Rangefinder

Abstract

The application provides a ranging device, which can be applied to the technical field of laser ranging. The ranging device comprises: a broadband light source configured to generate broadband light; a linear frequency modulation signal generating device configured to provide a reference signal by using a crystal oscillator, perform linear tracking on the reference signal in a phase-locked state by using a digital phase-locked loop, and output a linear frequency modulation signal with a frequency changing linearly with time; an electro-optical modulator configured to modulate the broadband light based on the linear frequency modulation signal to form modulated light; a ranging unit configured to form reference light and signal light based on the modulated light; send the signal light to a target to be measured, and receive measurement light formed by reflection of the signal light by the target to be measured; detect an interference signal of the reference light and the measurement light to obtain an amplitude spectrum of the interference signal; and determine the distance of the target to be measured based on the amplitude spectrum.

Description

Technical Field

[0001] At least one embodiment of this application relates to the field of laser ranging technology, and more specifically to a ranging device. Background Technology

[0002] Distance measurement is a core foundation of geometric metrology in fields such as aerospace, autonomous driving, and industrial robotics. Currently, industrial manufacturing is developing towards integration, miniaturization, and ultra-precision, leading to a continuously increasing demand for distance measurement technologies that combine large range and high stability.

[0003] Optical microwave scanning interferometry uses microwaves to modulate light waves and generates microwave scanning interference signals based on optical path difference to complete distance calculation. It can effectively avoid polarization fading and multimode interference defects that exist in pure optical ranging. However, existing optical microwave scanning interferometry systems are limited by hardware conditions and are difficult to apply in engineering. Summary of the Invention

[0004] In view of the above problems, this application provides a ranging device.

[0005] The aforementioned ranging device includes: a broadband light source configured to generate broadband light; a linear frequency modulated signal generating device configured to provide a reference signal using a crystal oscillator, linearly track the reference signal in a phase-locked state using a digital phase-locked loop, and output a linear frequency modulated signal whose frequency changes linearly with time; an electro-optic modulator configured to modulate the broadband light based on the linear frequency modulated signal to form modulated light; a ranging unit configured to form a reference light and a signal light based on the modulated light; transmit the signal light to the target to be measured, and receive the measurement light formed by the reflection of the signal light by the target to be measured; detect the interference signal of the reference light and the measurement light to obtain the amplitude spectrum of the interference signal; and determine the distance to the target to be measured based on the amplitude spectrum.

[0006] According to embodiments of this application, a linear frequency modulated signal generation device based on a crystal oscillator and a digital phase-locked loop is used to microwave modulate broadband light to form modulated light. A ranging unit then uses this modulated light to generate a reference light and a signal light. After propagating through a spatial optical path, the signal light is reflected back to the measurement light carrying the distance information of the target. The introduced time delay is used to detect the interference signal between the reference light and the measurement light, thereby obtaining the amplitude spectrum of the interference signal and determining the distance to the target. The ranging device of this application utilizes a linear frequency modulated signal generation device based on a crystal oscillator and a digital phase-locked loop to microwave modulate broadband light, which avoids dependence on a vector network analyzer, simplifies the system structure, and improves the integration and practicality of the optical microwave scanning interferometry system. Attached Figure Description

[0007] The above-mentioned contents, other objects, features and advantages of this application will become clearer from the following description of embodiments of this application with reference to the accompanying drawings.

[0008] Figure 1 A schematic diagram of a ranging device according to an embodiment of this application is shown.

[0009] Figure 2 A schematic diagram of a linear frequency modulation signal generating apparatus according to an embodiment of this application is shown.

[0010] Figure 3 A schematic diagram of a loop filter according to an embodiment of this application is shown.

[0011] Figure 4 A schematic diagram of a ranging device according to an embodiment of this application is shown.

[0012] Figure 5 A schematic diagram of a ranging device according to an embodiment of this application is shown. Detailed Implementation

[0013] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0014] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0015] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0016] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0017] Existing optical-borne microwave scanning interferometry systems suffer from hardware limitations, restricting their commercialization from laboratory prototypes. First, they are highly dependent on vector network analyzers, which serve as both signal generators and demodulation devices, but are bulky, expensive, and complex. Second, system integration and portability are limited. Constrained by the stringent requirements of vector network analyzers, miniaturization and high integration are difficult to achieve, resulting in high overall operating costs and making them unsuitable for field testing and portable detection terminals. Demodulation alternatives based on orthogonal demodulation or RMS detection typically still require vector network analyzers as signal generators, failing to fundamentally address the integration and miniaturization bottlenecks of optical-borne microwave scanning interferometry systems.

[0018] Therefore, there is an urgent need for an optical microwave scanning interferometric ranging scheme to solve the technical problems of existing systems, such as strong hardware dependence, low integration, and insufficient practicality.

[0019] This application proposes a ranging device that uses a linear frequency modulated signal generator to achieve optical microwave ranging, which can overcome the dependence on vector network analyzers and improve the integration level of the optical microwave scanning interferometric ranging system.

[0020] Figure 1 A schematic diagram of a ranging device according to an embodiment of this application is shown.

[0021] like Figure 1 As shown, the ranging device in this embodiment includes a broadband light source 1, a linear frequency modulation signal generating device 2, an electro-optic modulator 3, and a ranging unit 4.

[0022] A broadband light source 1 is configured to generate broadband light. A linear frequency modulated (LFM) signal generation device 2 is configured to provide a reference signal using a crystal oscillator, linearly track the reference signal in a phase-locked state using a digital phase-locked loop, and output a LFM signal whose frequency changes linearly with time. An electro-optic modulator 3 is configured to modulate the broadband light based on the LFM signal to form modulated light. A ranging unit 4 is configured to form a reference light and a signal light based on the modulated light; send the signal light to the target and receive the measurement light formed by the reflection of the signal light from the target; detect the interference signal of the reference light and the measurement light to obtain the amplitude spectrum of the interference signal; and determine the distance to the target based on the amplitude spectrum.

[0023] According to embodiments of this application, the crystal oscillator can be a crystal oscillator integrated on a circuit board. A high-precision onboard crystal oscillator can output a fixed standard frequency sine / square wave signal as a reference signal, providing a frequency reference for the digital phase-locked loop. In one example, the crystal oscillator can be an onboard temperature-compensated crystal oscillator.

[0024] According to embodiments of this application, a digital phase-locked loop can be programmed and configured through a preset program to achieve automatic stepping of linear frequency sweep, which is used to control the frequency division ratio of the feedback signal relative to the linear frequency modulation signal; the frequency division ratio is continuously and dynamically updated through preset programming logic, so that the digital phase-locked loop is always in a phase-locked closed-loop tracking state.

[0025] According to embodiments of this application, a linear frequency modulated signal generation device based on a crystal oscillator and a digital phase-locked loop is used to microwave modulate broadband light to form modulated light. A ranging unit then uses this modulated light to generate a reference light and a signal light. After propagating through a spatial optical path, the signal light is reflected back to the measurement light carrying the distance information of the target. The introduced time delay is used to detect the interference signal between the reference light and the measurement light, thereby obtaining the amplitude spectrum of the interference signal and determining the distance to the target. The ranging device of this application utilizes a linear frequency modulated signal generation device based on a crystal oscillator and a digital phase-locked loop to microwave modulate broadband light, which avoids dependence on a vector network analyzer, simplifies the system structure, and improves the integration and practicality of the optical microwave scanning interferometry system.

[0026] Figure 2 A schematic diagram of a linear frequency modulation signal generating apparatus according to an embodiment of this application is shown.

[0027] like Figure 2 As shown, the linear frequency modulation signal generating device of this application embodiment includes a crystal oscillator 21 and a digital phase-locked loop 22. The digital phase-locked loop 22 includes a frequency difference detection unit 221, a loop filter 222, a voltage-controlled oscillator 223, and a digital frequency divider 224.

[0028] According to an embodiment of this application, the frequency difference detection unit 221 is configured to receive a reference signal and a feedback signal, and output a current corresponding to the frequency difference between the reference signal and the feedback signal. The loop filter 222 is configured to filter the current and output a tuning voltage. The voltage-controlled oscillator 223 is configured to output a linear frequency modulated signal corresponding to the frequency of the tuning voltage. The digital frequency divider 224 is configured to perform dynamic frequency division processing on the linear frequency modulated signal to form a feedback signal.

[0029] According to an embodiment of this application, a frequency difference detection unit compares the frequency of the feedback signal and the reference signal. Based on the frequency difference between the reference signal and the feedback signal, a current corresponding to the magnitude of the frequency difference is output. This current is fed into a loop filter for filtering, resulting in a stable tuning voltage. This voltage is used to adjust the oscillation frequency of the voltage-controlled oscillator (VCO), enabling the VCO to output a linear frequency modulated (LFM) signal. The digital phase-locked loop structure of this embodiment avoids the transient response generated during the phase-locking process of traditional PLLs, eliminates the millisecond-level delay during frequency switching, and rapidly generates high-speed, multi-frequency LFM signals.

[0030] like Figure 2 As shown, according to an embodiment of this application, the digital frequency divider 224 includes a frequency divider 2241 and a controller 2242. The controller 2242 includes an integer register N and a fractional interpolator D.

[0031] Frequency divider 2241 is configured to receive the division ratio and perform dynamic frequency division processing on the linear frequency modulation signal according to the division ratio to form a feedback signal. Controller 2242 is configured to generate an integer division component and a fractional correction component of the division ratio according to the target frequency modulation parameters, and input the integer division component to the integer register N and the fractional correction component to the fractional interpolator D respectively. The sum of the integer division component and the fractional correction component is applied to frequency divider 2241 to achieve linear interval frequency scanning.

[0032] According to embodiments of this application, the digital frequency divider can be a fractional frequency divider, generating a division ratio with a decimal value, and capable of outputting dense, continuous frequency points. The digital frequency divider, through an integer register, a fractional interpolator, and real-time modification via software, can automatically step and continuously switch the division ratio, achieving automatic linear frequency sweep.

[0033] In one example, a digital frequency divider divides a linear frequency modulated (LFM) signal. The LFM signal frequency could be, for example, 2 GHz, with a target frequency of 4 GHz. The sweep starts at 2 GHz and ends at 4 GHz, with a total number of steps set to millions, for example, 1.08 million. The controller iteratively calculates the target frequency and the LFM signal frequency for each step in real time. The frequency of the LFM signal is denoted as B, and the current target output frequency in each step is denoted as A. A fixed frequency step size is calculated based on the total sweep bandwidth and the total number of steps. The step count starts from 0 and increments sequentially. Each time the count is updated, the current target output frequency A is calculated in real time. The real-time division ratio A / B is obtained by dividing the LFM signal frequency B by the current target output frequency A. This division ratio is then fed into the frequency divider to complete the frequency conversion output. Throughout the entire stepping process, the output frequency remains linearly variable, and the division ratio is updated synchronously with the target frequency at each step. After traversing all 1.08 million frequency points, a complete linear sweep is completed.

[0034] According to embodiments of this application, a digital frequency divider is used to continuously update the division ratio according to a preset program. This ensures that when the linear frequency modulation signal generating device drives the voltage-controlled oscillator to sweep the frequency along a preset linear slope, the digital phase-locked loop (PLL) maintains a continuously locked state. Compared to the traditional discrete point-by-point scanning method, which requires repeated phase-locking at each frequency point, the digital PLL of this application maintains closed-loop frequency tracking throughout the entire process, thus avoiding transient response problems such as frequency overshoot and damped ringing in principle. The driving mechanism of the frequency divider in this application limits stability constraints to the sweep frequency boundary range, transforming the millisecond-level phase-locking time of a single frequency point in point-by-point scanning into a picosecond-level division ratio update operation, thereby simultaneously achieving excellent sweep speed and extremely high sweep linearity.

[0035] like Figure 2 As shown, according to an embodiment of this application, the frequency difference detection unit 221 includes a frequency and phase detector 2211 and a charge pump 2212.

[0036] The frequency and phase detector 2211 is configured to receive a reference signal and a feedback signal, and output a pulse signal corresponding to the frequency difference between the reference signal frequency and the feedback signal. The charge pump 2212 is configured to output current based on the pulse signal.

[0037] Figure 3 A schematic diagram of a loop filter according to an embodiment of this application is shown.

[0038] In one example, the loop filter could be a three-stage cascaded active low-pass loop filter.

[0039] like Figure 3 As shown, according to an embodiment of this application, the loop filter includes an input filter circuit 2221, an operational amplifier circuit 2222, and an output filter circuit 2223.

[0040] Input filter circuit 2221 is configured to pre-smooth the current. Operational amplifier circuit 2222 is configured to amplify the current. Output filter circuit 2223 is configured to perform secondary smoothing filtering on the amplified current.

[0041] According to an embodiment of this application, the operational amplifier circuit 2222 includes an operational amplifier U1 and a feedback network RCW.

[0042] The non-inverting input of operational amplifier U1 is connected to the reference voltage Vr, and the inverting input is connected to the input filter circuit to receive the pre-smoothed filtered current. The output of operational amplifier U1 is connected to the output filter circuit to output the amplified current. The first terminal of the feedback network RCW is connected to the inverting input of operational amplifier U1, and the second terminal of the feedback network RCW is connected to the output of operational amplifier U1.

[0043] According to an embodiment of this application, the input filter circuit 2221 includes a first resistor R1 connected between a first terminal of the input filter circuit 2221 and a second terminal of the input filter circuit 2221; and a first capacitor C1 connected between the first terminal of the input filter circuit 2221 and ground GND.

[0044] According to an embodiment of this application, the feedback network RCW includes a second resistor R2 and a second capacitor C2 connected in series between a first end and a second end of the feedback network RCW; and a third capacitor C3 connected between the first end and the second end of the feedback network RCW.

[0045] According to an embodiment of this application, the output filter circuit 2223 includes a third resistor R3 connected between the first terminal and the second terminal of the output filter circuit 2223; and a fourth capacitor C4 and a fifth capacitor C5 connected in parallel between the second terminal of the output filter circuit 2223 and ground GND. The parallel fourth capacitor C4 and fifth capacitor C5 can be combined and equivalent in AC frequency domain theoretical calculations, forming the last stage of a three-stage cascaded active low-pass loop filter together with the third resistor R3. Furthermore, the output of the output filter circuit 2223 serves as the input to a voltage-controlled oscillator (VCO), and the fifth capacitor C5 can also be used for pre-filtering of the VCO.

[0046] Figure 4 A schematic diagram of a ranging device according to an embodiment of this application is shown.

[0047] According to embodiments of this application, such as Figure 4 As shown, the ranging unit 4 includes a reference path 41, a measurement path 42, a photodetector 43, and an amplitude demodulator 44.

[0048] Reference path 41 is configured to form a reference light based on a portion of the modulated light. Measurement path 42 is configured to form a signal light based on another portion of the modulated light; the signal light is sent to the target 50 under test, and the measurement light formed by the reflection of the signal light by the target 50 under test is received. Photodetector 43 is configured to detect the interference signal between the reference light and the measurement light and output a photocurrent. Amplitude demodulator 44 is configured to demodulate the amplitude of the photocurrent and output an amplitude spectrum to calculate the distance of the target 50 under test (relative to the ranging device) based on the amplitude spectrum.

[0049] Figure 5 A schematic diagram of a ranging device according to an embodiment of this application is shown.

[0050] In one specific embodiment, such as Figure 5As shown, the ranging unit 4 also includes a beam splitter 45, a coupler 46, a data processing module 47, a circulator 48, and a coaxial transceiver optical device 49. A slidingly adjustable reflector 51 is set on the experimental displacement stage 6 as the target to be measured.

[0051] In one example, photodetector 43 can be an APD (Avalanche Photodiode) type photodetector. Amplitude demodulator 44 can be an envelope detection circuit. Data processing module 47 can include an analog-to-digital converter chip and a logic computing unit with digital signal processing capabilities. Reflector 51 can be a hollow retroreflector.

[0052] The linear frequency modulation signal generating device 2 generates a linear frequency modulation signal.

[0053] The broadband light source 1 can output broadband light with a center wavelength of 1550nm. The electro-optic modulator 3 can be a Mach-Zehnder modulator, which can modulate the intensity of the linear frequency modulated signal onto the broadband light wave signal to form modulated light.

[0054] The modulated light is split into reference light by beam splitter 45.

[0055] The modulated light is split into signal light by beam splitter 45. After propagating through the spatial optical path, the signal light is reflected back to the measurement light carrying the distance information of the target. Due to the introduced time delay, the measurement light and the reference light are combined at coupler 46 to form an interference signal. In one example, beam splitter 45 is an optical fiber beam splitter, coupler 46 is an optical fiber coupler, circulator 48 is an optical fiber circulator, and coaxial transceiver optics 49 is a reflective collimator.

[0056] The ranging principle of the ranging device in this application embodiment is as follows.

[0057] An electro-optic modulator driven by a linear frequency modulation signal generator outputs intensity-modulated light, and the electric field intensity of the modulated light is... It can be represented as:

[0058] (1);

[0059] in, The amplitude of broadband light. For modulation depth, Indicates time, For transmission distance, The signal travels a longer distance than the speed of light in a vacuum. It will produce Lead time; The angular frequency of the linear frequency modulated signal. , It is the frequency of the linear frequency modulated signal. The initial phase of the linear frequency modulated signal. The electric field signal of broadband light, intensity and wavelength Distribution-related. Broadband light has multiple frequency components, and the interference components will cancel each other out.

[0060] Within the ranging unit, the reference light and the measuring light interfere. After the photodetector performs square-law detection on the interference signal, it outputs a photocurrent. Photocurrent It can be represented as:

[0061] (2);

[0062] in, For the responsivity of the photodetector, To measure the amplitude of light, Measuring the optical path length of light, For the amplitude of the reference light, The optical path length of the reference light; For optical interference terms, since the measurement distance is much greater than the coherence length of the optical interference, it is treated as a DC signal for broadband light sources. The amplitude spectrum after coherent superposition... It can be represented as:

[0063] (3);

[0064] Amplitude spectrum after coherent superposition Having an angular frequency that varies with the linear frequency modulated signal The linearly changing but periodically changing functional relationship, and the relationship between the period of angular frequency FSR and optical path difference OPD, can be expressed as:

[0065] (4);

[0066] Where FSR is the free spectral range, representing the amplitude envelope period in the frequency domain, and OPD is the optical path difference between the measurement light and the reference light, i.e., the distance to be measured. Therefore, by analyzing the amplitude spectrum after coherent superposition... The distance can be calculated by measuring the FSR.

[0067] The ranging device in this application utilizes a linear frequency modulation signal generation device based on a digital phase-locked loop, avoiding dependence on a vector network analyzer and simplifying the system structure. The linear frequency modulation signal generation device, composed of chip-level devices and discrete component loop filters, reduces hardware costs and system size, providing favorable conditions for the integrated development of portable and handheld ranging devices.

[0068] The linear frequency modulation signal generating device of the ranging device in this application embodiment is easier to achieve high reliability packaging and structural reinforcement compared to traditional large benchtop instruments. It has good environmental adaptability and can meet the application requirements of complex industrial sites.

[0069] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application.

[0070] The embodiments of this application have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of this application. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Without departing from the scope of this application, those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of this application.

Claims

1. A ranging device, characterized in that, The ranging device includes: A broadband light source is configured to produce broadband light; A linear frequency modulation signal generating device is configured to provide a reference signal using a crystal oscillator, linearly track the reference signal in a phase-locked state using a digital phase-locked loop, and output a linear frequency modulation signal whose frequency changes linearly with time. An electro-optic modulator is configured to modulate the broadband light based on the linear frequency modulation signal to form modulated light; The ranging unit is configured to form a reference light and a signal light based on the modulated light; send the signal light to the target to be measured and receive the measurement light formed by the reflection of the signal light by the target to be measured; detect the interference signal of the reference light and the measurement light to obtain the amplitude spectrum of the interference signal; and determine the distance to the target to be measured based on the amplitude spectrum.

2. The ranging device according to claim 1, characterized in that, The digital phase-locked loop includes: The frequency difference detection unit is configured to receive the reference signal and the feedback signal, and output a current corresponding to the frequency difference based on the frequency difference between the reference signal and the feedback signal. A loop filter is configured to filter the current and output a tuned voltage; A voltage-controlled oscillator is configured to adjust the oscillation frequency of the voltage-controlled oscillator according to the tuning voltage, and output the linear frequency modulation signal; A digital frequency divider is configured to dynamically divide the linear frequency modulated signal to form the feedback signal.

3. The ranging device according to claim 2, characterized in that, The digital frequency divider includes: The frequency divider is configured to receive the division ratio and perform dynamic frequency division processing on the linear frequency modulated signal according to the division ratio to form the feedback signal; The controller includes an integer register and a fractional interpolator. The controller is configured to generate an integer frequency division component and a fractional correction component of the frequency division ratio based on the target frequency modulation parameters, and input the integer frequency division component to the integer register, input the fractional correction component to the fractional interpolator, and apply the sum of the integer frequency division component and the fractional correction component to the frequency divider to achieve linear interval frequency scanning.

4. The ranging device according to claim 2, characterized in that, The frequency difference detection unit includes: A frequency and phase detector is configured to receive the reference signal and the feedback signal, and output a pulse signal corresponding to the frequency difference between the reference signal and the feedback signal. A charge pump is configured to output the current according to the impulse signal.

5. The ranging device according to claim 2, characterized in that, The loop filter includes: An input filter circuit is configured to pre-smooth the current. An operational amplifier circuit is configured to amplify the current; The output filter circuit is configured to perform secondary smoothing filtering on the amplified current.

6. The ranging device according to claim 5, characterized in that, The operational amplifier circuit includes: An operational amplifier, wherein the non-inverting input terminal of the operational amplifier is connected to a reference voltage, the inverting input terminal of the operational amplifier is connected to the input filter circuit to receive the pre-smoothed filtered current, and the output terminal of the operational amplifier is connected to the output filter circuit to output the amplified current; A feedback network is provided, with its first end connected to the inverting input of the operational amplifier and its second end connected to the output of the operational amplifier.

7. The ranging device according to claim 6, characterized in that, The input filtering circuit includes: A first resistor connected between the first terminal of the input filter circuit and the second terminal of the input filter circuit; A first capacitor connected between the first terminal of the input filter circuit and ground.

8. The ranging device according to claim 7, characterized in that, The feedback network includes: A second resistor and a second capacitor connected in series between the first end and the second end of the feedback network; A third capacitor is connected between the first end and the second end of the feedback network.

9. The ranging device according to claim 7, characterized in that, The output filtering circuit includes: A third resistor connected between the first terminal and the second terminal of the output filter circuit. A fourth capacitor and a fifth capacitor are connected in parallel between the second terminal of the output filter circuit and ground.

10. The ranging device according to claim 1, characterized in that, The ranging unit includes: A reference path is configured to form a reference light based on a portion of the modulated light; The measurement path is configured to form a signal light based on another portion of the modulated light; the signal light is sent to the target under test, and the measurement light formed by the reflection of the signal light by the target under test is received; A photodetector is configured to detect the interference signal between the reference light and the measurement light and output a photocurrent; An amplitude demodulator is configured to demodulate the photocurrent and output an amplitude spectrum to calculate the distance to the target under test based on the amplitude spectrum.

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