Signal detection apparatus, method, and applications

Abstract

Apparatus and associated method for unambiguously evaluating high-bandwidth, rapidly changing analog range data in real time using low-cost components that allow detection of the signal of interest using a sampling rate that is lower than the Nyquist rate required to directly evaluate the full range data bandwidth.

Claims

1. A range finding method, comprising: illuminating a surface of a distant object with a known frequency modulated laser beam over a known bandwidth, the laser beam comprising laser light; mixing a portion of the laser light reflected from the surface of the distant object with a portion of the original illumination laser light on a photodetector to generate a mixed light; generating a range-encoded beat signal having an unknown frequency .sub.RF within a known maximum range-dependent frequency bandwidth from the mixed light; and unambiguously determining the frequency of the range-encoded beat signal by: splitting the range-encoded beat signal having an unknown frequency .sub.RF into two halves of equal power; mixing one of the two split signals with a first signal having a known, fixed frequency .sub.LO1; and mixing the other of the two split signals with a second signal having a known, fixed frequency .sub.LO2; wherein the first known frequency .sub.LO1 is at least one-half of the known maximum range-dependent frequency bandwidth and the second known frequency .sub.LO2 is higher than .sub.LO1.

2. The method of claim 1, further comprising generating each of the first signal having a known, fixed frequency .sub.LO1 and the second signal having a known, fixed frequency .sub.LO2 using a corresponding at least one of a crystal oscillator, a Microelectromechanical System (MEMS) oscillator, a digital phase locked loop, and a frequency synthesizer.

3. The method of claim 1, wherein the step of mixing one of the two split signals with a first signal having a known, fixed frequency .sub.LO1 generates a sum of .sub.RF and .sub.LO1 and a difference of .sub.RF and .sub.LO1; wherein the step of mixing the other of the two split signals with a second signal having a known, fixed frequency .sub.LO2 generates a sum of .sub.RF and .sub.LO2 and a difference of .sub.RF and .sub.LO2; the method further comprising using a first low pass filter to block transmission of the sum of .sub.RF and .sub.LO1, and using a second low pass filter to block the transmission of the sum of .sub.RF and .sub.LO2.

4. The method of claim 3, further comprising determining the frequency of each of the difference of .sub.RF and .sub.LO1 and the difference of .sub.RF and .sub.LO2 using a corresponding at least one of an analog-to-digital converter and a phase-locked loop.

5. The method of claim 1, wherein the step of mixing one of the two split signals with a first signal having a known, fixed frequency .sub.LO1 generates a difference of .sub.RF and .sub.LO1, and wherein the step of mixing the other of the two split signals with a second signal having a known, fixed frequency .sub.LO2 generates a difference of .sub.RF and .sub.LO2, and the method further comprising comparing the difference of .sub.RF and .sub.LO1 and the difference of .sub.RF and .sub.LO1 to determine the frequency of the range-encoded beat signal, whereby a distance to the distant object is determined.

6. The method of claim 3, further comprising comparing the difference of .sub.RF and .sub.LO1 and the difference of .sub.RF and .sub.LO2 to determine the frequency of the range-encoded beat signal, whereby a distance to the distant object is determined.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic block diagram of an apparatus for unambiguously identifying an unknown time varying signal that occurs within a known signal bandwidth, according to an embodiment of the invention.

(2) FIG. 2 is a graphical illustration of a method embodiment of the invention.

DETAILED DESCRIPTION OF NON-LIMITING, EXEMPLARY EMBODIMENTS OF THE INVENTION

Definitions

(3) Sampling

(4) Sampling is the process of taking a time-varying continuous signal (e.g., analog signals such as electromagnetic waves; light, radio, etc.) and converting it into a discrete time signal (i.e., a digital signal) by measuring the continuous signal at discrete time intervals T.

(5) Bandwidth

(6) The continuous set of frequencies within which a signal of interest may lie.

(7) Nyquist Sampling Rate

(8) The minimum rate (1/T) at which a continuous signal needs to be sampled so that a discrete sequence of samples will capture all of the information within the continuous time signal of a finite bandwidth (all the different frequency components of the signal). The Nyquist theorem states that to capture all of the information within a time varying continuous signal of frequency, F, and to know everything about that signal one needs to take discrete samples of the signal at a rate at least 2F. Alternatively, if a signal may occur within a known bandwidth B, to find that signal one would have to sample at a rate of 2B; e.g., if the signal of interest may occur randomly within a set of frequencies ranging from 1-10 MHz, then to find that signal as it occurs one would need to sample the signal at >20 MHz.

(9) Aliasing

(10) The misidentification of the frequency of a signal is possible when a signal of frequency F is sampled at a rate less than 2F.

(11) RF Mixing

(12) When two or more time varying sinusoidal signals of frequency F1 and F2 are combined within an electronic mixer, the output of the mixer results in two separate signals; one at a frequency that is the sum F1+F2 of the two original signals, and another that is the difference F2F1 of the two original signals.

(13) Frequency Downconversion (Heterodyning)

(14) Using the phenomenon of RF mixing to reduce the frequency of a signal; i.e., combining a signal with an initially high frequency, F.sub.high, mixing it with a signal of known lower frequency F.sub.mix. The mixed output results in F.sub.high+F.sub.mix, which can be filtered out and ignored, while F.sub.highF.sub.mix becomes the signal of interest, where the resultant frequency F.sub.highF.sub.mix is typically much lower than the frequency F.sub.high and can thus be sampled at a lower rate.

(15) Photonic Mixing

(16) The analog of RF mixing using light. Monochromatic light such as laser light produced by a single frequency laser diode has a single emission frequency (=c/); i.e., all the photons emitted by the laser oscillate at the same frequency. For example, a laser with a wavelength of 1310 nm has an emission frequency of 229 THz. If two single frequency lasers with different emission frequencies F1 and F2 are combined on the surface of a semiconductor photodetector, a time varying sinusoidal current is generated in the semiconductor. Two signals make up the time varying sinusoidal current signal, one with a frequency of F1+F2 and one with a frequency of F1F2. For example, if the output of a laser with a wavelength of 1310 nm (F1229 THz) is combined with the output of a laser with an emission wavelength of 1310.1 nm (F2=228.99 THz), the photodetector will output a signal with a frequency of 229-228.99=0.01 THz=10 GHz. Since most photodetectors are not fast enough to respond beyond 100 GHz, the second signal with a frequency of 229+228.99=457.99 THz is simply filtered out or ignored by the detector.

(17) FMCW (or LFM) Laser Range Finding

(18) The phenomenon of photonic mixing can be used to produce a very fast and accurate range/distance measuring device. Single frequency laser diodes can be made to sweep their emission frequency by modulating their injection current and or varying their temperature with time. Typically, a linear saw-tooth modulation of the laser emission frequency is desired. By illuminating a distant object with part of the frequency modulated laser light, and recombining the light reflected with part of the original emission onto a photodetector, an interferometer is formed where the path length between the point of emission and the object being illuminated forms one branch, the other branch consisting of a fixed length waveguide within the system. The frequency of the signal (also known as a beat tone) generated from the optical mixing process on the photodetector is proportional to the target distance, enabling precise and high speed range measurements. Using a scanning mirror to project and collect the reflected laser light off several points on an object results in the creation of a depth map of the surface of the object from the scanning system.

(19) In digital logic design, it is possible to use a slower digital logic system to process the same amount or more data per time period as a high-speed system through the duplication of functional blocks that can run simultaneously within the IC. In this way, it is possible to use multiple lower-speed ADCs to sample the analog RF range information and process the slower data streams from these multiple ADCs simultaneously in digital logic, all at a slower clock rate. High-speed ADCs are expensive; ADCs that run at 50% of the speed of such a high-speed device can come at savings greater than 50%. Digital logic circuitry that can interface with multiple slower ADCs is less expensive than circuitry that can interface at higher speeds. This parallelization of using multiple slower ADCs to sample a signal, however, raises the number of components required in the electronic sampling subsystem.

(20) According to an exemplary embodiment of the invention, a beat frequency detection method enables a sampling system to evaluate the entire beat frequency bandwidth using a sampling rate that is lower than the Nyquist sampling rate required to directly and unambiguously sample this full bandwidth. In other words, the embodied invention enables one to instantaneously determine an unknown frequency, RF, that may occur anywhere within a large bandwidth, and changing on a sec time scale, using two known frequencies of local oscillators, LO1, LO2.

(21) It is well known that any discrete-time sampling system must operate at a sampling rate equal to at least twice the maximum frequency of the signal being sampled. In other words, a discrete-time sampling system will be incapable of unambiguously determining any input frequency that is greater than half the sampling rate of the system. For example, if a maximum sampling rate is 600 MHz, then the Nyquist frequency is 300 MHz; i.e., it is impossible to unambiguously determine the frequency of any signal higher than 300 MHz.

(22) A system that undertakes to sample an unknown signal with frequency wRF that occurs within a bandwidth B=max(RF) using the minimum sampling rate necessary would, according to the Nyquist theorem, be required to employ a sampling rate .sub.S2 max(.sub.RF). However, in laser ranging applications, it is frequently the case that the bandwidth B to be searched for .sub.RF would require .sub.S to be so large (>2 GS/s is common) as to be economically infeasible to implement. It would therefore appear desirable to limit .sub.S in some way. It is common practice to accomplish this through a heterodyne downconversion operation using a local oscillator with frequency .sub.LO such that .sub.S=2.sub.LO.

(23) It should be readily apparent that any signal with frequency .sub.RF>.sub.LO given .sub.RF<2.sub.LO can be downconverted to a signal with frequency .sub.RF.sub.LO through the use of heterodyne processing. Even if the frequency RF is not known prior to the heterodyne operation, it can be determined after the downconversion because .sub.LO is known. As known, the mixing process will also generate the sum frequency .sub.RF+.sub.LO, but since this result is always greater than .sub.RF, and thus of little value in a downconversion operation, it is usually eliminated with a low pass filter (LPF) prior to sampling. Further, given the requirements of the Nyquist theorem regarding bandlimiting the input signal, an LPF is a common component following a downconversion prior to a sampling stage. A sensible cutoff for an LPF for such a system would be .sub.LO since .sub.LO is chosen to maximize the bandwidth where the Nyquist rate is 2.sub.LO. However, if .sub.RF<.sub.LO, the downconverted result will be DC at maximum and leave only .sub.LO.sub.RF visible to the sampling system. Further, if the LPF of the downconversion block is set to pass frequencies at or below .sub.LO, it will be unclear whether the heterodyne result is the sum or difference since only one result will ever be visible to the sampling system (keeping in mind that RF is the unknown). Thus, it would appear that a system that attempts to sample an unknown signal RF by mixing it with a single local oscillator with frequency .sub.LO=.sub.S/2 will be unable to make an absolute determination of .sub.RF. For example, if .sub.LO=100 MHz and RF=50 MHz, then the resulting mixed products will be 150 MHz and 50 MHz. With an LPF at 100 MHz, only the 50 MHz signal will be visible to the sampling system. However there are two possible frequencies for .sub.RF that can produce a 50 MHz mixed product given a LPF=.sub.LO=100 MHz. If .sub.RF=50 MHz, .sub.RF+.sub.LO=150 MHz, which is filtered out, and .sub.LO.sub.RF=50 MHz, which is visible to the sampling system; but, if .sub.RF=150 MHz, then .sub.RF+.sub.LO=250 MHz, which is filtered out by the LPF=.sub.LO=100 MHz, and .sub.RF.sub.LO is also 50 MHz. Thus it is impossible for the sampling system to know whether the mixed product of .sub.RF and .sub.LO is only one frequency.

(24) The embodied invention makes it possible to resolve this ambiguity problem while still limiting .sub.S<2max(.sub.RF) by incorporating a second fixed local oscillator with frequency .sub.LO2=.sub.RLO+.sub., where .sub. is dictated by the SNR of the sampling system and the overall sampling rate, which is now .sub.S=2.sub.LO2 (.sub.=the lowest frequency desired to be sampled).

Example

(25) Let max(.sub.RF) be 1.0 GHz. Choose .sub.LO=max(.sub.RF)=500 MHz. Let .sub. be 100 MHz (i.e., the lowest sampling frequency of interest), so .sub.LO2 is 600 MHz. The system's sampling rate .sub.S=2.sub.LO2 is now 1.2 Giga-samples/sec (GS/s), thus only 1.2max(.sub.RF) and only 60% of the Nyquist rate required to sample max(.sub.RF) directly.

(26) FIG. 1 schematically shows a system 100 for carrying out the measurement. The system includes a two-way splitter 102 for splitting a signal of unknown frequency RF into two signals, .sub.F1, .sub.F2 of equal power, a first circuit 104-1 configured to generate an electrical signal of a known frequency .sub.LO1 that is at least half the known signal bandwidth, a second circuit 104-2 configured to generate an electrical signal of known frequency .sub.LO2 that is higher than .sub.LO1, a first mixer 106-1 adapted to mix one of the two split signals with unknown frequency .sub.RF with .sub.LO1, a second mixer 106-2 adapted to mix the other of the two split signals with unknown frequency .sub.RF with .sub.LO2, a first low pass filter 108-1 coupled to the output of the first mixer, and

(27) a second low pass filter 108-2 coupled to the output of the second mixer. The mixers may be MEMS chips, PLL-based digital chips, or other suitable components. The low pass filters filter the sum frequencies, .sub.LO+.sub.RF while passing the difference frequencies .sub.LO.sub.RF. The digital sampling system 110 need not be digital and may be any suitable frequency determination component (e.g., a phase lock loop).

(28) Per the model graph shown in FIG. 2, any frequency RF in a range of 0 Hz to 1.0 GHz can be unambiguously determined with a sample rate S=1.2 GS/s as follows:
.sub.LO=LPF([max(.sub.RF,.sub.LO])min(.sub.RF,.sub.LO])],.sub.LO)
.sub.LO2=LPF([max(.sub.RF,.sub.LO2])min(.sub.RF,.sub.LO2])],.sub.LO2).

(29) Determination of the frequency .sub.RF is now accomplished by evaluating the frequencies .sub.LO and .sub.LO2 at any given time and comparing their relative magnitudes. Circuitry used to determine .sub.LO and .sub.LO2, and hence determining .sub.RF may include analog to digital conversion circuits, a phase locked loop together with an analog to digital conversion circuit, or any other means familiar to one skilled in the art.

(30) The use of the terms a and an and the and similar references in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms comprising, having, including, and containing are to be construed as open-ended terms (i.e., meaning including, but not limited to,) unless otherwise noted. The term connected is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening.

(31) The recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.

(32) All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., such as) provided herein, is intended merely to better illuminate embodiments of the invention and does not impose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(33) While the invention is susceptible to various modifications, and alternative forms, specific examples thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the invention is not to be limited to the particular forms or methods disclosed, but to the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.

(34) It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.