Abstract:
A method and apparatus for minimizing image interference using a very low intermediate frequency image rejection receiver is disclosed. Image interference detection and avoidance by frequency plan adjustment within the very low intermediate frequency receiver image rejection receiver minimizes rejection requirements for an image reject mixer. Image rejection can be measured by switching between in-phase and out-of-phase image reject mixer output ports. Rejection may also be measured by creating a tri-mode image reject mixer. A set of linear equations allows rejection to be measured without requiring control of input signals and additional mixers.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to co-pending U.S. provisional application entitled, “Very Low Frequency Image Rejection Receiver with Image Interference Detection and Avoidance”, having Ser. No. 60/532,568, filed Dec. 29, 2003, which is entirely incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is generally related to radio frequency receivers and, more particularly, is related to an apparatus and method for mitigating image interference using a very low intermediate frequency (VLIF) receiver.  
       BACKGROUND OF THE INVENTION  
       [0003]     Many conventional radio receivers for use in portable communication devices, such as cellular telephones, are of the super-heterodyne type in which a radio signal to be received is first down-converted to an intermediate frequency (IF), which is still within the radio frequency range, and then further down-converted to a base-band signal having both in-phase and quadrature-phase components from which the information contained in the signal may be recovered. A conventional super-heterodyne receiver architecture is shown in  FIG. 2 . However, direct conversion receivers and very low IF receivers reduce costs by eliminating both a relatively high performance, and therefore, expensive, surface acoustic wave band-pass filter (for allowing the wanted IF signal to pass while blocking all unwanted IF signal enabling channels) and one of the two radio frequency local oscillators (LO) required in super-heterodyne receivers.  
         [0004]     Direct conversion receivers immediately down-convert the received radio signal to a base-band signal, thus completely eliminating the IF stage. However, such receivers suffer from the formation of a very large unwanted DC component interfering with the base-band signal. That DC component is formed largely by leakage from the LO being received at the receiver antenna together with the unwanted signal, and also by offsets of the amplifiers and mixers in the receivers. A typical direct-down conversion receiver is shown in  FIG. 3 .  
         [0005]     Undesired signals that cause a response at the IF frequency in addition to the desired signal are known as spurious responses. Spurious responses must be filtered out before reaching mixer stages and the heterodyne receiver. One spurious response is known as an image frequency, or image. An RF filter (known as a preselector filter) is required for protection against the image unless an image reject mixer is used. Image reject mixers reduce the image component during the mixing process and thus provide protection against the image.  
         [0006]     Image rejection is integral to VLIF receivers. A typical VLIF receiver architecture is shown in  FIG. 1 . In VLIF receivers, sampling of the received signal is often performed directly at the intermediate frequency. VLIF receivers, because of the elimination of components related to multiple IFs, are generally lower cost than conventional super-heterodyne receivers. However, the cost savings is not achieved without problems such as spurious responses due to interference at the image frequency.  
         [0007]     A principal difficulty in implementing a VLIF architecture is the design of an image rejection receiver with sufficient attenuation to reject the image. Front end filters, although effective for image rejection super-heterodyne architectures, due to the higher first IF, do not provide sufficient attenuation at VLIF image frequencies. That is often addressed by a single side-band mixer (image reject mixer). The image reject mixer uses phase cancellation to reject the image, while down-converting the desired signal.  
         [0008]     To sufficiently reject the image, image reject mixers often employ complex tuning systems with feedback. Defining sufficient rejection depends on the signal levels likely to be present in the image band, and the desired signal-to-image ratio.  
         [0009]     Thus, to further reduce the costs and facilitate the implementation of VLIF architectures, it would be desirable to reduce the rejection required from the image reject mixer. Hence, an unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.  
       SUMMARY OF THE INVENTION  
       [0010]     Embodiments of the present invention provide an apparatus and method for mitigating image interference using VLIF architecture. Instead of only trying to maximize rejection, the invention minimizes the amount of power in the image band. Thus, the method can be used independently, or in conjunction with maximizing rejection.  
         [0011]     Briefly described, a preferred embodiment of the apparatus can be implemented as follows. The apparatus includes an image reject mixer that has two possible outputs, one output for the desired signal, and another output for the image. The image reject mixer is incorporated in the architecture of a VLIF receiver. The VLIF receiver includes a dynamically adjustable frequency plan. The dynamically adjustable frequency plan includes at least two adjustable frequency sources. A measuring device is used to measure the power at the two outputs of the image reject mixer. An algorithm is used in calculations for detecting and avoiding image interference.  
         [0012]     In yet another embodiment of the invention, the VLIF receiver includes three modes, an upper side band (USB) mode, a lower side band (LSB) mode and a double-side band (DSB) mode. In this embodiment, the image detection and avoidance provided by the VLIF receiver includes a rejection measurement capability. Also in this embodiment, by using a system of linear equations, the rejection may be measured without requiring control of the input signals, or additional mixers.  
         [0013]     In another embodiment of the invention, an image detection and avoidance receiver is disclosed with rejection measurement capability and rejection tuning. In this method, amplitude and phase adjustment capabilities are provided.  
         [0014]     In one embodiment of the invention, an alternative method is used for measuring the image level. In this embodiment, to measure the power in the image band, a local oscillator is tuned to place the image frequency in the signal band of the mixer.  
         [0015]     In another embodiment, the apparatus may include an image reject VLIF receiver with a dynamic frequency plan connected to a DSP. Image interference may be detected indirectly by received signal metrics, such as signal levels and noise levels. Based on that measurement, the frequency plan could be changed in an effort to minimize noise levels.  
         [0016]     Embodiments of the present invention can also be viewed as providing methods for mitigating image interference. In that regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: measuring power at the dual outputs of an image reject mixer; providing at least two adjustable frequency sources; employing and adjusting a frequency plan based on a signal-to-image interference ratio; and controlling the at least two adjustable frequency sources via the results from measuring the power at the dual outputs of the image reject mixer.  
         [0017]     Other systems, methods, features and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Many aspects of the invention can be better understood with reference to the following drawings. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
         [0019]      FIG. 1  is a block diagram of a VLIF receiver architecture;  
         [0020]      FIG. 2  is a block diagram of a super-heterodyne receiver architecture;  
         [0021]      FIG. 3  is a block diagram of a direct down-conversion receiver architecture;  
         [0022]      FIG. 4  is a block diagram of a preferred embodiment of a receiver with image detection and avoidance;  
         [0023]      FIG. 5  is a flow diagram for evaluating image detection and avoidance;  
         [0024]      FIG. 6  is a graphical representation of an alternative method for measuring power in an image band by tuning an oscillator to place the image and the signal band of the image reject mixer;  
         [0025]      FIG. 7  is a block diagram of an embodiment of the invention with image detection and avoidance, and having rejection measurement capability;  
         [0026]      FIG. 8  is a block diagram of an embodiment of the invention using an image detection and avoidance receiver with rejection measure capability and rejection tuning; and  
         [0027]      FIG. 9  is a flow diagram of an embodiment of the invention illustrating image detection and avoidance with receiver tuning. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]      FIG. 4  illustrates a VLIF image rejection receiver  400  with image detection and avoidance. The VLIF receive  400  includes an image reject mixer  402  with dual outputs, one output  414  for a desired frequency signal and another output  412  for an image frequency signal. A measuring device for measuring power at the dual outputs of the image reject mixer  402  can be implemented using a suggested frequency plan. Immediately prior to a receiver operation, an image detection test would be performed. If a weak power level in the image band was detected, the receiver could proceed with the current frequently plan. If a strong image was detected, the frequency plan would be adjusted, and the image would be measured again. The cycle would continue until a frequency plan with a weak image was identified. An algorithm  500  illustrating how to implement the measuring device for sampling the frequency plan is illustrated in  FIG. 5 .  
         [0029]     In  FIG. 5 , the algorithm  500  begins with the step of selecting an initial frequency plan while avoiding frequencies with known interference potential  502 . Then, in step  504  a pre-receive image check is started. Next, a digital signal processor  408  sets a double pole double throw switch  404  to measure a desired signal and power, at step  506 . In step  508 , the digital signal processor sets the double pole double throw switch  404  to measure the image frequency signal and power. After the previous measurements, in step  510 , the digital signal processor  408  calculates the image interference by dividing by the worse case rejection. The digital signal processor  408  then, in step  512 , calculates the signal-to-image interference ratio. Step  514  addresses the issue as to whether the signal-to-image interference ratio is acceptable. If the ratio is not acceptable, then in step  516  the instruction is to add this frequency plan to the list of potentially bad frequency plans, select a new frequency plan at step  518 , and return to the beginning of pre-receiving the image check, step at  504 . If the signal-to-image interference ratio is acceptable in step  514 , then in step  520  the receiver signal is immediately accepted.  
         [0030]     As mentioned above, the digital signal processor  408 , along with the double pole double throw switch  404 , are used as a controller device for controlling the frequency plans used in the algorithm  500  for image detection and avoidance. Interestingly, a practical implementation could be to limit the amount of frequency plan iterations before eventually exiting the loop and reporting a failure (not shown).  
         [0031]     In an alternative method  600 , measuring the power in the image band could be achieved by tuning an oscillator  416  to place the image frequency signal in the signal band of the image reject mixer  402 . ( FIG. 6 ). This method has potential limitations in high interference-to-signal ratio environments. Because the very large interferer is now in the image band, limited image rejection ratio makes it appear as though the image band of the desired receiver frequency plan has an interferer, despite the fact that it was actually clear. This could lead to unnecessary frequency plan retuning. Thus, the approach using the image reject mixer  402  with the dual outputs, is preferred.  
         [0032]     Simply measuring the power in the image band only determines image interference if the level of rejection is known, or can be sufficiently well estimated. If that is not the case, a measurement of rejection is also required. Measurement of rejection requires that the receiver  400  include three modes, upper-side band (USB) mode, lower-side band (LSB) mode, and double-side band (DSB) mode.  
         [0033]      FIG. 7  illustrates a VLIF with image detection and avoidance and rejection measurement capability  700 . To put the image reject mixer  402  in LSB mode, the double pole double throw switch  404  is cross-connected and the single pole single throw switch  706  is closed. To put the image reject mixer  402  in USB mode, the double pole double throw switch  404  is connected straight through and the single pole single throw switch  706  is closed. To put the image reject mixer  402  in DSB mode, the single pole single throw switch  706  is opened, and the double pole double throw switch  404  can be cross-connected or connected straight through. By measuring received power in the three modes, a linear system of equations is identified from which rejection can be calculated.  
         [0034]     That system of equations for identifying rejection are as follows:  
         [0035]     Equation 1: Power measured when the mixer is configured in USD mode. 
 
 P   USB  Mode= P   signal   +P   image /Rejection 
 
         [0036]     Equation 2: Power measured when the mixer is configured in LSB mode. 
 
 P   LSB  Mode= P   image   +P   signal /Rejection 
 
         [0037]     Equation 3: Power measured when the mixer is configured in DSB mode. 
 
 P   DSB  Mode=P image   /K+P   signal   /K   g,  where K is the known additional loss factor introduced by only using one of the two branches of the mixer. 
 
         [0038]     With rejection quantified, image interference can be calculated without assumptions.  
         [0039]     With the aforementioned method to measure rejection, closed loop tuning is now possible. Closed loop tuning requires amplitude  802  and phase  804  adjustment capabilities (see  FIG. 8 ). In situations with very stringent image rejection specifications, closed loop tuning along with image detection and avoidance could provide optimal performance by maximizing rejection and minimizing the image.  
         [0040]      FIG. 9  illustrates another algorithm for configuring closed loop tuning, detection and avoidance  900  in a receiver. In step  902 , one tunes to the desired signal and receives everything in USB, LSB and DSB modes. Then, in step  904 , image interference is calculated using Equations 1, 2 and 3 (as referenced above) without assumptions on rejection. The question is then raised in step  906  as to whether the signal-to-image interference ratio is acceptable. If the answer to step  906  is no, then in step  908 , the frequency plan is adjusted ‘N’ times to find an acceptable image interference. If the results to the question of step  906  are positive, then in step  910 , one begins to receive the signal as soon as possible. Interestingly, in step  912 , the question is asked whether an acceptable image interference is obtained. If the answer to that question is positive, then one moves on to step  910 . If the answer to step  912  is negative, then in step  914 , the most favorable frequency plan identified is acquired. In step  916 , RF phase, RF amplitude, and IF phase are tuned to optimize rejection. The output of that step is then forwarded to step  920  to tune to the desired signal and receive in all USB, LSB and DSB modes. The image interference is then calculated using Equations 1, 2 and 3 without assumptions on rejection, step  922 . The results of step  922  are then forwarded to a query step  924  where the question is asked as to whether the signal-to-image interference ratio is acceptable. If the answer to that question is positive, then one returns to step  910 . If the results of step  924  are negative, then in step  926 , the question is asked whether there is still room for improvement. If the answer to step  926  is yes, then one is transferred to step  918  which makes intelligent tuning choices based upon previous trials and then forwards those results to step  916 . If the answer to step  926  is negative, then one is returned to step  908  where frequency plans are again tried ‘N’ times until one is found acceptable.  
         [0041]     In another embodiment of the invention, the apparatus may include an image reject VLIF receiver with a dynamic frequency plan connected to a DSP. Image interference is detected indirectly by received signal metrics, such as signal levels and noise levels. In the case of image interference, a normal signal level with a high noise level could be indicative of image interference. Based on that measurement, the frequency plan could be changed in an effort to minimize noise in the image band, which could be observed as a return of the noise level to normal levels. However, this approach has limitations, primarily because of the indirect nature of the image interference measurement, as other reception impairments could be mistaken for image interference.  
         [0042]     Furthermore, by including image interference detection and avoidance by frequency plan adjustment within a VLIF receiver architecture, rejection requirements for the image reject mixer are minimized and thereby, minimizing costs and facilitating implementation. By switching between in-phase and out-of-phase mixer output ports, image rejection can be measured. Thus, rejection requirements for the image reject mixer are minimized.  
         [0043]     It should be emphasized that the above described embodiments of the present invention, particularly, any preferred embodiments are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above described embodiments of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims.