Abstract:
Apparatus and methods for removing dc offsets in feedback loops such as may be used in communication circuits are disclosed. A comparator may be used to sample the output of the feedback loop, with the comparator output applied to a DSP module. The DSP module is configured to determine a dc offset in the output signal and generate an offset correction signal, which may then be applied to the input of the feedback loop to adjust the de offset.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/052,971, entitled DC OFFSET REMOVAL USING DIGITAL FEEDBACK, filed May 13, 2008, the content of which is incorporated by reference herein in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to radio receivers. More particularly, but not exclusively, the present invention relates to apparatus and methods for the removal of dc offsets that degrade performance. 
     BACKGROUND 
     A radio receiver selects a desired signal amongst several potentially strong interfering signals. To operate properly, it must minimize circuit effects that add unwanted artifacts to the received signal such as distortion, noise, and dc offsets. In practice, it&#39;s possible for these artifacts to degrade performance and even saturate the receiver. 
     A typical radio receiver is shown in  FIG. 1 . It uses a single RF downconverter to translate the received signal to baseband. In practice, the baseband frequency equals dc (for direct conversion architectures) or lies near dc (low-IF approach). At baseband, the A/D converters sample the received signal and convert it into digital data. An automatic gain control (AGC) system in the digital MODEM allows the receiver to process a wide variety of signal levels. 
     The direct conversion and low-IF receiver architectures allow highly integrated radio solutions. Unfortunately, these approaches also suffer from a number of problems. Since direct conversion schemes center the baseband signal at dc, these receivers are sensitive to dc offsets and even order distortion. Narrowband signals like GSM/EDGE present a greater challenge. 
     Traditionally, the radio receiver includes multiple feedback loops to remove dc offsets at critical points and to avoid saturation of its high-gain stages. The feedback loops affect the system&#39;s response and must be stable. Moreover, the resulting response cannot affect the desired signal. This can be challenging since the receiver&#39;s gain changes dynamically. 
     In practice, the dc offset correction loop generally uses analog signal processing techniques. Unfortunately, the analog circuits show sensitivities to signal levels, process parameters, supply voltages, and layout effects. As such, they oftentimes need to be redesigned when conditions change. 
     It therefore would be advantageous to eliminate as much analog signal processing as possible and rely on digital methods to remove the dc offsets. 
     SUMMARY 
     In one or more embodiments of the present invention, apparatus and methods that exploit digital techniques to remove dc offsets in feedback loops such as may be used in communication devices are described. 
     In one aspect, the present invention is directed to an apparatus for removing a dc offset in a feedback loop comprising a comparator coupled to an output of the feedback loop to sample a feedback loop output signal, a digital signal processing (DSP) module coupled to the comparator to determine a dc offset level in the output signal and a digital to analog (D/A) converter coupled to the DSP module to provide an offset compensation signal based on the dc offset level. 
     In another aspect, the present invention is directed to a method for reducing a dc offset in a feedback loop, comprising sampling an output signal of the feedback loop with a comparator to generate a comparator output signal, providing the comparator output signal to a DSP module, determining, in the DSP module, a dc offset level in the output signal and generating an offset compensation signal based on the dc offset level. 
     Additional aspects of the present invention are described below in conjunction with the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following is a brief description of the drawings wherein: 
         FIG. 1  shows a diagram of radio receiver implementing a direct conversion or low-IF architecture; 
         FIG. 2   a  shows a diagram of a simple feedback loop to remove dc offsets; 
         FIG. 2   b  depicts the system response for the simple dc feedback loop; 
         FIG. 3  shows a digital feedback loop to remove dc offsets based on an A/D converter; 
         FIG. 4   a  shows one embodiment of a digital feedback loop, in accordance with aspects of the present invention; 
         FIG. 4   b  shows one embodiment of a digital feedback loop with an added dither signal, in accordance with aspects of the present invention; 
         FIG. 5  displays the spectrum of a GSM modulated signal; 
         FIG. 6   a  illustrates the timing related to dc offset removal in a GSM/EDGE half-duplex system; and 
         FIG. 6   b  shows one embodiment of a modified digital algorithm for dc feedback, in accordance with aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A simple feedback loop to remove dc offsets is shown in  FIG. 2   a . It uses an integrator to drive the feedback point and minimize the dc output level. The resulting transfer function equals 
                     T   ⁡     (   s   )       =       A   ⁡     (   s   )         1   +       A   ⁡     (   s   )       ⁢     fb   ⁡     (   s   )                     (   1   )               
where A(s) models the forward gain and fb(s) represents the feedback path. Since the feedback fb(s) includes an integrator, the overall system response shown in  FIG. 2   b  reveals a notch at dc. Note that the response and stability of the dc feedback loop depends on A(s). If A(s) varies, then the overall system response of the dc feedback loop moves. (This is commonplace if the forward path includes a variable gain amplifier controlled by the receiver&#39;s AGC system.) More importantly, the ability to reduce dc offsets changes—it solely depends on the dc loop gain given by A(s) fb (where s equals 0 or dc)—with
 
 a ( s )·( dc  after cal/ dc  before cal)=1/(1 +A ( s )· fb )  (2)
 
This behavior is unacceptable. To compensate and reduce this effect, the feedback function fb(s) must track the forward gain A(s). As a result, the design of the integrator depends on the circuits comprising A(s).
 
     It is possible to sense the dc offset using the alternative digital system shown in  FIG. 3 . The system relies on a pair of companion data converters. A digital algorithm determines the dc output level and supplies the appropriate feedback signal to reduce it. In practice, the A/D converter also samples any potentially large interfering signals as well as the desired signal. This means the A/D converter must have a wide dynamic range. It&#39;s common and necessary that the A/D converters interfacing to the digital modem meet this requirement. 
     A typical radio receiver generally includes two or three variable gain amplifiers (VGAs) and filter stages (LPFs) as illustrated in  FIG. 1 . These stages respond to the AGC system and reduce the range of signal levels applied to the A/D converter. In practice, the high gain of these stages amplifies dc offsets and potentially saturates the receiver at multiple points. To combat this, the dc level must be sensed and removed at critical points in the receiver. Unfortunately, this requires more than the A/D converters at the interface to the digital modem. 
       FIG. 4   a  shows one embodiment of a feedback loop in accordance with aspects of the present invention to remove dc offsets, that relies on analog functions and an innovative digital signal processing (DSP) algorithm implemented on a DSP module including memory and a processor, DSP or other programmable device. It replaces the A/D converter and samples the output signal with a single comparator. The comparator is configured to determine the sign of the differential output signal. The DSP module includes an algorithm to integrate and average the samples to provide an improved measure of the output dc level. The resulting measure may then be used to adjusts a D/A converter to cancel the output dc level. 
     One embodiment of a DSP module and processing algorithm in accordance with aspects of the present invention is further detailed as follows. The comparator output drives a moving average filter (MAF) that accumulates the samples over a window of time (This realizes the integration function needed to resolve the dc level). Its output then directs an up/down counter that stores the data applied to the D/A converter. The feedback factor fb(s) may be selected based on the comparator&#39;s sample rate, the MAF filter&#39;s length, and the counter&#39;s clock frequency. Moreover, these parameters provide a straightforward way to adjust the gain of the feedback path (to compensate for changes in the forward path A(s)). For example, increasing the comparator&#39;s sample rate or the counter&#39;s clock frequency may be done to adjust the gain of the feedback factor higher, while extending the MAF&#39;s filter length may be done to adjust the feedback factor lower). As a result, the response of the system may be set to remain constant at different forward path A(s) gain settings. In practice, this may be implemented in a straightforward fashion if the AGC system is used to control fb(s) along with A(s). 
     The DSP algorithm causes the feedback loop to operate as a discrete-time, non-linear system. This advantageously allows the system to accelerate loop settling, minimize the low frequency notch, add hysteresis, and quiet the dc correction. For example, the up/down control depends on the MAF filter output—a large output directs the counter to step multiple values while a very small (noise-like) output disables the counter. The result is an intelligent feedback loop that is well suited to dynamic wireless applications. 
     Use of an analog comparator greatly simplifies the design of the dc feedback loop. Its requirements are both easy and straightforward compared to an analog integrator or A/D converter. Moreover, in some embodiments it&#39;s possible to relax the requirements further by using a dither signal as shown in  FIG. 4   b . The dither signal modulates the output signal&#39;s dc offset and makes the signal busy. It may be realized with a pseudo-random number generator that has a Gaussian distribution with a mean value equal to zero. The dither signal allows the feedback loop to resolve small dc offsets and eases the design of the analog comparator. The dither signal may also extend settling time for the loop. 
     It is important to recognize that narrowband signals such as GSM/EDGE cannot tolerate a notch at dc. This is because their power spectral density is actually concentrated at dc as shown in  FIG. 5 . Fortunately, GSM/EDGE systems employ half-duplex communications with slotted mode operation. As a result, in accordance with one embodiment, the dc offset can be removed prior to an active receive slot as shown in the timing diagram of  FIG. 6   a . The dc offset is measured without the received signal and the appropriate correction level is found. The level is then held during the active slot. This may be accomplished by freezing the up/down counter. Alternatively, a simple successive approximation routine (SAR) can be employed as shown in the embodiment illustrated in  FIG. 6   b . The SAR approach is fairly straightforward and is generally faster. It tests each bit of a register, starting with the MSB, until it finds the closest code that toggles the output level. The code corresponds to the required correction value. 
     One implementation of a digital feedback loop in accordance with aspects of the present invention has been applied to a direct conversion radio receiver with WCDMA signals present. It cancels dc offsets at three critical points and operates continuously to reduce dc offsets to less than 5 mV even with interfering signals as strong as 400 mVpp. Moreover, this offset level is maintained at different gain settings. 
     One implementation of the digital feedback loop has also been used in a direct conversion radio receiver configured for EDGE operation. In this application, it reduces dc offset levels to less than 5 mV during the slot prior to the receive burst (active slot). Embodiments of these circuits and algorithms reduce analog complexity in dc feedback correction loops. Moreover, the algorithms include the flexibility to adapt the feedback loops to typical gain changes in the radio receiver. The result is a simpler and more robust solution. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.