Speed and distance measuring system and method

Abstract

The application relates to a velocity and distance measuring system and method. The system comprises a laser; a signal source electrically connected with a Mach-Zehnder modulator, a velocity processing device and a distance processing device, and used for outputting a modulated electric signal; a Mach-Zehnder modulator used for modulating an outgoing light beam based on the modulated electric signal to obtain a modulated light beam; an optical channel device used for making the modulated light beam incident on a target to be measured and receiving a return light beam; a photoelectric component used for generating a first electric signal and a second electric signal based on the outgoing light beam, the modulated light beam and the return light beam; a velocity processing device used for acquiring the velocity of the target to be measured through the modulated electric signal and the first electric signal; and a distance processing device used for acquiring the distance of the target to be measured through the modulated electric signal and the second electric signal. The system calculates the velocity and the distance through the velocity processing device and the distance processing device respectively, discards the traditional distance measuring mode, reduces the single-point measuring time of the FMCW laser radar, improves the point frequency and enriches the scanning mode.

Description

Technical Field

[0001] This application relates to the field of laser technology, and in particular to a speed and distance measurement system and method. Background Technology

[0002] LiDAR has been widely used in the field of autonomous driving. Traditional ToF (Time of Flight) LiDAR can only measure the distance to the target and cannot directly obtain the target's speed. Therefore, FMCW (Frequency Modulated Continuous Wave) LiDAR has a greater advantage. It can not only measure the distance to the target, but also obtain the speed of the moving target from the physical point, thereby reducing the dependence on deep learning and reducing the computing power requirements of the computing platform.

[0003] Traditional methods often employ external or internal modulation schemes. However, both of these schemes suffer from long single-point detection times and difficulty in increasing the number of points generated in the point cloud. Summary of the Invention

[0004] Therefore, it is necessary to provide a speed and distance measurement system and method to address the aforementioned technical problems.

[0005] In a first aspect, this application provides a speed and distance measuring system, the system comprising:

[0006] A laser is used to emit an outgoing beam of light;

[0007] The signal source is electrically connected to the Mach-Zehnder modulator, speed processing device, and distance processing device, and is used to output modulated electrical signals.

[0008] The Mach-Zehnder modulator modulates the output beam based on a modulation electrical signal, and outputs a modulated beam.

[0009] An optical path device is used to direct a modulated beam onto the target under test and to receive the echo beam; the echo beam returns from the target under test.

[0010] An optoelectronic component for generating a first electrical signal and a second electrical signal based on an outgoing beam, a modulated beam, and an echo beam;

[0011] A speed processing device is used to acquire the speed of the target under test by modulating an electrical signal and a first electrical signal.

[0012] A distance processing device is used to obtain the distance to the target being measured by modulating an electrical signal and a second electrical signal.

[0013] In one embodiment, the modulated electrical signal output by the signal source is a sawtooth wave, and the modulated beam output by the Mach-Zehnder modulator is a carrier-suppressed double-sideband optical signal.

[0014] In one embodiment, the optoelectronic component includes:

[0015] The first coupler is used to mix the echo beam with the outgoing beam to obtain the first coherent beat frequency signal;

[0016] The first detector is used to convert the first coherent beat frequency signal into a first electrical signal;

[0017] The second coupler is used to mix the echo beam with the modulated beam to obtain the second coherent beat frequency signal;

[0018] The second detector is used to convert the second coherent beat frequency signal into a second electrical signal.

[0019] In one embodiment, the speed processing device includes a first mixer and a low-pass filter.

[0020] In one embodiment, the distance processing device includes a second mixer and a bandpass filter.

[0021] Secondly, this application also provides a method for measuring speed and distance. The method includes:

[0022] The laser emits an output beam, and the signal source outputs a sawtooth wave.

[0023] The outgoing beam is modulated by a Mach-Zehnder modulator loaded with a sawtooth wave, thereby generating a carrier-suppressed double-sideband optical signal; the bias voltage of the Mach-Zehnder modulator is set at the minimum transmission point;

[0024] A carrier-suppressed double-sideband optical signal is incident on the target under test to obtain an echo beam.

[0025] The first and second electrical signals are obtained based on the echo beam, the outgoing beam, and the carrier-suppressed double-sideband optical signal.

[0026] The velocity of the target is obtained based on the first electrical signal and the sawtooth wave, and the distance of the target is obtained based on the second electrical signal and the sawtooth wave.

[0027] In one embodiment, the carrier-suppressed double-sideband optical signal is incident on the target under test to obtain an echo beam, including:

[0028] The carrier-suppressed double-sideband optical signal is transmitted to the target under test after passing through a circulator and a two-dimensional scanning device. The beam reflected by the target under test passes through the two-dimensional scanning device and the circulator again to obtain the echo beam.

[0029] In one embodiment, obtaining a first electrical signal and a second electrical signal based on the echo beam, the emitted beam, and the carrier-suppressed double-sideband optical signal includes:

[0030] The echo beam is coupled with the outgoing beam to obtain the first coherent beat frequency signal;

[0031] The first coherent beat frequency signal is converted into a first electrical signal by the first detector;

[0032] The echo beam is coupled with the carrier-suppressed double-sideband optical signal to obtain the second coherent beat frequency signal;

[0033] The second coherent beat frequency signal is converted into a second electrical signal by the second detector.

[0034] In one embodiment, obtaining the velocity of the target based on the first electrical signal and the sawtooth wave includes:

[0035] The first electrical signal and the sawtooth wave are mixed to obtain the first difference frequency signal;

[0036] The first difference frequency signal is low-pass filtered to obtain the self-mixing difference frequency signal;

[0037] The self-mixed difference frequency signal is acquired by a low-speed analog-to-digital converter and then subjected to a fast Fourier transform in a field-programmable gate array to obtain the velocity of the target under test.

[0038] In one embodiment, obtaining the distance to the target based on the second electrical signal and the sawtooth wave includes:

[0039] The second electrical signal is mixed with the sawtooth wave to obtain the second difference frequency signal;

[0040] The distance to the target is obtained by bandpass filtering the second difference frequency signal.

[0041] The aforementioned velocity and distance measurement system includes a laser for emitting an outgoing beam; a signal source electrically connected to a Mach-Zehnder modulator, a velocity processing device, and a distance processing device for outputting a sawtooth wave; a Mach-Zehnder modulator that modulates the outgoing beam based on the sawtooth wave to obtain a carrier-suppressed double-sideband optical signal; an optical path device for incidenting the carrier-suppressed double-sideband optical signal onto the target and receiving the echo beam, wherein the echo beam returns from the target; an optoelectronic component for generating a first electrical signal and a second electrical signal based on the outgoing beam, the carrier-suppressed double-sideband optical signal, and the echo beam; a velocity processing device for obtaining the velocity of the target through the sawtooth wave and the first electrical signal; and a distance processing device for obtaining the distance of the target through the sawtooth wave and the second electrical signal. This velocity and distance measurement system uses sawtooth-wave modulated light as the outgoing beam and employs velocity and distance processing devices to calculate velocity and distance respectively, reducing the single-point measurement time of the FMCW lidar, increasing the point frequency, and enriching the scanning methods, thereby improving the system's measurement accuracy, detection speed, and dynamic response capability. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of a ranging and speed measuring system in one embodiment;

[0043] Figure 2 This is a time-frequency relationship diagram of a sawtooth wave LFM signal in one embodiment;

[0044] Figure 3 This is a schematic diagram of a Mach-Zehnder modulator generating a carrier-suppressed double-sideband signal based on the outgoing beam and the modulated electrical signal in one embodiment.

[0045] Figure 4 This is a time-frequency relationship diagram of a Mach-Zehnder modulated carrier-suppressed double-sideband modulated (LFM) optical signal in one embodiment;

[0046] Figure 5 Here is a frequency spectrum of a carrier-suppressed double-sideband modulated optical signal using a Mach-Zehnder modulator in one embodiment;

[0047] Figure 6 This is a detailed schematic diagram of a ranging and speed measuring system in one embodiment;

[0048] Figure 7 This is a schematic diagram of a dual-sideband FMCW lidar for ranging and velocity measurement of stationary targets in one embodiment.

[0049] Figure 8 This is a schematic diagram of a dual-sideband FMCW lidar for ranging and velocity measurement of moving targets in one embodiment.

[0050] Figure 9 This is a schematic diagram of the speed and distance measurement method in one embodiment;

[0051] Figure 10 This is a schematic diagram of the speed and distance measurement method in another embodiment. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0053] Currently, the basic working principle of FMCW lidar is to emit LFM (Linear Frequency Modulation) lasers and detect the frequency values ​​of the beat frequency signals of the local oscillator light and the reflected light to calculate the distance and speed of the target.

[0054] There are currently two commonly used methods for speed and distance measurement with FMCW radar:

[0055] The first method uses internal modulation, producing a triangular LFM waveform. Signal processing employs ADC (Analog-to-Digital Converter) acquisition and FFT (Fast Fourier Transform) calculation. In this method, the internal modulation process occurs simultaneously with the laser oscillation establishment, making it impossible to simultaneously achieve both narrow linewidth and high tuning rate. With a laser linewidth less than 500kHz, achieving a 5GHz modulation bandwidth requires at least several hundred microseconds. Adding the time for ADC acquisition and FFT calculation, the single-point detection time is long, resulting in a low laser spot frequency.

[0056] If an internal modulation scheme is used, the LFM waveform must be a triangular wave. To increase the spot frequency, the modulation bandwidth must be reduced to shorten the modulation time, but this sacrifices range resolution. Furthermore, internal modulation involves linear wavelength changes, resulting in nonlinear frequency changes (i.e., frequency modulation nonlinearity), which reduces measurement accuracy and resolution, requiring more complex correction algorithms, further increasing the single-point detection time. The internal modulation scheme cannot achieve both reduced single-point detection time to improve angular resolution and guaranteed range resolution.

[0057] The second method uses external modulation, with the LFM waveform being a triangular wave. The signal processing unit employs ADC acquisition and FFT calculation. External modulation does not disrupt the laser oscillation setup process and avoids the inherent contradiction between narrow linewidth and high tuning rate. Carrier-suppressed single-sideband modulation can be achieved using an electro / silicon-optical modulator.

[0058] The beat frequency of the local oscillator and reflected light signals is in the range of 10MHz-1GHz. High-speed ADC acquisition and FFT calculation require tens of microseconds, resulting in a persistently high single-point detection time. Increasing the point frequency could be achieved by adding laser emission channels, similar to internal modulation; however, considering the complexity and maturity of the technology, there is currently no such solution in the industry.

[0059] To address the issue of long single-point measurement time, the only solution is to physically arrange multiple laser emission channels in the vertical direction and use a one-dimensional scanning method with a polyhedral rotating mirror. When the single-point measurement time is less than 10 microseconds, a two-dimensional scanning method such as a MEMS (MicroElectromechanical System) galvanometer can be used. However, physically arranging multiple laser emission channels in the vertical direction will greatly increase the number of points generated in the point cloud.

[0060] Although the LFM waveform, which is a triangular wave with single-sideband modulation, can achieve simultaneous distance and velocity measurement, the two difference frequencies belong to different time sweep bands, which increases the difficulty of obtaining the real-time distance and velocity of rapidly accelerating targets.

[0061] Therefore, the speed and distance measurement system provided in this application embodiment can be applied to automotive radar systems. This embodiment illustrates the application of this method to a terminal; it is understood that this method can also be applied to a server, and further to a system including both a terminal and a server, and implemented through interaction between the terminal and the server.

[0062] In one embodiment, such as Figure 1 As shown, a speed and distance measurement system is provided, including:

[0063] Laser 102 is used to emit an outgoing beam; signal source 104 is electrically connected to Mach-Zehnder modulator 106, velocity processing device 112, and distance processing device 114, and is used to output a modulated electrical signal; Mach-Zehnder modulator 106 modulates the outgoing beam based on the modulated electrical signal and outputs a modulated beam; optical path device 108 is used to direct the modulated beam onto the target under test and receive the echo beam; the echo beam returns from the target under test; photoelectric component 110 is used to generate a first electrical signal and a second electrical signal based on the outgoing beam, the modulated beam, and the echo beam; velocity processing device 112 is used to obtain the velocity of the target under test through the modulated electrical signal and the first electrical signal; distance processing device 114 is used to obtain the distance of the target under test through the modulated electrical signal and the second electrical signal.

[0064] The signal source 104 can be a radio frequency (RF) signal source, which is a signal generator capable of generating radio frequency signals (RF signals). The signal source 104 emits a modulated electrical signal, which can optionally be a voltage signal.

[0065] A Mach-Zehnder modulator (MZM) is a device that uses the principle of optical interference to modulate optical signals. It typically consists of two branched optical fibers or waveguides, one serving as a reference path and the other transmitting the signal light to be modulated. By controlling the phase difference between the reference path and the signal light path, amplitude modulation of the optical signal can be achieved.

[0066] For example, the signal source 104 is electrically connected to the Mach-Zehnder modulator 106. After the signal source 104 outputs a modulated electrical signal, this modulated electrical signal becomes the electrode drive signal in the Mach-Zehnder modulator 106. Based on the aforementioned optical signal modulation principle of the Mach-Zehnder modulator 106, after the outgoing light beam emitted by the laser 102 is incident on the Mach-Zehnder modulator 106, the Mach-Zehnder modulator 106 converts the radio frequency signal of the modulated electrical signal into a modulation signal for the light intensity of the incident light beam, so that the phase and amplitude of the transmitted light wave change with the time evolution of the radio frequency signal, thereby forming a light wave with a changing frequency.

[0067] The target to be measured is typically an object such as a vehicle, pedestrian, obstacle, or building. The echo beam refers to the beam that is reflected back to the speed and distance measuring system after the emitted beam is incident on the target. This echo beam carries information such as the target's position, distance, speed, and reflectivity.

[0068] Specifically, refer to Figure 1 Laser 102 emits an outgoing beam, signal source 104 outputs a modulated electrical signal, and Mach-Zehnder modulator 106 modulates the outgoing beam based on the modulated electrical signal to obtain a modulated beam. The modulated beam passes through optical path device 108 and is incident on the target under test, where an echo beam is received. Optoelectronic component 110 generates a first electrical signal and a second electrical signal based on the outgoing beam, modulated beam, and echo beam. Velocity processing device 112 quickly obtains the velocity of the target under test through the modulated electrical signal and the first electrical signal, and distance processing device 114 quickly obtains the distance of the target under test through the modulated electrical signal and the second electrical signal.

[0069] The aforementioned speed and distance measurement system, based on the modulation of the emitted beam from the laser by a Mach-Zehnder modulator, can utilize the modulation characteristics of the Mach-Zehnder modulator to simultaneously measure the speed and distance of the target. The speed processing device acquires the speed of the target by modulating an electrical signal and a first electrical signal, while the distance processing device acquires the distance of the target by modulating an electrical signal and a second electrical signal. By having the speed processing device and the distance processing device calculate the speed and distance separately, the traditional method of calculating distance is abandoned, reducing the single-point measurement time of the FMCW lidar, increasing the point frequency, and enriching the scanning methods.

[0070] In one embodiment, the modulated electrical signal output by the signal source is a sawtooth wave, and the modulated beam output by the Mach-Zehnder modulator is a carrier-suppressed double-sideband optical signal.

[0071] For example, signal source 104 emits such as Figure 2 The sawtooth wave signal shown can optionally be a voltage signal. (Refer to...) Figure 2 From the time-frequency relationship diagram of the sawtooth wave LFM signal, the frequency of the sawtooth wave generated by the signal source can be obtained as f. M B = f2 - f1, f M = (Bt) / T, where T is the sawtooth wave period, B is the bandwidth, f2 is the ending frequency, f1 is the starting frequency, and t is the time.

[0072] The signal source 104 is electrically connected to the Mach-Zehnder modulator 106. After the signal source 104 outputs a sawtooth waveform radio frequency signal voltage, this radio frequency signal voltage becomes the electrode driving signal in the Mach-Zehnder modulator 106. Based on the aforementioned optical signal modulation principle of the Mach-Zehnder modulator 106, after the outgoing light beam emitted by the laser 102 is incident on the Mach-Zehnder modulator 106, the Mach-Zehnder modulator 106 converts the sawtooth waveform radio frequency signal voltage into a modulation signal for the light intensity of the incident light beam. This causes the phase and amplitude of the transmitted light wave to change with the time evolution of the radio frequency signal, thereby forming a light wave with a changing frequency.

[0073] like Figure 3 The diagram shown is a schematic of a Mach-Zehnder modulator 106 generating a carrier-suppressed double-sideband signal based on the outgoing beam and sawtooth wave in one embodiment.

[0074] Reference Figure 3 , Figure 3 The attached diagram on the left shows a block diagram of a laser and a Mach-Zehnder modulator. It can be seen that the output beam from laser 102 is incident on Mach-Zehnder modulator 106, where the frequency of the output laser is f. c In this embodiment, the bias voltage of the Mach-Zehnder modulator 106 is set at the minimum transmission point, and the sawtooth wave LFM signal generated by the signal source 104 is applied to the Mach-Zehnder modulator 106 as a modulation electrical signal. Specifically, the frequency of the sawtooth wave generated by the signal source is f. M This generates a carrier-suppressed double-sideband optical signal. (Refer to...) Figure 3 Right side, Figure 3 The upper right side shows a double-sideband modulated optical signal. Figure 3 The lower right side shows the carrier-suppressed double-sideband optical signal, f C -f M f is the optical frequency of the -1st order sideband. C +f M This represents the optical frequency of the +1st order sideband. A comparison between the carrier-suppressed double-sideband optical signal and the double-sideband modulated optical signal shows that f... c The corresponding optical power is lower than that of the ±1st order sideband.

[0075] like Figure 4 The figure shows the time-frequency relationship of a carrier-suppressed double-sideband modulated (LFM) optical signal from a Mach-Zehnder modulator 106. Figure 4 In the middle, f c -f1 represents the maximum frequency of the -1st order sideband light. c -f2 is the minimum frequency of the -1st order sideband light, f c +f1 is the minimum frequency of the +1st order sideband light, f c +f2 is the maximum value of the +1st order sideband optical frequency.

[0076] In one embodiment, the frequency spectrum of a Mach-Zehnder modulator carrier-suppressed double-sideband modulated optical signal is as follows: Figure 6 As shown, f c -f0 is the -1st order sideband optical frequency, f c +f0 is the +1st order sideband optical frequency.

[0077] In this embodiment, the sawtooth wave signal is modulated by the Mach-Zehnder modulator 106, which is equivalent to generating another sawtooth wave that is symmetrical to the original sawtooth wave but with the opposite slope. This can be canceled out in the self-mixing process, eliminating the need for algorithm correction of speed measurement errors caused by frequency modulation nonlinearity. This improves the calculation speed and distance measurement speed, achieving the purpose of speed and distance measurement more efficiently. Furthermore, using the sawtooth wave as the modulation electrical signal of the Mach-Zehnder modulator can obtain a modulated beam of carrier-suppressed double-sideband optical signal, which can not only achieve simultaneous measurement of distance and speed but also make the extracted signal more accurate. Combined with analog and digital mixed signal processing, it can also solve the industry pain points of long single-point detection time and difficulty in increasing the number of point cloud output points.

[0078] In one embodiment, such as Figure 6 As shown, the optoelectronic component 110 includes: a first coupler 602 for mixing the echo beam and the emitted beam to obtain a first coherent beat frequency signal; a first detector 604 for converting the first coherent beat frequency signal into a first electrical signal; a second coupler 606 for mixing the echo beam and the modulated beam to obtain a second coherent beat frequency signal; and a second detector 608 for converting the second coherent beat frequency signal into a second electrical signal.

[0079] Laser 102 can be a narrow linewidth laser. The emitted laser light is split into two parts by a first beam splitter. The first part enters a Mach-Zehnder modulator 106 to obtain a modulated beam, and the second part serves as a reference beam entering a first coupler 602 for coherent beat frequency. The modulated beam passes through a second beam splitter. The first part enters an optical path device 108 and is incident on the target under test, returning from the target. The second part enters a second coupler 606 and coherently beats with the echo beam. The echo beam, after passing through the optical path device 108, is split into two parts by a third beam splitter. The first part enters a first coupler 602 and mixes with the emitted beam for coherent beat frequency, obtaining a first coherent beat frequency signal, which is then converted into a first electrical signal by a first detector 604. The second part enters a second coupler 606 and coherently beats with the modulated beam, which is then converted into a second electrical signal by a second detector 608.

[0080] Coherent beat frequency is an optical measurement method based on the principle of interference. It uses the interference effect of two coherent beams to measure physical quantities and can be used to measure very small distance or displacement changes. The first detector and the second detector can be photodetectors, which are used to convert the received optical signals into electrical signals.

[0081] In this embodiment, the echo beam and the emitted beam are mixed by a first coupler and a first detector to obtain a first coherent beat frequency signal, which is then further converted into a first electrical signal, laying the groundwork for further distance measurement. Similarly, the echo beam and the modulated beam are mixed by a second coupler and a second detector to obtain a second coherent beat frequency signal, which is then further converted into a second electrical signal, laying the groundwork for further velocity measurement. In other words, by introducing the coherent beat frequency signal, the received signal contains the phase information of the interferometric light, thereby enabling the received value of the measured physical quantity to have high precision and high resolution.

[0082] In one embodiment, such as Figure 6 As shown, the speed processing device 112 includes a first mixer 610 and a low-pass filter 612.

[0083] The first electrical signal output from the first detector 604 and the modulated electrical signal output from the signal source 104 are mixed by the first mixer 610 to obtain the first difference frequency signal. The first difference frequency signal contains intermediate frequency signals of ±1st order sidebands and a self-mixed signal generated in the first mixer 610. After filtering by the low-pass filter 612, the self-mixed difference frequency signal of the ±1st order sideband intermediate frequency signal is obtained. This self-mixed difference frequency signal is only related to speed and is independent of distance, and is a low-frequency signal on the order of several megahertz. After being acquired by a low-speed ADC, the speed information can be calculated by performing FFT (Fast Fourier Transform) operations in the FPGA (Field Programmable Gate Array).

[0084] In this embodiment, the speed processing device includes a first mixer and a low-pass filter. The first mixer mixes a first electrical signal with a modulated electrical signal to obtain a first difference frequency signal. The low-pass filter extracts a self-mixed difference frequency signal from the first difference frequency signal. Speed ​​information is obtained based on the self-mixed difference frequency signal to achieve the speed measurement purpose. Based on the characteristics of the self-mixed difference frequency signal—it is only related to speed, independent of distance, and only on the order of a few megahertz—a low-pass filter is selected to extract the speed signal more accurately, thus better achieving the speed measurement purpose and resulting in high-precision speed detection results.

[0085] like Figure 6 As shown, the distance processing device 114 includes a second mixer 614 and a bandpass filter 616.

[0086] The second electrical signal output from the second detector 608 and the modulated electrical signal output from the signal source 104 are mixed by the second mixer 614 to obtain the second difference frequency signal. This difference frequency signal is then filtered by the bandpass filter 616 to obtain the +1st order sideband difference frequency signal. The frequency of this signal changes linearly with time, rather than being a fixed value. The sawtooth wave LFM signal starts at time t1, and is detected at time t2. The time difference is calculated, and the distance can be calculated using the formula.

[0087] The formula for the distance processing device 114 is as follows:

[0088] R = 1 / 2 × c × Δt

[0089] Where R is the distance to the target, Δt is the time difference Δt = t2 - t1, and c is the speed of light in a vacuum.

[0090] In this embodiment, the distance processing device includes a second mixer and a bandpass filter. The second mixer mixes the second electrical signal with the modulated electrical signal to obtain a second difference frequency signal. The bandpass filter extracts the difference frequency signal of the +1st order sideband from the second difference frequency signal. The bandpass filter is selected based on the characteristic that the frequency of the difference frequency signal of the +1st order sideband changes linearly with time rather than being a fixed value, so that the distance signal can be extracted more accurately, and the ranging purpose can be better achieved, so that the obtained distance detection result has high precision.

[0091] In one embodiment, the optical path device includes a circulator.

[0092] In one embodiment, the principle of dual-sideband FMCW (Frequency Modulated Continuous Wave) lidar for ranging and velocity measurement of stationary targets is as follows: Figure 7 As shown, the solid line represents the modulated beam emitted to the target, and the dashed line represents the echo beam. R Let be the intermediate frequency signal frequency when the target is stationary, τ be the delay time between the echo signal and the transmitted signal, R be the distance of the radar to the target, T be the period of a complete sawtooth wave, τ = 2R / c, f R It is obtained by mixing the modulated beam and the echo beam.

[0093] In one embodiment, the principle of dual-sideband FMCW lidar for ranging and velocity measurement of moving targets is as follows: Figure 8 As shown. The solid line represents the modulated beam emitted to the target, and the dashed line represents the echo beam. f R f is the intermediate frequency signal value when the target is stationary. d Let τ be the Doppler frequency shift caused by the target's motion, τ be the delay time between the echo signal and the transmitted signal, R be the distance of the radar to the target, T be the period of a complete sawtooth wave, and τ = 2R / c. R -fd f R +f d It is obtained by mixing the modulated beam and the echo beam.

[0094] In one embodiment, such as Figure 6 As shown, the laser frequency emitted by laser 102 is f c The beam is split into two parts by the first beam splitter. The first part enters the Mach-Zehnder modulator 106 to obtain the modulated beam, and the second part enters the first coupler 602 as a reference beam for coherent beat frequency.

[0095] The frequency of the modulated electrical signal output by signal source 104 is f M f M =f1+(f2-f1)t / T, where T is the period of a complete sawtooth wave, f1 is the starting frequency, f2 is the ending frequency, T is the period of a complete sawtooth wave, and t is time.

[0096] The emitted beam and the modulation electrical signal are applied to the Mach-Zehnder modulator 106 to generate a modulated beam, f C -f M f is the optical frequency of the -1st order sideband. C +f M The optical frequency is the +1st order sideband frequency. The generated modulated beam is amplified by an erbium-doped fiber amplifier (EDFA) and then split into two parts by a second beam splitter. The first part is emitted to the target under test after passing through a circulator and a two-dimensional scanning device, and returns from the target to obtain the echo beam. The second part serves as a reference beam and enters the second coupler 606 for coherent beat frequency. The optical frequencies of the echo beams are (f... C +f M )-(f R -f d ), (f C -f M )+(f R +f d ), where f R f is the intermediate frequency signal value when the target is stationary. d The Doppler shift caused by the motion of the target.

[0097] The echo beam is split into two parts by the third beam splitter. The first part enters the first coupler 602 and mixes with the outgoing beam to perform coherent beat frequency, obtaining a first coherent beat frequency signal, which is then converted into a first electrical signal by the first detector 604. The second part enters the second coupler 606 and performs coherent beat frequency with the modulated beam, which is then converted into a second electrical signal by the second detector 608. The frequency of the first electrical signal is f. M -(f R -fd ), f M -(f R +f d The frequency of the second electrical signal is f. R -f d f R +f d .

[0098] The first electrical signal output by the first detector 604 and the modulated electrical signal output by the signal source 104 are mixed by the first mixer 610 to obtain the first difference frequency signal. up f down Frequency mixing, where f up f is the intermediate frequency signal value in the upsweep band. down f is the intermediate frequency signal value in the downsweep band. up =f R -f d f down =f R +f d The frequency of the first difference frequency signal is f. R f d , where f R =(f up +f down ) / 2, f d =(f down -f up ) / 2. After filtering by a low-pass filter 612, the self-mixing difference frequency signal of the ±1st order sideband intermediate frequency signal is obtained, with a frequency of f. d After being acquired by a low-speed ADC, the speed information can be calculated by performing FFT operations in a field-programmable gate array.

[0099] The second electrical signal output from the second detector 208 and the modulated electrical signal output from the signal source 104 are mixed by the second mixer 614 to obtain the second difference frequency signal, the frequency of which is f. M -(f R -f d ), f M -(f R +f d The difference frequency signal f of the +1st order sideband is then obtained by filtering it through a bandpass filter 616. M -(f R -f d After being processed by a signal amplifier, envelope detector, and TDC (Time Digital Converter), the sawtooth wave LFM signal starts at time t1 and is detected at time t2. The time difference Δt = t2 - t1 is calculated, and the distance can be calculated using the formula R = 1 / 2 * c * Δt.

[0100] In another embodiment, laser 102 emits an output beam with a frequency of f. c Signal source 104 outputs a sawtooth wave with a frequency of f. M f M =f1+(f2-f1)t / T; The Mach-Zehnder modulator 106 modulates the output beam based on a sawtooth wave, thereby obtaining a carrier-suppressed double-sideband optical signal. At this time, the optical frequency of the -1st order sideband is f. C -f M The optical frequency of the +1st order sideband is f C +f M The generated carrier-suppressed double-sideband optical signal is amplified by an erbium-doped fiber amplifier (EDFA) and then split into two parts by a second beam splitter. The first part passes through optical path device 108, specifically a circulator and a two-dimensional scanning device, and is emitted to the target under test. The signal is then reflected back from the target, resulting in an echo beam. The optical frequencies of the echo beams are (f... C +f M )-(f R -f d ), (f C -f M )+(f R +f d ), where f R f is the intermediate frequency signal value when the target is stationary. d The second part serves as the reference light entering the second coupler 206 for coherent beat frequency. The frequency is (f... C +f M )-(f R -f d ), (f C -f M )+(f R +f d The echo beam is split into two parts by the third beam splitter. The first part enters the first coupler 602 and is coupled with a frequency of f. C The emitted beams are mixed and coherently beat to obtain a first coherent beat frequency signal, which is then converted into a first electrical signal by the first detector 604. The frequency of the first electrical signal is f. M -(f R -f d ), f M -(f R +f d The second part enters the second coupler 606 and is connected to a frequency of f. C -f M and f C +f M The modulated beam undergoes coherent beat frequency modulation, and is then converted into a second electrical signal by the second detector 608. The frequency of the second electrical signal is f.R -f d f R +f d The first electrical signal output by the first detector 604 has a frequency f that is the same as the frequency output by the signal source 104. M The modulated electrical signal is mixed by the first mixer 610 to obtain the first difference frequency signal. up f down Frequency mixing, f up =f R -f d f down =f R +f d The frequency of the first difference frequency signal is f. R f d , where f R =(f up +f down ) / 2, f d =(f down -f up The difference frequency signal, obtained by filtering the ±1st order sideband intermediate frequency signal after passing through a low-pass filter 612, is f. d After being acquired by a low-speed ADC, the velocity information is calculated using an FFT operation in a field-programmable gate array. The second electrical signal output by the second detector 608 has a frequency f that is the same as the frequency output by the signal source 104. M The modulated electrical signal is mixed by the second mixer 614 to obtain the second difference frequency signal, the frequency of which is f. M -(f R -f d ), f M -(f R +f d The difference frequency signal f of the +1st order sideband is then obtained by filtering it through a bandpass filter 616. M -(f R -f d After being processed by a signal amplifier, envelope detector, and TDC (Time Digital Converter), the sawtooth wave LFM signal starts at time t1 and is detected at time t2. The time difference Δt = t2 - t1 is calculated, and the distance can be calculated using the formula R = 1 / 2 * c * Δt.

[0101] The aforementioned velocity and distance measurement system separates velocity and distance calculations. It acquires the target's velocity using modulated electrical signals and a first electrical signal, and its distance using modulated electrical signals and a second electrical signal. This eliminates traditional distance calculation methods, reduces the single-point measurement time of FMCW lidar, increases the point frequency, and enriches the scanning methods. In distance measurement, it completely abandons high-speed ADC acquisition and FFT calculation, adopting a method similar to the time-of-flight method in ToF lidar, resulting in a superior distance measurement method. For velocity measurement, a mixer is used to down-frequency the echo signal, employing low-speed ADC acquisition and FFT calculation, saving significant acquisition and processing time.

[0102] Each module in the aforementioned speed and distance measurement system can be implemented entirely or partially through software, hardware, or a combination thereof. These modules can be embedded in or independent of the processor in a computer device, or stored in the computer device's memory as software, so that the processor can call and execute the corresponding operations of each module.

[0103] It should be understood that although the various devices in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these devices are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these devices, and they may be executed in other orders. Moreover, at least some of the devices in the flowcharts of the embodiments described above may include multiple devices or multiple stages. These devices or stages are not necessarily completed at the same time, but may be executed at different times. The execution order of these devices or stages is not necessarily sequential, but may be performed alternately or in turn with other devices or at least a portion of devices or stages in other devices.

[0104] Based on the same inventive concept, this application also provides a speed and distance measurement method implemented based on the above-described speed and distance measurement system. This embodiment illustrates the method by applying it to a terminal; however, it is understood that the method can also be applied to a server, and further to a system including both a terminal and a server, and implemented through interaction between the terminal and the server.

[0105] like Figure 9 As shown, in one embodiment, the method includes the following steps:

[0106] Step 902: Control the laser to emit an outgoing beam and control the signal source to output a sawtooth wave.

[0107] In one embodiment, the laser emitted from the laser is split into two parts by a first beam splitter. The first part enters a Mach-Zehnder modulator for signal generation, and the second part serves as a reference beam entering a first coupler for coherent beat frequency modulation. The modulated electrical signal output by the signal source includes a linear frequency modulated sawtooth wave, wherein the linear frequency modulated waveform is a double-sideband modulation of the sawtooth wave. There are two difference frequency signals in the same time period, with the ±1st order sidebands having opposite frequencies. The deviation caused by the nonlinearity of the LFM signal is opposite and can be canceled out in self-mixing.

[0108] Step 904: The emitted beam is modulated by a Mach-Zehnder modulator loaded with a sawtooth wave to generate a carrier-suppressed double-sideband optical signal; the bias voltage of the Mach-Zehnder modulator is set at the minimum transmission point.

[0109] In one embodiment, the bias voltage of the Mach-Zehnder modulator is set at the minimum transmission point, and the sawtooth wave linear frequency modulated signal generated by the radio frequency signal source is loaded onto the Mach-Zehnder modulator as a modulating electrical signal to generate a carrier-suppressed double-sideband optical signal.

[0110] Step 906: The carrier-suppressed double-sideband optical signal is incident on the target to be tested to obtain the echo beam.

[0111] In one embodiment, the generated double-sideband optical signal is amplified by an erbium-doped fiber amplifier and then split into two parts by a second beam splitter. The first part is transmitted to the target under test after passing through a circulator and a two-dimensional scanning device, and the second part is used as reference light to enter the second coupler for coherent beat frequency.

[0112] Step 908: Obtain the first electrical signal and the second electrical signal based on the echo beam, the outgoing beam, and the carrier-suppressed double-sideband optical signal.

[0113] In one embodiment, the echo beam is split into two parts by a third beam splitter after passing through a two-dimensional scanning device and a circulator. The first part enters the first coupler and coherently beats with the outgoing beam, and is then converted into a first electrical signal by the first detector. The second part enters the second coupler and coherently beats with the modulated beam, and is then converted into a second electrical signal by the second detector.

[0114] Step 910: Obtain the velocity of the target based on the first electrical signal and the sawtooth wave, and obtain the distance of the target based on the second electrical signal and the sawtooth wave.

[0115] In one embodiment, the first electrical signal output by the first detector and the signal output by the radio frequency signal source are mixed by the first mixer to obtain a first difference frequency signal. The first difference frequency signal includes intermediate frequency signals of ±1st order sidebands and a self-mixed signal generated in the first mixer. After filtering by a low-pass filter, the self-mixed difference frequency signal of the ±1st order sideband intermediate frequency signals is obtained. This self-mixed difference frequency signal is only related to speed and not to distance, and is a low-frequency signal on the order of several megahertz. After being acquired by a low-speed analog-to-digital converter, the speed information can be calculated by performing a fast Fourier transform in a field-programmable gate array. The second electrical signal output by the second detector and the signal from the radio frequency signal source are mixed by the second mixer to obtain a second difference frequency signal, which is then filtered by a band-pass filter to obtain a difference frequency signal of +1st order sideband. The frequency of this signal changes linearly with time, rather than being a fixed value. The sawtooth wave linear frequency modulated signal starts at time t1, and is detected at time t2. The time difference Δt = t2 - t1. The distance R can be calculated using the formula R = 1 / 2 * c * Δt.

[0116] The aforementioned speed and distance measurement method, by introducing a sawtooth wave as the LFM signal, addresses the issue that the deviation caused by the nonlinearity of the LFM signal is opposite and can be canceled out in self-mixing. Therefore, no algorithm is needed to correct the speed measurement error caused by frequency modulation nonlinearity. Furthermore, using a sawtooth wave not only allows for simultaneous measurement of distance and speed, but also solves the industry pain points of long single-point detection time and difficulty in increasing the number of point cloud outputs by combining analog and digital mixed signal processing. By modulating the sawtooth wave signal with a Mach-Zehnder modulator to generate a carrier-suppressed double-sideband optical signal, the speed of speed and distance calculation can be improved, achieving the purpose of speed and distance measurement more efficiently. Finally, by separating speed and distance calculation, the speed of the target is obtained by modulating the electrical signal and the first electrical signal, and the distance of the target is obtained by modulating the electrical signal and the second electrical signal. This eliminates the traditional method of calculating distance, reduces the single-point measurement time of FMCW lidar, increases the point frequency, and enriches the scanning methods.

[0117] In one embodiment, the carrier-suppressed double-sideband optical signal is incident on the target under test to obtain an echo beam, including:

[0118] The carrier-suppressed double-sideband optical signal is transmitted to the target under test after passing through a circulator and a two-dimensional scanning device. The beam reflected by the target under test passes through the two-dimensional scanning device and the circulator again to obtain the echo beam.

[0119] In this embodiment, the carrier-suppressed double-sideband optical signal is transmitted to the target after passing through a circulator and a two-dimensional scanning device to obtain an echo beam, thereby linking the velocity and distance measuring device with the target.

[0120] In one embodiment, obtaining a first electrical signal and a second electrical signal based on the echo beam, the emitted beam, and the carrier-suppressed double-sideband optical signal includes:

[0121] The echo beam is coupled with the outgoing beam to obtain a first coherent beat frequency signal; the first coherent beat frequency signal is converted into a first electrical signal by a first detector; the echo beam is coupled with a carrier-suppressed double-sideband optical signal to obtain a second coherent beat frequency signal; the second coherent beat frequency signal is converted into a second electrical signal by a second detector.

[0122] In this embodiment, a first coherent beat frequency signal is obtained by intersecting the echo beam and the emitted beam, and then a first electrical signal is obtained from the first coherent beat frequency signal, laying the groundwork for calculation speed. In this embodiment, a second coherent beat frequency signal is obtained by intersecting the echo beam and the modulated beam, and then a second electrical signal is obtained from the second coherent beat frequency signal, laying the groundwork for distance calculation. By introducing the coherent beat frequency signal, the received signal contains the phase information of the interferometric light, thereby enabling the received value of the measured physical quantity to have high precision and high resolution measurement results.

[0123] In one embodiment, obtaining the velocity of the target based on the first electrical signal and the sawtooth wave includes:

[0124] The first electrical signal and the sawtooth wave are mixed to obtain the first difference frequency signal; the first difference frequency signal is low-pass filtered to obtain the self-mixed difference frequency signal; the self-mixed difference frequency signal is acquired by a low-speed analog-to-digital converter and then subjected to a fast Fourier transform in a field-programmable gate array to obtain the velocity of the target under test.

[0125] In this embodiment, a first difference frequency signal is obtained by mixing a first electrical signal with a modulated electrical signal. This first difference frequency signal is then low-pass filtered to obtain a self-mixed difference frequency signal. This signal is then acquired by a low-speed analog-to-digital converter and subjected to a Fast Fourier Transform (FFT) in a field-programmable gate array (FPGA) to obtain the speed of the target, thus achieving the purpose of speed measurement. The speed measurement employs a mixer to down-frequency the echo signal, and uses a low-speed ADC for acquisition and FFT calculation, saving significant acquisition and processing time.

[0126] In one embodiment, obtaining the distance to the target based on the second electrical signal and the sawtooth wave includes:

[0127] The second electrical signal is mixed with the sawtooth wave to obtain the second difference frequency signal; the second difference frequency signal is bandpass filtered to obtain the distance to the target.

[0128] In this embodiment, the distance to the target is obtained by mixing the second electrical signal with the modulated electrical signal and bandpass filtering, thereby achieving the purpose of ranging.

[0129] In another embodiment, such as Figure 10 As shown, a method for measuring speed and distance is provided.

[0130] Step 1002: Control the laser to emit an outgoing beam and control the signal source to output a sawtooth wave.

[0131] Step 1004: The outgoing beam is modulated by a Mach-Zehnder modulator loaded with a sawtooth wave to generate a carrier-suppressed double-sideband optical signal; the bias voltage of the Mach-Zehnder modulator is set at the minimum transmission point.

[0132] Step 1006: The carrier-suppressed double-sideband optical signal is transmitted to the target under test after passing through the circulator and the two-dimensional scanning device. The beam reflected by the target under test passes through the two-dimensional scanning device and the circulator again to obtain the echo beam.

[0133] Step 1008: The echo beam is coupled with the outgoing beam to obtain the first coherent beat frequency signal.

[0134] Step 1010: The first coherent beat frequency signal is converted into a first electrical signal by the first detector.

[0135] Step 1012: The echo beam is coupled with the carrier-suppressed double-sideband optical signal to obtain the second coherent beat frequency signal.

[0136] Step 1014: The second coherent beat frequency signal is converted into a second electrical signal by the second detector.

[0137] Step 1016: The first electrical signal and the sawtooth wave are mixed to obtain the first difference frequency signal.

[0138] Step 1018: Perform low-pass filtering on the first difference frequency signal to obtain the self-mixing difference frequency signal.

[0139] Step 1020: After the self-mixed difference frequency signal is acquired by a low-speed analog-to-digital converter, it is subjected to a fast Fourier transform in a field-programmable gate array to obtain the velocity of the target under test.

[0140] Step 1022: The second electrical signal is mixed with the sawtooth wave to obtain the second difference frequency signal.

[0141] Step 1024: Bandpass filter the second difference frequency signal to obtain the distance to the target.

[0142] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, etc., and are not limited to these.

[0143] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0144] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A speed and distance measuring system, characterized in that, include: A laser is used to emit an outgoing beam of light; The signal source is electrically connected to the Mach-Zehnder modulator, speed processing device, and distance processing device, and is used to output a sawtooth wave as a modulated electrical signal. The Mach-Zehnder modulator has a bias voltage set at the minimum transmission point to modulate the sawtooth wave onto the output beam, and outputs a carrier-suppressed double-sideband optical signal as the modulated beam. An optical path device is used to direct the modulated beam onto the target under test and receive the echo beam; the echo beam is returned from the target under test. An optoelectronic component for generating a first electrical signal and a second electrical signal based on the emitted beam, the modulated beam, and the echo beam; A speed processing device includes a first mixer and a low-pass filter, used to mix the first electrical signal with the sawtooth wave to obtain a first difference frequency signal, and then obtain a self-mixed difference frequency signal from the first difference frequency signal through the low-pass filter, and obtain the speed of the target under test based on the self-mixed difference frequency signal; The distance processing device includes a second mixer and a bandpass filter, used to mix the second electrical signal with the sawtooth wave to obtain a second difference frequency signal, and then filter it through the bandpass filter to obtain a distance-related difference frequency signal, and obtain the distance of the target under test based on the difference frequency signal.

2. The system according to claim 1, characterized in that, The optoelectronic component includes: A first coupler is used to mix the echo beam with the outgoing beam to obtain a first coherent beat frequency signal; A first detector is used to convert the first coherent beat frequency signal into a first electrical signal; The second coupler is used to mix the echo beam with the modulated beam to obtain a second coherent beat frequency signal; The second detector is used to convert the second coherent beat frequency signal into a second electrical signal.

3. The system according to claim 2, characterized in that, The laser can be a narrow linewidth laser. The emitted laser is split into two parts by the first beam splitter. The first part of the laser enters the Mach-Zehnder modulator to obtain the modulated beam, and the second part of the laser enters the first coupler as a reference beam for coherent beat frequency.

4. The system according to claim 3, characterized in that, The modulated beam passes through the second beam splitter. The first part of the modulated beam enters the optical path device and is incident on the target under test, and returns from the target under test. The second part of the modulated beam enters the second coupler and coherently beats with the echo beam.

5. The system according to claim 4, characterized in that, The echo beam is split into two parts by the third beam splitter after passing through the optical path device. The first part of the echo beam enters the first coupler and mixes with the output beam to perform coherent beat frequency, thereby obtaining the first coherent signal. It is then converted into the first electrical signal by the first detector. The second part of the echo beam enters the second coupler and performs coherent beat frequency with the modulated beam. It is then converted into the second electrical signal by the second detector.

6. A method for measuring speed and distance, characterized in that, The method applicable to the speed and distance measuring system according to any one of claims 1-5 includes: Control the laser to emit an output beam, and control the signal source to output a sawtooth wave; A sawtooth wave is modulated onto the outgoing beam using a Mach-Zehnder modulator, and the output carrier-suppressed double-sideband optical signal is used as the modulated beam; the bias voltage of the Mach-Zehnder modulator is set at the minimum transmission point. The modulated beam is incident onto the target under test using an optical path device to obtain an echo beam. The echo beam, the emitted beam, and the modulated beam are input into the optoelectronic component to obtain a first electrical signal and a second electrical signal. Based on the first mixer and low-pass filter in the speed processing device, the first electrical signal is mixed with the sawtooth wave to obtain the first difference frequency signal, and then the self-mixed difference frequency signal is obtained from the first difference frequency signal through the low-pass filter. The speed of the target under test is obtained according to the self-mixed difference frequency signal. Based on the second mixer and bandpass filter in the distance processing device, the second electrical signal is mixed with the sawtooth wave to obtain a second difference frequency signal, which is then filtered by the bandpass filter to obtain a distance-related difference frequency signal. The distance of the target to be measured is obtained based on the difference frequency signal.

7. The method according to claim 6, characterized in that, The carrier-suppressed double-sideband optical signal is incident on the target to obtain the echo beam, which includes: The carrier-suppressed double-sideband optical signal is transmitted to the target under test after passing through a circulator and a two-dimensional scanning device. The beam reflected by the target under test passes through the two-dimensional scanning device and the circulator again to obtain the echo beam.

8. The method according to claim 6, characterized in that, The process of obtaining the first and second electrical signals based on the echo beam, the emitted beam, and the carrier-suppressed double-sideband optical signal includes: The echo beam is coupled with the emitted beam to obtain a first coherent beat frequency signal; The first coherent beat frequency signal is converted into a first electrical signal by the first detector; The echo beam is coupled with the carrier-suppressed double-sideband optical signal to obtain a second coherent beat frequency signal; The second coherent beat frequency signal is converted into a second electrical signal by the second detector.

9. The method according to claim 6, characterized in that, The process of obtaining the velocity of the target under test based on the first electrical signal and the sawtooth wave includes: The first electrical signal and the sawtooth wave are mixed to obtain the first difference frequency signal; The first difference frequency signal is low-pass filtered to obtain a self-mixing difference frequency signal; The self-mixing difference frequency signal is acquired by a low-speed analog-to-digital converter and then subjected to a fast Fourier transform in a field-programmable gate array to obtain the velocity of the target under test.

10. The method according to claim 6, characterized in that, The process of obtaining the distance to the target based on the second electrical signal and the sawtooth wave includes: The second electrical signal is mixed with the sawtooth wave to obtain the second difference frequency signal; The distance to the target under test is obtained by bandpass filtering the second difference frequency signal.

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