Optical spatial frequency measurement

• US 6,501,258 B1
• Filed: 09/28/2001
• Issued: 12/31/2002
• Est. Priority Date: 09/28/2001
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Levitt, Hal L, Kith, Sokhom, Reynolds, Dale L.
• Primary Examiner(s): Oda, Christine
• Assistant Examiner(s): He, Amy
• Application Number:
  US09/965,249
• Time in Patent Office: 459 Days
• Field of Search: 324/76.37, 324/76.36, 324/76-7-, 359/191, 359/305, 359/310, 356/303, 356/450, 356/345, 356/346, 356/347, 375/130, 375/140, 375/316
• US Class Current: 324/76.37
• CPC Class Codes: G01R 23/17 with optical or acoustical auxiliary devices
• CPC Class Codes: G01S 7/021 Auxiliary means for detecting or identifying radar signals or the like, e.g. radar jamming signals

Abstract

A signal processing apparatus for providing electrical frequency measurements of input signals. In one embodiment, a power splitter divides an RF input signal into first and second input signals. An RF channelizer having a filter bank receives the first input signal and determines a coarse frequency measurement. An optical modulator modulates an optical carrier signal with the second input signal, and a beam splitter divides the modulated carrier signal into a plurality of optical signals that each feed into one of a plurality of optical frequency discriminators (“OFD”). Each OFD uses a fiber optic delay line and spatial domain sampling techniques to extract a phase measurement. An ambiguity resolver receives the phase measurement from each OFD and determines a single, absolute phase measurement, which is translated to a fine frequency measurement. A frequency encoder combines the coarse and fine frequency measurements to produce a final frequency measurement having increased resolution.

Claims

1. A signal processing apparatus for providing electrical frequency measurement of an input signal, comprising:

a splitter dividing the input signal into a first input signal and a second input signal having a same frequency as the first input signal;

a channelizer separating the first input signal into a plurality of frequency channels and determining a coarse frequency measurement corresponding to one of the frequency channels in which signal activity is detected;

a fine frequency measurement unit receiving the second input signal, applying spatial domain sampling to extract a phase measurement, and converting the phase measurement to a fine frequency measurement; and

a frequency encoder combining the coarse frequency measurement and the fine frequency measurement into an increased resolution frequency measurement.

2. The signal processing apparatus of claim 1, further comprising:

a saturating amplifier receiving the second input signal before the fine frequency measurement unit receives the second input signal, the saturating amplifier raising a signal level of the second input signal to a saturation point of the saturating amplifier; and

a bandpass filter receiving the second input signal with the raised signal level and outputting the second input signal with the raised signal level to the fine frequency measurement unit after removing out-of-band frequency components produced by the saturating amplifier.

3. The signal processing apparatus of claim 1, wherein the channelizer is an RF channelizer, comprising:

a power splitter equally dividing the first input signal into a plurality of output signals;

a filter bank having a plurality of filters, each filter receiving one of the output signals to divide the first input signal into separate frequency channels;

a plurality of signal .activity detectors, each signal activity detector receiving a signal from one of the filters and determining an amplitude of the signal from the respective filter;

a plurality of comparators, each comparator receiving the amplitude from one of the signal activity detectors and generating an output signal indicating signal activity in a filter by determining whether the amplitude exceeds a predetermined reference threshold value; and

a subband encoder receiving the output signals from the comparators and determining the coarse frequency corresponding to the filter having signal activity.

4. The signal processing apparatus of claim 1, wherein the fine frequency measurement unit comprises:

a laser producing a carrier signal;

an optical modulator receiving the carrier signal and the second input signal;

a beam splitter dividing the modulated carrier signal into a plurality of optical signals;

a plurality of optical frequency discriminators, each one of the optical frequency discriminators receiving one of the optical signals and generating a phase measurement; and

a fine frequency extraction unit receiving each phase measurement and outputting the fine frequency measurement.

5. The signal processing apparatus of claim 4, wherein each one of the optical frequency discriminators comprises:

a two-way beam splitter dividing the optical signal received into two signals that travel along two separate paths, one of the paths incurring a time delay relative to the second path, and the two signals interfering spatially to produce an intensity interference pattern;

a plurality of photodetectors upon which the interference pattern is projected, the photodetectors producing electrical output signals; and

a phase extraction unit receiving the electrical output signals and outputting the phase measurement.

6. The signal processing apparatus of claim 4, wherein the fine frequency extraction unit comprises:

a plurality of phase offset correction units, each phase offset correction unit receiving one of the phase measurements and providing offset correction for the respective phase measurement;

an ambiguity resolver receiving the offset corrected phase measurements, resolving phase ambiguity across the offset corrected phase measurements to generate a single unambiguous phase measurement; and

a phase-to-frequency converter, transforming the single phase measurement to the fine frequency measurement.

7. The signal processing apparatus of claim 1, wherein the frequency encoder comprises:

an RF band start lookup table providing a starting frequency for an RF band number provided from RF band conversion; and

an adder that sums the coarse frequency measurement, the fine frequency measurement, and the starting frequency.

8. The signal processing apparatus of claim 1, further comprising:

an antenna receiving the input signal propagating in free-space; and

a downconverter receiving the input signal from the antenna and translating a frequency of the input signal to an intermediate frequency, and sending the downconverted input signal to the splitter.

9. A signal processing apparatus for providing electrical frequency measurement of an input signal, comprising:

an antenna receiving the input signal propagating in free-space;

a downconverter receiving the input signal from the antenna and translating a frequency of the input signal to an intermediate frequency;

a splitter dividing the downconverted input signal into a first input signal and a second input signal having a same frequency as the first input signal;

a channelizer separating the first input signal into a plurality of frequency channels and determining a coarse frequency measurement corresponding to one of the frequency channels in which signal activity is detected;

a fine frequency measurement unit receiving the second input signal, applying spatial domain sampling to extract a phase measurement, and converting the phase measurement to a fine frequency measurement; and

a frequency encoder combining the coarse frequency measurement and the fine frequency measurement into an increased resolution frequency measurement.

10. A signal processing method for providing electrical frequency measurement of an input signal, comprising:

dividing the input signal into a first input signal and a second input signal having a same frequency as the first input signal;

determining a coarse frequency measurement by separating the first input signal into a plurality of frequency channels, the coarse frequency measurement corresponding to one of the frequency channels in which signal activity is detected;

determining a fine frequency measurement by applying spatial domain sampling to the second input signal to extract a phase measurement, and converting the phase measurement to the fine frequency measurement; and

providing an increased resolution frequency measurement by combining the coarse frequency measurement and the fine frequency measurement.

11. The signal processing method of claim 10, further comprising:

raising a signal level of the second input signal to a saturation point before determining the fine frequency measurement; and

determining the fine frequency measurement after removing out-of-band frequency components from the second input signal with the raised signal level.

12. The signal processing method of claim 10, wherein determining the coarse frequency measurement comprises:

dividing the first input signal into a plurality of channelized output signals and sending each output signal to a separate filter of a channelized filter bank;

determining a signal level of each channelized output signal;

comparing each signal level with a predetermined reference threshold value; and

selecting the coarse frequency measurement corresponding to the strongest signal level that also exceeds the predetermined threshold value.

13. The signal processing method of claim 10, wherein determining the fine frequency measurement comprises:

producing an optical carrier signal;

modulating the optical carrier signal with the second input signal;

dividing the modulated carrier signal into a plurality of optical signals;

sending each one of the optical signals to one of a plurality of optical frequency discriminators;

generating a phase measurement for each optical frequency discriminator;

performing phase offset correction for each phase measurement;

resolving phase ambiguity across the offset corrected phase measurements to generate a single phase measurement, and transforming the single phase measurement to the fine frequency measurement.

14. The signal processing method of claim 13, wherein generating a phase measurement for each frequency discriminator comprises:

dividing each optical signal into two signals that travel along two separate paths, one of the paths incurring a time delay relative to the second path, and the two signals interfering spatially to produce an intensity interference pattern; and

applying spatial domain sampling to the interference pattern to produce a phase measurement.

15. The signal processing method of claim 10, wherein providing an increased resolution frequency measurement comprises:

determining a starting frequency for an RF band number provided from RF band conversion; and

summing the coarse frequency measurement, the fine frequency measurement, and the starting frequency.