Abstract:
Apparatus, systems, and methods implementing techniques for reducing spurious components are described. According to one aspect, a wideband polyphase filter filters an input signal that has an associated first frequency. The wideband polyphase filter has poles corresponding to a first filter frequency and a second filter frequency, where the two filter frequencies are different. According to another aspect, a mixer mixes the filtered signal with a local-oscillator signal at a second frequency to produce an upconverted signal, where the second frequency is substantially an integer multiple of the first frequency.

Description:
RELATED APPLICATION 
     This application is a continuation of and claims the benefit of priority under 35 USC 120 to U.S. application Ser. No. 10/829,801, filed Apr. 21, 2004, now U.S. Pat. No. 7,613,249 the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The following disclosure relates to electrical circuits and signal processing. 
     A wireless communications transmitter typically converts an information-bearing baseband signal from around DC to a high frequency referred to as the carrier frequency (e.g., a frequency in the microwave or RF band) that is suitable for wireless transmission. In many systems, this frequency upconversion process takes place in multiple stages. The baseband signal is first upconverted to an intermediate frequency (f IF ) that is higher than the bandwidth of the baseband signal. At the intermediate frequency, the signal is amplified and filtered before the signal is upconverted to the carrier frequency (f c ) for transmission. 
     In an ideal transmitter, all transmitted signal energy is confined to a dedicated frequency channel, and no energy is emitted outside the channel to interfere with other wireless systems. In practical realizations, out-of-band spurious emissions often are generated by transmitters due to, for example, local-oscillator (LO) harmonics, image generation, and intermodulation. 
     Spurious emissions caused by the mechanisms mentioned above can fall into restricted frequency bands and result in an emissions violation. Conventional transceivers can use highly selective external filters (e.g., SAW filters) to suppress undesirable spurious emissions. External filters add to the overall cost and size of the transceiver. 
     SUMMARY 
     In one aspect, the invention features an apparatus including a wideband polyphase filter, which filters an input signal that has an associated first frequency. The wideband polyphase filter has poles corresponding to a first filter frequency and a second filter frequency, where the two filter frequencies are different. A mixer mixes the filtered signal with a local-oscillator signal at a second frequency to produce an upconverted signal, where the second frequency is substantially an integer multiple of the first frequency. 
     In another aspect, the invention features an apparatus including a filtering means, which filters an input signal that has an associated first frequency. The filtering means has poles corresponding to a first filter frequency and a second filter frequency, where the two filter frequencies are different. A mixing means mixes the filtered signal with a local-oscillator signal at a second frequency to produce an upconverted signal, where the second frequency is substantially an integer multiple of the first frequency. 
     In one aspect, the invention features a wireless transceiver that includes a transmitter to transmit a modulated carrier signal. The transmitter includes a communications circuit, where the communications circuit includes a wideband polyphase filter that filters an input signal. The input signal has an associated first frequency, and the wideband polyphase filter has poles corresponding to a first filter frequency and a second filter frequency, where the two filter frequencies are different. The communications circuit also includes a mixer that mixes the filtered signal with a local-oscillator signal at a second frequency to produce an upconverted signal, where the second frequency is substantially an integer multiple of the first frequency. 
     In another aspect, the invention features a wireless transceiver that includes a transmitting means to transmit a modulated carrier signal. The transmitting means includes an upconversion means, where the upconversion means includes a filtering means that filters an input signal. The input signal has an associated first frequency, and the filtering means has poles corresponding to a first filter frequency and a second filter frequency, where the two filter frequencies are different. The upconversion means also includes a mixing means that mixes the filtered signal with a local-oscillator signal at a second frequency to produce an upconverted signal, where the second frequency is substantially an integer multiple of the first frequency. 
     In yet another aspect, the invention features a process for reducing spurious components in an upconverted signal, where the process includes filtering an input signal that has an associated first frequency to produce an in-phase filtered signal and a quadrature filtered signal. The quadrature filtered signal is substantially ninety degrees out of phase with the in-phase filtered signal at first and second filter frequencies, where the two filter frequencies are different. The in-phase filtered signal is mixed to a second frequency to produce an in-phase upconverted signal, where the second frequency is substantially an integer multiple of the first frequency. The quadrature filtered signal is also mixed to the second frequency to produce a quadrature upconverted signal. 
     Particular implementations may include one or more of the following features. The first frequency can correspond to a fundamental frequency of an intermediate-frequency local-oscillator signal. The first filter frequency can correspond to a desired signal in the input signal, and the second filter frequency can correspond to a spurious component in the input signal. The first filter frequency can be substantially equal to the first frequency, and the second filter frequency can be a non-unity integer multiple of the first frequency. 
     The filtered signal can include an in-phase component and a quadrature component, the local-oscillator signal can include an in-phase component and a quadrature component, and the upconverted signal can include an in-phase component and a quadrature component. The mixer can mix the in-phase component of the filtered signal with the in-phase component of the local-oscillator signal to produce the in-phase component of the upconverted signal and mix the quadrature component of the filtered signal with the quadrature component of the local-oscillator signal to produce the quadrature component of the upconverted signal. A circuit can combine the quadrature component of the upconverted signal and the in-phase component of the upconverted signal to produce an output signal. 
     The apparatus, system, or method can be compliant with any of IEEE standards 802.11, 802.11a, 802.11b, 802.11g, 802.11i, 802.11n, and 802.16. 
     In one aspect, the invention also features a wideband polyphase filter that has one or more poles corresponding to a first frequency and one or more poles corresponding to a second frequency, where the second frequency is different than the first frequency. The first frequency can correspond to a fundamental frequency of an intermediate-frequency local-oscillator signal, and the second frequency can correspond to a frequency of a spurious component. 
     In another aspect, the invention features a process for reducing spurious components in an upconverted signal. The process includes first mixing an input signal to a first frequency, thereby producing an intermediate signal. The intermediate signal is mixed to a second frequency, thereby producing an upconverted signal, where the second frequency is different than the first frequency. The first frequency and the second frequency are selected such that a spurious component of the intermediate signal generated in the first mixing falls, when upconverted, on a same frequency as another component in the upconverted signal. The second frequency can be selected to be an integer multiple of the first frequency. 
     Implementations can include one or more of the following advantages. A method, apparatus, and system are disclosed that can be used to reduce a number of spurious components in an output signal of a transmitter. Intermodulation products typically will overlap existing spurious components instead of creating new spurious components. The method, apparatus, and system can also attenuate spurious components in the output signal. The method, apparatus, and system can substantially remove critical spurious components from the output signal using internal filters and can reduce the cost and/or size of a communications transmitter or receiver. 
     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 block diagram of a system for upconverting a signal. 
         FIG. 1B  is a graph of signals generated by mixing a baseband signal with an upconversion signal. 
         FIG. 2  is a schematic of a polyphase filter circuit. 
         FIG. 3  is a block diagram of a system for upconverting a signal while reducing spurious components. 
         FIG. 4  is a graph of the phase difference between the in-phase and quadrature outputs of a polyphase filter. 
         FIG. 5  is a flowchart of a process to reduce spurious components in a transmitter. 
         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. 1A  is a block diagram of a conventional system  100  that can be used to convert a baseband signal to a radio-frequency (RF) signal. An in-phase component of the baseband signal is applied to a terminal  104 , and a quadrature component of the baseband signal is applied to a terminal  106 . A mixer  120  mixes the in-phase component of the baseband signal with an LO signal at an intermediate frequency from a signal source  110  to create an in-phase intermediate-frequency (IF) signal  122 . A mixer  125  mixes the quadrature component of the baseband signal with an LO signal at the intermediate frequency from a signal source  115  to create a quadrature IF signal  124 . The LO signal from signal source  115  is at a same frequency as the LO signal from signal source  110 , but is shifted in phase by ninety degrees. For example, the LO signal from signal source  110  can be of the form of a cosine wave at intermediate frequency f IF  and the LO signal from signal source  115  can be of the form of a sine wave at f IF . Mixers  120  and  125  will hereafter be referred to collectively as the first mixer, the in-phase and quadrature components of the baseband signal will be referred to collectively as the baseband signal, and the LO signals from signal sources  110  and  115  will be referred to collectively as the first LO signal unless otherwise noted. 
     The first mixer can generate spurious components (e.g., due to clock harmonics) when mixing the baseband signal with the first LO signal. Because the mixer function is approximately a square wave, the output signal will have a fundamental frequency component at the intermediate frequency and harmonics at odd integer multiples of the intermediate frequency. Referring to  FIG. 1B , a frequency spectrum  105  represents the magnitude signals at the output of the first mixer. Frequency spectrum  105  includes a desired signal  172  at the intermediate frequency. Frequency spectrum  105  also includes copies  174 - 178  at odd harmonics of the intermediate frequency. Copies  174 - 178  are spurious components, since only desired signal  172  is desired to be transmitted. In some implementations, a signal at a frequency other than the fundamental frequency of the first LO signal may be the desired signal. 
     Referring again to  FIG. 1A , the in-phase  122  and quadrature  124  IF signals are combined by an adder  130 . Depending on the implementation used, adder  130  can add the in-phase and quadrature IF signals or subtract one of the IF signals from the other. A combined IF signal  132  is filtered by a highly selective bandpass filter  135  (e.g., an external SAW filter). Filter  135  reduces spurious components due to LO harmonics and leaves the desired component at the intermediate frequency. A filtered IF signal  137  from filter  135  is provided to a polyphase filter  140 . 
       FIG. 2  illustrates a filter circuit  200  that can be used to implement polyphase filter  140  ( FIG. 1A ). Filter circuit  200  has differential inputs  210  and  220 , differential outputs  230  and  240 , and differential outputs  250  and  260 . Filter circuit  200  is constructed by cascading several stages of RC-CR networks (stages  270 ,  280 , and  290 ). Each of stages  270 ,  280 , and  290  has an RC time constant corresponding to a pole of filter circuit  200 . At each pole frequency of filter circuit  200 , the amplitude of the output signal at differential outputs  230  and  240  is the same as the amplitude of the output signal at differential outputs  250  and  260 , but the phases of the output signals are separated by ninety degrees. The pole frequencies of filter circuit  200  typically are placed close to the IF frequency of system  100  ( FIG. 1A ) to provide matching amplitudes and an accurate ninety degree phase separation at the intermediate frequency even when process variations occur in the R and C values of stages  270 ,  280 , and  290 . Typically, at frequencies away from the intermediate frequency the amplitudes of the output signals will not be matched or the phase difference between the output signals will deviate from ninety degrees (depending on whether the polyphase filter is driven by a current or by a voltage). A conventional implementation of a polyphase filter is therefore a narrowband structure exhibiting a ninety-degree phase separation over a relatively narrow range of frequencies. 
     Referring again to  FIG. 1A , polyphase filter  140  outputs two signals at the intermediate frequency—one in-phase filtered signal  142  and one quadrature filtered signal  144 . The quadrature filtered signal  144  is substantially ninety degrees out of phase with the in-phase filtered signal  142  at frequencies corresponding to the poles of the polyphase filter  140 . The filtered signals ( 142 ,  144 ) from the output of polyphase filter  140  are converted to RF signals by a mixer  160  and a mixer  165 . Mixer  160  mixes the in-phase filtered signal  142  with an LO signal from a signal source  150  to create an in-phase signal  162 . Mixer  165  mixes the quadrature filtered signal with an LO signal from a signal source  155  to create a quadrature signal  164 . The LO signals from signal sources  150  and  155  have the same frequency (f) but the signal from signal source  155  is shifted in phase by ninety degrees relative to the signal from signal source  150 . For example, the LO signal from signal source  150  can be of the form of a cosine wave at f RF , and the mixer signal from signal source  155  can be of the form of a sine wave at f RF . Mixers  160  and  165  will hereafter be referred to collectively as the second mixer, and the LO signals from signal sources  150  and  155  will be referred to collectively as the second LO signal. 
     Signals  162  and  164  each contain two components—a desirable component at the carrier frequency f=f RF +f IF , and an image at f RF +f IF . Because of the phase shifts between signals  142  and  144  and between the signals from signal sources  150  and  155 , the desirable components of signals  162  and  164  are out of phase with each other, and the image components of signals  162  and  164  are in phase with each other. When signal  164  is subtracted from signal  162  to produce an output signal  170 , the image components cancel and only the desirable component at the carrier frequency is left. Polyphase filter  140 , mixer  160 , and mixer  165  form an image-rejection mixer structure. 
     System  100  relies on filter  135  to remove spurious components due to LO harmonics. If the spurious components are not filtered by filter  135 , they can pass through the image-rejection mixer structure and reach output signal  170 . In addition, because the narrow-band polyphase filter  140  typically does not produce ninety degree phase separation or matched amplitudes at the spurious component frequencies, images of the spurious components are not cancelled by the image-rejection mixer structure, resulting in spurious components in output signal  170 . 
       FIG. 3  illustrates a system  300  that can be used to substantially reduce spurious emissions without relying on an external filter. System  300  significantly reduces the number of spurious components and can selectively remove spurious components that would otherwise fall into undesirable frequency bands (e.g., restricted bands). Referring to  FIG. 1A  and  FIG. 3 , system  300  is similar to system  100 , but has some important differences. For example, filter  135  (e.g., an external high-selectivity bandpass filter) is replaced by an optional internal filter  335 . Filter  335  mildly attenuates spurious components due to LO harmonics and can be, for example, a low-selectivity low-pass filter. 
     In system  100 , the intermediate frequency (f IF ) and the frequency of the second LO signal (f RF ) can be chosen independently of each other, except that the sum of the intermediate frequency and the frequency of the second LO signal should be equal to the desired channel frequency (f C ). The requirement that f IF +f RF =f C  assumes that system  100  uses high-side mixing, where f IF  is lower than f C . If system  100  uses low-side mixing (where f IF  is higher than f C ), the difference between the intermediate frequency and the frequency of the second LO signal should be equal to f C . If the spurious components due to LO harmonics were not removed by filter  135 , all of the spurious components and the associated images generated in the second mixer would typically fall on distinct frequencies, resulting in a large number of spurious components in output signal  170 . 
     In one implementation of system  300 , a fixed integer-ratio relation (hereafter referred to as a ratio-based LO frequency plan) between the frequencies of the second and first LO signals (f RF  and f IF ) is imposed (i.e., f RF =K*f IF , and f C =f RF +f IF =(K+1)f IF , where K is a positive integer). The fixed integer-ratio reduces the number of spurious components. When there is a fixed integer-ratio relating f RF  to f IF , a spurious component in the output signal  370  due to the N th  LO harmonic in the first mixer (at frequency f RF +N*f IF =(K+N)*f IF ) will overlap an image generated in the second mixer due to the M th  LO harmonic of the first mixer (at frequency |fRF−M*f IF |=|K−M|*f IF ) whenever K+N=|K−M|. The total number of spurious components in output signal  370  therefore can be reduced. 
     The constant ratio K is typically chosen to be a power of two (i.e., 1, 2, 4, . . . ) so that the first LO signal can be generated by dividing down the second LO signal. Therefore, only a single local oscillator is needed in system  300 . Two factors to consider when choosing the value of K are the number of spurious components (which decreases with decreasing K) and whether the resulting frequency plan will generate spurious components in undesirable frequency bands. 
     System  300  includes a wideband polyphase filter  340 , which selectively attenuates critical spurious components. A conventional implementation of a polyphase filter, as described in the context of  FIG. 2 , involves cascading several RC-CR networks with pole frequencies that are close to a single frequency. A wideband polyphase filter includes one or more additional RC-CR sections with poles away from the primary pole location. By placing one or more secondary poles at frequencies of critical spurious components, the amplitude matching and phase splitting properties of the polyphase filter can function at one or more spurious component frequencies, which sharply attenuates corresponding spurious component images in output signal  370 .  FIG. 4  shows the phase response of wideband polyphase filter  340 . 
     In principle, any spurious component image that is located at a frequency lower than the carrier frequency can be removed in a high-side mixing system using the ratio-based LO frequency plan (discussed above in reference to system  300 ) by strategically placing a secondary pole in the wideband polyphase filter. For example, if an LO frequency ratio K of four is chosen, f IF =2.4 GHz/5=480 MHz, f RF =4*f IF =1.92 GHz, the desired output signal is at 2.4 GHz (=f RF +f IF ), and spurious components can occur at frequencies N*480 MHz, where N is any integer. If the spurious component located at 3*480 MHz=1.44 GHz falls in a restricted band, the spurious component should be maximally attenuated. The spurious component at 1.44 GHz originates from two sources: an image of the desired signal generated in the second mixer (since |f RF −f IF |=4−1|*480 MHz=1.44 GHz), and an image generated in the second mixer due to the 7 th  LO harmonic of the first mixer (since |f RF −7*f IF |=|4−7|*480 MHz=1.44 GHz). The former image is rejected by the image-rejection mixer structure, since the primary poles of wideband polyphase filter  340  are placed around f IF . The latter image can be attenuated if a secondary pole of wideband polyphase filter  340  is placed at the frequency of the 7 th  LO harmonic (3.36 GHz). 
     Referring to  FIG. 5 , a process is shown whereby a baseband signal can be converted to an RF signal. A baseband signal is mixed with an LO signal at an intermediate frequency f IF  (step  510 ) to produce an IF signal. The IF signal may contain spurious components at integer multiples of f IF . The IF signal is filtered with a wideband polyphase filter (step  520 ). The wideband polyphase filter has at least one pole at the intermediate frequency and at least one pole at a frequency that corresponds to a spurious component. The wideband polyphase filter outputs an in-phase filtered signal and a quadrature filtered signal. The quadrature filtered signal is substantially ninety degrees out of phase from the in-phase filtered signal at the intermediate frequency and at the frequency of the pole corresponding to the spurious component. 
     In step  530 , an in-phase signal is produced by mixing the in-phase filtered signal with an in-phase LO signal at a radio frequency that is an integer multiple of the intermediate frequency. A quadrature signal is produced by mixing the quadrature filtered signal with a quadrature LO signal at the radio frequency, where the quadrature LO signal is substantially ninety degrees out of phase with the in-phase LO signal. 
     The quadrature signal produced in step  530  is combined with the in-phase signal produced in step  530  (step  540 ). For example, the quadrature signal can be subtracted from the in-phase signal. The combining removes images from the resulting signal that correspond to the frequencies in the IF signal at which poles are located. For example, if one or more poles of the wideband polyphase filter are located at the intermediate frequency and one or more poles are located at seven times the intermediate frequency when the radio frequency is four times the intermediate frequency, the images in the signal output from step  540  at three times the intermediate frequency will be removed. The image at three times the intermediate frequency can be removed by placing a pole at the intermediate frequency, and the other image that falls on three times the intermediate frequency can be removed by placing a pole at seven times the intermediate frequency. 
     The described spurious component reduction system and method can be used in a wide range of applications. Referring to  FIG. 6 , the system and method can be used in a wireless transceiver  600  (hereafter referred to as transceiver  600 ). The transmit path of transceiver  600  includes digital-to-analog converters (DACs)  605  and  606  that supply in-phase and quadrature components of a baseband signal to a mixer  120  and a mixer  125  respectively. Mixer  120  modulates a signal generated by a signal source  610  with the in-phase baseband signal and mixer  125  modulates a signal generated by a signal source  615  with the quadrature baseband signal, where the signal generated by signal source  615  is substantially ninety degrees out of phase with the signal generated by signal source  610  and at the same frequency (the intermediate frequency). The in-phase and quadrature modulated signals are combined in an adder  130  and the combined signal is optionally filtered by a lowpass filter  335  to attenuate spurious components in the combined signal. 
     The filtered signal is filtered by a wideband polyphase filter  340  to produce an in-phase filtered signal and a quadrature filtered signal. Wideband polyphase filter  340  has poles corresponding to the frequency of the signal generated by signal source  610  and corresponding to the frequency of a spurious component in the filtered signal. A mixer  160  modulates the in-phase filtered signal with a signal generated by a signal source  650  to produce an in-phase RF signal. A mixer  165  modulates the quadrature filtered signal with a signal generated by a signal source  655  to produce a quadrature RF signal. The signal generated by signal source  655  is substantially ninety degrees out of phase with the signal generated by signal source  650 . In one implementation, the signals generated by signal sources  650  and  655  are both at a frequency that is an integer multiple of the intermediate frequency. An adder  170  subtracts the quadrature RF signal from the in-phase RF signal to attenuate images in the RF signal. An amplifier  620  amplifies the RF signal and transmits the RF signal using an antenna  630 . 
     The receive path of transceiver  600  includes a receiver  640  and an analog-to-digital converter  645 . Transceiver  600  can be IEEE 802 compliant with the following standards: 802.11, 802.11a, 802.11b, 802.11g, 802.11i, 802.11n, and 802.16. 
     This application describes a method, apparatus, and system that can be used to reduce spurious emission from a transmitter without relying on external filters. The method, apparatus, and system can include one or both of the following aspects: first, a ratio-based LO frequency plan can be used to reduce the total number of spurious components generated; second, a wide-band polyphase filter can be employed to selectively remove critical spurious components around the carrier frequency. Various implementations have been described. These and other implementations are within the scope of the following claims. For example, the method, apparatus, and system described above can be used with different transceiver architectures. The polyphase filter in the method, apparatus, and system can also include multiple poles at multiple frequencies corresponding to multiple spurious components.