Abstract:
An adaptive interference canceller employing digital synthesis techniques and a digital to analog converter for eliminating narrow band RFI.

Description:
THE FIELD OF THE INVENTION 
     The present invention is directed generally towards interference cancellers, and more particularly towards an interference canceller which uses digital synthesis techniques to eliminate narrow band radio frequency interference (RFI). 
     BACKGROUND OF THE INVENTION 
     Conventional narrow band RFI cancellers such as line-interference cancellers eliminate the RFI present in a signal in order to improve the clarity and strength of the signal. Such devices are well known in the art and employ analog techniques to analyze the spectrum of a signal, estimate the level of RFI present in the signal, and then subtract this estimated RFI from the signal. 
     Such devices are used, for example, as part of communications receivers to eliminate the narrow band RFI generated by power lines. These conventional devices, however, suffer from a significant drawback in that the amount of RFI which can be eliminated is limited. Accordingly, there is a need for a device which can eliminate greater amounts of RFI. Therefore, it would be desirable to provide a canceller which can overcome the drawback of conventional cancellers by employing digital synthesis techniques to eliminate narrow band RFI. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided an adaptive RFI canceller using digital synthesis and a digital to analog converter for eliminating narrow band RFI. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawing in which: 
     FIG. 1 shows an exemplary embodiment of an adaptive RFI canceller using digital control techniques according to this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 there is shown an exemplary embodiment of an adaptive RFI canceller  10  using digital control techniques according to this invention. Analog to digital converter (A/D)  11  converts an analog signal from programmable gain amplifier (PGA)  14  into a digital signal. RFI predictor  12 , analyzes the spectrum of this digital signal using Fast Fourier Transforms, or other similar mathematical methods, to determine the level of RFI present therein. RFI predictor  12  then digitally synthesizes this signal using linear predictive coding, or other similar techniques, to replicate the undesired RFI. RFI predictor  12  is preferably implemented as a filter driven by an adaptive algorithm. 
     D/A  13  converts the synthesized digital signal into an analog signal which subtractor  15  subtracts from the amplified signal output by PGA  14 . Cancelling the RFI before A/D conversion in this manner prevents A/D overload. Although a large attenuation is initially used in PGA  14  to accommodate the RFI, as greater amounts of RFI are cancelled by canceller  10 , controller  16  is able to increase the gain of PGA  14 , thereby improving its input resolution. This cancellation need not be exact, but only sufficient to reduce the dynamic range of the amplified signal output from PGA  14 . Accordingly, canceller  10  need not eliminate all of the RFI. In an alternative embodiment, controller  16  can be coupled to predictor  12 . 
     Changes in the frequency, amplitude or phase of the RFI input are detected by RFI predictor  12 . Determining the amount of RFI present and synthesizing a signal to replicate the undesired RFI by employing digital techniques, rather than by employing analog techniques as is conventionally done, improves the amount of RFI which can be eliminated from the amplified signal output from PGA  14 . Canceller  10  is able to eliminate between 6 to 12 dB of RFI. 
     Numerous modifications to and alternative embodiments of the present invention will be apparent to those skilled to the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. Details of the structure may be varied substantially without departing from the spirit of the invention and the exclusive use of all modifications which come within the scope of the appended claims is reserved.