Abstract:
Apparatus, systems, and methods implementing techniques for filtering signals are described. A filter circuit receives an input signal and produces a corresponding filtered signal. The filter circuit has a transfer function that relates the filtered signal to the input signal. The transfer function includes at least one pole and at least one zero, where at least one of the zeros corresponds to a first frequency, and at least one of the poles corresponds to a second frequency. The apparatus also includes a negative-transconductance circuit that is coupled to the filter circuit and that increases a magnitude of a component of the filtered signal that corresponds to the second frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to commonly assigned U.S. Provisional Patent Application No. 60/515,297, filed on Oct. 29, 2003, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The following disclosure relates to electrical circuits and signal processing. 
     A signal in a transmit or receive path of a communications transceiver can contain undesired spurious tones, and the spurious tones can degrade the quality of the signal. Spurious tones can be caused, for example, by a local-oscillator signal or a signal at a mixer input coupling to a mixer output. Spurious tones can also be caused by clock harmonics in a local-oscillator signal or noise from other parts of the communications transceiver capacitively coupling to a signal conduit. A received signal can contain spurious tones from a remote transmitter. A communications transceiver can filter signals in the transmit or receive path to attenuate spurious tones. Some conventional transceivers use external filters to attenuate spurious tones in a transmitted or received signal. Integrated communications transceivers can use on-chip filters instead of external filters to save space and to lower component costs. 
     Conventional on-chip filters in a transceiver can include a serial or parallel connection of a capacitor and an inductor, hereafter referred to as a serial or parallel LC circuit, respectively.  FIG. 1A  shows a graph  110  of the magnitude of an impedance of parallel LC circuit  120  versus frequency. The impedance of circuit  120  shown in graph  110  is present between terminals  135  and  140 . Graph  110  has a peak  115  where a capacitor  125  and an inductor  130  resonate. The frequency at which peak  115  is located corresponds to a pole in a transfer function of circuit  120 . When an input current is passed through terminals  135  and  140  and an output voltage is measured between terminals  135  and  140 , tones at or near the frequency at which peak  115  is located are passed with less attenuation than tones at frequencies other than the frequency at which peak  115  is located. When inductor  130  and capacitor  125  are ideal components, the impedance of peak  115  is infinite. Because inductor  130  and capacitor  125  typically include parasitic resistance, the impedance of peak  115  is typically finite. 
       FIG. 1B  shows a graph  150  of the magnitude of an impedance of serial LC circuit  160  versus frequency. The impedance of circuit  160  shown in graph  150  is present between terminals  175  and  180 . Graph  150  has a dip  155  where a capacitor  165  and an inductor  170  resonate. The frequency at which dip  155  is located corresponds to a zero in a transfer function of circuit  160 . When an input current is passed through terminals  175  and  180  and an output voltage is measured between terminals  175  and  180 , tones at or near the frequency of dip  155  are attenuated. 
     Impedance in a conventional LC circuit can be tuned to attenuate a spurious tone (e.g., a tone at the frequency of dip  155  in  FIG. 1B ). However, when a transmitted signal in a wireless transmitter contains spurious tones, a single LC circuit may not attenuate spurious tones adequately (for example, to satisfy Federal Communications Commission regulations), so multiple LC circuits can be cascaded. Cascaded LC circuits that adequately attenuate spurious tones can also attenuate a desired signal significantly. If a spurious tone is close in frequency to the desired signal, conventional LC circuits may not be able to adequately attenuate the spurious tone while preserving the desired signal. 
     SUMMARY 
     In one aspect, the invention features an apparatus that includes a first filter circuit, which receives an input signal and produces a corresponding filtered signal. The first filter circuit has a transfer function that relates the filtered signal to the input signal. The transfer function includes at least one pole and at least one zero, where at least one of the zeros corresponds to a first frequency, and at least one of the poles corresponds to a second frequency. The apparatus also includes a negative-transconductance circuit that is coupled to the first filter circuit and that increases a magnitude of a component of the filtered signal that corresponds to the second frequency. 
     In another aspect, the invention features an apparatus that includes a first filtering means, which receives an input signal and produces a corresponding filtered signal. The first filtering means has a transfer function that relates the filtered signal to the input signal. The transfer function includes at least one pole and at least one zero, where at least one of the zeros corresponds to a first frequency, and at least one of the poles corresponds to a second frequency. The apparatus also includes a boosting means that is coupled to the first filtering means and that increases a magnitude of a component of the filtered signal that corresponds to the second frequency. 
     In one aspect, the invention features a wireless transceiver that includes a receiver, which receives a modulated carrier signal. The receiver includes a first filter circuit that receives an input signal and produces a corresponding filtered signal. The first filter circuit has a transfer function that relates the filtered signal to the input signal. The transfer function includes at least one pole and at least one zero, where at least one of the zeros corresponds to a first frequency, and at least one of the poles corresponds to a second frequency. The receiver also includes a negative-transconductance circuit that is coupled to the first filter circuit and that increases a magnitude of a component of the filtered signal that corresponds to the second frequency. 
     In another aspect, the invention features a wireless transceiver that includes a receiver means, which receives a modulated carrier signal. The receiver means includes a first filtering means that receives an input signal and produces a corresponding filtered signal. The first filtering means has a transfer function that relates the filtered signal to the input signal. The transfer function includes at least one pole and at least one zero, where at least one of the zeros corresponds to a first frequency, and at least one of the poles corresponds to a second frequency. The receiver means also includes a boosting means that is coupled to the first filtering means and that increases a magnitude of a component of the filtered signal that corresponds to the second frequency. 
     In yet another aspect, the invention features a method for filtering a signal. An input signal is filtered to produce a corresponding filtered signal using a first filter circuit. The first filter circuit has a transfer function relating the filtered signal to the input signal. The transfer function includes at least one pole and at least one zero, where at least one of the zeros corresponds to a first frequency, and at least one of the poles corresponds to a second frequency. A magnitude of a component of the filtered signal that corresponds to the second frequency is increased using a negative-transconductance circuit. 
     Particular implementations may include one or more of the following features. The first filter circuit can include passive components and an input transistor, where the input transistor receives the input signal and produces a corresponding current in the passive components, which produces the filtered signal. The passive components can include an inductor formed from a bond wire. The passive components can include a first inductor connected in series between a DC voltage source and an output node and a first capacitor connected in series between the DC voltage source and the output node. The passive components can also include a second inductor connected in series to a second capacitor, where the second inductor and second capacitor are connected in series between the DC voltage source and the output node. A pole and a zero of the transfer function can be tunable. 
     A second filter circuit can be included to filter a second input signal and produce a corresponding second filtered signal. The second filter circuit can have a second transfer function that relates the second filtered signal to the second input signal, where the second transfer function is substantially similar to the transfer function of the first filter circuit. The negative-transconductance circuit can also increase a magnitude of a component of the second filtered signal corresponding to the second frequency. 
     The negative-transconductance circuit can include a first transistor with a gate, a source, and a drain, where the gate is coupled to the second filter circuit, the source is coupled to a biasing circuit, and the drain is coupled to the first filter circuit. The negative-transconductance circuit can also include a second transistor with a gate, a source, and a drain, where the gate is coupled to the first filter circuit, the source is coupled to the biasing circuit, and the drain is coupled to the second filter circuit. 
     The first filter circuit and the negative-transconductance circuit can be fabricated monolithically on a semiconductor substrate. The transfer function can include two or more zeros. The transfer function can include two or more poles. The first frequency can correspond to a spurious tone in the input signal. The second frequency can correspond to a desired signal in the input signal. The apparatus can be compliant with one or more of IEEE standards 802.11, 802.11a, 802.11b, 802.11e, 802.11g, 802.11h, 802.11i, 802.11n, and 802.16. 
     Implementations can include one or more of the following advantages. A method and system are disclosed that can be used to filter a signal to attenuate spurious tones. A desired signal can be passed through the system substantially unattenuated. The method and system can be adjusted to adapt to changing signal conditions. The method and system can attenuate a spurious tone that is close in frequency to a desired signal while passing the desired signal substantially unattenuated. The method and system can allow an on-chip filter to be used in a transceiver and can reduce or eliminate a need for off-chip filtering, saving space and money. 
     These general and specific aspects may be implemented using an apparatus, a system, a method, or any combination of apparatus, systems, and methods. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a prior art impedance graph for a parallel LC circuit. 
         FIG. 1B  is a prior art impedance graph for a serial LC circuit. 
         FIG. 2A  is a schematic of a combination of a parallel LC circuit and a series LC circuit. 
         FIG. 2B  is an impedance graph for the circuit in  FIG. 2A   
         FIG. 3  is a filter circuit including a negative-transconductance circuit. 
         FIG. 4  is a schematic of a negative-transconductance circuit. 
         FIG. 5  is a flowchart of a process for filtering signals. 
         FIG. 6  is a block diagram of a wireless transceiver. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 2A  shows a filter circuit  200  consisting of two LC circuits connected in parallel. A capacitor  210  and an inductor  220  are connected in series to form a series LC circuit. A capacitor  230  and an inductor  240  are connected in parallel to form a parallel LC circuit. Capacitors  210  and  230  can be variable or fixed capacitors. Inductors  220  and  240  can be variable or fixed inductors. Inductors  220  and  240  can be monolithic inductors fabricated on a silicon substrate. In one implementation, inductor  220  and/or inductor  240  are formed using bond wires. In another implementation, inductor  240  is not included in filter circuit  200 . 
     Filter circuit  200  has an impedance between a terminal  250  and a terminal  260  that has at least one zero and at least one pole at non-infinite and non-zero frequencies. The impedance between terminals  250  and  260  is hereafter referred to as the impedance of filter circuit  200 . Filter circuit  200  includes a transconductance cell  270  that converts an input voltage applied at a terminal  275  into a current. Transconductance cell  270  drives the passive components between terminals  250  and  260  (e.g., capacitors  210  and  230  and inductors  220  and  240 ) with the current. When a direct-current (DC) voltage is applied to terminal  250  and an alternating-current (AC) voltage is applied to terminal  275 , transconductance cell  270  drives the passive components with a varying current, producing an AC output voltage at terminal  260 . 
     The frequency of a pole in the transfer function of filter circuit  200  (hereafter referred to as the frequency of the pole) and the frequency of a zero in the transfer function of filter circuit  200  (hereafter referred to as the frequency of the zero) are determined by the values of capacitors  210  and  230  and by the values of inductors  220  and  240 . When any pole or zero is mentioned hereafter in this specification, the pole or zero that is referred to will be at a non-infinite and non-zero frequency. Other poles or zeros can exist at infinite or zero frequency. 
     Referring to  FIG. 2A  and  FIG. 2B , a graph  280  shows the magnitude of an impedance of filter circuit  200  versus frequency. Graph  280  has a dip  290  corresponding to a zero in the transfer function of filter circuit  200 . Graph  280  also has a peak  285  corresponding to a pole in the transfer function of filter circuit  200 . 
     Referring to filter circuit  300  in  FIG. 3 , a differential input signal is applied to transconductance cells  270 ( a ) and  270 ( b ) through terminals  275 ( a ) and  275 ( b ), respectively. In one implementation, transconductance cells  270 ( a ) and  270 ( b ) are single-transistor circuits. In another implementation, transconductance cells  270 ( a ) and  270 ( b ) are circuits including multiple transistors. Filter circuits  200 ( a ) and  200 ( b ) are similarly configured to filter circuit  200  (FIG.  2 A) and have been described above in the context of  FIG. 2A  and  FIG. 2B . In one implementation, a filter circuit  200 ( a ) or  200 ( b ) with one zero and one pole that includes different components or components in different configurations than filter circuit  200  in  FIG. 2A  can be used. In another implementation, a filter circuit with multiple zeros and/or poles can be used in place of filter circuit  200 ( a ) or filter circuit  200 ( b ). Terminals  260 ( a ) and  260 ( b ) of filter circuits  200 ( a ) and  200 ( b ) are connected to a negative-transconductance cell  350 . Terminals  250 ( a ) and  250 ( b ) are coupled to a DC voltage source. In one implementation, terminals  250 ( a ) and  250 ( b ) are coupled to a DC voltage source through other circuits and are not directly connected to the DC voltage source. 
     Filter circuits  200 ( a ) and  200 ( b ) produce signal voltages at terminals  260 ( a ) and  260 ( b ), respectively. The signal voltages at terminals  260 ( a ) and  260 ( b ) correspond to the magnitudes of the impedances of filter circuits  200 ( a ) and  200 ( b ). The impedances of filter circuits  200 ( a ) and  200 ( b ) are low at the frequency of the zero and are high at the frequency of the pole. When a broadband signal (e.g., white noise) is input to transconductance cells  270 ( a ) and  270 ( b ), the AC currents generated by transconductance cells  270 ( a ) and  270 ( b ) are equal (assuming that transconductance cells  270 ( a ) and  270 ( b ) have a uniform frequency response). A given current flowing through a low impedance generates a low voltage, while the given current flowing through a high impedance generates a high voltage. Therefore, the AC output voltages at terminals  260 ( a ) and  260 ( b ) are low at the frequency of the zero, and the AC output voltages at terminals  260 ( a ) and  260 ( b ) are high at the frequency of the pole. Variation of the impedance of filter circuits  200 ( a ) and  200 ( b ) with frequency causes filter circuits  200 ( a ) and  200 ( b ) to have frequency-selective properties. 
     Negative-transconductance cell  350  substantially increases the AC output voltages at terminals  260 ( a ) and  260 ( b ) of an AC signal at the frequency of the pole by providing a negative impedance to filter circuits  200 ( a ) and  200 ( b ), while increasing output voltages at other frequencies less substantially. Therefore, negative-transconductance cell  350  improves the spurious tone rejection of filter circuits  200 ( a ) and  200 ( b ). Filter circuits  200 ( a ) and  200 ( b ) may have parasitic impedance (e.g., due to non-ideal components). An impedance corresponding to the real part of the impedance of the serial connection of capacitors  210 ( a ),  210 ( b ) with inductors  220 ( a ),  220 ( b ) can also be present in filter circuits  200 ( a ) and  200 ( b ) at the frequency of the pole. The positive real-valued impedance in filter circuits  200 ( a ) and  200 ( b ) removes energy from signals in filter circuits  200 ( a ) and  200 ( b ) by converting the energy to heat. Negative-transconductance cell  350  presents a negative-valued impedance in parallel with the positive impedance by injecting energy into filter circuits  200 ( a ) and  200 ( b ). The negative impedance substantially increases the AC output voltages at terminals  260 ( a ) and  260 ( b ) of an AC signal at the frequency of the pole. At frequencies away from the pole, the negative impedance has little effect. 
     As was discussed in the context of  FIG. 2A , filter circuits  200 ( a ) and  200 ( b ) can include variable components. The frequencies of the zero and the pole of the transfer functions of filter circuits  200 ( a ) and  200 ( b ) can be tuned by varying the values of the variable components. The frequencies of the pole and the zero of filter circuits  200 ( a ) and  200 ( b ) can be tuned using an amplitude detection circuit (e.g., to find the poles and zeros), a phase-locked loop with an oscillator, or a frequency-locked loop with an oscillator. The frequencies of the pole and the zero of filter circuits  200 ( a ) and  200 ( b ) can also be tuned by adjusting the values of the capacitors in filter circuits  200 ( a ) and  200 ( b ) using a capacitor calibration circuit. In one implementation, an amplitude detection circuit is used to measure an amplitude of an output of a filter circuit so that the pole(s) and/or zero(s) of the filter circuit can be placed at desired frequencies by adjusting one or more variable components (e.g., variable capacitors) included in the filter circuit. 
     For example, the following procedure can be used to tune a filter circuit having one zero and one pole where the zero of the filter circuit depends on the value of a first variable component, but not on the value of a second variable component, and the pole of the filter circuit depends on at least the second variable component. In the first step of the procedure, the value of the first variable component is varied while the filter is provided with a signal at the desired frequency of the zero. When the amplitude detection circuit detects that the output amplitude of the filter circuit is at a lowest point, the frequency of the zero of the filter circuit has been tuned to the desired frequency of the zero (assuming that the value of the first variable component can be varied over a wide enough range to tune the zero of the filter circuit to the desired frequency of the zero). 
     In the second step of the procedure, the filter is provided with a signal at the desired frequency of the pole, and the value of the first variable component is kept at the value determined in the first step while the value of the second variable component is varied. When the amplitude detection circuit detects that the output amplitude of the filter circuit is at a greatest point, the frequency of the pole of the filter circuit has been tuned to the desired frequency of the pole (assuming that the value of the second variable component can be varied over a wide enough range to tune the pole of the filter circuit to the desired frequency of the pole). This procedure can be used with many different kinds of filter circuits and can be extended to tune multiple poles and/or zeros of the filter circuit. 
     In another implementation, filter circuits  200 ( a ) and  200 ( b ) each have more than one pole and/or more than one zero. In a filter with multiple poles, poles can be placed at frequencies near to the frequencies of other poles to increase the passband of filter circuit  300 . In a filter circuit with multiple zeros, zeros can be placed at frequencies near to the frequencies of other zeros to increase the attenuation of a frequency by filter circuit  300 , or additional zeros can be placed at various frequencies to attenuate multiple spurious tones. 
     The frequency of the pole(s) can be adjusted to match the frequency of the desired signal. The frequency of the zero(s) can be adjusted to match the frequency of a/plural spurious tone(s). The desired frequency and spurious tones can be monitored, and the frequency of the respective pole(s) and zero(s) adjusted. For example, if the frequency of a zero corresponds to the frequency of a first spurious tone, but the first spurious tone disappears from the input signal of filter circuit  300  or becomes less important than a second spurious tone, the frequency of a zero can be adjusted to correspond to the frequency of the second spurious tone. 
       FIG. 4  shows a negative-transconductance cell  350 . Negative-transconductance cell  350  includes transistors  410  and  420 . The gate of transistor  410  is connected to the drain of transistor  420 , and the gate of transistor  420  is connected to the drain of transistor  410 . The sources of transistors  410  and  420  are connected to a tail current source  450 . Negative-transconductance cell  350  can be connected to other circuits at terminals  430  and  440 . Referring to  FIG. 3  and  FIG. 4 , terminal  430  of negative-transconductance cell  350  is connected to terminal  260 ( a ), and terminal  440  of negative-transconductance cell  350  is connected to terminal  260 ( b ). In one implementation, negative-transconductance cell  350  can be implemented using different components or configurations than the circuit shown in  FIG. 4 . Negative-transconductance cell  350  can be calibrated to provide a predefined amount of negative impedance to filter circuits  200 ( a ) and  200 ( b ) by adjusting the current provided by tail current source  450 . Negative-transconductance cell  350  can be calibrated using an amplitude-detection circuit or a Q-detection circuit that outputs a current or voltage that has a known relationship to the impedances of filter circuits  200 ( a ) and  200 ( b ) at the frequency of the pole(s). For example, a Q-detection circuit can measure the amplitude of the voltage at terminal  260 ( a ) or terminal  260 ( b ) and can divide the amplitude of the voltage by the current biasing transconductance cell  270 ( a ) or transconductance cell  270 ( b ), respectively. The resulting quantity is proportional to Q. 
     Referring to  FIG. 3  and  FIG. 5 , a process  500  is shown for filtering a signal (e.g., by using filter circuit  300 ). One or more filter circuits (e.g., filter circuits  200 ( a ) and  200 ( b )) are provided and tuned (step  510 ) so that the frequency of a pole of the filter circuits corresponds to the frequency of a desired signal in the input signal, and the frequency of a zero of the filter circuits corresponds to the frequency of a spurious tone in the input signal. A negative-transconductance cell (e.g., negative transconductance cell  350 ) is provided and calibrated (step  520 ) to decrease the attenuation of the desired signal. In one implementation, the negative-transconductance cell is calibrated so that a spurious tone at the frequency of the zero is attenuated by at least 20 dB more than the desired signal is attenuated. When an input signal is applied to the filter circuits, a spurious tone at the frequency of the zero is attenuated (step  530 ) while the desired signal is passed with a smaller amount of attenuation (step  540 ). The negative-transconductance cell increases the amplitude of the AC voltage of the desired signal (e.g., at terminals  260 ( a ) and  260 ( b )) (step  550 ). 
     A filter circuit with negative transconductance can be used in a wide range of applications. Referring to  FIG. 6 , a filter circuit with negative transconductance can be used as an RF filter circuit  300  in a wireless transceiver  600  (hereafter referred to as transceiver  600 ). RF filter circuit  300  can filter both received and transmitted signals. The receive path of transceiver  600  includes a low-noise RF amplifier  610  for amplifying an RF input signal. A mixer  620  modulates a signal generated by signal source  630  with the amplified RF input signal from the output of RF amplifier  610  to create a baseband signal. A baseband filter circuit  635  filters the baseband signal. The filtered baseband signal is then amplified by gain stage  640  and is converted into a digital signal by an analog-to-digital converter  650 . The transmit path of transceiver  600  includes digital-to-analog converter (DAC)  660  and a transmitter  670 . 
     Transceiver  600  can be IEEE 802 compliant with the following standards: 802.11, 802.11a, 802.11b, 802.11e, 802.11g, 802.11h, 802.11i, 802.11n, and 802.16. 
     Various implementations have been described. These and other implementations are within the scope of the following claims.