Patent Publication Number: US-8542784-B2

Title: Spur mitigation for radio frequency receivers utilizing a free-running crystal

Description:
FIELD OF THE INVENTION 
     This application relates generally to receivers and more particularly to radio frequency (RF) receivers that use crystal resonators. 
     BACKGROUND 
     There exist two commonly implemented front-end architectures in radio frequency (RF) receiver design; namely, the homodyne architecture and the superheterodyne architecture. The homodyne architecture down-converts a desired carrier (or channel) in a received signal directly from RF to baseband, whereas the superheterodyne architecture down-converts a desired carrier (or channel) in a received signal to one or more intermediate frequencies (IFs) before down-conversion to baseband. In general, each of these front-end architectures typically employ an antenna to receive a signal, a low noise amplifier (LNA) to provide gain to the signal, and one or more down-conversion and filtering stages. 
     In both front-end architectures, the down-conversion stage(s) include a mixer for mixing the received signal with a local oscillator (LO) signal to down-convert the desired carrier in the received signal to baseband or some non-zero IF for further processing. The LO signal in the homodyne architecture is ideally tuned to have a frequency identical to the desired carrier such that the carrier is down-converted to baseband. The LO signal in the superheterodyne architecture (or at least one LO signal in the superheterodyne architecture), on the other hand, is ideally tuned to have a frequency that is offset from the frequency of the desired carrier by an amount equal to the chosen IF such that the carrier is down-converted to the IF. 
     There is often a small frequency error (or offset), however, in the LO signal from its ideal frequency. This error causes the desired carrier to be down-converted to a frequency position other than what is expected (i.e., to a frequency position other than at baseband in a homodyne architecture and to a frequency position other than at the chosen IF in the superheterodyne architecture). Proper recovery of the information modulated onto the desired carrier generally requires that the carrier be down-converted (very close) to the expected frequency location. 
     Therefore, automatic frequency correction is often employed at the receiver to estimate and compensate for any frequency error in the LO signal, such that the desired carrier is down-converted (very close) to its expected frequency position (i.e., very close to baseband in a homodyne architecture and very close to the chosen IF in the superheterodyne architecture).  FIG. 1  illustrates a conventional homodyne receiver  100  that performs automatic frequency correction. As illustrated in  FIG. 1 , conventional homodyne receiver  100  includes a front-end  105  for performing amplification, down-conversion, and filtering, and a baseband processing section  110  for performing decoding or demapping. 
     Front-end  105  specifically includes an antenna  115 , a low-noise amplifier (LNA)  120 , a mixer  125 , a phase-locked loop (PLL)  130 , a digitally controlled crystal oscillator (DCXO)  135 , a crystal resonator  140 , a low-pass filter  145 , an analog-to-digital converter (ADC)  150 , and a digital signal processor (DSP)  155 . In operation, antenna  115  is configured to receive an RF signal that includes a desired carrier. The desired carrier can be positioned within a frequency band defined by a particular communications standard. For example, the desired carrier can be positioned within a frequency band defined by the Global System for Mobile Communications (GSM) standard. 
     After being received, the RF signal is provided to LNA  120 , which provides sufficient amplification to the RF signal to overcome the noise of subsequent stages in front-end  105 , for example. The amplified RF signal is then mixed by mixer  125  with a LO signal provided by PLL  130 . PLL  130  provides the LO signal based on a reference oscillator signal provided by DCXO  135  and crystal resonator  140  (i.e., PLL  130  provides the LO signal as some multiple or fractional multiple of the reference oscillator signal). The LO signal is ideally controlled by PLL  130  to have a frequency equal to the desired carrier such that the mixing operation, performed by mixer  125 , results in the carrier being down-converted to baseband. The down-converted signal is then filtered by low-pass filter  145  to remove unwanted frequency components, converted to a digital signal (i.e., a sequence of discrete values) by ADC  150 , and processed by DSP  155 . 
     Baseband processing section  110  receives the down-converted and filtered signal from DSP  155  and performs further processing. As illustrated in  FIG. 1 , baseband processing section  110  includes baseband processor  160  and an automatic frequency controller (AFC)  165 . Baseband processor  160  is configured to perform decoding or demapping to recover information transmitted over the carrier. AFC  165  is configured to estimate and compensate for any frequency error in the LO signal provided by PLL  130 , such that the desired carrier is down-converted (very close) to its expected frequency position (i.e., very close to baseband in the homodyne architecture illustrated in  FIG. 1 ). 
     AFC  165  can estimate the frequency error using, for example, the down-converted carrier or the information recovered from the carrier by baseband processor  160 . The estimated frequency error can then be used by AFC  165  to adjust a frequency at which crystal resonator  140  oscillates and, thereby, the frequency of the reference oscillator signal. Specifically, AFC  165  can adjust the frequency at which crystal resonator  140  oscillates using DCXO  135 . For example, DCXO  135  can include a tunable capacitor coupled in parallel (or series) with crystal resonator  140 . In general, adding and removing capacitance across a crystal resonator, such as crystal resonator  140 , will respectively cause the resonance of the crystal resonator to shift upward and downward. 
     As noted above, the reference oscillator signal provided by DCXO  135  and crystal resonator  140  is used by PLL  130  as a reference signal to generate the LO signal used by mixer  125 . Thus, the frequency of the reference oscillator signal provided by DCXO  135  and crystal resonator  140  can be adjusted by AFC  165  to compensate for the estimated frequency error in the LO signal provided by PLL  130 . 
     Although adjusting the frequency of the reference oscillator signal presents a viable solution for reducing the frequency error in the LO signal provided by PLL  130 , this solution has drawbacks. One notable drawback is that the reference oscillator signal provided by DCXO  135  and crystal resonator  140  potentially can no longer serve as a reference clock for other functionalities supported by the device containing homodyne receiver  100 . For example, many communication devices, such as cellular phones, provide support for wireless local area network (WLAN) and Global Positioning System (GPS) functionalities in addition to cellular communication functionalities. Even though the reference oscillator signal of a single crystal resonator, such as crystal resonator  140 , can be used to support each of these additional functionalities, the sudden changes in frequency of the reference oscillator signal that are caused by AFC  165  to support one functionality are not acceptable to many of the other functionalities, such as GPS. 
     Therefore, what is needed is a system and method for performing automatic frequency correction in a receiver without adjusting the resonance of the crystal resonator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
         FIG. 1  illustrates a conventional homodyne receiver that performs automatic frequency correction. 
         FIG. 2  illustrates a homodyne receiver that performs automatic frequency correction, according to embodiments of the present invention. 
         FIG. 3  illustrates potential leakage paths within a homodyne receiver, according to embodiments of the present invention. 
         FIG. 4  illustrates a leakage canceler, according to embodiments of the present invention. 
         FIG. 5  illustrates a homodyne receiver, according to embodiments of the present invention. 
         FIG. 6  illustrates a DSP with a rate adaptor, according to embodiments of the present invention. 
     
    
    
     The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention, including structures, systems, and methods, may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the invention. 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     1. EXEMPLARY OPERATING ENVIRONMENT 
       FIG. 2  illustrates an exemplary operating environment in which features of the present invention can be implemented.  FIG. 2  specifically illustrates an exemplary homodyne receiver  200  with nearly the same configuration as homodyne receiver  100 , illustrated in  FIG. 1 . However, in homodyne receiver  200 , the resonance of crystal resonator  140  is not being adjusted by AFC  165 . Rather, AFC  165  is configured to adjust PLL  130  to correct for any estimated frequency error in the LO signal produced by PLL  130 . 
     Because AFC  165  is no longer adjusting the resonance of crystal resonator  140 , the reference oscillator signal produced by DCXO  135  and crystal resonator  140  in  FIG. 2  is not locked to any particular carrier in the received signal and can be said to be free-running. The reference oscillator signal can therefore serve as a reference clock to support multiple, different functionalities of the device containing receiver  200  (even those functionalities that are sensitive to sudden changes in the reference oscillator signal). For example, such functionalities can include cellular communications, Bluetooth, GPS, and WLAN to name a few. 
     In one embodiment, PLL  130  includes a variable frequency oscillator and a phase detector (not shown). Using the phase detector, PLL  130  compares the phase of the reference oscillator signal with the phase of a divided down version of the output signal from its variable frequency oscillator, which is divided down in frequency according to some divider ratio. The signal from the phase detector is then used to control the variable frequency oscillator to keep the phases matched. The output of the variable frequency oscillator can be used as the LO signal and its frequency can be controlled to be some multiple (or fractional multiple) of the reference oscillator signal by adjusting the divider ratio. In one embodiment, AFC  165  can adjust this divider ratio of PLL  130  to compensate for any estimated frequency error in the LO signal. 
     The receiver architecture in  FIG. 2  allows the reference oscillator signal produced by DCXO  135  and crystal resonator  140  to be used as a reference clock to support multiple different functionalities of the device containing homodyne receiver  200 . However, the reference oscillator signal is now no longer compensated and locked to the frequency of the desired carrier, and therefore can produce several challenging sources of noise to homodyne receiver  200 . These challenging sources of noise include in-band spurs and long-term drift and they are described further below together with potential solutions for mitigating them. 
     2. IN-BAND SPURS 
     Oscillator signals and their harmonics are a common source of spurious interference in RF receivers.  FIG. 3  depicts several potential leakage paths for the reference oscillator signal, produced by DCXO  135  and crystal resonator  140 , to leak into the input path of mixer  125  and cause spurious interference with the signal received by antenna  115 . It is often very difficult to address every potential leakage path that may exist in an implementation of homodyne receiver  200 . Therefore, the best approach to avoid interference from the reference oscillator signal is to devise a frequency plan in which the frequency of the reference oscillator signal and the frequencies of its stronger harmonics do not fall in the RF bands of interest of a signal received by antenna  115 . However, such a frequency plan is often impractical, impossible, or undesirable to implement due to cost and/or design constraints. When frequency planning is unsuccessful, spurious interference can occur. 
     For example, in a second-generation (2G) cellular system, each downlink frequency band includes multiple carriers (or channels) whose frequencies are 0.2 MHz apart. One of the downlink frequency bands used in 2G cellular systems is the Extended GSM-900 band that spans the spectrum from 800-915 MHz and 925-965 MHz. Given the 0.2 MHz spacing between the carriers, it is clear that every integer number between 880 and 915 or between 925 and 960 is a frequency of a particular carrier in the Extended GSM-900 band. These numbers can easily fall on an integer multiple of a typical operating frequency for crystal resonator  140 . For instance, assuming homodyne receiver  200  is configured to operate in a 2G GSM cellular system and DCXO  135  and crystal resonator  140  provide a reference oscillator with a fundamental frequency of 26 MHz, the 36 th  harmonic of the reference oscillator signal will be at 936 MHz. This harmonic can leak into the input path of mixer  125  as illustrated in  FIG. 3  and interfere with the carrier at 936 MHz in a 2G signal received by antenna  115 . 
     In homodyne receiver  100 , illustrated in  FIG. 1 , the 36 th  harmonic of the reference oscillator signal provided by DCXO  135  and crystal resonator  140  would be frequency-locked to the carrier at 936 MHz because AFC controller  165  is adjusting the frequency of the reference oscillator signal to compensate for any frequency error between the carrier signal and the reference oscillator signal. Consequently, the 36 th  harmonic of the reference oscillator signal at 936 MHz will appear at DC in the output of mixer  125  (assuming the carrier at 936 MHz is presently being demodulated) and can be eliminated using standard DC offset cancellation techniques. 
     In homodyne receiver  200 , illustrated in  FIGS. 2 and 3 , however, the 36 th  harmonic of the reference oscillator signal provided by DCXO  135  and crystal resonator  140  is no longer frequency-locked to the carrier at 936 MHz because AFC controller  165  is adjusting PLL  130  and not the frequency of the reference oscillator signal. In homodyne receiver  200 , the reference oscillator signal produced by DCXO  135  and crystal resonator  140  is free-running. Consequently, the 36 th  harmonic of the reference oscillator signal at 936 MHz can appear as a spur not at DC within the down-converted signal. Rather, any spur caused by the 36 th  harmonic of the reference oscillator signal will appear at the estimated frequency error (or offset) determined by AFC  165  in the down-converted signal. Standard DC offset cancellation techniques therefore cannot be so readily implemented. Because the sensitivity requirement of a 2G GSM receiver is quiet stringent (around −110 dBm), these in-band spurs can severely degrade performance. 
       FIG. 4  illustrates a leakage canceler  400  configured to eliminate (or reduce) these in-band spurs, according to embodiments of the present invention. As illustrated in  FIG. 4 , leakage canceler  400  includes a complex mixer  405 , a DC offset canceler  410 , a complex mixer  415 , and a direct digital synthesizer (DDS)  420 . The operation of leakage canceler  400  is described below with continued reference to homodyne receiver  200  illustrated in  FIGS. 2 and 3 . It should be noted, however, that leakage canceler  400  is not limited to use in homodyne receiver  200  and can be used in any reasonable receiver that implements automatic frequency correction and has a free-running crystal. In one embodiment, leakage canceler  400  can be implemented in DSP  155  in receiver  200 . 
     In general, leakage canceler  400  is configured to use the frequency error determined by AFC  165  (denoted by Δf in  FIG. 4 ) to compensate for a harmonic of the reference oscillator signal that couples to the input path of mixer  125  and falls in the band of the desired carrier at the output of mixer  125 . More specifically, using complex mixer  405 , leakage canceler  400  frequency-shifts the down-converted signal provided at the output of mixer  125  by the estimated frequency error determined by AFC  165  such that the unwanted spur is frequency shifted to baseband. Leakage canceler  400  then uses one or more conventional DC offset correction techniques, implemented by DC offset canceler  410 , to reduce or eliminate the spur in the frequency-shifted signal. Once the spur has been reduced or eliminated, leakage canceler  400  re-positions the frequency-shifted signal, using mixer  415 , back to (or at least near) its original frequency position. 
     In one embodiment, leakage canceler  400  is configured to receive the down-converted signal provided at the output of mixer  125  as a sequence of complex digital samples. The complex digital samples each have an in-phase component (I) and a quadrature component (Q), Complex mixer  405  mixes the complex digital samples of the down-converted signal with a complex oscillator signal produced by DDS  420  to frequency-shift the signal by the estimated frequency error determined by AFC  165 . 
     In order to produce the complex oscillator signal, the estimated frequency error determined by AFC  165  is provided to DDS  420 . In one embodiment, DDS  420  includes a numerically controlled oscillator (NCO), a sine/cosine look-up table, and a phase accumulator. The frequency of the complex oscillator signal produced by DDS  420  essentially depends on two variables: a frequency of a reference clock signal used by DDS  420  (not shown) and the estimated frequency error, which acts like a “tuning word” to DDS  420 . The estimated frequency error provides the main input into the phase accumulator of DDS  420 . The phase accumulator computes a phase angle or address for the sine/cosine look-up table, which outputs the digital amplitude corresponding to the sine/cosine of the phase angle. The value of the accumulator is incremented based on the magnitude of the estimated frequency error with each cycle of the reference clock signal. If the estimated frequency error is large, the phase accumulator will step quickly though the sine/cosine look-up table and thus generate a high frequency sine/cosine oscillator signal. On the other hand, if the estimated frequency error is small, the phase accumulator will take many more steps to step through the sine/cosine look-up table and therefore generate a comparatively lower frequency sine/cosine oscillator signal. Both complex mixers  405  and  415  can utilize the complex oscillator signal, produced by DDS  420 , as illustrated in  FIG. 4 . 
     It should be noted that other implementations of DDS  420  are possible, as would be appreciated by one of ordinary skill in the art. For example, as opposed to using a straight sine/cosine look-up table, the Coordinate Rotation Digital Computer (CORDIC) algorithm can be used in the implementation of DDS  420 . 
     3. LONG-TERM DRIFT 
     Referring to  FIG. 2 , after mixer  125  down-converts a desired carrier in the signal received by antenna  115 , the down-converted signal is optionally low-pass filtered by low pass filter  145 , converted to a digital signal by ADC  150 , and processed by DSP  155 . DSP  155  can include any additional logic to perform additional processing on the down-converted signal prior to being received by baseband processing section  110  and can include, for example, mechanism(s) for buffering the down-converted signal or mechanism(s) for performing clock domain crossing. DSP  155  typically provides samples of the down-converted signal to baseband processing section  110  at a rate that is a multiple of the symbol rate used to modulate the desired carrier. The symbol rate is the number of symbol changes made to the desired carrier per unit time and can be measured, for example, in symbols per second. Each symbol can represent or convey one or more bits of information. 
     Typically, it is desirable to operate ADC  150  and DSP  155  at the same rate at which the samples of the down-converted signal are provided to baseband processing section  110  (i.e., at the given rate that is a multiple of the symbol rate used to modulate the desired carrier). However, it is often impractical to operate ADC  150  and DSP  155  at this rate. More specifically, in order to operate ADC  150  and DSP  155  at the same rate at which the samples of the down-converted signal are provided to baseband processing section  110 , a dedicated PLL for the ADC is generally required, which increases cost/area of the implementation of receiver  200  and introduces more phase noise into the system. 
     Moreover, since the jitter requirement in baseband processing section  110  is usually much less stringent than the jitter requirement of front-end  105  in many receivers, a low-cost ring oscillator PLL can be used to generate a clock for use by baseband processing section  110  that has a frequency which is a multiple of the symbol rate used to modulate the desired carrier. Because this clock is very jittery, it generally cannot be used by ADC  150 . 
     Therefore, one potential implementation is illustrated in  FIG. 5 .  FIG. 5  specifically illustrates a homodyne receiver  500  with nearly the same configuration as homodyne receiver  200 , illustrated in  FIG. 2 . However, in homodyne receiver  500  an additional clocking structure is depicted for ADC  150 , DSP  155 , and baseband processor  160 . 
     In the clocking structure depicted in  FIG. 5 , ADC  150  and a first portion of DSP  155  are clocked using the LO signal provided by PLL  130 . A second portion of DSP  155  and baseband processor  160 , on the other hand, are clocked using a second LO signal provided by PLL  505 . In one embodiment, PLL  505  is implemented using a low-cost ring oscillator. PLL  505  uses the reference oscillator signal provided by DCXO  135  and free-running crystal resonator  140  to provide the second LO signal with a fundamental frequency that is a multiple of the symbol rate used to modulate the desired carrier in the signal received by antenna  115 . 
     In order to transfer the down-converted signal received by antenna  115  across the two clock domains (i.e., across the clock domain defined by the LO signal provided by PLL  130  and the clock domain defined by the LO signal provided by PLL  505 ), DSP  155  can include a rate adaptor  610  as illustrated in  FIG. 6 . Rate adaptor  610  sits between variable rate logic  605 , clocked at a rate defined by the LO signal provided by PLL  130 , and fixed rate logic  615 , clocked at a rate defined by the LO signal provided by PLL  505 . Rate adaptor  610  is configured to adapt the rate of the down-converted signal received from variable rate logic  605  to the rate of fixed rate logic  615  according to the following adaptor conversion ratio: 
                       f   IN       f   OUT       =     M   N             (   1   )               
where M and N are programmable integers and f IN  and f OUT  represent the input and output rate of rate adaptor  610 , respectively.
 
     Because PLL  505  is not frequency-locked to PLL  130 , however (given that frequency correction is performed on PLL  130  by AFC  165 ), rate adaptor  610  cannot guarantee that the rate at which it provides the down-converted signal to fixed rate logic  615  is exactly the rate at which fixed rate logic  615  expects to receive the down-converted signal (i.e., at a rate equal to the rate defined by the LO signal provided by PLL  505 ). Any deviation in the output rate of rate adaptor  610  and the rate at which fixed rate logic  615  expects to receive the down-converted signal can result in long-term drift and, where a FIFO is implemented in the logic following rate adaptor  610 , FIFO overflow/underflow. FIFO overflow/underflow indicates lost samples and can degrade the performance of receiver  500 . 
     One solution for the long-term drift issue is that the estimated frequency error, provided by AFC  165  to PLL  130 , can be used to compensate for any rate mismatch between the output rate of rate adaptor  610  and the expected rate at which fixed rate logic  615  expects to receive the down-converted signal. More specifically, and in one embodiment, the estimated frequency error, provided by AFC  165  to PLL  130 , can be used to modify the M parameter and/or the N parameter of the adaptor conversion ratio of rate adaptor  610  to compensate for any rate mismatch. For example, using equation (1) above, the amount of correction can be calculated as: 
                       f   IN         f   OUT     +     Δ   ⁢           ⁢   f         =         M   +     Δ   ⁢           ⁢   M       N     ⁢           ⁢   or             (   2   )                 Δ   ⁢           ⁢   M     =     -     M     1   +       f   OUT       Δ   ⁢           ⁢   f                     (   3   )               
where Δf is the estimated frequency error and ΔM is the modification to be made to M in order to compensate for any rate mismatch that exists.
 
     4. CONCLUSION 
     It should be noted that although features of the present invention were described above with respect to exemplary homodyne receivers, these features are equally applicable to superheterodyne receivers as would be appreciated by one of ordinary skill in the art. 
     It is to be appreciated that the Detailed Description section, and not the Abstract section, is intended to be used to interpret the claims. The Abstract section may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, is not intended to limit the present invention and the appended claims in any way. 
     The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
     The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.