High-precision stepping frequency ground penetrating radar transmitting and receiving system based on double phase-locked loops

Abstract

The invention discloses a high-precision stepping frequency ground penetrating radar transmitting and receiving system based on double phase-locked loops, and belongs to the technical field of underground target detection. The system comprises a radio frequency signal transmitting circuit and a local oscillator signal generating circuit which are respectively used for transmitting a radio frequency signal outwards and transmitting a generated reference local oscillator signal to a receiving part; wherein the radio frequency signal transmitting circuit comprises a first phase-locked loop connected with the band-pass filter bank, and the local oscillator signal generating circuit comprises a second phase-locked loop connected with the band-pass filter bank; the receiving part receives an external echo signal and mixes the external echo signal with the reference local oscillator signal to output an intermediate frequency analog signal; an intermediate frequency analog signal is digitally sampled by the intermediate frequency sampling part to obtain a sampling signal, and the sampling signal is transmitted to the control communication part; and the power supply part supplies power to the whole system. According to the invention, the distance resolution is improved, and the overall structure complexity of the circuit is low.

Description

Technical Field

[0001] The invention belongs to the technical field of underground target detection, and in particular relates to a high-precision stepped frequency ground penetrating radar transceiver system based on a dual phase-locked loop. Background Art

[0002] Ground penetrating radar (GPR) uses the interaction between electromagnetic waves and different underground objects to obtain underground target information. Because of its advantages of non-destructive detection, convenience, speed, and high resolution, it has become one of the important means of underground exploration. In the application of ground penetrating radar, in order to obtain higher distance resolution, the transmitted signal is often required to have a large bandwidth. Traditional impulse ground penetrating radar is difficult to achieve high resolution at a certain depth because it is difficult to take into account the wide instantaneous bandwidth and high transmission power of the signal. Although linear frequency modulation ground penetrating radar does not have this problem, it has high requirements for the linearity between signal frequency and time, and the signal with a relatively large bandwidth often has poor amplitude flatness, which will affect the high-precision imaging results. In contrast, the stepped frequency ground penetrating radar can not only take into account the detection depth and high distance resolution due to the discreteness of its signal in the frequency domain and time domain, but also its signal is easier to generate and control, and the hardware circuit design is relatively simple, which has a high research significance.

[0003] Many stepped frequency ground penetrating radar systems are built based on vector network analyzers. The stepped frequency signals are directly generated and received by the vector network analyzer. The operation is simple and the construction is convenient. However, the instrument is not dedicated to the realization of ground penetrating radar. Therefore, the ground penetrating radar realized in this way is often large, bulky, and costly, which is not conducive to the application and promotion of stepped frequency ground penetrating radar. There are also stepped frequency ground penetrating radar systems designed with direct digital synthesizer (DDS). The output signal of DDS is flexible and controllable, and the whole system is small and flexible. However, the highest frequency of the DDS output signal is often not high, so it is impossible to generate a sufficiently wide stepped frequency signal, and the generated signal has large spurious, which is difficult to meet the high-resolution requirements of high-precision detection. In addition, there is also a mixed frequency design scheme using a combination of DDS, mixer, multiplier and phase-locked loop. Although this scheme can generate a sufficiently wide stepped frequency signal, due to the combination of multiple devices, its circuit structure is often complex and its reliability is poor. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a high-precision stepped frequency ground penetrating radar transceiver system based on a dual phase-locked loop, which adopts a dual phase-locked loop transmitter and a digital fixed intermediate frequency structure receiver. Not only can the operating frequency band reach 0.5~3.5GHz, which improves the distance resolution, but also the overall circuit structure is relatively simple and small in size. The frequency and gain of each signal frequency point can also be adjusted according to actual conditions, which can better adapt to a variety of application scenarios.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] A high-precision stepped frequency ground penetrating radar transceiver system based on a dual phase-locked loop, the system comprises a transmitting part, a receiving part, an intermediate frequency sampling part, a control communication part and a power supply part, wherein:

[0007] The transmitting part includes a radio frequency signal transmitting circuit and a local oscillator signal generating circuit, which are respectively used to transmit the radio frequency signal to the outside and transmit the generated reference local oscillator signal to the receiving part; wherein the radio frequency signal transmitting circuit includes a first phase-locked loop connected to the band-pass filter group, and the local oscillator signal generating circuit includes a second phase-locked loop connected to the band-pass filter group;

[0008] The receiving part is used to receive the external echo signal, mix it with the reference local oscillator signal and output an intermediate frequency analog signal;

[0009] The intermediate frequency analog signal is digitally sampled by the intermediate frequency sampling part to obtain a sampling signal, and the sampling signal is transmitted to the control communication part;

[0010] The power supply part provides power for the entire system.

[0011] The beneficial effects of the present invention are:

[0012] The broadband phase-locked loop used in the present invention can easily output high-quality sinusoidal signals of different frequencies under control, and its operating frequency band is relatively wide, and the upper limit of the output signal frequency is higher than that of DDS, and it can directly generate high-quality, ultra-wideband stepped frequency signals;

[0013] The present invention uses two phase-locked loops with the same reference source to output radio frequency and local oscillator signals with a fixed intermediate frequency difference under control, and the two signals do not lose correlation. Compared with the method of splitting the power into two paths after the output of a single phase-locked loop, one path is directly used as the transmitted radio frequency signal, and the other path is obtained through orthogonal modulation to obtain the local oscillator signal with a fixed intermediate frequency difference, the dual phase-locked loop method can greatly simplify the circuit structure and improve the signal quality;

[0014] The present invention adopts a segmented filtering method of a bandpass filter group, which can realize bandpass filtering without harmonic omission for ultra-wideband step frequency signals. Such a circuit structure and selected components can reduce the circuit area under the premise of ensuring high-performance filtering;

[0015] The present invention adopts a digital fixed intermediate frequency structure receiver. First, this structure greatly suppresses DC interference and improves the dynamic range and sensitivity of the receiver. Then, a controllable gain device is adopted to perform individual gain control on each frequency signal, thereby improving the flexibility of the system. Finally, the intermediate frequency signal is sampled and processed by digital demodulation, thereby reducing circuit complexity and improving signal processing quality and flexibility. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A schematic diagram of a high-precision stepped frequency ground penetrating radar transceiver system based on a dual phase-locked loop according to the present invention;

[0017] Figure 2 It is a schematic diagram of the structure of the transmitting part of the present invention;

[0018] Figure 3 It is a schematic diagram of the structure of the receiving part of the present invention;

[0019] Figure 4 It is a schematic diagram of the structure of the bandpass filter bank of the present invention. DETAILED DESCRIPTION

[0020] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0021] The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops proposed in the present invention adopts a dual phase-locked loop transmitter and a digital fixed intermediate frequency structure receiver, and can achieve high-precision detection within at least 2.5m underground with a resolution better than 5cm.

[0022] Among them, the signal source of the transmitter selects the phase-locked loop solution. Compared with the DDS solution, which has problems such as low high frequency, insufficient bandwidth, and large signal spurious, and the mixed frequency solution has a complex circuit structure, the phase-locked loop solution has great advantages. First, the output signal range of the broadband phase-locked loop can cover the required frequency band, and the circuit structure is relatively simple. Secondly, the signal quality it generates is high and the spurious suppression is good. Finally, it can flexibly adjust the frequency, which is convenient for the generation and adjustment of the step frequency signal.

[0023] The receiver adopts a digital fixed intermediate frequency structure. Compared with the zero intermediate frequency structure receiver, there are problems such as DC offset and other DC interference signals that affect the high-resolution performance, and the analog fixed intermediate frequency structure receiver has problems such as amplitude and phase imbalance and complex circuit structure. The digital fixed intermediate frequency structure receiver has great advantages. First, the intermediate frequency signal is easy to be sampled by the analog-to-digital converter (ADC), and the subsequent processing can be completely realized by software, simplifying the circuit structure. Secondly, the whole process of digital demodulation is more flexible and controllable, and it is easy to implement. Finally, digital demodulation can avoid problems such as amplitude and phase imbalance in analog circuit demodulation and improve signal processing quality.

[0024] The present invention uses two phase-locked loops with the same reference signal to generate radio frequency and local oscillator signals respectively. Compared with a single phase-locked loop power splitting into two paths and then orthogonally modulating one path to obtain a local oscillator signal, the dual phase-locked loop has certain advantages. On the one hand, the frequency of the phase-locked loop output signal can be flexibly controlled and the signal spurious is small, and two signals with a fixed frequency difference that meet the requirements can be generated. On the other hand, there is no additional circuit for orthogonal demodulation and related problems, the circuit structure is simple and the signal quality is better.

[0025] As shown in Figure 1, the present invention consists of five parts: a transmitting part, a receiving part, an intermediate frequency sampling part, a control communication part, and a power supply part. Among them, the transmitting part is used to transmit the radio frequency signal to the outside, and transmit the generated reference local oscillator signal to the receiving part; the receiving part is used to receive the external echo signal, and mix it with the reference local oscillator signal to output the intermediate frequency analog signal; the intermediate frequency analog signal is digitally sampled by the intermediate frequency sampling part to obtain the sampling signal and send it to the control communication part; the above three parts all work under the control of the control communication part, which also communicates and transmits data with the computer through the network cable; the power supply part provides power for the entire system.

[0026] The transmitting part mainly completes the transmission of RF signals and the generation of local oscillator signals. The structure is shown in Figure 2, including a transmitter and a transmitting antenna. The transmitting antenna adopts a horn double-ridge antenna. The transmitter includes a reference signal generating circuit, a RF signal transmitting circuit and a local oscillator signal generating circuit. The reference signal generating circuit is composed of a crystal oscillator, a π-type attenuation circuit, a low noise amplifier, a low-pass filter and a three-way power divider. The 50MHz temperature-compensated crystal oscillator generates the clock signal of the entire system, and generates a reference signal after being attenuated by the π-type attenuation circuit, amplified by the low noise amplifier and filtered by the 50MHz low-pass filter. The reference signal is sent to the phase-locked loop (PLL) of the RF signal transmitting circuit and the local oscillator signal generating circuit by the three-way power divider as a reference input signal, and the last one is used as the external clock of the FPGA (field programmable gate array) core board to ensure that the control signal timing is synchronized with the signal source timing, and also provide a synchronous clock for controlling the intermediate frequency sampling. The RF signal transmitting circuit includes a first phase-locked loop, a bandpass filter group and a power amplifier cascaded in sequence. The 50MHz reference signal enters the first phase-locked loop of the RF signal transmission circuit. Under the control of the FPGA core board, the first phase-locked loop outputs the first step frequency signal of 0.5~3.5GHz and the step amount is 20MHz. After the harmonics are filtered out by the bandpass filter group, the RF signal is finally amplified and output by the power amplifier. The local oscillator signal generation circuit is composed of the second phase-locked loop and the bandpass filter group. The 50MHz reference signal enters the second phase-locked loop of the local oscillator signal generation circuit. Under the control of the FPGA core board, the second phase-locked loop outputs the second step frequency signal of 0.52~3.52GHz and the step amount is 20MHz. After the harmonics are filtered out by the bandpass filter group, it is used as the reference local oscillator signal for receiving mixing.

[0027] The receiving part mainly completes the amplification, filtering and mixing of the echo signal to obtain the intermediate frequency analog signal output. The structure is shown in Figure 3, including a receiver and a receiving antenna. The receiving antenna adopts the same horn double-ridge antenna as the transmitting antenna. The receiver includes a radio frequency processing circuit and an intermediate frequency processing circuit. The radio frequency processing circuit consists of a first-stage low noise amplifier, a digital attenuator, a second-stage low noise amplifier, a bandpass filter group and a mixer that are cascaded in sequence. The received echo signal is first amplified by the first-stage low noise amplifier, then attenuated by the digital attenuator according to the control signal, and then amplified by the second-stage low noise amplifier to obtain sufficient receiving gain. Finally, the harmonics are filtered out by the bandpass filter group, and then enter the mixer to multiply with the reference local oscillator signal to obtain a mixed signal. The intermediate frequency processing circuit consists of a low-pass filter and a digital gain amplifier (DGA). The mixed signal is subjected to the first low-pass filtering to obtain a fixed intermediate frequency signal of 20MHz, and then the power is adjusted by the DGA, and finally the intermediate frequency analog signal is output after the second low-pass filtering.

[0028] Among them, the bandpass filter group mentioned in the transmitting and receiving parts is a combined circuit, which uses the time discrete characteristics of the step frequency signal and adopts a multi-channel segmented filtering method to complete the bandpass filtering of the ultra-wideband step frequency signal with a relative bandwidth of 150% without harmonic omission. The structure is shown in Figure 4, which consists of 2 4-channel high-speed switches and 8 patch filters. The 8 patch filters are divided into 4 groups, each group includes 1 low-pass filter and 1 high-pass filter, and the 2 4-channel high-speed switches are arranged on both sides of the 4 groups of patch filters. The patch filters use the HFCN and LFCN series high and low pass filters of Mini-Circuits. Not only are each only 1206 package size, making the entire circuit area small, but also they have excellent performance.

[0029] The intermediate frequency sampling part mainly completes the digital sampling of the intermediate frequency analog signal. It includes the ADC sampling circuit. The 20MHz intermediate frequency analog signal is sampled into a 14-bit digital signal at a rate of 100Msps by the ADC, and finally transmitted to the FPGA core board for subsequent processing.

[0030] The control and communication part mainly completes the control of each module of the entire ground penetrating radar and communicates and transmits data with the computer. Including the FPGA core board. The FPGA core board uses the AC7020 core board of Heijin, which is equipped with the ZYNQ7000 series FPGA of Xilinx. The FPGA core board communicates with the computer through the network cable, and controls the phase-locked loop according to the instructions of the computer to realize the emission, stop and parameter setting of the step frequency signal. At the same time, it controls the bandpass filter group, digital attenuator and DGA to realize independent processing of each frequency point. The sampling data of the intermediate frequency signal is also transmitted from the FPGA core board to the computer through the network cable.

[0031] The power supply part mainly provides various stable and reliable power supply voltages for each device. The power supply part adopts a two-stage step-down method. The external input 9~36V DC is first stepped down to 6.5V, 6V and 5V DC by the first-stage DC-DC converter, and then converted to 6V, 5V and 3.3V DC by the second-stage low dropout regulator (LDO). Finally, after being isolated by magnetic beads, low ripple power is provided to each device to ensure the good operation of the RF device.

[0032] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops, characterized in that: The system includes a transmitting part, a receiving part, an intermediate frequency sampling part, a control communication part and a power supply part, wherein: The transmitting part includes a radio frequency signal transmitting circuit and a local oscillator signal generating circuit, which are respectively used to transmit the radio frequency signal to the outside and transmit the generated reference local oscillator signal to the receiving part; wherein the radio frequency signal transmitting circuit includes a first phase-locked loop connected to the band-pass filter group, and the local oscillator signal generating circuit includes a second phase-locked loop connected to the band-pass filter group; The receiving part is used to receive the external echo signal, mix it with the reference local oscillator signal and output an intermediate frequency analog signal; The intermediate frequency analog signal is digitally sampled by the intermediate frequency sampling part to obtain a sampling signal, and the sampling signal is transmitted to the control communication part; The power supply part provides power for the entire system.

2. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loop according to claim 1, characterized in that: The transmitter also includes a reference signal generating circuit, the temperature compensated crystal oscillator of the reference signal generating circuit is used to generate a clock signal for the entire system, and the reference signal is generated after being attenuated by a π-type attenuation circuit, amplified by a low-noise amplifier, and filtered by a low-pass filter. The reference signal is sent by a three-way power divider to the first phase-locked loop and the second phase-locked loop of the radio frequency signal transmitting circuit and the local oscillator signal generating circuit as a reference input signal, and is sent to the FPGA core board as an external clock.

3. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loop according to claim 2, characterized in that: The radio frequency signal transmitting circuit comprises a first phase-locked loop, a band-pass filter group and a power amplifier which are cascaded in sequence. The reference signal is output as a first step frequency signal through the first phase-locked loop. The first step frequency signal is filtered out of harmonics through the band-pass filter group and output as a radio frequency signal after being amplified by the power amplifier.

4. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 1, characterized in that: The local oscillator signal generating circuit includes a second phase-locked loop and a bandpass filter group which are cascaded in sequence. The reference signal outputs a second step frequency signal through the second phase-locked loop. The second step frequency signal outputs a reference local oscillator signal after harmonics are filtered out by the bandpass filter group.

5. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 1, characterized in that: The receiving part includes a radio frequency processing circuit and an intermediate frequency processing circuit.

6. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 5, characterized in that: The echo signal received by the RF processing circuit is first amplified by a first-stage low-noise amplifier, then attenuated by a digital attenuator according to a control signal, then amplified by a second-stage low-noise amplifier, and finally filtered out harmonics by a bandpass filter group. The signal enters a mixer and is multiplied by the reference local oscillator signal to obtain a mixed signal.

7. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 6, characterized in that: The intermediate frequency processing circuit obtains a fixed intermediate frequency signal by low-pass filtering the mixed signal for the first time, adjusts the power through a digital variable gain amplifier, and then outputs an intermediate frequency analog signal through low-pass filtering for the second time.

8. A high-precision stepped frequency ground penetrating radar transceiver system based on a dual phase-locked loop according to any one of claims 1, 3, 4 or 6, characterized in that: The bandpass filter group adopts a multi-channel segmented filtering method, including 2 4-channel high-speed switches and 8 patch filters. The 8 patch filters are divided into 4 groups, each group includes 1 low-pass filter and 1 high-pass filter, and the 2 4-channel high-speed switches are arranged on both sides of the 4 groups of patch filters.

9. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 1, characterized in that: The intermediate frequency sampling part uses ADC to sample the intermediate frequency analog signal at a rate of 100Msps into a 14-bit digital signal.

10. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 1, characterized in that: The control communication part is also used to control various parts of the entire ground penetrating radar transceiver system and to communicate and transmit data to the computer.

11. The high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 1, characterized in that: The power supply part adopts a two-stage step-down method. The external input 9~36V DC power is first stepped down to 6.5V, 6V and 5V DC power by the first-stage DC-DC converter, and then converted into 6V, 5V and 3.3V DC power respectively by the second-stage low-voltage difference linear regulator. Finally, it is isolated by magnetic beads to provide low-ripple and high-quality power supply to each device.

12. A high-precision stepped frequency ground penetrating radar transceiver system based on dual phase-locked loops according to claim 3 or 4, characterized in that: The first step frequency signal frequency is 0.5~3.5GHz, the step size is 20MHz, and the second step frequency signal frequency is 0.52~3.52GHz, the step size is 20MHz.

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