Abstract:
A digitally controlled filter tuning method and corresponding WLAN (Wireless Local Area Network) communication devices and integrated circuit chips are provided. A WLAN communication signal is filtered by a tunable filter. A cut-off frequency of the tunable filter is tuned by a feedback loop. Tuning the cut-off frequency includes comparing by a comparator an output signal emitted by the tunable filter to a reference signal and emitting by the comparator a comparator signal indicative of the difference between the output signal and the reference signal. Further, tuning the cut off frequency comprises receiving the comparator signal by a tuning controller and setting by the tuning controller the cut-off frequency of the tunable filter based on the comparator signal by applying a digital tuning word to the tunable filter. The described filter tuning technique may reduce product and manufacturing costs while providing enhanced tuning accuracy.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to WLAN (Wireless Local Area Network) communication devices and corresponding methods and integrated circuit chips, and in particular to the filter tuning in such WLAN communication devices. 
     2. Description of the Related Art 
     A wireless local area network is a flexible data communication system implemented as an extension to, or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility. 
     Today, most WLAN systems use spread spectrum technology, a wideband radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems. 
     The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. Further extensions exist. 
     Examples of these extensions are the IEEE 802.11a, 802.11b and 802.11g standards. The 802.11a specification applies to wireless ATM (Asynchronous Transfer Mode) systems and is used in access hubs. 802.11a operates at radio frequencies between 5 GHz and 6 GHz. It uses a modulation scheme known as OFDM (Orthogonal Frequency Division Multiplexing) that makes possible data speeds as high as 54 Mbps, but most commonly communications take place at 6 Mbps, 12 Mbps or 24 Mbps. The 802.11b standard uses a modulation method known as CCK (Complementary Code Keying) which allows high data rates and is less susceptible to multipath propagation interference. The 802.11g standard can use data rates of up to 54 Mbps in the 2.4 GHz frequency band using OFDM. Since both 802.11g and 802.11b operate in the 2.4 GHz frequency band, they are completely interoperable. The 802.11g standard defines CCK-OFDM as optional transmit mode that combines the access modes of 802.11and 802.11b, and which can support transmission rates of up to 22 Mbps. 
     In any transmit mode, a WLAN communication device, i.e. transmitter, receiver or transceiver, needs to filter the communication signal in order to eliminate unwanted interference and noise. In a WLAM receiver, filtering of a received communication signal is accomplished to remove signals with frequencies outside of a determined frequency range to avoid overloading of the receiver, and in particular any signal falling within the image frequency, i.e. the frequency that results, when downconverted by a mixer, to the same intermediate or baseband frequency as the desired communication signal. In a WLAN transmitter, filtering is used to ensure that the transmitter only emits signals within the allowed frequency range by removing other spurious signals that may be introduced into the communication signal, e.g. due to imperfections in the transmitter circuitry. 
     In order to achieve the desired filtering, there is a need to adjust such filters after manufacturing by initially tuning them to the desired frequency response. This includes tuning the cut-off frequency (or frequencies) of the filter above or below which signals can pass the filter. Since many WLAN communication devices operate at a number of different channels in a given frequency band, continuous tuning of the cut-off frequency during operation of the WLAN communication device is also required. Especially when frequency hopping techniques are used, the tuning circuitry needs to allow for quickly adapting the cut-off frequency to a new channel frequency. Further, continuous tuning is needed for compensating for a cut-off frequency drift caused, e.g., by temperature coefficients of filter components that change in the ambient or operating temperature. 
     Many conventional WLAN communication systems use a master-slave tuning technique for achieving real time cut-off frequency tuning. In the master-slave architecture, a master oscillator is implemented using circuits similar to those employed within a slave filter to be tuned. Both circuits receive the same frequency control input which is derived by phase locking the master oscillator to an external reference. Thus, when the frequency of the master oscillator is set, the passband frequency of the slave filter is properly tuned. However, substantial additional circuitry is required for implementing the master-slave tuning technique. Therefore, conventional WLAN communication devices often suffer from the problem of increased power consumption. In addition, those WLAN communication devices have the disadvantage of causing high manufacturing and product costs. 
     In order to overcome the problems arising in master-slave tuning systems, the self-tuning technique was developed. In this approach, the filter is periodically taken offline and tuned directly. Yet, since filtering of the communication signal is interrupted while the filter cut-off frequency is being tuned, prior art WLAN communication devices applying the self-tuning technique often have difficulties in achieving efficient data rates. Further, the filtering accuracy in those devices is decreased due to, e.g., cut-off frequency drift, during the time interval between the individual tuning interruptions. 
     SUMMARY OF THE INVENTION 
     An improved filter tuning method and corresponding WLAN communication devices and integrated circuit chips are provided that may overcome the disadvantages of the conventional approaches. In particular, embodiments may allow for lowering the power consumption caused by the tuning circuitry. Other embodiments offer the advantage of reduced product and manufacturing costs. In further embodiments, the achievable communication data rate is increased. In still other embodiments, filter tuning accuracy is enhanced. 
     In one embodiment, a WLAN communication device is provided comprising a tunable filter for filtering a WLAN communication signal and a feedback loop arranged to tune a cut-off frequency of the tunable filter. The feedback loop comprises a comparator and a tuning controller. The comparator is arranged to compare an output signal emitted by the tunable filter to a reference signal and to emit a comparator signal indicative of the difference between the output signal and the reference signal. The tuning controller is arranged to receive the comparator signal and to set the cut-off frequency of the tunable filter based on the comparator signal by applying a digital tuning word to the tunable filter. 
     In another embodiment, an integrated circuit chip for performing WLAN communication is provided comprising a tunable filter circuit for filtering a WLAN communication signal and a feedback loop circuit arranged to tune a cut-off frequency of the tunable filter circuit. The feedback loop circuit comprises a comparator circuit and a tuning control circuit. The comparator circuit is arranged to compare an output signal emitted by the tunable filter circuit to a reference signal and to emit a comparator signal indicative of the difference between the output signal and the reference signal. The tuning control circuit is arranged to receive the comparator signal and to set the cut-off frequency of the tunable filter circuit based on the comparator signal by applying a digital tuning word to the tunable filter circuit. 
     In a further embodiment, a method of operating a WLAN communication device is provided. A WLAN communication signal is filtered by a tunable filter. A cut-off frequency of the tunable filter is tuned by a feedback loop. Tuning the cut-off frequency of the tunable filter comprises comparing by a comparator an output signal emitted by the tunable filter to a reference signal and emitting by the comparator a comparator signal indicative of the difference between the output signal and the reference signal. Further, tuning the cut-off frequency of the tunable filter comprises receiving the comparator signal by a tuning controller and setting by the tuning controller the cut off frequency of the tunable filter filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are incorporated into and form a part of the specification for the purpose of explaining the principles of the invention. The drawings are not to be construed as limiting the invention to only the illustrated and described examples of how the invention can be made and used. Further features and advantages will become apparent from the following and more particular description of the invention, as illustrated in the accompanying drawings, wherein: 
         FIG. 1  is a block diagram illustrating components of a filter tuning implementation in a WLAN communication device according to an embodiment; 
         FIG. 2  is a flow diagram illustrating a filter tuning process according to an embodiment; and 
         FIG. 3  is a flow diagram illustrating the tuning word optimization in the filter tuning process of  FIG. 2  according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The illustrative embodiments of the present invention will be described with reference to the figure drawings 
     Referring now to the drawings and in particular to  FIG. 1 , components of a filter tuning implementation in a WLAN communication device according to an embodiment are shown. While the WLAN communication device is in a processing mode, a communication signal is passed from a previous processing stage acting on the communication signal within the WLAN communication device through a filter  130 . The resulting filtered communication signal is forwarded from the filter  130  to a next processing stage within the WLAN communication device for further processing of the filtered communication signal. The previous processing stage and the next processing stage may comprise, e.g., mixers, amplifiers, A/D converters, etc. 
     According to the present embodiment, the WLAN communication device comprises switches  110 ,  120 ,  140 ,  150  for connecting the filter  130  either to the previous/next processing stage or to a feedback loop for tuning the filter cut-off frequency. When the WLAN communication device transitions from the processing mode to a filter tuning mode, the switches  110 ,  120 ,  140 ,  150  may disconnect the filter  130  from the previous/next processing stage and connect the filter  130  to the tuning feedback loop and vice versa. In the present embodiment, the switches  110 ,  120 ,  140 ,  150  are analog switches. The switches  110 ,  120 ,  140 , may be operated independently from each other or simultaneously. For example, the filter pair  110 ,  120 , the filter pair  140 ,  150 , or all four switches  110 ,  120 ,  140 ,  150  may be operated simultaneously. 
     For tuning the cut-off frequency of the filter  130 , the filter  130  may be connected to the tuning feedback loop. In the present embodiment, the tuning feedback loop comprises a comparator  160 , a tuning controller  170  and a current generator  180 . The current generator  180  may provide a test signal to the filter  130  over the switches  110 ,  120 . According to the embodiment, the current generator  180  produces current pulses having a defined current level that are used as the test signal. The current pulses may be generated periodically, e.g., at 20 MHz based on a clock signal provided to the current generator  180 . The tuning feedback loop may further comprise a comparator  160  which may receive over the switches  140 ,  150  an output signal of the filter  130  which may correspond to the filtered test signal while the WLAN communication device is in the filter tuning mode. The output signal of the filter  130  may undergo further processing, e.g., amplification or frequency conversion, before being input to the comparator  160 . According to the present embodiment, the comparator  160  is a high speed comparator. 
     The comparator  160  may further receive a reference signal and compare the output signal of the filter  130  to the reference signal. This may include measuring the level of the output signal, which may be an AC signal, and comparing the measured level to the level of the reference signal. For this purpose, the comparator 160 may include level detecting and comparing subunits. The reference signal may be, e.g., a DC voltage signal or a DC current signal derived from a bandgap-source, which may set defined switching thresholds within the comparator  16 O. The comparator  160  may emit a comparator signal indicative of the result of comparing the output signal and the reference signal, i.e. of the difference between those two signals. For instance, the comparator signal may be a current or voltage signal. 
     The comparator signal may be provided to the tuning controller  170 . In the present embodiment, the tuning controller  170  is a digital block. Based on the comparator signal, the tuning controller  170  may generate a digital tuning word and provide the digital tuning word the filter  130 . Further, the digital tuning word may be supplied to further filters or circuits within the WLAN communication device in parallel. In the described embodiment, the digital tuning word is a five bit digital tuning word. Other digital tuning words may also be used. The tuning controller  170  may comprise a counter for increasing and/or decreasing the digital tuning word based on a clock signal provided to the tuning controller  170 . 
     The clock signal may be generated by a clock oscillator within the WLAN communication device. According to the present embodiment, the clock signal is generated at a frequency of 20 MHz. Alternatively, other frequencies may be used for the clock signal. The tuning controller  170  may forward the clock signal to the current generator  180 . In other embodiments, the clock signal may be provided to the tuning controller  170  and the current generator  180  in parallel. In still other embodiments, different clock signals may be provided to the tuning controller  170  and the current generator  180 . This may be accomplished, e.g., by using different clock oscillators or by passing the clock signal through frequency multipliers or dividers before providing it to the tuning controller  170  and/or the current generator  180 . 
     The tuning controller  170  may further be provided with an enable signal initiating the tuning controller  170  to start the counter. Upon reception of the enable signal or upon having started the counter, the tuning controller  170  may in turn output an enable confirmation signal. Further, the tuning controller  170  may receive a disable signal causing the tuning controller  170  to stop the counter. Alternatively, the tuning controller  170  may stop the counter individually, for example once the comparator signal indicates that the output signal of the filter  130  equals the reference signal. Upon having received the disable signal or having stopped the counter, the tuning controller  170  may emit a disable confirmation signal. 
     Once the filter  130  is properly tuned, i.e. the comparator signal indicates that the output signal of the filter  130  corresponds to the reference signal, the tuning controller  170  may output a validation signal indicating that the actual digital tuning word is valid. In other embodiments, the digital tuning word may be provided to the further filters or circuits only when the tuning controller  170  has output the validation signal. For this purpose, switches may be interposed between the tuning controller  170  and the further filters or circuits within the WLAN communication device. 
     In addition to the components shown in FIG. 1, the WLAN communication device may comprise one or more further controllers for setting the switches  110 ,  120 ,  140 ,  150 , providing the reference signal to the comparator  160 , exchanging the enable signal and/or the enable confirmation signal with the tuning controller  170 , receiving the validation signal from the tuning controller  170 , and/or exchanging the disable signal and/or the disable confirmation signal with the tuning controller  170 . In further embodiments, the switches  110 ,  120 ,  140 ,  150  may be controlled, e.g., by the tuning controller  170 . 
     Turning now to  FIG. 2 , a flow diagram of the filter tuning process according to an embodiment is shown. In step  210 , the reference signal may be provided to the comparator  160 . According to the present embodiment, a voltage signal is used for the reference signal. The clock signal may be provided to the tuning controller  170  in step  220 . 
     In step  230 , the counter within the tuning controller  170  may be started. This step may be preceded by receiving the enable signal by the tuning controller  170 . Further, the tuning controller  170  may emit an enable confirmation signal upon having received the enable signal or upon having started the counter in step  230 . 
     In step  240 , the tuning feedback loop may be closed. This may be achieved by setting the switches  110 ,  120 ,  140 ,  150  to disconnect the filter  130  from the previous processing stage and the next processing stage and to connect the filter  130  to the feedback loop comprising the comparator  160 , the tuning controller  170  and the current generator  180 . Once the tuning feedback loop has been closed in step  240 , the digital tuning word provided by the tuning controller  170  to the filter  130  may be optimized in step  250 . This step will be described in more detail with reference to  FIG. 3 . 
     Upon having optimized the digital tuning word in step  250 , the tuning feedback loop may be opened in step  26 O. This may comprise setting the switches  110 ,  120 ,  140 ,  150  to reconnect the filter  130  with the previous processing stage and the next processing stage of the communication signal. In step  270 , the counter within the tuning controller  170  may be stopped. As indicated above, this may be achieved by the tuning controller  170  individually or upon a disable signal provided to the tuning controller  170 . Upon having received the disable signal or upon having stopped the counter in step 270, the tuning controller  170  may output a disable conformation signal. 
     In step  280 , it may be determined whether the cut-off frequency of the filter  130  is to be retuned. Retuning may be performed, e.g., periodically at a certain repetition rate or when particular process variables have changed, for example when the WLAN communication device has switched to another communication channel. Retuning may be initiated, for instance, by the tuning controller  170 , an additional separate controller, or a detector identifying a change in the reference signal. 
     If the determination in step  280  yields that the cut-off frequency of the filter  130  is to be retuned, the filter tuning scheme may return to step  230  for restarting the counter in the tuning controller  170 . Otherwise, the filter tuning process may be complete at this point. 
     The sequence of steps shown in  FIG. 2  has been chosen for illustration purposes only and is not to be understood as limiting the invention. For instance, steps  210  to  240  may be performed in a different order. Accordingly, steps  260  and  270  may be performed in the inverse order. Further, the reference signal provided to the comparator  160  in step  210  may be disabled once the digital tuning word has been optimized in step  250  and reenabled if the necessity for retuning the cut-off frequency of the filter  130  has been determined in step  280 . 
       FIG. 3  shows step  250  of optimizing the digital tuning word in more detail. In step  310 , the digital tuning word may be provided by a tuning controller  170  to the filter  130 . The start value of the digital tuning word may be set in step  320  so as to maximize the cut-off frequency of the filter  130 . In step  330 , the filter output signal may be measured by the comparator  160 . This may include measuring the level of the output signal. Further, the level of the reference signal provided to the comparator  160  may be measured. The comparator  160  may compare the output signal of the filter  130  to the reference signal in step  340 . According to the present embodiment, step  340  comprises comparing the levels of the output signal and the reference signal. 
     In step  350 , it may be determined whether the level of the output signal is higher than the level of the reference signal. According to the present embodiment, step  350  is performed by the comparator  160 . If the level of the output signal is higher than the level of the reference signal, the tuning controller  170  may decrease the digital tuning word in step  370 . In the present embodiment, decreasing the digital tuning word causes the cut-off frequency of the filter  130  to decrease as well. The sequence of steps  370 ,  330 ,  340  and  350  may be repeated until the level of the output signal equals the level of the reference signal. Once this is the case, the tuning controller  170  may output the validation signal indicating that the corresponding digital tuning word is valid. 
     In other embodiments, the start value of the digital tuning word may be set in step  320  to minimize the cut-off frequency of the filter  130 . In such embodiments it may be determined in step  350  whether the level of the output signal is lower than the level of the reference signal, and if this is the case, the tuning controller may increase the digital tuning word in step  370 . 
     The filter  130  may be a low pass or high pass filter removing signals having frequencies that are above or below the cut-off frequency, respectively. Alternatively, the filter  130  may also be a bandpass filter or a bandstop filter blocking signals outside or inside a certain frequency band, respectively. For bandpass and bandstop filters, the presented filter tuning method may be applied for tuning a center frequency and or the corner frequencies of the filter frequency band. 
     As apparent from the above description of embodiments, an improved method and apparatus for adjusting the frequency response of the filter  130  depending on the process variations has been presented. The discussed filter tuning may simplify the manufacturing of corresponding WLAN communication devices as well as improve their accuracy and increase their operating range. 
     The input of the filter  130  may be connected with a current generator  180  using an analog switch  110 ,  120 . The current generator  180  may provide current pulses with a frequency of, e.g., 20 MHz with a defined current level. 
     The digital block  170  may generate a digital tuning word of, for example, five bits, which may adjust the filter  130  to the highest cut-off frequency. The output signal of the filter  130  may be measured by the high speed comparator  160 . The comparator  160  may compare the output level to the reference level. 
     When the output level is higher than reference level, the digital word may be decreased. The comparator may measure the level again. This procedure may be repeated until the reference level is detected. The counter in the digital block may be stopped and the five bit output word may be valid. 
     As discussed above, a high speed comparator  160  may be used for comparing the output signal of the filter  130  to the reference signal, allowing for the time during which the filter  130  is disconnected from the communication signal path for tuning purposes to be significantly reduced. Further, the tuning process may be accelerated by digitally controlling the filter tuning. This may be accomplished by providing e digital tuning word to the filter  130 , which may be generated by a digital block. Moreover, the digitized filter tuning control may increase the filter tuning accuracy. 
     While the invention has been described with respect to the physical embodiments constructed in accordance therewith, it will be apparent to those skilled in the art that various modifications, variations and improvements of the present invention may be made in the light of the above teachings and within the purview of the appended claims with out departing from the scope of the invention. In addition, those areas in which it is believed that those of ordinary skill in the art are familiar, have not been described herein in order to not unnecessarily obscure the invention described herein. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrative embodiments, but only by the scope of the appended claims.