. The combination of all these interferers, Tx and jammers, will result in an important Signal-to-Noise Ratio (SNR) degradation due to the limited linearity of the Rx channel such as cross-modulation noise, inter-modulation products noise, and the like. 
         [0004]    Typically, regarding the Rx path, a Low-Noise Amplifier (LNA) is the first element to degrade the SNR and the performance is managed by its IIP3 specification. However, the mixer is also a non-linear block and the interference levels have to be managed to avoid further SNR degradation. Normally, an inter-stage filter is added between the LNA and the mixer to limit the impact of these interferers. The inter-stage filter is usually a non-integrated Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) filter, which presents the disadvantages of cost and Bill-Of-Materials (BOM) increase. Additionally, two main factors limit the integration of such filters, namely, the resonance frequency sensitivity to temperature and process variations, and the Q-factor or quality-factor limitations (amount of resistance to resonance). 
         [0005]    For example, to solve the above drawbacks of the prior art, various solutions of tracking the resonance frequency have been advanced. In one solution, namely, in U.S. Pat. No. 6,940,348 B1, the LC pass-band filter (a filter having an inductor L and a capacitor C) is matched with an auxiliary oscillator and using the locking loop, performs its control directly on the auxiliary oscillator and not on the LC filter itself. The control information is then applied on the LC filter in a second step. However, the LC filter is not included into any feedback control loop, which results in a number of drawbacks. 
         [0006]    First, the matching between the LC filter and the oscillator&#39;s tank presents a certain degree of error or mismatch due to process and temperature gradients. Thus, even if the control applied on the oscillator itself is perfect, this same control cannot be true for the LC filter because the control loop cannot correct or eliminate the mismatch error between the LC filter and the tank&#39;s oscillator. 
         [0007]    Secondly, a good matching between the LC filter and the oscillator&#39;s tank requires placing each one extremely close to each other. The oscillator&#39;s tank, being a strong source of electromagnetic radiation, is able to magnetically couple with the LC filter and considerably pollute the receiver input with many undesirable frequency interferers that will imply high degrading consequences on the receiver sensitivity. 
         [0008]    Therefore, in view of these concerns and drawbacks, there is a continuing need for developing a new and improved system and method for controlling and tuning the resonance frequency of a filter which would avoid the disadvantages and above mentioned problems while being cost effective and simple to implement. 
       SUMMARY OF THE INVENTION 
       [0009]    One aspect of the invention is to provide a new method and system of tracking and controlling a resonance frequency of a band-pass filter. The new method and system accurately tracks and controls the resonance frequency of a filter, and provides a solution for the sensitivity limitation against temperature and process variations. In particular, the invention can include a method of providing a phase sensing module to obtain a phase difference between the input and output and a negative feed-back control architecture that can be used to tune the filter&#39;s resonance accurately over the input RF frequency. A phase sensing module, according to one embodiment of the present invention, comprises a local oscillator (LO) frequency that equals the input RF frequency, a frequency modulator to down-convert the input and output channels to DC zero frequency terms, and a low-pass filter used to obtain the desired DC terms. A negative feed-back control architecture includes a comparator of which one input is connected to the ground. When the input RF frequency moves away, the control loop tracks the new RF frequency position and automatically re-tunes the filter&#39;s resonance over the new RF frequency. 
         [0010]    One or more of the following features of the present invention may also be included. 
         [0011]    In one aspect of the invention, the method of tracking and controlling a resonance frequency of a band-pass filter also includes suppressing the down-converted terms resulting from a plurality of interferers. 
         [0012]    In another aspect of the invention, the tuning of the phase includes using a negative feedback control acting on the resonance frequency of the band-pass filter. 
         [0013]    In yet another aspect of the invention, the comparator is operated either in a single-ended or in a differential mode. 
         [0014]    Embodiments of the invention may also have one or more of the following advantages. 
         [0015]    The invention can provide accurate resonance tuning of a filter over a given RF frequency. Further, it can provide broadband frequency operation, which gives the possibility of a single implemented solution compatible with a multi-bands/multi-mode application. 
         [0016]    Moreover, the invention can provide immunity against temperature and process variations and gradients, and full integration capability, allowing the removal of on-board SAW filters, which in turn favors the BOM and PCB (Printed Circuit Board) area reduction. 
         [0017]    Additionally, the invention can make possible low-power and simple integrable functions, as well as autonomous functionality regarding annexed-to-Rx circuitries (Tx, Baseband, and the like). Further, the invention can provide a wider application range. Therefore, advantageously, the architecture of the invention operates and accomplishes the control of the filters even when the applied signal is a frequency channel or band or has a certain frequency extension. 
         [0018]    Furthermore, the invention can provide in the application a direct control on the LC filter. Consequently, there is no additional mismatch or resultant error components, thereby ensuring higher control and accuracy aptitude. 
         [0019]    Another benefit of the present invention is that the invention uses the input frequency as a reference frequency to lock the filter, so there is no need for an additional “reference frequency”. And by placing the pass-band filter at the input of a receiver chain, the present invention provides the added advantage of keeping only the needed frequency band and filtering all the undesirable spectral components. 
         [0020]    These and other aspects of the invention will become apparent from and elucidated with reference to the embodiments described in the following detailed description, drawings and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The aforementioned and other features and objects of the present invention and the manner of attaining them will become more apparent, and the invention itself will be best understood, by reference to the following description of one or more embodiments taken in conjunction with the accompanying drawings, wherein: 
           [0022]      FIG. 1  illustrates a filter response in gain and in phase, showing how the phase of a pass-band filter presents a zero-singularity at its resonance frequency; 
           [0023]      FIG. 2A  shows the principle of a control loop allowing the filter resonance tuning over the input RF frequency; 
           [0024]      FIG. 2B  shows the principle of a control loop allowing the filter resonance tuning over the input RF frequency where a comparator block is operated in a differential mode; 
           [0025]      FIG. 3A  is an example of the filter resonance control loop implementation, illustrating the implementation of an improved method and system according to one of the embodiments of the present invention; 
           [0026]      FIG. 3B  is an example of the filter resonance control loop implementation where a comparator block is operated in a differential mode, according to one of the embodiments of the present invention; 
           [0027]      FIG. 4  illustrates the advantage in overcoming parasitic and unwanted down-converted DC and low frequency terms, according to one of the embodiments of the present invention; 
           [0028]      FIG. 5  is an example of implementation of a broad-band tunable filter, according to one of the embodiments of the present invention; 
           [0029]      FIGS. 5A-5B  are examples of a singled-ended first order LC filter; 
           [0030]      FIGS. 5C-5D  are examples of LC-filters in differential configurations; 
           [0031]      FIG. 6  illustrates an exemplary wireless communication device using an external SAW filter; and 
           [0032]      FIG. 7  illustrates another exemplary device incorporating an LC filter and its control block according to the present invention. 
       
    
    
       [0033]    The Figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
       DETAILED DESCRIPTION 
       [0034]    Specific embodiments of the present invention are hereafter described in detail with reference to the accompanying Figures. Like elements in the various Figures are identified by like reference numerals for consistency. Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention. 
         [0035]    Referring to  FIG. 1 , a filter response in gain and in phase is illustrated in a graph  100 . The phase of a pass-band filter (LC filter) presents a zero-singularity at its resonance frequency. One principle of the invention is based on the phase sensing and annulling of the filter. When the resonance frequency is different from the input RF signal frequency, the difference of phase between the input and the output signals throughout the filter will be different from zero. 
         [0036]    Referring now in addition to  FIG. 2A , the principle of a control loop allowing the filter resonance tuning over the input RF frequency is illustrated. In other words,  FIG. 2A  shows a circuit architecture  200  of a resonance frequency of a filter tracking and controlling solution. The circuit architecture  200  requires the cooperation of three modules: a phase sensing module  202 , a negative feed-back control circuit which includes a comparator  204 , and a filter  206 . 
         [0037]    In addition to the phase difference, the input and the output signals, i.e., the phase difference sensing  202 , by forcing this difference to be equal to zero via a negative feed-back control acting on resonance frequency of the filter  206 , the resonance of the filter  206  can be accurately tuned over the input RF frequency. Moreover, when the RF frequency moves away, the control loop tracks the new RF frequency position to automatically re-tune the filter&#39;s resonance over the new RF frequency. 
         [0038]    The comparator block  204  is able to operate in either single-ended mode or in a differential mode at its input side. In  FIG. 2A , the comparator  204  operates in a single ended mode. Referring in addition to  FIG. 2B , a comparator block  204 B is shown operated in a differential mode. In the differential case, no connection to ground is needed at the comparator&#39;s input. The phase sensing module  202 B provides the phase information of input and output signals which are connected to the inputs of the comparator  204 B, respectively. The comparator  204 B forces its inputs to be equal and then the output of the comparator  204 B is sent back to the filter  206 B to tune the resonance frequency. 
         [0039]    Referring now to  FIG. 3A , a filter resonance control loop implementation according to one embodiment of the present invention is illustrated. A phase sensing module  300  primarily includes two frequency modulators  302 ,  304  used to down-convert the frequency of an input signal, a low-pass filter  310  used in the input terminal, two frequency modulators  306 ,  308  used to down-convert the frequency of the output signal, and a low-pass filter  312  used in the output terminal. 
         [0040]    Considering the following signal at the filter&#39;s input: 
         [0041]    A 1 ·cos(ω RF ·t+φ 1 ): input RF signal 
         [0000]    
       
         
           
             
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         [0042]    Then, at the filter&#39;s output, the results are: 
         [0043]    A 2 ·cos(ω RF ·t+φ 2 ): output RF signal 
         [0000]    
       
         
           
             
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         [0044]    The above scenario assumes that a LO (Local Oscillator) frequency equals the input RF frequency based upon the well-known Zero-IF architecture or similar architectures. One embodiment of the present invention accurately tracks and controls the resonance frequency of a filter, as is hereafter described. 
         [0045]    First, referring still to  FIG. 3A , the input and output signals are sent into the frequency modulators  302 ˜ 304 ,  306 ˜ 308 , respectively. The down-conversion of the input and output RF frequencies, which is implemented in the frequency modulators  302 ˜ 304 ,  306 ˜ 308 , provides DC terms representing the difference of phase between the RF signals and the LO, namely, (φ LO −φ 1 ) &amp; (φ LO −φ 2 ). 
         [0046]    Second, the DC terms then are sent into the low-pass filters  310 ,  312  respectively. The low-pass filters  310 ,  312  suppress the high frequency mixed term (2*ω RF ) and all the down-converted terms resulting from the interferers (ω j ±ω LO ) &amp; (ω i ±ω LO ). The cut-off frequency of the low-pass filter can be set low so that it can eliminate all non-linearity products to modulate the DC terms. This can also lower the linearity requirements of the mixers as well as their power consumption level. 
         [0047]    Third, the desired DC terms are then sent to the comparator  314  of which one input is connected to the ground. After comparison, the output of comparator  314  is sent back to the filter  316  to tune the phase. 
         [0048]    Consequently, taking the down-converted terms at the low-pass filter  310 ,  312  outputs, the following is obtained: 
         [0000]      A 1 ·cos(ω RF ·t+φ 1 )*sin(ω LO ·t+φ LO )         V a ∝A 1 ·sin(φ LO −φ 1 ) 
         [0000]      A 1 ·cos(ω RF ·t+φ 1 )*cos(ω LO ·t+φ LO )         V b ∝A 1 ·cos(φ LO −φ 1 ) 
         [0000]      A 2 ·cos(ω RF ·t+φ 2 )*cos(ω LO ·t+φ LO )         V c ∝A 2 ·cos(φ LO −φ 2 ) 
         [0000]      A 2 ·cos(ω RF ·t+φ 2 )*sin(ω LO ·t+φ LO )         V d ∝A 2 ·sin(φ LO −φ 1 ) 
         [0049]    And when V x =V b *V d −V a *V c , then, V x ∝cc sin(φ 2 −φ 1 ), and the condition of the control loop becomes: 
         [0000]      V x =0         φ 2 =φ 1    
         [0050]    As a result, the filter resonance frequency becomes equal to the input RF frequency. 
         [0051]    The comparator block  314  is able, according to one embodiment of the present invention, to operate in either single-ended mode or in a differential mode at its input side. In  FIG. 3A , the comparator  314  operates in a single ended mode. Referring now in addition to  FIG. 3B , a comparator operating in a differential mode is also a possibility. No connection to ground is needed in this case at the input of the comparator  314 B. The multiplier provides the information on the phase difference ‘V x ’ as a differential signal directly connected to the input of the comparator  314 . The comparator  314 B forces its inputs to be equal, as follows: 
         [0000]        V   b   *V   d   =V   a   *V   c             V   b   *V   d   −V   a   *V   c =0 
         [0052]    And if V x =V b *V d −V a *V c , then, V x ∝sin(φ 2 −φ 1 ), and the condition of the control loop becomes: 
         [0000]      V x =0         φ 2 =φ 1    
         [0053]    As a result, the filter resonance frequency becomes equal to the input RF frequency. 
         [0054]    Referring now in addition to  FIG. 4 , one advantage in overcoming parasitic and unwanted DC and low frequency terms, according to one of the embodiments of the present invention, is illustrated. The use of a Local Oscillator (LO) signals to convert the channels to DC at the filter&#39;s input and output overcomes the issue of parasitic and unwanted DC and low frequency terms. 
         [0055]    After having the input and output channels down-converted to DC by the 
         [0056]    LO signals, the use of a low-pass filter with a very low cut-off frequency assists in keeping the wanted DC term, or a very limited frequency region, around each wanted DC term. Since each DC term is proportional to the phase information, respectively of the input and output channel centers, this permits, in the next step, the present invention to obtain the phase difference information related to the channel centers and accomplish the filter control, even when the input signal has a certain frequency extension. 
         [0057]    The use of the low-pass filters  310 ,  312  of  FIG. 3A , with a very low cut-off frequency, is linked to the Zero IF architecture, where the LO is equal to the central frequency of the received band or channel. The same proposed principle and implementation is equally valid in the case of a Near Zero IF architecture, with the exception that the previously mentioned low-pass filters  310 ,  312  are replaced by pass-band filters, e.g., poly-phase filters, having a very low cut-off frequency. 
         [0058]    Referring now to  FIG. 5 , an example of implementation of a broad-band tunable filter is illustrated in diagram  500 , according to one embodiment of the present invention. The control loop acts as an analogue-tuning loop. A digital tuning, ensuring a broadband operating frequency range is possible using high-Q switched capacitors  502 . This can be feasible by applying a digital pre-calibration phase, before the analogue tuning phase. 
         [0059]    Referring now to  FIGS. 5A and 5B , diagram  500 A shows an example of a single-ended first order LC filter. A higher order can be naturally used depending on the rejection specification and requiring in this case more components. The capacitors  504  and  505  are optional and are used to free the input and output DC levels. The presence of the capacitors  504  and  505  should not intervene into the resonance behavior. Components  501 ,  502  and  503  are the components which set the resonance behavior. The resonance control may be achieved in one or two steps. First, in the case of a wide tuning range, a first pre-calibration process, i.e., coarse tuning, is used to make the filter&#39;s resonance very close to the wanted resonance frequency. This can be achieved by a digital programming or tuning, preceding the fine control loop. The component  502  is shown in  FIGS. 5A and 5B  in the diagrams  500 A and  500 B, as a means allowing such a digital pre-calibration phase. This pre-calibration can be done using switched capacitors, as shown in  FIG. 5A , but it may also be possible using a digitally programmable inductor, such as that shown in  FIG. 5B  and as would be known by one skilled in the relevant art. Secondly, the fine control loop is initiated, and by acting on “Vtune” (analogue tuning), a very accurate resonance tuning over the input RF frequency is obtained. Such an analogue control may be done using a Varicap, as in diagram  500 A or an analogously programmable inductor as in diagram  500 B. 
         [0060]    In the case of a limited tuning range, the first digital pre-calibration phase may not be useful or necessary. In this instance, the analogue tuning loop can be directly initiated. 
         [0061]    Additionally, all combinations among  501 ,  502  and  503  components are allowed irrespective of whether inductors or capacitors are used. The sole condition is to globally provide an equivalent LC tank in order to ensure the resonance behavior. In effect, in diagram  500 A, for example, the  502  component can be replaced with a digitally programmable inductor, or the  503  component can be replaced by an analogously programmable inductor, giving two further derivatives. The same is applicable for  FIG. 5B , in diagram  500 B, where the component  502  may be replaced by a switched-capacitors tank, or the component  503  may be replaced by a Varicap, providing two further derivatives. 
         [0062]    Referring to  FIGS. 5C and 5D , the different mode of use of LC filters are illustrated. Whereas  FIGS. 5A and 5B  in diagrams  500 A and  500 B provide single-ended configurations, differential configurations are also possible. There are particular advantages by the use of differential configurations. There is no need for a ground reference connection in a differential mode. All of the foregoing remarks concerning the control steps such as coarse/fine tuning and the freedom of choosing the LC variable components are equally valid in applications using the differential mode as well. 
         [0063]    Referring in addition to  FIG. 6 , a wireless communication device  600  according to one embodiment of the present invention is shown. The device  600  includes a receiver  602 , using an external SAW filter  604 . Figure shows an integrated RF front-end chain in the receiver  602 , showing a Zero-IF IC receiver architecture. 
         [0064]    Referring to  FIG. 7 , another wireless communication device  700  is illustrated, which incorporates an LC filter and its control block according to the present invention. In the device  700 , an integrated RF front-end chain in the receiver  702  is shown, using an integrated LC filter  704  with its control block. 
         [0065]    While there has been illustrated and described what are presently considered to be the preferred embodiments of the present invention, it will be understood by those of ordinary skill in the art that various other modifications may be made, and equivalents may be substituted, without departing from the true scope of the present invention. 
         [0066]    For example, although the foregoing description used the name local oscillator, this may be understood as embodying a unique frequency plan, by using the input frequency as a reference frequency to lock the filter. Thus, there is no need for additional reference frequencies. Further, a local oscillator does not denote that the oscillator is placed either locally or nearby the LC filter. The LO may be place anywhere or as far from the LC filter as desired. 
         [0067]    Additionally, many modifications may be made to adapt a particular situation to the teachings of the present invention without departing from the central inventive concept described herein. Furthermore, an embodiment of the present invention may not include all of the features described above. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed, but that the invention include all embodiments falling within the scope of the appended claims and their equivalents.