Complex Fourier coefficient extractor

• US 6,307,895 B1
• Filed: 04/01/1998
• Issued: 10/23/2001
• Est. Priority Date: 04/01/1998
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Spezio, Anthony, Krantz, Brian, Alexander, Edward, Levitt, Hal
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Fan, Chieh M.
• Application Number:
  US09/052,676
• Time in Patent Office: 1,301 Days
• Field of Search: 324/76.37, 324/76.36, 375/130, 375/140, 375/316, 356/303, 356/450, 342/192, 359/191, 359/305, 359/310
• US Class Current: 375/316
• CPC Class Codes: G01R 23/17 with optical or acoustical auxiliary devices

Abstract

A system for extracting the complex Fourier coefficients from an unknown RF signal to classify the RF signal is disclosed. The extraction system utilizes optical signal processing in combination with the generation of electronic timing and control signals. The system employs an optical processor comprising a Bragg cell to yield the Fourier coefficients including the amplitude, phase and frequency parameters thereof. The system further includes a parallel arrangement that provide for parallel processing to reduce the computational time needed to determine the complex Fourier coefficients including the amplitude, phase and frequency parameters thereof.

Claims

1. An apparatus for extracting complex Fourier coefficients from an RF signal of interest so as to characterize said RF signal, said RF signal having a leading edge and modulation, said extracting apparatus comprising:

(a) receiving means for receiving said RF signal and generating first and second timing signals in response to said leading edge of said RF signal, said receiving means downconverting and demodulating said RF signal to a baseband so as to remove a carrier frequency but leaving a modulation voltage, said downconverted, demodulated RF signal serving as an output of said receiving means;

(b) means for generating a signal of interest and a reference signal for a two acoustic column Bragg cell device, said signal of interest and reference signal generating means receiving said downconverted, demodulated RF signal and said first timing signal, said signal of interest and reference signal generating means including a local oscillator;

(c) an optical processor comprising said Bragg cell, a laser, a Fourier lens, and an array of spatially positioned means each responsive to intercepted optical signals to correspondingly produce n electrical signals that represent a pattern of said intercepted optical signals, said Bragg cell having first and second acoustic columns respectively connected to said signal of interest and reference signal of said signal of interest and reference signal generating means; and

(d) means for receiving said pattern of said intercepted optical signals and extracting therefrom said complex Fourier coefficients.

2. The apparatus according to claim 1, wherein said optical processor includes an arrangement comprising:

(i) said laser generating a coherent light beam;

(ii) said first and second acoustic columns of the Bragg cell each being arranged to intercept said laser beam and providing respective optical outputs;

(iii) said Fourier lens being arranged to intercept both of said optical outputs of said Bragg cell and providing corresponding optical outputs; and

(iv) said means responsive to optical signals of said Fourier lens and being arranged to intercept said optical outputs of said Fourier lens and providing corresponding electrical output signals that represent a pattern of said optical outputs of said Fourier lens.

3. The apparatus according to claim 2 further comprising a beam splitter interposed between said laser and said first and second columns of said Bragg cell.

4. The apparatus for extracting complex Fourier coefficients according to claim 1, wherein said extracting means comprises:

(i) means for sampling and holding and separately receiving each of said electrical output signals, each of said sampling and holding means being connected and responsive to said second timing signal and providing an electrical analog output in response to the presence of said second timing signal,

(ii) analog-to-digital (A/D) conversion means for separately receiving the output of each of said sampling and holding means and correspondingly providing a digital signal thereof; and

(iii) means for examining the digital signals provided by said A/D conversion means and determining the complex Fourier coefficients contained therein including frequency and phase parameters.

5. The apparatus for extracting complex Fourier coefficients according to claim 2, wherein said optical outputs of said Fourier lens covers a predetermined region and wherein said means responsive to optical signals and arranged to intercept said output of said Fourier lens comprises:

(i) a plurality of optical fibers arranged to cover said predetermined region of said optical output of said Fourier lens; and

(ii) a plurality of detector amplifier combinations arranged in correspondence with said plurality of optical fibers and providing said corresponding electrical output signals that represent said pattern of said optical outputs.

6. The apparatus for extracting complex Fourier coefficients according to claim 2, wherein said means responsive to optical signals and arranged to intercept said output of said Fourier lens comprises:

(i) a beam splitter for dividing said intercepted optical outputs of said Fourier lens into at least three components and providing outputs thereof; and

(ii) at least three column arrangements of photodetector elements arranged to respectively receive said at least three output components of said beam splitter and provide corresponding electrical output signals that represent said pattern of said optical outputs of said Fourier lens.

7. The apparatus for extracting complex Fourier coefficients according to claim 6, further comprising at least three lenslets interposed between said beam splitter and said at least three column arrangements and respectively arranged to intercept optical signals of said at least three output components of said beam splitter.

8. The apparatus for extracting complex Fourier coefficients according to claim 2, wherein said optical outputs of said Fourier lens covers a predetermined region and said means responsive to optical signals and arranged to intercept said output of said Fourier lens comprises:

(i) a row-column arrangement of photodetectors arranged to cover said predetermined region of said optical output of said Fourier lens and providing said corresponding electrical output signals that represent said pattern of said intercepted optical output of said Fourier lens.

9. An apparatus for extracting complex Fourier coefficients from an RF signal of interest so as to characterize said RF signal, said RF signal having a leading edge and modulation, said extracting apparatus comprising:

(a) receiving means for receiving said RF signal and generating first and second timing signals in response to said leading edge of said RF signal, said receiving means downconverting and demodulating said RF signal to a baseband so as to remove a carrier frequency but leaving a modulation voltage, said downconverted, demodulated RF signal serving as an output of said receiving means;

(b) means for generating a signal of interest and a reference signal for a two acoustic column Bragg cell device, said signal of interest and reference signal generating means receiving said downconverted, demodulated RF signal and said first timing signal;

(c) an optical processor comprising said Bragg cell, a laser, a Fourier lens, and an array of spatially positioned means each responsive to intercepted optical signals to correspondingly produce n electrical signals that represent a pattern of said intercepted optical signals, said Bragg cell having first and second acoustic columns respectively connected to said signal of interest and reference signal of said signal of interest and reference signal generating means; and

(d) means for receiving said pattern of said intercepted optical signals and extracting therefrom said complex Fourier coefficients;

wherein said means for generating said signal of interest and said reference signal comprises first, second and third branches which, in turn, respectively comprise;

(i) said first branch comprising means for receiving said downconverted, demodulated RF signal, means for delaying said received downconverted, demodulated RF signal by a predetermined amount, a mixer having first and second inputs and an output with the first input receiving said delayed downconverted, demodulated RF signal and with said output providing said signal of interest of said Bragg cell device;

(ii) said second branch comprising an oscillator with an output and pseudorandom noise (PRN) generator with an output, said oscillator and said pseudorandom noise (PRN) generator both being activated in response to said first timing signal and with said output of said oscillator being applied to said pseudorandom noise (PRN) generator which is responsive thereto, said second branch further comprising a mixer having first and second inputs and an output with the first input receiving said output of said pseudorandom noise (PRN) generator and said output of said mixer providing said reference signal of said Bragg cell device; and

(iii) said third branch comprising a local oscillator having first and second outputs with said first output connected to said second input of said mixer of said first branch and said second output connected to said second input of said mixer of said second branch.

10. The apparatus for extracting complex Fourier coefficients according to claim 9, wherein said pseudorandom noise (PRN) generator provides output whose Fourier series is at evenly spaced intervals representative of a comb spectrum.

11. The apparatus for extracting complex Fourier coefficients according to claim 10, wherein said pseudorandom noise (PRN) generator provides a signal whose Fourier series is a spectral comb:

5, 10 . . . 40 MHz.