Abstract:
A system and method is provided for reducing signal distortion and saturation within an RF receiver which may be operated in an environment under the presence of interfering signals such as in a WiMAX environment. In an embodiment, a first gain stage and a second gain stage are selectively lowered to predetermined lower levels, assuring that if there is a blocker present, it would not cause signal distortion and saturation in the receiver. The loss of the gain in the first gain stage and second gain stage is compensated by a third gain stage which selectively amplifies the signals of interest. If a blocker is not detected, the maximum allowable gain of the first gain stage and the second gain stage is set to a predetermined upper limit allowing for maximum receiver sensitivity. Accordingly, with this system and method a direct conversional receiver can operate in the presence of interfering signals without signal distortion and saturation.

Description:
COPYRIGHT AND LEGAL NOTICES 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyrights whatsoever. 
     BACKGROUND INFORMATION 
     The present invention relates to communications technology and more particularly to radio frequency (RF) receiver systems and the methods of such receivers. 
     An RF receiver is an electronic circuit that enables a particular radio signal to be separated from all others being received and converted into a format suitable for further processing. In an RF receiver unwanted frequency products can exist with wanted signals at the receiver inputs. For instance, it may be desired for an RF receiver to operate in an environment with interferers, one example may be the Worldwide Interoperability for Microwave Access (WiMAX) signal, which is used, among other applications, in connecting WiFi hotspots with each other and to other parts of the internet. These unwanted signals are known as interferers or block signals. If these out of band signals are not eliminated or attenuated, signal distortion and saturation within the receiver may result. 
     Thus, there is a need for a system and method for reducing signal distortion and saturation within an RF receiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is illustrated in the figures of the accompanying drawings, which are meant to be exemplary and not limiting, and in which like references are intended to refer to like or corresponding parts. 
         FIG. 1  shows a direct conversional receiver architecture such as may be used in an embodiment of the invention. 
         FIG. 2  shows a flow diagram of a method in accordance with an embodiment of the invention. 
         FIG. 3  depicts different regions of an MS Detector output in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A system and method is provided for reducing signal distortion and saturation within an RF receiver.  FIG. 1  depicts a direct conversional receiver architecture such as may be used in an embodiment of the invention. Such architecture may comprise an input antenna  100 , a low noise amplifier (LNA)  105 , at least one mixing node  110  and  115 , at least one variable gain amplifier (VGA)  125  and  127 , at least one mean square detector (MS Detector)  150  and  155 , at least one low pass filter with variable gain stage (LPF)  130  and  135 , at least one analog to digital converter (ADC)  140  and  145 , a digital automatic gain control circuit (AGC)  160 , and a local oscillator (LO)  120 . 
     External signals, which may include in-band and out of band signals, are received by input antenna  100  and coupled to the input of an LNA  105  which has an adjustable gain. The gain of LNA  105  may be adjustable by an output signal of the AGC  160 . The outputs of the LNA  105  and the LO  120  are supplied to mixer  110  which provides a conversion of the RF signal to the base band. Mixer  115  provides a similar purpose except that it is 90° out of phase with mixer  110 . A variable gain amplifier  125  receives the output of the mixer  110  signal and the output of the AGC  160 . Here, the gain allowable is increased or decreased, depending on the output of the AGC  160 . The analog MS Detector  150  detects the power of the output of VGA  125 . This output represents the combined gain of the LNA  105  and VGA  125 . In one embodiment, the LNA  105  power may range from a minimum −9 dB to 21 dB. The VGA  125  power may range from 3 dB to 30 dB. Thus, the combined gain of LNA  105  and VGA  125  may be from −6 dB to 51 dB. Based upon the power level at the output of VGA  125 , the analog MS Detector  150  passes a signal to the AGC  160 , which is indicative of the combined power level of the LNA  105  and VGA  125 . 
       FIG. 3  depicts an embodiment of the different output signals of the MS Detectors  150  and  155 . The output signals may indicate the operation of the LNA  105  and VGA  125  in an extended upper level (EUL)  310 , an upper level (UL)  320 , a lower level (LL)  330 , and an extended lower level (ELL)  340 . The EUL  310  represents the highest combined power output level of the LNA  105  and VGA  125  with respect to UL  320 , LL  330 , and EUL  340 . It may be indicative that the combined output power of the LNA  105  and VGA  125  is too strong. MS Detector  150  output level UL indicates a combined output power of the LNA  105  and VGA  125  to be lower than EUL  310  but higher than LL  330 . MS Detector  150  output level LL  330  indicates a combined output power output of the LNA  105  and VGA  125  to be lower than UL  320  but higher than ELL  340 . The ELL  340  represents the lowest output power level of the LNA  105  and VGA  125  which may indicate that the power is too weak. Both the UL  320  and LL  330  indicate desired output power levels. In one embodiment, MS Detector  150  could use two bits to represent the combined output power level of the LNA  105  and VGA  125 . For example, 11=EUL  310  (signal is too strong), 10=UL  320  (signal is strong), 01=LL  330  (signal is good), and 00=ELL  340  (signal is too weak). 
     Referring back to  FIG. 1 , the LPF  130  receives the output signal of VGA  125  and filters out undesired signals. In addition, the LPF  130  has a gain component that can increase, decrease, or leave the gain the same for the filtered signal. Next, the filtered signal at the output of LPF  130  is processed by an analog to digital converter ADC  140 . The digital output of ADC  140  is representative of sampled data  1170 . 
     The parallel path from LNA  105  through mixer  115 , VGA  127 , MS Detector  155 , LPF  134 , and ADC  145 , provides substantially the same functionality as the path of mixer  110 , VGA  125 , MS Detector  150 , LPF  135 , and ADC  140  as described above, except that it is 90° out of phase. For instance, in an embodiment, mixer  110  may be a sine mixer while mixer  115  may be a cosine mixer. Thus, output Q  180  is 90° out of phase with output  1170 . 
     In an embodiment of the present invention, the digital AGC  160  monitors an indication of the combined gain of the LNA  105  and VGA  125  through the output of the MS Detector  150 . In addition, an indication of the gain of the LPF  130  is monitored from the output of the ADC  140 . Complete input signal strength detection is enabled by monitoring an indication of the combined gain of the LNA  105  and VGA  127  through the output of MS Detector  155  and by monitoring an indication of the gain of the LPF  135  from the output of the ADC  145 . Based on these inputs and since AGC  160  tries to keep the power at the output of the VGA  125  within the desired range, as shown in  FIG. 3 , AGC  160  may set the gains of the system to the maximum allowable gain for the LNA  105  and the maximum allowable gain for VGA  125  accordingly. The LPF  130  gain can also be set by the AGC  160  to amplify the in-band signal to keep its output within its own desired range. By dynamically controlling the maximum and minimum allowable gains setting under the presence or blockers or interferers, the effect of the blocking signal can be minimized or eliminated. The methodology for the maximum and minimum allowable gain settings is described below. 
       FIG. 2  shows a flow diagram of a method of blocker detection that enables reduction in signal distortion and saturation in accordance with an embodiment of the present invention. In step  200 , the receiver comes out of power off state. Next, in  205 , the maximum combined allowable gain of the LNA  105  and VGA  125  is set to a lower than maximum predetermined limit which is used during the presence of a blocker and herein called predetermined lower blocker limit. In one embodiment, the combined predetermined lower blocker limit may be 12, from a decimal range of 0 to 19, where 0 is the lowest allowable gain setting and 19 is the maximum allowable gain setting. The combined gain range of the LNA  105  and VGA  125  in dB may be between −6 dB and 51 dB with 3 dB steps. For example, −6 dB would be the 0 setting and 51 dB would be the 19 setting on the decimal range. The default value, during normal operation, may be 19, which would set the maximum combined allowable gain of the LNA  105  and VGA  125  to an upper limit. The upper limit is above the lower blocker limit and may be 51 dB. This maximum combined allowable gain to be used by the AGC  160  may be set by a register pointing to appropriate fields in a programmable lookup table. The lookup table may specify a maximum allowable gain for the LNA  105  and VGA  125  separately. By limiting the gain during power on, it is assured that there is no signal distortion and saturation in the receiver, in case of the presence of a blocker. 
     In steps  210  to  230 , it is determined if there is no blocker present. In step  210 , one waits for an interrupt signal. An interrupt may be a sampling event at a predetermined interval. The interrupt duty cycle may be programmable. For example, in one embodiment, it could have a minimum period of 310 μs and a maximum of 1.269 s. In step  215 . It is determined if the MS Detector  150  level is too weak. A weak signal may be represented by an ELL  340  signal. If the MS Detector  150  signal is higher than ELL  340 , as in LL  330 , UL  320 , or EUL  310 , then a first counter  220  is set to 0, and the method continues with step  210 . A weak MS Detector signal may be indicative that a blocker is not present since a strong blocker would increase the power level at the output of the VGA  125 . The first counter is incremented by one in step  222 . A decision is made if the first counter has reached a predefined number of times of blocker not being present. For example, in one embodiment, if the first counter has not reached 10, then the method continues with step  210 . If step  225  determines that the first counter has reached 10, then the first counter is reset in step  230 . The first counter provides hysteresis in order to assure that the MS Detector  215  level is stable and not fluctuating. In one embodiment, the first counter could be a programmable 6 bit counter. 
     Moving past step  230  is indicative that a blocker is not present. Steps  240  to  265  will check if blocker is present. In step  235 , the maximum allowable gain of the combined LNA  105  and VGA  125  is increased to an upper limit. In one embodiment, this combined upper limit may be 19, from a range of 0 to 19. Step  240  waits for an interrupt. The interrupt duty cycle may be programmable and may be the same interrupt as in  210 . In step  245 , it is determined whether the LPF  130  gain is greater than a predetermined upper threshold. In one embodiment the predetermined upper threshold is 6 dB, which may be programmable. The range of gain for LPF  130  may be 0 dB to 24 dB with 3 dB increments. If the gain is not greater than a predetermined upper threshold, a second counter is reset to 0 and the method continues with step  240 . If LPF  130  gain is greater than the predetermined upper threshold, then step  255  determines if the combined gain of the LNA  105  and the VGA  125  is less than a predetermined lower threshold. In one embodiment, the predetermined lower threshold may be programmable and set to 49 dB from the maximum of 51 dB. If not, then the second counter is reset in step  250  and the method continues with a wait for another interrupt in step  240 . The combined gain tested for in the combined gain of the LNA  105  and the VGA  125  in step  255  is programmable, depending on the sensitivity required. Further, the maximum allowable gain of LNA  105  and VGA  125  may be independently programmable. For example, the LNA  105  gain may have a range in hex from 1 to 6, while the VGA may have a programmable range in hex from 1 to A. A low combined gain of the LNA  105  and the VGA  125  may be indicative that a blocker is present. A high LPF  130  gain may be indicative that the in-band desired signal is weaker than the blocker and was needed to be incremented to achieve the desired operating range. In step  260  the second counter is incremented. If the LPF  130  gain remains greater than 6 dB while the combined LNA  105  and VGA  125  gain is below 49 dB for a predetermined and programmable count of 2, step  265  leads to step  270  where the second counter is reset to 0 and the method continues at step  205  where the maximum gain allowable by the AGC  160  for the LNA  105  and VGA  125  is set to the lower lower blocker limit. Otherwise, the method continues with step  240  to wait for another interrupt. As in the first counter, the second counter adds hysteresis to assure that the determination of the presence of a blocker is stable. The order of step  245  and step  255  may be interchanged. 
     Although the present invention has been described with reference to particular examples and embodiments, it is understood that the present invention is not limited to those examples and embodiments. For example, in an alternate embodiment, step  235  may follow step  200 . Further, gains and ranges have been provided for explanation purposes only. The present invention as claimed, therefore, includes variations from the specific examples and embodiments described herein, as will be apparent to one of skill in the art. Except to the extent necessary or inherent in the processes themselves, no particular order to steps or stages of methods or processes described in this disclosure, including the figures is implied. In many cases the order of process steps may be varied without changing the purpose, effect or import of the methods described.