CN109039471A - A kind of numerical model analysis demodulation method applied to high-rate laser communication - Google Patents

Abstract

The invention discloses a digital-analog hybrid demodulation method applied to high-speed laser communication, which includes: roughly compensating the obtained carrier optical signal; performing photoelectric conversion on the compensated carrier optical signal to generate an electrical signal; The electrical signal and the local oscillator signal are quadrature mixed to obtain the baseband quadrature signal; the baseband quadrature signal is divided into two signals; the first signal is provided to the FPGA through the limiter, and the FPGA decodes the first signal The baseband data is obtained by tuning; the second channel signal is subjected to analog processing to obtain the phase detection signal of the residual carrier; the phase detection signal of the residual carrier is provided to FPGA after A/D conversion; and the residual signal after A/D conversion is processed by FPGA The phase detection signal of the carrier is captured and tracked, and then the carrier is reconstructed, and the local oscillator signal is obtained by quadrature modulation based on the reconstructed carrier and the analog phase-locked loop PPL.

Description

A digital-analog hybrid demodulation method applied to high-speed laser communication

technical field

The invention relates to the technical field of satellite communication, in particular to a digital-analog hybrid demodulation method applied to high-speed laser communication.

Background technique

The space laser communication system requires space-based terminals to use laser as a medium for high-speed communication. It needs to deal with large dynamic high-speed signal processing scenarios, and requires reducing complexity and power consumption as much as possible to adapt to power-constrained systems.

At present, there are two systems of coherent and non-coherent receivers for space high-speed laser communication: non-coherent is mainly represented by intensity modulation and direct detection, which has low receiving sensitivity and low spectrum utilization; while coherent communication is limited by the amplification of local oscillator light It has the advantage of sensitivity, and high-order modulation can be selected to improve spectrum utilization.

Coherent demodulation technology is mainly divided into two categories: analog homodyne optical phase-locked loop technology and digital optical heterodyne technology. The analog homodyne optical phase-locked loop technical solution is similar to the traditional microwave receiver solution, except that the traditional voltage-controlled oscillator (VCO) is replaced by a tunable optical local oscillator, and the carrier loop frequency estimation, frequency discrimination and phase discrimination, loop The channel filtering and local oscillator optical adjustment are controlled by analog devices. This solution has relatively high requirements on the stability and drift rate of the laser and is easily affected by the environment; the digital optical heterodyne technology solution first obtains the intermediate frequency through optical mixing. The signal is then sampled into the digital chip through a high-speed analog-to-digital converter (ADC) to complete a series of operations such as capture, tracking, clock recovery, and data demodulation. This solution requires a high-speed analog-to-digital converter (ADC) and a high-speed parallel processing chip. The algorithm has high complexity and high power consumption.

In addition, the carrier Doppler frequency of laser communication between low-orbit satellites can reach about 10 GHz, and the corresponding system symbol rate needs to reach the order of tens of Gbps, and the sampling rate of the analog-to-digital converter (ADC) needs to meet the "Nyquis Specific theory" (that is, the sampling frequency of the ADC is at least twice the signal frequency) can be used to sample the transmitted symbols, so the traditional digital processing method is no longer applicable, and the traditional carrier synchronization method is also difficult to apply.

Therefore, there is an urgent need for a new type of digital-analog hybrid demodulation device and method applied to high-speed laser communication to solve the problem in the existing technology of laser communication that it is difficult to achieve high-efficiency demodulation of large dynamic high-speed laser signals, and a high-speed analog-to-digital converter is required (ADC) and high-speed parallel processing chips, high algorithm complexity, high power consumption and other issues.

Contents of the invention

Aiming at the problems existing in the prior art of laser communication that it is difficult to realize high-efficiency demodulation of large dynamic high-speed laser signals, high-speed analog-to-digital converters (ADCs) and high-speed parallel processing chips, high algorithm complexity, and high power consumption are required, according to the present invention An embodiment of provides a digital-analog hybrid demodulation method applied to high-speed laser communication, including:

Coarsely compensate the acquired carrier optical signal;

Perform photoelectric conversion on the compensated carrier optical signal to generate an electrical signal;

Perform quadrature mixing on the generated electrical signal and the local oscillator signal to obtain the baseband quadrature signal;

Divide the baseband quadrature signal into two signals;

Provide the first signal to the FPGA through the limiter,

FPGA demodulates the first signal to obtain baseband data;

The second channel signal is subjected to analog processing to obtain the phase detection signal of the residual carrier;

providing the phase detection signal of the residual carrier to the FPGA after A/D conversion; and

FPGA captures and tracks the phase detection signal of the residual carrier after A/D conversion, and then reconstructs the carrier, and performs quadrature modulation based on the reconstructed carrier and the analog phase-locked loop PPL to obtain the local oscillator signal.

In an embodiment of the present invention, the method for roughly compensating the acquired carrier optical signal further includes;

Satellite orbit information to obtain carrier Doppler signal;

amplifying the obtained carrier Doppler signal through an optical amplifier; and

The amplified carrier Doppler signal is coarsely compensated in the optical mixer by adjusting the optical local oscillator generated by the local oscillator laser.

In an embodiment of the present invention, the method of photoelectrically converting the compensated carrier optical signal to generate an electrical signal is to convert the optical signal output by the optical mixer through a balanced detector, a band-pass filter, and a low-noise amplifier. Photoelectric conversion is performed to generate an electrical signal.

In an embodiment of the present invention, the splitting of the baseband quadrature signal into two paths is to divide the baseband quadrature signal into two paths by using a power divider.

In an embodiment of the present invention, the specific method for the FPGA to demodulate the first signal to obtain the baseband data is to obtain the baseband data through demodulation by a decision feedback equalizer (DFE) and a clock data recovery (CDR) module.

In an embodiment of the present invention, the method for the FPGA to capture the phase detection signal of the residual carrier after the A/D conversion further includes:

Set the frequency control word through the software to control the D/A output carrier frequency;

detection of error signals at two local oscillator frequencies;

If no error signal is detected at both LO frequencies, then adjust the LO frequency to continue sweeping across the entire processing bandwidth; and

If error signals are continuously detected at two local oscillator frequencies, it is judged that the frequency of the error signal is obtained.

In an embodiment of the present invention, the frequency control word is Among them, M is the phase control word length.

In an embodiment of the present invention, the method for detecting the error signal within two local oscillator frequencies is to perform N-point FFT transformation on the error shaping signal, and search for the peak point m exceeding the threshold, if the amplitude of the peak point and the second peak point is If the ratio is greater than 10, it is determined that an error signal is detected, and its frequency can be expressed as Among them, N and M represent FFT length and phase control word length respectively, and f _s is the A/D sampling frequency.

In an embodiment of the present invention, the method for determining and obtaining the frequency of the error signal is as follows:

Let the local oscillator frequencies be f _ref,1 and f _ref,2 (f _ref,2 >f _ref,1 ), the corresponding peaks are P ₁ and P ₂ , and the estimated frequencies of the error signal are f ₁ and f ₂ ;

If P ₁ >P ₂ , then the error signal frequency f=f _ref,1 +f ₁ ;

If P ₁ <P ₂ , then the error signal frequency f=f _ref,2 −f ₂ .

The invention provides a digital-analog hybrid demodulation method applied to high-speed laser communication. First, the carrier Doppler information is obtained by using the satellite orbit information, and the carrier Doppler is roughly compensated by adjusting the optical local oscillator, and the optical mixer The output laser signal undergoes photoelectric conversion to generate an electrical signal; then it performs quadrature mixing with the locally generated local oscillator signal to obtain a baseband quadrature signal; the baseband quadrature signal is divided into two paths, and one path is sent to the FPGA through a limiter The baseband data is demodulated by decision feedback equalizer (DFE), clock data recovery (CDR) and other modules, and the other channel is simulated to obtain the phase detection signal of the residual carrier, which is collected to FPGA through A/D In the process, the reconstructed residual carrier is generated, and the local local oscillator frequency is adjusted to achieve the purpose of carrier synchronization. The digital-analog hybrid demodulation method based on the invention can realize high-efficiency demodulation of large dynamic high-speed laser signals without high-speed A/D conversion.

Description of drawings

In order to further clarify the above and other advantages and features of various embodiments of the present invention, a more particular description of various embodiments of the present invention will be presented with reference to the accompanying drawings. It is understood that the drawings depict only typical embodiments of the invention and therefore are not to be considered limiting of its scope. In the drawings, the same or corresponding parts will be denoted by the same or similar symbols for clarity.

Fig. 1 shows a schematic structural diagram of a receiver of a digital-analog hybrid demodulation method applied to high-speed laser communication according to an embodiment of the present invention.

Fig. 2 shows a flow chart of a digital-analog hybrid demodulation method applied to high-speed laser communication according to a specific embodiment of the present invention.

Fig. 3 shows a flow chart of carrier capture of a digital-analog hybrid demodulation method applied to high-speed laser communication according to a specific embodiment of the present invention.

Detailed ways

In the following description, the present invention is described with reference to various examples. One skilled in the art will recognize, however, that the various embodiments may be practiced without one or more of the specific details, or with other alternative and/or additional methods, materials, or components. In other instances, well-known structures, materials, or operations are not shown or described in detail so as not to obscure aspects of the various embodiments of the invention. Similarly, for purposes of explanation, specific quantities, materials and configurations are set forth in order to provide a thorough understanding of embodiments of the invention. However, the invention may be practiced without these specific details. Furthermore, it should be understood that the various embodiments shown in the drawings are illustrative representations and are not necessarily drawn to scale.

In this specification, reference to "one embodiment" or "the embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. The appearances of the phrase "in one embodiment" in various places in this specification are not necessarily all referring to the same embodiment.

It should be noted that the embodiments of the present invention describe the method steps in a specific order, but this is only for the convenience of distinguishing the steps, and does not limit the order of the steps. In different embodiments of the present invention, according to the method adjustment to adjust the order of each step.

The invention provides a digital-analog hybrid demodulation method applied to high-speed laser communication. First, the carrier Doppler information is obtained by using the satellite orbit information, and the carrier Doppler is roughly compensated by adjusting the optical local oscillator, and the optical mixer The output laser signal undergoes photoelectric conversion to generate an electrical signal; then it performs quadrature mixing with the locally generated local oscillator signal to obtain a baseband quadrature signal; the baseband quadrature signal is divided into two paths, and one path is sent to the FPGA through a limiter The baseband data is demodulated by decision feedback equalizer (DFE), clock data recovery (CDR) and other modules, and the other channel is simulated to obtain the phase detection signal of the residual carrier, which is collected to FPGA through A/D In the process, the reconstructed residual carrier is generated, and the local local oscillator frequency is adjusted to achieve the purpose of carrier synchronization. The digital-analog hybrid demodulation method based on the invention can realize high-efficiency demodulation of large dynamic high-speed laser signals without high-speed A/D conversion.

A digital-analog hybrid demodulation method applied to high-speed laser communication based on an embodiment of the present invention will be described below with reference to the accompanying drawings 1-2. Fig. 1 shows a schematic diagram of the receiver structure of a digital-analog hybrid demodulation method applied to high-speed laser communication according to an embodiment of the present invention; Fig. 2 shows a kind of receiver used in a specific embodiment of the present invention Flow chart of a digital-analog hybrid demodulation method for high-speed laser communication; FIG. 3 shows a flow chart of carrier capture for a digital-analog hybrid demodulation method applied to high-speed laser communication provided according to a specific embodiment of the present invention.

First, in step 201, rough compensation is performed on the laser signal. As shown in Figure 1, after the satellite orbit information obtains the carrier Doppler signal information, the obtained carrier Doppler signal is amplified and processed by the optical amplifier, and then the optical local oscillator is generated by adjusting the local oscillator laser. In the process, the amplified carrier Doppler signal is roughly compensated to obtain the compensated carrier Doppler signal.

Next, in step 202, photoelectric conversion is performed on the compensated carrier Doppler signal to generate an electrical signal. In one embodiment of the present invention, as shown in FIG. 1 , the laser signal output by the optical mixer is photoelectrically converted through a balanced detector, a band-pass filter and a low-noise amplifier to generate an electrical signal.

Then, in step 203, quadrature mixing is performed on the generated electrical signal and the locally generated local oscillator signal through a quadrature mixer to obtain a baseband quadrature signal. In one embodiment of the present invention, as shown in Figure 1, the local oscillator signal is carried out by digital direct frequency synthesis DDS and analog phase-locked loop PPL to carry out quadrature mixing modulation and then produce in frequency multiplication mode, with a large frequency dynamic range, The advantages of low phase noise and fast adjustment speed.

Next, in step 204, the baseband quadrature signal is divided into two paths. In one embodiment of the present invention, a power divider is used to split the baseband quadrature signal into two paths. One output is to the high-speed serial receiving module in the FPGA, and the other is output to the carrier capture and tracking module.

Then, in step 205, one of the signals is provided to the high-speed serial receiving module in the FPGA through the limiter.

Next, in step 206, baseband data is obtained through demodulation. Since the data rate to be processed by the present invention reaches tens of Gbps, conventional A/D chips and FPGAs cannot process such high-rate signals at all. In one embodiment of the present invention, as shown in Figure 1, a processing scheme based on the high-speed serial receiving mode is designed, the baseband quadrature signal enters the high-speed serial receiving module in the FPGA through the limiter, and is equalized by decision feedback The baseband data is demodulated by the DFE, CDR and other modules, and further processed in the FPGA, thereby omitting the A/D sampling. At the same time, the data is processed in parallel through the high-speed serial receiving module, which reduces the need for FPGA handles clock requirements.

Then, in step 207, the other signal is subjected to analog processing to obtain a phase detection signal of the residual carrier. In one embodiment of the present invention, as shown in FIG. 1 , analog processing is performed through an analog multiplier and a low-pass filter.

Next, in step 208, the phase detection signal of the residual carrier is provided to the carrier capture and tracking module of the FPGA after A/D conversion.

Finally, in step 209, the carrier acquisition and tracking module of the FPGA captures and tracks the phase detection signal, then performs carrier reconstruction, and adjusts the local oscillator signal based on the reconstructed carrier to achieve carrier synchronization.

According to a specific embodiment of the present invention, when providing the above-mentioned digital-analog hybrid demodulation method applied to high-speed laser communication, the main technical contents involved include:

1) IF quadrature mixing technology

It means that after the broadband signal is amplified by the amplifier and the local oscillator signal generated by the local oscillator laser is input into the optical mixer for quadrature mixing, the input signal is down-converted and the carrier Doppler frequency shift is eliminated, and the output is two-way Baseband quadrature signals. The local oscillator signal is generated by digital direct frequency synthesis DDS and analog phase-locked loop PPL through quadrature modulation, which has the advantages of large frequency dynamic range, low phase noise and fast adjustment speed.

2) Power splitter

It refers to the use of a power divider to divide the baseband quadrature signal into two paths. The quadrature mixer outputs the baseband quadrature signal, which is divided into two paths after passing through the power divider, one path is output to the high-speed serial receiving module in the FPGA, and the other path is output to the carrier capture and tracking module.

3) Analog phase detection and low-pass filtering

It refers to performing analog phase detection on the baseband quadrature signal and performing low-pass filtering on the phase detection error. The baseband quadrature signal can be expressed as:

I=cos(Δωt) (1)

Q＝sin(Δωt) (2)

Where Δω is the frequency residual, t is the time, then the phase detection error can be expressed as:

Δθ(t)=sin(2Δωt) (3)

After the analog phase detection, the phase detection error is low-pass filtered to assist the subsequent carrier capture and tracking.

4) A/D sampling

It means that the phase detection error of the phase detection signal is sampled by A/D. After the phase detection error is sampled by A/D, it can be expressed as:

Among them, f _s is the A/D sampling frequency, n is the serial number of the sampling point, Ω is the normalized digital frequency, ε is the DC bias.

5) Offset elimination

It means to eliminate the DC bias. Since carrier estimation and tracking are very sensitive to DC offset, DC offset needs to be removed first before processing. A high-pass filter needs to be used for DC blocking processing, and the digital phase detection error of the processed phase detection signal can be expressed as:

Δθ(n)=sin(Ωn) (5)

6) Carrier capture

It refers to the carrier capture of the phase detection signal. In a specific embodiment of the present invention, considering that the residual Doppler frequency range is as high as hundreds of MHz, the operating frequency of A/D and D/A is too high for direct sampling and regeneration, which also puts huge pressure on FPGA processing. Therefore the present invention proposes a kind of scanning strategy, the whole frequency capture range is divided into a plurality of frequency sections in the simulation part by the mode of software control local oscillator frequency, in each section Doppler frequency is searched, thus A/D chip only A single frequency segment needs to be sampled, which greatly reduces the pressure of A/D sampling and the processing rate of FPGA.

Since the real signal is generated from the complex signal during phase detection, the frequency symbol information is lost, so when using the conventional FFT transformation for carrier capture, only the absolute deviation of the frequency can be obtained, and it is impossible to judge whether it is a positive frequency or a negative frequency. To this end, Fig. 3 shows a carrier capture flow chart of a digital-analog hybrid demodulation method applied to high-speed laser communication provided according to a specific embodiment of the present invention. As shown in Fig. 3, the frequency estimation algorithm based on FFT is revised as follows:

A) Control the D/A output carrier frequency by setting the frequency control word through the software. In the entire frequency range, set the frequency control word to control the D/A output carrier frequency in steps. The step value is half the bandwidth of the low-pass filter B , which can be expressed as:

Δθ(n)=sin(Ωn) (5)

f _ref ＝f _c -f _d,max ,f _c -f _d,max +Δf,...,f _c +f _d,max -Δf,f _c +f _d,max (6)

Δf=B/2 (7)

Among them, f _ref is the local oscillator frequency of the quadrature mixer, f _c is the set carrier frequency, f _d,max is the upper limit of the Doppler frequency, Δf is the frequency step value, and B represents the bandwidth of the low-pass filter B .

B) Perform N-point FFT transformation on the error shaping signal, search for the peak point m exceeding the predetermined threshold, if the amplitude ratio of the peak point and the sub-peak point is greater than 10, then it is determined that an error signal is detected, and its frequency can be expressed as:

Where f _s is the A/D sampling frequency.

C) If error signals are not detected at both local oscillator frequencies, then adjust the local oscillator frequency and continue to scan within the entire processing bandwidth;

D) If error signals are detected at two local oscillator frequencies continuously, set the local oscillator frequencies as f _ref,1 and f _ref,2 (f _ref,2 >f _ref,1 ), and the corresponding peak values are P ₁ and P ₂ , the estimated frequency of the error signal is f ₁ and f ₂ , then the frequency of the error signal can be judged as follows:

If P ₁ >P ₂ , the error signal frequency:

f=f _ref,1 +f ₁ (9)

Conversely, if P ₁ <P ₂ , the error signal frequency

f = f _ref,2 - f ₂ (10)

E) The corresponding frequency control word is Among them, M is the phase control word length.

7) Carrier tracking

It refers to the carrier tracking of the phase detection signal. In one embodiment of the present invention, after the carrier is captured successfully, the capture scan is stopped and the carrier tracking is started. Since the D/A working clock should be more than 3 times the signal frequency, generating a carrier with a frequency of several hundred MHz has high requirements on D/A. In order to reduce the working clock of D/A, in one embodiment of the present invention, the mode of combination of analog/digital is adopted, and the frequency of the regenerated carrier is compressed to one-eighth of the original in the digital domain, and then passed in the analog processing The method of frequency doubling reproduces the real frequency, thereby reducing the requirement for the D/A operating frequency.

In a specific embodiment of the present invention, in order to reduce D/A working frequency, carry out special processing to frequency in tracking, its method is as follows:

A) the frequency control word of the carrier regeneration module E is set to the frequency control word determined by the acquisition unit;

B) Inputting the error shaping signal into the loop filter unit C for filtering;

C) Divide the loop filter output by 8, and input it to the carrier regeneration module E as a frequency control word.

8) Quadrature modulation

After the carrier is reconstructed, the local oscillator signal is generated by quadrature modulation based on the reconstructed carrier signal and the analog phase-locked loop PPL, which has the advantages of large frequency dynamic range, low phase noise, and fast adjustment speed. Since the frequency control word is reduced by 8 times in carrier tracking, the signal needs to be multiplied by 4 after quadrature modulation to generate a real control frequency.

9) High-speed serial reception

It refers to the high-speed serial reception of data by the FPGA pair. Since the data rate to be processed by the present invention reaches tens of Gbps, conventional A/D chips and FPGAs cannot process such high-rate signals at all. In one embodiment of the present invention, a processing scheme based on the high-speed serial receiving mode is designed, and the baseband orthogonal signal is carried out in the high-speed serial receiving module in the FPGA through the limiter, after processing such as decision feedback equalization and time synchronization , to obtain the baseband data, further processing in the FPGA, thereby omitting the A/D sampling, and at the same time, the data is processed in parallel through the high-speed serial receiving module, which reduces the requirements for the FPGA processing clock.

Based on the digital-analog hybrid demodulation method applied to high-speed laser communication provided by the present invention, firstly, the carrier Doppler information is obtained by using the satellite orbit information, and the carrier Doppler is roughly compensated by adjusting the optical local oscillator, and the optical mixing The laser signal output by the frequency converter performs photoelectric conversion to generate an electrical signal; then it performs quadrature mixing with the locally generated local oscillator signal to obtain a baseband quadrature signal; the baseband quadrature signal is divided into two paths, and one path is sent to the The baseband data is demodulated by decision feedback equalizer (DFE), clock data recovery (CDR) and other modules, and the other channel is simulated to obtain the phase detection signal of the residual carrier, which is collected by A/D In the FPGA, the reconstructed residual carrier is generated, and the local local oscillator frequency is adjusted to achieve the purpose of carrier synchronization. The digital-analog hybrid demodulation method based on the invention can realize high-efficiency demodulation of large dynamic high-speed laser signals without high-speed A/D conversion.

While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to those skilled in the relevant art that various combinations, modifications and changes can be made thereto without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention disclosed herein should not be limited by the above-disclosed exemplary embodiments, but should be defined only in accordance with the appended claims and their equivalents.

Claims (9)

1. A digital-analog hybrid demodulation method applied to high-speed laser communication, comprising: Coarsely compensate the acquired carrier optical signal; Perform photoelectric conversion on the compensated carrier optical signal to generate an electrical signal; Perform quadrature mixing on the generated electrical signal and the local oscillator signal to obtain the baseband quadrature signal; Divide the baseband quadrature signal into two signals; Provide the first signal to the FPGA through the limiter; FPGA demodulates the first signal to obtain baseband data; The second channel signal is subjected to analog processing to obtain the phase detection signal of the residual carrier; providing the phase detection signal of the residual carrier to the FPGA after A/D conversion; and FPGA captures and tracks the phase detection signal of the residual carrier after A/D conversion, and then reconstructs the carrier, and performs quadrature modulation based on the reconstructed carrier and the analog phase-locked loop PPL to obtain the local oscillator signal. 2. The method according to claim 1, wherein the method for performing coarse compensation to the acquired optical carrier signal further comprises; Satellite orbit information to obtain carrier Doppler signal; amplifying the obtained carrier Doppler signal through an optical amplifier; and The amplified carrier Doppler signal is coarsely compensated in the optical mixer by adjusting the optical local oscillator generated by the local oscillator laser. 3. The method according to claim 1, characterized in that, the described carrier light signal obtained after compensation is carried out to photoelectric conversion, and the method for generating the electrical signal is to pass through a balanced detector, a band-pass filter and a low noise amplifier to light The optical signal output by the mixer undergoes photoelectric conversion to generate an electrical signal. 4. The method according to claim 1, wherein said dividing the baseband quadrature signal into two signals is dividing the baseband quadrature signal into two paths by using a power divider. 5. The method according to claim 1, characterized in that, the specific method that the FPGA demodulates the first road signal to obtain baseband data is to solve the problem by a decision feedback equalizer (DFE) and a clock data recovery (CDR) module. Tune to obtain baseband data. 6. method as claimed in claim 1, is characterized in that, the method that described FPGA captures the phase detection signal of residual carrier after A/D conversion further comprises: Set the frequency control word through the software to control the D/A output carrier frequency; detection of error signals at two local oscillator frequencies; If no error signal is detected at both LO frequencies, then adjust the LO frequency to continue sweeping across the entire processing bandwidth; and If error signals are continuously detected at two local oscillator frequencies, it is judged that the frequency of the error signal is obtained. 7. The method according to claim 6, wherein the frequency control word is Among them, M is the phase control word length. 8. The method according to claim 6, wherein the method for detecting the error signal in two local oscillator frequencies is to carry out N-point FFT transformation to the error shaping signal, and search for the peak point m exceeding the threshold, if the peak value point and the amplitude ratio of the sub-peak point is greater than 10, then it is determined that an error signal is detected, and its frequency can be expressed as Among them, N and M represent FFT length and phase control word length respectively, and f _s is the A/D sampling frequency. 9. The method according to claim 6, wherein the method for obtaining the error signal frequency in the judgment is as follows: Let the local oscillator frequencies be f _ref,1 and f _ref,2 (f _ref,2 >f _ref,1 ), the corresponding peaks are P ₁ and P ₂ , and the estimated frequencies of the error signal are f ₁ and f ₂ ; If P ₁ >P ₂ , then the error signal frequency f=f _ref,1 +f ₁ ; If P ₁ <P ₂ , then the error signal frequency f=f _ref,2 −f ₂ .