High resolution frequency analyzer and vector spectrum analyzer

• US 5,576,978 A
• Filed: 05/18/1994
• Issued: 11/19/1996
• Est. Priority Date: 05/18/1994
• Status: Expired due to Term

• Current Assignee: Advantest Corporation
• Original Assignee: Advantest Corporation
• Inventors: Kitayoshi, Hitoshi
• Primary Examiner(s): Trammell, James P.
• Assistant Examiner(s): VO, HIEN XUAN
• Application Number:
  US08/246,252
• Time in Patent Office: 916 Days
• Field of Search: 364/576, 364/485, 364/484, 364/577, 364/514 B, 364/572, 364/726, 364/724.06, 364/725, 364/485, 324/76.19, 324/76.21, 324/76.26
• US Class Current: 702/77
• CPC Class Codes: G01R 23/16 Spectrum analysis; Fourier analysis

Abstract

A high resolution frequency analyzer analyzes frequency spectra of an incoming analog signal based on a fast Fourier transformer at ultra-high resolution without increasing the sampling number or time data of the analog signal. This series of input time data representing the analog signal is provided to the fast Fourier transformer through a subtractor. The analyzer distinguishes a spread spectrum, due to fluctuations, from a true line spectrum in the frequency spectra output by the fast Fourier transformer. If the spectrum is found to be the true line spectrum, then true frequency, amplitude and phase of the line spectrum are accurately obtained by an interpolator. The line spectrum is accumulated by an accumulator and inversely converted by an inverse Fourier transformer provided in a feedback loop so as to feedback time base data to the subtractor. The time base data is subtracted by the input time data so that the remainder data is Fourier transformed. The line spectrum accumulated by the accumulator is vector-added to the output of the fast Fourier transformer, which establishes a high resolution frequency spectrum to be displayed on a display device.

Claims

1. A high resolution frequency analyzer for obtaining frequency spectrum of an incoming signal and displaying the frequency on a display, comprising:

a subtractor for subtracting input digital data from the other input digital data and generating resulted digital data at its output;

a fast Fourier transformer receiving said resulted digital data from said subtractor for transforming discrete digital data to a frequency spectrum;

an interpolation judgment circuit which distinguishes a line spectrum from a dispersed spectrum in each frequency spectrum obtained by said fast Fourier transformer and determines that said frequency spectrum be interpolated when a true line spectrum is found;

a spectrum interpolator which produces frequency domain data by estimating a true frequency, amplitude and phase of said frequency spectrum that has been determined by said interpolation judgment circuit to be interpolated;

an inverse Fourier transformer which inversely converts said frequency domain data produced by said spectrum interpolator into time base data, said inverse Fourier transformer being provided in a feedback loop to provide said time base data to said subtractor;

a first accumulator for accumulating said frequency domain data produced by said spectrum interpolator; and

an adder for vector-summing said frequency domain data accumulated by said accumulator to said frequency spectrum from said fast Fourier transformer and providing the resulted frequency spectrum to said display.

2. The high resolution frequency analyzer as defined in claim 1 further includes:

an AD converter for successively converting an incoming analog signal to be frequency analyzed to a series of digital data; and

a memory for successively storing said digital data and generating said digital data, said digital data being applied to said subtractor.

3. The high resolution frequency analyzer as defined in claim 1 further includes a window function weighting circuit which is provided between said subtractor and said fast Fourier transformer for multiplying said digital input data series by predetermined window function.

4. The high resolution frequency analyzer as defined in claim 1 further includes a second accumulator which is provided in said feedback loop between said subtractor and said inverse Fourier transformer for accumulating said time base data from said inverse Fourier transformer.

5. The high resolution frequency analyzer as defined in claim 1, wherein said first accumulator provides said frequency domain data produced by said spectrum interpolator both to said inverse Fourier transformer and said adder.

6. The high resolution frequency analyzer as defined in claim 1, wherein said interpolation judgment circuit which distinguishes said line spectrum from said dispersed spectrum by monitoring three adjacent frequency spectra (n-1, n, n+1) from said fast Fourier transformer, when a phase difference between said spectra (n-1, n+1) is within π

/36 (radian), said frequency spectrum (n) is regarded as a line spectrum so that the estimation of said frequency domain data is proceeded by said interpolator.

7. A vector spectrum analyzer for obtaining vector frequency spectrum of an incoming signal and displaying the vector frequency on a display, comprising:

a subtractor for subtracting input digital data from the other input digital data and generating resulted digital data at its output;

a fast Fourier transformer receiving said resulted digital data from said subtractor for transforming discrete digital data to a frequency spectrum;

an interpolation judgment circuit which distinguishes a line spectrum from a dispersed spectrum in each frequency spectrum obtained by said fast Fourier transformer and determines that said frequency spectrum be interpolated when a true line spectrum is found;

a spectrum interpolator which produces frequency domain data by estimating a true frequency, amplitude and phase of said frequency spectrum that has been determined by said interpolation judgment circuit to be interpolated;

an inverse Fourier transformer which inversely converts said frequency domain data produced by said spectrum interpolator into time base data, said inverse Fourier transformer being provided in a feedback loop to provide said time base data to said subtractor;

a accumulator for accumulating said frequency domain data produced by said spectrum interpolator;

an adder for vector-summing said frequency domain data accumulated by said accumulator to said frequency spectrum from said fast Fourier transformer and providing the resulted frequency spectrum to said display;

a spectrum data memory for storing said frequency domain data accumulated by said accumulator;

a read-out controller for controlling a read-out operation in said spectrum data memory to extract amplitude data, frequency data, and phase data of each frequency spectrum in said frequency domain data;

wherein each of said amplitude data, frequency data and phase data is separately displayed on said display in a time sequence manner.

8. The vector spectrum analyzer as defined in claim 7 further includes:

an amplitude time change memory for storing said amplitude data from said spectrum data memory with respect to time sequence;

a frequency time change memory for storing said frequency data from said spectrum data memory with respect to time sequence; and

a phase time change memory for storing said phase data from said spectrum data memory with respect to time sequence.

9. The vector spectrum analyzer as defined in claim 7 wherein each of said amplitude time change memory, frequency time change memory and phase time change memory is formed of a plurality of memories to produce a time varying display of said amplitude, frequency and phase data on said display.

10. The vector spectrum analyzer as defined in claim 7, wherein said interpolation judgment circuit which distinguishes said line spectrum from said dispersed spectrum by monitoring three adjacent frequency spectra (n-1, n, n+1) from said fast Fourier transformer, when a phase difference between said spectra (n-1, n+1) is within π

/36 (radian), said frequency spectrum (n) is regarded as a line spectrum so that the estimation of said frequency domain data is proceeded by said interpolator.