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Narrow band spectrum analysis method and apparatus

  • US 5,691,924 A
  • Filed: 07/28/1995
  • Issued: 11/25/1997
  • Est. Priority Date: 12/09/1994
  • Status: Expired due to Term
First Claim
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1. A method of zoom processing a time domain frequency signal digitized at a sampling frequency comprising:

  • providing a zoom processor having an input device, a memory, a processor, and an output device,acquiring the digitized time domain frequency signal with the input device of the zoom processor, to produce an acquired signal,lowering the frequency of the acquired signal with the processor of the zoom processor, by;

    inputting a sine table into the zoom processor using the input device of the zoom processor, where the value of each entry in the sine table is based on one full period being represented by a predetermined number of entries, the entries being twenty-four bit numbers,storing the sine table in the memory of the zoom processor,solving the equation ƒ

    c

    s =(X+Δ

    )/Ψ

    for X and Δ

    using the processor of the zoom processor, where X is a positive integer less than or equal to 800, Ψ

    is the predetermined number, and Δ

    is a non-negative number less than one,selecting the η

    X sine table entry using the processor of the zoom processor when Δ

    equals zero, the value of which entry equals sin (2π





    c

    s)!,interpolating by Δ

    between the η

    X sine table entry and η

    (X+1) sine table entry using the processor of the zoom processor, to determine an interpolated value when Δ

    does not equal zero, which interpolated value equals sin (2π





    c

    s)!, andcalculating cos (2π





    c

    s)! using the processor of the zoom processor according to the equation cos(θ

    )=sin(θ



    /2), andmultiplying the acquired signal by {cos (2π





    c

    s)!+j*sin (2π





    c

    s)!}, where η

    is a consecutive data point number, j is an imaginary number having a value of (-1)1/2, ƒ

    c is a frequency of the acquired signal, and ƒ

    s is the sampling frequency used to produce the digitized time domain frequency signal, to produce a shifted signal,low-pass filtering the shifted signal, to produce a low-pass filtered signal, andoutputting the low-pass filtered signal with the output device of the zoom processor, to produce a zoom processed signal.

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