Abstract:
Systems are disclosed that utilize AC-coupled filtering to reduce DC offset, with aspects dynamically correcting instantaneous DC offset generated from the AC-coupled filtering. A DC offset correction circuit for an radio frequency (rf) receiver, comprising a switchable high pass filter configured to receive a signal, the switchable high pass filter including a plurality of high pass filters connected in parallel, wherein at least one of the plurality of high pass filters has a tunable corner frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. application Ser. No. 10/850,774, filed on May 21, 2004, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to wireless communication systems and methods, and more specifically to DC offset cancellation for a wireless receiver. 
         [0003]    Wireless network technology conveniently allows a mobile user to wirelessly connect to a wired network, such as an enterprise&#39;s local area network (LAN). Heterodyne receivers and direct downconversion receivers are generally used to receive wireless transmissions. In a heterodyne receiver, a radio frequency (RF) signal is mixed with a signal from a local oscillator to produce and intermediate frequency (IF) signal. Through various IF stages with mixers, oscillators, and filters, the IF signal eventually is downconverted to a baseband signal for further processing. 
         [0004]    Direct downconversion receivers (DCRs) directly downconvert a received RF signal to a baseband signal without additional signal processing at various IF stages. DCRs have a local oscillator operating at the carrier frequency used to downconvert signals to baseband, resulting in a simpler architecture. The downconverted signal is amplified by a gain to achieve optimal signal strength before presentation to a digital detector or demodulator for further processing. 
         [0005]    Processing of received signals may be negatively impacted by DC offset. DC offset may occur due to, for example, current leakage from a receiver&#39;s local oscillator (LO), signal reflections, temperature changes, and other causes. 
         [0006]    An AC-coupled high pass filter can provide a straightforward approach to correct DC offset, with the filter removing low frequency and DC signal components. Such high pass filters are more effective in removing effective DC offset when the corner frequency of the filter is closer to the frequencies carrying information. Unfortunately, filters with high corner frequencies are also more likely to attenuate signals at the information carrying frequencies. Accordingly, some have proposed use of a high pass filter with a corner frequency switchable from a first higher frequency used during initial reception to a second lower frequency during reception of other portions of a signal. Use of such filters, however, may result in a substantially instantaneous change, or distortion, in perceived signal strength and DC offset by downstream processing components. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The invention provides methods and systems that utilize AC-coupled filtering to reduce DC offset, with aspects dynamically correcting instantaneous DC offset generated from the AC-coupled filtering. In one aspect, the invention provides a DC offset correction circuit for an radio frequency (rf) receiver, comprising a switchable high pass filter configured to receive a signal, the switchable high pass filter including a plurality of high pass filters connected in parallel, wherein at least one of the plurality of high pass filters has a tunable corner frequency. 
         [0008]    In another aspect of the invention, the invention comprises a dc offset correction circuit for a radio frequency (rf) receiver, comprising: a plurality of high pass filters connected in parallel, wherein each of the plurality of high pass filters has a tunable corner frequency, wherein the tunable corner frequency of each of the plurality of high pass filters is controlled by an independently controlled control signal. 
         [0009]    These and other aspects of the invention are more fully appreciated in view of this disclosure including the associated figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    In the drawings, like reference characters generally refer to the same parts throughout the different drawings. 
           [0011]      FIG. 1  is a block diagram of an embodiment of a receiver in a communication system in accordance with aspects of the invention. 
           [0012]      FIG. 2  schematically depicts an embodiment of an N-corner switchable high pass filter used in a receiver in accordance with aspects of the present invention. 
           [0013]      FIG. 3  illustrates sampling of a preamble field in an RF signal with N equally spaced sample points. 
           [0014]      FIG. 4  schematically depicts an implementation of an embodiment of a high pass filter useful in an N-corner switchable high pass filter in accordance with aspects of the present invention. 
           [0015]      FIG. 5  is a block diagram of an embodiment of an N-corner switchable high pass filter useful in a receiver in accordance with aspects of the present invention. 
           [0016]      FIG. 6  is a block diagram of a further N-corner switchable high pass filter useful in a receiver in accordance with aspects of the present invention. 
           [0017]      FIG. 7  is a timing diagram illustrating switching of an N-corner switchable high pass filter in accordance with aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  illustrates a direct conversion receiver (DCR)  10  in a communication system in accordance with aspects of the invention. In typical implementations, the communication system of  FIG. 1  operates in a radio frequency based network. In one embodiment, a source (not shown) provides a data stream in a digital format. The source in some embodiments is a computer system, a personal digital device that has wireless communication capability, or other digital device of subsystem of such a device. The source provides information in a digital form to, for example, a QAM (quadrature amplitude modulation) modulator (not shown). The QAM modulator receives the information in digital form and modulates the digital signal to form an analog signal stream. The QAM modulator then provides the analog signal stream to a transmitter (not shown), which transmits the analog signal stream to the receiver. 
         [0019]    In the receiver, an RF antenna  10  receives an analog signal stream. The received signal stream is provided to a mixer  12 . The mixer  12  mixes a local oscillator (LO) signal with the received signal. The LO signal is provided by a local oscillator (not shown). The LO signal is at the received RF signal&#39;s center frequency, so the mixer  12  downconverts the RF signal into a baseband signal. Very often, the received signal is in quadrature, and two mixers are used to generate I-channel and Q-channel signals. In other embodiments the mixer downconverts the signal to an IF signal. 
         [0020]    An amplifier  14  receives the downconverted signal and provides an amplified downconverted signal. In some embodiments, the amplifier  14  is a variable gain amplifier, with the magnitude of the gain determined, for example, by an automatic gain control circuit. 
         [0021]    The amplified downconverted signal is passed through a N-corner switchable high pass filter  16 . The N-corner switchable high pass filter includes N, with N more than two, corner frequencies. In some embodiments, in operation the corner frequency is successively lowered during reception of a preamble field of a data stream. After filtering by the N-corner switchable high pass filter the filtered amplified downconverted signal is provided to an analog to digital converter (ADC)  18 . In some embodiments, such as where the signal received by the filter is an IF signal, further downconversion may first be performed. 
         [0022]    The ADC converts the analog baseband signal to a digital signal, and forwards the digital signal to a digital baseband processor  20 . The digital baseband processor performs additional processing, such as demapping, framing, and other processing. 
         [0023]      FIG. 2  schematically depicts an embodiment of an N-corner switchable high pass filter in accordance with aspects of the invention. The filter is used as part of a receiver system such as the system of  FIG. 1 . The filter of  FIG. 2  includes N switchable high pass filters  505 ,  510 ,  515 . In one embodiment, the high pass filters have identical, virtually identical, or substantially similar components, where each of the filters has controllable corner frequencies. In some embodiments, the corner frequencies are controlled by control signals triggered at different times. The N high pass filters, HPF 1 , HPF 2 , . . . , HPF N , are interconnected in parallel, sharing a common input  500 . The outputs of the filters, HPF 1 , HPF 2 , . . . , HPF N , are summed together in a summer  517  and are linearly scaled by the sampling factor N by a divider (or attenuator)  520 . 
         [0024]    Each high pass filter includes a capacitor and a first resistor RI connected in series, with the resistor RI coupled to ground and the output of the filter taken between the capacitor and the resistor. A second resistor R 2  is coupled in parallel to the resistor RI by a switch. 
         [0025]    Each high pass filter is therefore a high pass RC filter, with a corner frequency inversely dependent on the capacitance C and the resistance R. When the switch is closed the resistance is 
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         [0000]    When the switch is open the resistance is RI, which will generally be greater than 
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         [0000]    As the corner frequency inversely depends on the resistance, the higher effective resistance when the switch is open reduces the corner frequency of the filter. 
         [0026]    Taking the high pass filter HPF 1  as an example, HPF 1  includes a capacitor  530  and a resistor RI  540  connected in series. A further resistor R 2   550  is coupled in parallel to the resistor  540  by a switch  560 . The high pass filter HPF 1  has a higher corner frequency when the switch  560  is closed. The high pass filter HPF 1  has a lower corner frequency when the switch  560  is open. The other high pass filters HPF 2 , . . . , HPF N , are configured and operate in the same manner. 
         [0027]    In operation, the switches of the HPF 1 , HPF 2 , . . . , HPF N  are initially closed and then sequentially opened over a course of one period T of a received signal. For example, for HPF 1 , the switch is open at t 0 , the switch for HPF 2  is open at t 1 , and the switch for HPF N  is open at t N-1  where t 0 &lt;t 1 &lt; . . . &lt;t N-1 . More particularly, in some embodiments switches are open at N equally spaced sampling points, such that t 1-t0 =t 2 −t 1 , etc. 
         [0028]    In an exemplary embodiment the filter of, for example,  FIG. 2  is used in an 802.11 wireless receiver. Corner frequencies for the filter are switched at N equally spread sample points within a period of a preamble of a received signal. Thus, for the filter of  FIG. 2 , the N switches are open at N equally spaced sample points within one period of the preamble. 
         [0029]      FIG. 3  illustrates an example of a preamble field of an RF signal at the receiver  10  in a time domain and a frequency domain. In an exemplary IEEE 802.11 compliant frame structure, an RF signal comprises a number of frames, where each of the frames has a preamble field and a data field. For an IEEE 802.11g-based communication system, the preamble signal components are periodic at every T=800 ns. 
         [0030]    It is believed that a method of operation of sequentially opening the switches of the filter at N equally spaced sampling points within a preamble field reduces instantaneous DC offset caused by switching corner frequencies. For an 802.11g signal, the preamble sequence of the signal may be represented as 
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         [0000]    . When sampled by N equally spaced sampled points, ω=2π/T and t=nT/N. Substituting these values for ω and t provides 
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         [0032]    For 802.11g implementations, M=2, 4, 6. The smallest common denominator for 2M=4, 8, 12, is 24. Thus, for 24 sampling points the RMS error reduction due to switching corner frequencies is 100%. Similarly, for 2 sampling points 20 the RMS error reduction is 50%, for 4 sampling points the RMS error reduction is 66%, and for 8 samplings the RMS error reduction is 83%. Although N=24 provides an optimal error reduction rate of 100% I it is believed that N=4, with an error reduction of 66%, provides an acceptable rate of error reduction. 
         [0033]    Accordingly, in various embodiments a high pass filtering system is used in a receiver. The filtering system has a plurality of corner frequencies, with the number of corner frequencies a multiple of the number of fundamental frequencies of a sampled signal. Each corner frequency is assigned a sampling point within a period of at least one of the frequencies. 
         [0034]      FIG. 4  schematically depicts a further embodiment of a switchable high pass filter in accordance with aspects of an AC-coupled DC offset correction circuit in accordance with the invention. The filter of  FIG. 4  uses a switched capacitance resistor. A switched capacitance resistor uses two switches to charge and discharge, respectively, a capacitor. Non-overlapping clocks of the same frequency, but differing in phase, are used to open and close each switch. The equivalent resistance of the switched capacitance resistor is inversely proportional to the frequency of the non-overlapping clocks and the capacitance. Use of a switched capacitance resistor obviates the need to use a resistor, which may use excess space and be subject to process variations in an integrated circuit. Further, as process variations are expected to affect capacitors in similar ways, the corner frequency of an RC filter using a switched capacitance resistor is largely immune to process variations. 
         [0035]    The filter of  FIG. 4  includes a capacitor  410  and a first resistance  412  in series, with the output of the filter taken between the capacitor and the first resistance. A second resistance  414  is coupled in parallel to the first resistance by a switch  426 . The first resistance is provided by a switched capacitor. The switched capacitor is formed by coupling a capacitor  418  with a first switch  420  coupling a node of the capacitor  418  to the output of the capacitor  410  and a second switch  422  coupling the same node to common. In some embodiments, the capacitor  418  is a cascade of capacitors connected in series. The first and second switches are each set to an on or off state by applying clock signals φ 1  and φ 2 , to the switches  420  and  422  respectively. The clock signals are non-overlapping clock signals with the same frequency. 
         [0036]    As illustrated in  FIG. 4 , the switches are transistors, and more specifically the switches  420  and  422  are NMOS transistors, receiving the clock signals φ 1  and φ 2 . In some embodiments, the switches may be, but are not limited to, PMOS transistors, bipolar junction transistors (BJTs), MESFETs, or other types of a transistors. 
         [0037]    The second resistance is provided by a resistive element  424 . In one embodiment, the resistive element  424  is a resistor. In some embodiments, the resistive element  424  is a transistor or a series of transistors set to operate in their linear range. A switch  426  allows current to flow through the resistive element  424 . In one embodiment, the switch  426  is an NMOS transistor, as illustrated in  FIG. 4 . In some embodiments, the switch may be, but is not limited to, a PMOS transistor, a bipolar junction transistor (BJT), a MESFET, or other types of a transistor. The switch  426  changes its state by application of a corner switching signal to its gate. The filter has a higher corner frequency when the corner switching signal closes the switch and couples the second resistance to the output node. 
         [0038]      FIG. 5  is a block diagram depicting an embodiment of an N-corner switchable high pass filter in accordance with aspects of the present invention. In one embodiment, the switchable high pass filter has N selectable corner frequencies, where N is greater than two. In some embodiments, the switchable high pass filter comprises N number of high pass filters, where N is greater than two. In these embodiments, the high pass filters are switchable from a first corner frequency to a second corner frequency. In one embodiment, the high pass filters have identical, virtually identical, or substantially similar components, where each of the filters has controllable corner frequencies. In some embodiments, the corner frequencies are controlled by control signals triggered at different times. As illustrated in  FIG. 5 , N number of switchable high pass filters are distributed over two blocks  620  and  640 . In some other embodiments, N number of switchable high pass filters are distributed over K number of blocks, where K can be any integer between 1 and N. Referring back to the blocks  620  and  640  of  FIG. 5 , within each block, the high pass filters may be arranged in parallel, in series, or in a combination of a parallel and serial arrangements. In some embodiments, within each block, at least some of the high pass filters are arranged in series and at least some of high pass filters are arranged in parallel. 
         [0039]      FIG. 6  is a block diagram depicting a further embodiment of a switchable high pass filter used as dynamic AC-coupled DC offset correction circuit in accordance with aspects of the present invention. The filter includes a first switchable high pass filter formed by a series of switchable high pass filters,  700 ,  720 ,  740  interspersed by amplifiers, or buffers,  710 ,  730 ,  750 . 
         [0040]    The output of the last buffer in the series is provided to four parallel switchable high pass filters  760 ,  770 ,  780 ,  790 . The output of each of the parallel switchable high pass filters is provided to a summer  795 . The switchable high pass filters may be, for example, the filters illustrated in  FIG. 2  or  4 . 
         [0041]    The corner frequency is lowered for the series of filters at a first time, t 0 , as illustrated in  FIG. 5 . Changing the corner frequency of the series of filters effectively results in a somewhat instantaneous change in the signal output from the series of filters. This change may be viewed as a change in the DC offset of the received signal, particularly by other circuitry such as AGCs. Effects of the change are reduced, however, by subsequent passage of the signal through the parallel filters. 
         [0042]    The corner frequency of the filters in parallel is lowered sequentially for the filters  76 ,  770 ,  780 , and  790  at times t 1 , t 2 , t 3 , and t 4 , respectively. Preferably times t 1 , t 2 , t 3  and t 4  are within one fundamental period of a received signal. 
         [0043]      FIG. 7  schematically depicts a signal diagram showing operation, for example, of the filter of  FIG. 6  in the system, for example, of  FIG. 1 . In one embodiment, during reception of a preamble of a frame, an AGC in a receiver adjusts the gain applied to the received signal via, for example, an amplifier to provide optimal signal strength. This occurs at least during a gain adjustment period  800 , and may also occur thereafter. During the gain adjustment period  800  the AGC corrects a substantial portion of DC offset. 
         [0044]    Referring also to  FIG. 7  upon completion of the gain adjustment period, the corner frequencies of the filters  710 ,  720 ,  740  are lowered at time t 0 . The parallel filters  760 ,  770 ,  780 ,  790  reduce instantaneous DC offset error introduced by lowering the corner frequencies of the filters  700 ,  720 ,  740 . The corner frequency of the filter  760  is lowered at time t 1 . 200 ns later at time t 2 , the corner frequency of the filter  770  is lowered. 200 ns later at time t 3 , the corner frequency of the filter  780  is lowered and 200 ns later at time t 4 , the corner frequency of the filter  790  is lowered. 200 ns is used to separate lowering of corner frequencies as one fundamental period T in a 802.11g-based frame is 800 ns. 
         [0045]    Variations/modifications, and other implementations of what is described herein will occur to those of ordinary skill in the art without departing from the spirit and the scope of the invention as claimed. Although the invention has been described with respect to certain embodiments, it should be recognized that the invention includes the claims and their equivalents supported by this disclosure.