Coherent detection system and demodulation method thereof

Abstract

The embodiment of the invention discloses a coherent detection system and a demodulation method thereof. The coherent detection system comprises a signal generation module, a transmitting module, a receiving module, an IQ demodulation module and a processing module. The signal generation module outputs a local signal and a detection signal, the local signal is transmitted to a first input end of the IQ demodulation module, and the detection signal is transmitted to the transmitting module; the transmitting module transmits a detection signal to a to-be-detected target, and a frequency shift detection signal reflected by the to-be-detected target is received by the receiving module and then is transmitted to a second input end of the IQ demodulation module; the IQ demodulation module outputs a first difference frequency signal and a second difference frequency signal to the processing module, and the processing module determines the magnitude and positive and negative values of difference frequency according to the first difference frequency signal and the second difference frequency signal; wherein the difference frequency is a frequency difference value of the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90-degree phase shift. According to the technical scheme of the embodiment of the invention, the magnitude and positive and negative values of the difference frequency can be obtained.

Description

Technical Field

[0001] The present invention relates to the field of coherent detection, and in particular to a coherent detection system and a demodulation method thereof. Background Art

[0002] Coherent detection systems are widely used in radar, communication, sensing and other fields. The basic principle is: the original signal generated by the signal module is divided into two, one of which is transmitted as a local signal within the system, and the other is transmitted as a detection signal by the transmitting module, and then captured by the receiving module after line transmission / target reflection. The local signal is mixed with the received detection signal in the coherent module, and finally the information is extracted and restored through signal demodulation. Since the local signal and the detection signal are of the same source, its detection performance and anti-interference ability have obvious advantages over the incoherent detection system.

[0003] Generally speaking, the signal after passing through the coherent module is the difference frequency (beat frequency) signal of the local signal and the (received) detection signal. When the detection signal undergoes the transmission and reception process, it is possible that the frequency is offset (for example, in the radar system, the detected target has a certain movement speed. According to the Doppler phenomenon, the received detection signal has a frequency offset compared to the transmission). When demodulating the signal, it is necessary to determine the specific value of the frequency offset. Although the frequency offset can be determined by Fourier transform and other operations, the direction of the frequency offset cannot be obtained (that is, it is impossible to determine whether the frequency of the detection signal is higher or lower than that of the local signal). This is because the single-channel difference frequency signal is a real signal, and the real signal is symmetrical about the zero point in the Fourier frequency domain, that is, the positive frequency and the negative frequency have the same performance, so the positive and negative nature of the difference frequency signal cannot be determined. Summary of the invention

[0004] The embodiment of the present invention provides a coherent detection system and a demodulation method thereof. By designing an IQ demodulation module to replace the original coherent module in the system, two difference frequency signals (local signal and detection signal) can be output, and there is a fixed 90° phase shift between the two difference frequency signals. By processing the two real number signals, an asymmetric spectrum can be obtained, so that the positive and negative values ​​of the frequency can be uniquely determined.

[0005] According to one aspect of the present invention, there is provided a coherent detection system, comprising a signal generating module, a transmitting module, a receiving module, an IQ demodulation module and a processing module;

[0006] The signal generating module is used to output a local signal and a detection signal, the local signal is transmitted to the first input end of the IQ demodulation module, and the detection signal is transmitted to the transmitting module;

[0007] The transmitting module is used to transmit the detection signal to the target to be measured, and the frequency shift detection signal reflected by the target to be measured is received by the receiving module and then transmitted to the second input end of the IQ demodulation module;

[0008] The output end of the IQ demodulation module is connected to the processing module, and the IQ demodulation module outputs the first difference frequency signal and the second difference frequency signal to the processing module, and the processing module is used to determine the magnitude and positive and negative values ​​of the difference frequency according to the first difference frequency signal and the second difference frequency signal;

[0009] The difference frequency is the frequency difference between the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90° phase shift.

[0010] Optionally, the processing module is specifically configured to assemble a complex signal according to the first difference frequency signal and the second difference frequency signal to obtain an asymmetric frequency spectrum, thereby determining the size and positive and negative values ​​of the difference frequency.

[0011] Optionally, the processing module determines the magnitude and positive and negative values ​​of the difference frequency using the following algorithm:

[0012] Let the time domain expressions of the local signal and the frequency shift detection signal be x Lo (t) and x Si (t), the instantaneous frequencies of the local signal and the frequency shift detection signal are f Lo (t) and f Si (t), then:

[0013] x Lo (t) = sin(2πf Lo t)

[0014] x Si (t) = sin(2πf Si t);

[0015] After the local signal and the IQ demodulation module, two orthogonal first difference frequency signal I(t) and second difference frequency signal Q(t) are generated:

[0016] I(t)=cos[2π(f Lo -f Si )t]

[0017] Q(t)=sin[2π(f Lo -f Si )t];

[0018] The first difference frequency signal and the second difference frequency signal are constructed into a complex signal:

[0019]

[0020] The spectrum of the complex signal is analyzed using Fourier transform:

[0021]

[0022] Where δ is the pulse impulse function, which means that only in the actual difference frequency f Lo -f Si There is an energy pulse at the difference frequency to distinguish the positive and negative frequency information.

[0023] Optionally, the signal generating module includes a first output terminal and a second output terminal, the first output terminal outputs the local signal, and the second output terminal outputs the detection signal.

[0024] Optionally, the signal generating module includes a signal source and a beam splitter, the output end of the signal source is connected to the input end of the beam splitter, and the beam splitter is used to split the output signal of the signal source into the local signal and the detection signal.

[0025] Optionally, the transmitting module includes a transmitting lens group, and the transmitting lens group is used to collimate the local signal and then output it to the target to be measured;

[0026] The receiving module comprises a receiving lens group, and the receiving lens group is used to converge the frequency shift detection signal to the second input end of the IQ demodulation module.

[0027] Optionally, the IQ demodulation module includes an IQ coupler, a first photodetector, and a second photodetector;

[0028] The first input end of the IQ coupler receives the local signal, the second input end of the IQ coupler receives the frequency shift detection signal, the first output end and the second output end of the IQ coupler are both connected to the first photodetector, and the third output end and the fourth output end of the IQ coupler are both connected to the second photodetector.

[0029] Optionally, the coherent detection system is a Doppler vibrometer system or a frequency modulated continuous wave radar system.

[0030] According to another aspect of the present invention, a demodulation method of a coherent detection system is provided, which is applicable to the above-mentioned coherent detection system, and the demodulation method comprises:

[0031] The signal generation module outputs a local signal and a detection signal, wherein the local signal is transmitted to the first input end of the IQ demodulation module, and the detection signal is transmitted to the transmission module;

[0032] The transmitting module transmits the detection signal to the target to be measured, and the frequency shift detection signal reflected by the target to be measured is received by the receiving module and then transmitted to the second input end of the IQ demodulation module;

[0033] The processing module receives the first difference frequency signal and the second difference frequency signal output by the IQ demodulation module, and determines the magnitude and positive and negative values ​​of the difference frequency according to the first difference frequency signal and the second difference frequency signal;

[0034] The difference frequency is the frequency difference between the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90° phase shift.

[0035] Optionally, determining the magnitude and the positive and negative value of the difference frequency according to the first difference frequency signal and the second difference frequency signal includes:

[0036] Let the time domain expressions of the local signal and the frequency shift detection signal be x Lo (t) and x Si (t), the instantaneous frequencies of the local signal and the frequency shift detection signal are f Lo (t) and f Si (t), then:

[0037] x Lo (t) = sin(2πf Lo t)

[0038] x Si (t) = sin(2πf Si t);

[0039] After the local signal and the IQ demodulation module, two orthogonal first difference frequency signal I(t) and second difference frequency signal Q(t) are generated:

[0040] I(t)=cos[2π(f Lo -f Si )t]

[0041] Q(t)=sin[2π(f Lo -f Si )t];

[0042] The first difference frequency signal and the second difference frequency signal are constructed into a complex signal:

[0043]

[0044] The spectrum of the complex signal is analyzed using Fourier transform:

[0045]

[0046] Where δ is the pulse impulse function, which means that only in the actual difference frequency f Lo -f Si There is an energy pulse at the difference frequency to distinguish the positive and negative frequency information.

[0047] The coherent detection system provided by the embodiment of the present invention includes a signal generation module, a transmitting module, a receiving module, an IQ demodulation module and a processing module. The local signal and the detection signal are outputted by the signal generation module, the local signal is transmitted to the first input end of the IQ demodulation module, and the detection signal is transmitted to the transmitting module; the detection signal is transmitted to the target to be measured by the transmitting module, and the frequency shift detection signal reflected by the target to be measured is transmitted to the second input end of the IQ demodulation module after being received by the receiving module; the first difference frequency signal and the second difference frequency signal are outputted to the processing module by the IQ demodulation module, and the processing module determines the size and positive and negative value of the difference frequency according to the first difference frequency signal and the second difference frequency signal; wherein the difference frequency is the frequency difference between the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90° phase shift. Thus, the problem that the positive and negative nature of the difference frequency signal cannot be determined in the existing coherent detection system is solved.

[0048] It should be understood that the contents described in this section are not intended to identify the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0050] Figure 1 It is a structural block diagram of a coherent detection system in the prior art;

[0051] Figure 2 for Figure 1 The signal schematic diagram of the coherent detection system shown;

[0052] Figure 3 A structural block diagram of a coherent detection system provided by an embodiment of the present invention;

[0053] Figure 4 for Figure 3 The signal schematic diagram of the coherent detection system shown;

[0054] Figure 5 A structural block diagram of another coherent detection system provided by an embodiment of the present invention;

[0055] Figure 6 A structural block diagram of another coherent detection system provided by an embodiment of the present invention;

[0056] Figure 7 A structural block diagram of another coherent detection system provided by an embodiment of the present invention;

[0057] Figure 8 A schematic flow chart of a demodulation method for a coherent detection system is provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0058] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0059] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0060] Figure 1 is a structural block diagram of a coherent detection system in the prior art, refer to Figure 1 The coherent detection system includes a signal generating module 1, a transmitting module 2, a receiving module 3, a coherent module 4 and a processing module 5. The signal generating module 1 divides the original signal into two, one of which is transmitted in the system as a local signal, and the other is transmitted as a detection signal by the transmitting module 2, and then captured by the receiving module 3 after line transmission / reflection by the target to be measured. The local signal is mixed with the received detection signal in the coherent module 4, and finally the information is extracted and restored by the processing module 5 after signal demodulation.

[0061] Figure 2 for Figure 1The signal schematic diagram of the coherent detection system shown in FIG. 4 shows that the signal after passing through the coherent module 4 is the difference frequency (beat frequency) signal of the local signal and the (received) detection signal. When the detection signal undergoes the transmission and reception process, it may be offset in frequency, and its time domain waveform is as follows: Figure 2 As shown in the figure above. When demodulating the signal, it is necessary to determine the specific value of the frequency offset. Although the frequency offset can be determined through Fourier transform and other operations, the direction of the frequency offset cannot be obtained (that is, it is impossible to determine which frequency is higher between the detection signal and the local signal). This is because the single-channel difference frequency signal is a real signal, and the real signal is symmetrical about the zero point in the Fourier frequency domain, that is, the positive frequency and the negative frequency have the same performance, as shown in Figure 2 As shown in the figure below, it is impossible to determine the positive or negative nature of the difference frequency signal.

[0062] In order to solve the above problems, Figure 3 A structural block diagram of a coherent detection system provided by an embodiment of the present invention, refer to Figure 3 The coherent detection system includes a signal generating module 10, a transmitting module 20, a receiving module 30, an IQ demodulation module 40 and a processing module 50; the signal generating module 10 is used to output a local signal and a detection signal, the local signal is transmitted to the first input end of the IQ demodulation module 40, and the detection signal is transmitted to the transmitting module 20; the transmitting module 20 is used to transmit the detection signal to the target 60 to be measured, and the frequency shift detection signal reflected by the target 60 to be measured is received by the receiving module 30 and then transmitted to the second input end of the IQ demodulation module 40; the output end of the IQ demodulation module 40 is connected to the processing module 50, and the IQ demodulation module 40 outputs a first difference frequency signal and a second difference frequency signal to the processing module 50, and the processing module 50 is used to determine the size and positive and negative values ​​of the difference frequency according to the first difference frequency signal and the second difference frequency signal; wherein the difference frequency is the frequency difference between the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90° phase shift.

[0063] During specific implementation, the embodiment of the present invention does not limit the type of coherent detection system, and any coherent detection system using the beat frequency principle is applicable. The signal emitted by the signal generation module 10 may be a microwave, a sound wave, a laser, etc. Optionally, the coherent detection system may be a Doppler vibrometer system (LDV) or a frequency modulated continuous wave radar system (FMCW). IQ demodulation is a technology widely used in digital communication systems, which uses two orthogonal carrier signals to simultaneously transmit two independent signals. In IQ demodulation, one signal is called the I (in-phase) component and the other is called the Q (orthogonal) component. This method can effectively utilize spectrum resources and play an important role in many modern communication systems. The basic idea of ​​IQ demodulation is to divide the data into two paths, perform carrier modulation separately, and then the two carriers are orthogonal to each other. In this way, two signals can be transmitted simultaneously at the same frequency, thereby improving the efficiency of spectrum utilization. In IQ demodulation, the I component is multiplied by a cosine wave, and the Q component is multiplied by a sine wave and inverted, and then the two results are added to obtain the final modulated signal. The embodiment of the present invention utilizes the principle of IQ demodulation and utilizes an IQ demodulation module to replace the coherent module in the traditional coherent detection system, thereby outputting a first difference frequency signal and a second difference frequency signal with a fixed phase difference of 90°. Optionally, the processing module 50 is specifically used to assemble a complex signal according to the first difference frequency signal and the second difference frequency signal to obtain an asymmetric spectrum, thereby determining the size and positive and negative values ​​of the difference frequency.

[0064] Optionally, the processing module 50 determines the magnitude and positive and negative values ​​of the difference frequency using the following algorithm:

[0065] Let the time domain expressions of the local signal and the frequency shift detection signal be x Lo (t) and x Si (t), the instantaneous frequencies of the local signal and the frequency-shifted detection signal are f Lo (t) and f Si (t), then:

[0066] x Lo (t) = sin(2πf Lo t)

[0067] x Si (t) = sin(2πf Si t);

[0068] After the local signal passes through the IQ demodulation module, two orthogonal first difference frequency signals I(t) and second difference frequency signals Q(t) are generated:

[0069] I(t)=cos[2π(f Lo -f Si )t]

[0070] Q(t)=sin[2π(fLo -f Si )t];

[0071] The first difference frequency signal and the second difference frequency signal are constructed into a complex signal:

[0072]

[0073] Use Fourier transform to analyze the spectrum of a complex signal:

[0074]

[0075] Where δ is the pulse impulse function, which means that only in the actual difference frequency f Lo -f Si There is an energy pulse at f Si -f Lo A peak appears at the difference frequency to distinguish the positive and negative frequency information of the difference frequency.

[0076] For example, Figure 4 for Figure 3 The signal schematic diagram of the coherent detection system shown in the figure, wherein the upper figure is the time domain signal, including two orthogonal signals, and the lower figure is the frequency domain signal. By constructing the two difference frequency signals into a complex signal, an asymmetric spectrum can be obtained, so that the positive and negative values ​​of the frequency can be uniquely determined.

[0077] The technical solution of the embodiment of the present invention is to output a local signal and a detection signal through a signal generating module, the local signal is transmitted to the first input end of the IQ demodulation module, and the detection signal is transmitted to the transmitting module; the detection signal is transmitted to the target to be measured through the transmitting module, and the frequency-shifted detection signal reflected by the target to be measured is received by the receiving module and then transmitted to the second input end of the IQ demodulation module; the first difference frequency signal and the second difference frequency signal are output to the processing module through the IQ demodulation module, and the processing module determines the size and positive and negative values ​​of the difference frequency according to the first difference frequency signal and the second difference frequency signal, thereby solving the problem that the positive and negative values ​​of the difference frequency signal cannot be determined in the existing coherent detection system.

[0078] Continue to refer Figure 3 Optionally, the signal generating module 10 includes a first output terminal and a second output terminal, the first output terminal outputs a local signal, and the second output terminal outputs a detection signal.

[0079] In this embodiment, the signal generating module 10 directly has two output terminals, and outputs a local signal and a detection signal simultaneously, wherein the local signal and the detection signal are coherent signals.

[0080] In another embodiment, in order to improve the coherence of the local signal and the detection signal, the local signal and the detection signal may be implemented by splitting the same signal beam. Figure 5A structural block diagram of another coherent detection system provided by an embodiment of the present invention, referring to Figure 5 Optionally, the signal generating module 10 includes a signal source 11 and a beam splitter 12, the output end of the signal source 11 is connected to the input end of the beam splitter 12, and the beam splitter 12 is used to split the output signal of the signal source 11 into a local signal and a detection signal.

[0081] By arranging the beam splitter 12 to divide the output signal of the signal source 11 into two beams, it can be ensured that the local signal and the detection signal are completely identical, thereby improving the coherence of the local signal and the detection signal.

[0082] Figure 6 A structural block diagram of another coherent detection system provided by an embodiment of the present invention, referring to Figure 6 Optionally, the transmitting module 20 includes a transmitting lens group 21, which is used to collimate the local signal and output it to the target 60 to be measured; the receiving module 30 includes a receiving lens group 31, which is used to converge the frequency shift detection signal to the second input end of the IQ demodulation module 40.

[0083] The transmitting lens group 21 and the receiving lens group 31 each include at least one converging lens for adjusting the transmission direction of the signal and improving the performance of the coherent detection system. The specific structures of the transmitting lens group 21 and the receiving lens group 31 can be designed according to actual conditions, and the embodiment of the present invention does not limit this.

[0084] Figure 7 A structural block diagram of another coherent detection system provided by an embodiment of the present invention, referring to Figure 7 Optionally, the IQ demodulation module 40 includes an IQ coupler 41, a first photodetector 42 and a second photodetector 43; the first input end of the IQ coupler 41 receives a local signal, the second input end of the IQ coupler 41 receives a frequency shift detection signal, the first output end and the second output end of the IQ coupler 41 are both connected to the first photodetector 42, and the third output end and the fourth output end of the IQ coupler 41 are both connected to the second photodetector 43.

[0085] Among them, the structure of the IQ coupler 41 is a two-input and four-output coupler, the first input end and the second input end receive the local signal and the frequency shift detection signal respectively, and the four output signals can be divided into two groups, defined as I and Q. According to the phase principle of the coupler, there is a 90° phase difference between the I and Q signals, and the first photodetector 42 and the second photodetector 43 can be balanced detectors BPD. By processing the two signals through the processing module 50, the size and positive and negative values ​​of the difference frequency can be obtained.

[0086] Figure 8A schematic flow chart of a demodulation method for a coherent detection system provided in an embodiment of the present invention, the demodulation method is applicable to any coherent detection system provided in the above embodiments, refer to Figure 8 , the demodulation methods include:

[0087] S110: The signal generation module outputs a local signal and a detection signal. The local signal is transmitted to the first input end of the IQ demodulation module, and the detection signal is transmitted to the transmission module.

[0088] S120, the transmitting module transmits the detection signal to the target to be measured, and the frequency-shifted detection signal reflected by the target to be measured is received by the receiving module and then transmitted to the second input end of the IQ demodulation module.

[0089] S130: The processing module receives the first difference frequency signal and the second difference frequency signal output by the IQ demodulation module, and determines the magnitude and the positive and negative value of the difference frequency according to the first difference frequency signal and the second difference frequency signal.

[0090] The difference frequency is the frequency difference between the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90° phase shift.

[0091] Optionally, determining the magnitude and the positive and negative value of the difference frequency according to the first difference frequency signal and the second difference frequency signal includes:

[0092] Let the time domain expressions of the local signal and the frequency shift detection signal be x Lo (t) and x Si (t), the instantaneous frequencies of the local signal and the frequency-shifted detection signal are f Lo (t) and f Si (t), then:

[0093] x Lo (t) = sin(2πf Lo t)

[0094] x Si (t) = sin(2πf Si t);

[0095] After the local signal passes through the IQ demodulation module, two orthogonal first difference frequency signals I(t) and second difference frequency signals Q(t) are generated:

[0096] I(t)=cos[2π(f Lo -f Si )t]

[0097] Q(t)=sin[2π(f Lo -f Si )t];

[0098] The first difference frequency signal and the second difference frequency signal are constructed into a complex signal:

[0099]

[0100] Use Fourier transform to analyze the spectrum of a complex signal:

[0101]

[0102] Where δ is the pulse impulse function, which means that only in the actual difference frequency f Lo -f Si There is an energy pulse at the difference frequency to distinguish the positive and negative frequency information.

[0103] The demodulation method of the coherent detection system provided in the embodiment of the present invention has the same or corresponding technical effect as the coherent detection system, which will not be described in detail here.

[0104] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A coherent detection system, characterized in that: It includes a signal generation module, a transmission module, a receiving module, an IQ demodulation module and a processing module; The signal generating module is used to output a local signal and a detection signal, the local signal is transmitted to the first input end of the IQ demodulation module, and the detection signal is transmitted to the transmitting module; The transmitting module is used to transmit the detection signal to the target to be measured, and the frequency shift detection signal reflected by the target to be measured is received by the receiving module and then transmitted to the second input end of the IQ demodulation module; The output end of the IQ demodulation module is connected to the processing module, and the IQ demodulation module outputs the first difference frequency signal and the second difference frequency signal to the processing module, and the processing module is used to determine the magnitude and positive and negative values ​​of the difference frequency according to the first difference frequency signal and the second difference frequency signal; The difference frequency is the frequency difference between the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90° phase shift.

2. The coherent detection system according to claim 1, characterized in that: The processing module is specifically used to assemble a complex signal according to the first difference frequency signal and the second difference frequency signal to obtain an asymmetric frequency spectrum, thereby determining the size and positive and negative values ​​of the difference frequency.

3. The coherent detection system according to claim 2, characterized in that: The processing module determines the magnitude and positive and negative values ​​of the difference frequency using the following algorithm: Let the time domain expressions of the local signal and the frequency shift detection signal be x Lo (t) and x Si (t), the instantaneous frequencies of the local signal and the frequency shift detection signal are f Lo (t) and f Si (t), then: x Lo (t)=sin(2πf Lo t) x Si (t)=sin(2πf Si t); After the local signal and the IQ demodulation module, two orthogonal first difference frequency signal I(t) and second difference frequency signal Q(t) are generated: I(t)=cos[2π(f Lo -f Si )t] Q(t)=sin[2π(f Lo -f Si )t]; The first difference frequency signal and the second difference frequency signal are constructed into a complex signal: The spectrum of the complex signal is analyzed using Fourier transform: Where δ is the pulse impulse function, which means that only in the actual difference frequency f Lo -f Si There is an energy pulse at the difference frequency to distinguish the positive and negative frequency information.

4. The coherent detection system according to claim 1, characterized in that: The signal generating module comprises a first output terminal and a second output terminal, the first output terminal outputs the local signal, and the second output terminal outputs the detection signal.

5. The coherent detection system according to claim 1, characterized in that: The signal generating module comprises a signal source and a beam splitter, wherein the output end of the signal source is connected to the input end of the beam splitter, and the beam splitter is used for splitting the output signal of the signal source into the local signal and the detection signal.

6. The coherent detection system according to claim 1, characterized in that: The transmitting module includes a transmitting lens group, and the transmitting lens group is used to collimate the local signal and then output it to the target to be measured; The receiving module comprises a receiving lens group, and the receiving lens group is used to converge the frequency shift detection signal to the second input end of the IQ demodulation module.

7. The coherent detection system according to claim 1, characterized in that: The IQ demodulation module includes an IQ coupler, a first photodetector and a second photodetector; The first input end of the IQ coupler receives the local signal, the second input end of the IQ coupler receives the frequency shift detection signal, the first output end and the second output end of the IQ coupler are both connected to the first photodetector, and the third output end and the fourth output end of the IQ coupler are both connected to the second photodetector.

8. The coherent detection system according to claim 1, characterized in that: The coherent detection system is a Doppler vibration measurement system or a frequency modulated continuous wave radar system.

9. A demodulation method for a coherent detection system, characterized in that: Applicable to the coherent detection system according to any one of claims 1 to 8, the demodulation method comprising: The signal generation module outputs a local signal and a detection signal, wherein the local signal is transmitted to the first input end of the IQ demodulation module, and the detection signal is transmitted to the transmission module; The transmitting module transmits the detection signal to the target to be measured, and the frequency shift detection signal reflected by the target to be measured is received by the receiving module and then transmitted to the second input end of the IQ demodulation module; The processing module receives the first difference frequency signal and the second difference frequency signal output by the IQ demodulation module, and determines the magnitude and positive and negative values ​​of the difference frequency according to the first difference frequency signal and the second difference frequency signal; The difference frequency is the frequency difference between the local signal and the frequency shift detection signal, and the first difference frequency signal and the second difference frequency signal have a fixed 90° phase shift.

10. The demodulation method of the dry detection system according to claim 9, characterized in that: Determining the magnitude and positive and negative values ​​of the difference frequency according to the first difference frequency signal and the second difference frequency signal includes: Let the time domain expressions of the local signal and the frequency shift detection signal be x Lo (t) and x Si (t), the instantaneous frequencies of the local signal and the frequency shift detection signal are f Lo (t) and f Si (t), then: x Lo (t)=sin(2πf Lo t) x Si (t)=sin(2πf Si t); After the local signal and the IQ demodulation module, two orthogonal first difference frequency signal I(t) and second difference frequency signal Q(t) are generated: I(t)=cos[2π(f Lo -f Si )t] Q(t)=sin[2π(f Lo -f Si )t]; The first difference frequency signal and the second difference frequency signal are constructed into a complex signal: The spectrum of the complex signal is analyzed using Fourier transform: Where δ is the pulse impulse function, which means that only in the actual difference frequency f Lo -f Si There is an energy pulse at the difference frequency to distinguish the positive and negative frequency information.

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