Moon detection cubesat measurement and control responder system and emission signal coherent carrier generation method thereof

Abstract

The invention discloses a lunar exploration cubesat measurement and control responder system and a transmitting signal coherent carrier generation method thereof, and the system comprises a frequency mixer, a receiving local oscillator phase-locked loop, an intermediate frequency sampling analog-to-digital converter, a voltage-controlled oscillator, a digital-to-analog converter, a transmitting local oscillator phase-locked loop and a carrier tracking loop. Wherein the frequency mixer generates an intermediate-frequency signal according to radio frequency input and a local oscillation signal, the intermediate-frequency signal is output to the intermediate-frequency sampling analog-to-digital converter after band-pass filtering, and the intermediate-frequency sampling analog-to-digital converter outputs the intermediate-frequency signal to the carrier tracking loop after the intermediate-frequency signal is subjected to down-conversion processing of an X-band radio frequency signal; the carrier tracking loop outputs a digital signal to the digital-to-analog converter to generate an analog voltage for controlling the voltage-controlled oscillator; the voltage-controlled oscillator controls the output frequency through the voltage of the digital-to-analog converter; receiving a reference signal output by the local oscillator phase-locked loop according to the voltage-controlled crystal oscillator, performing phase-locked frequency multiplication processing and generating a local oscillator signal; and the transmitting local oscillator phase-locked loop performs phase-locked frequency multiplication processing on the reference signal. According to the invention, in an analog-digital combination mode, a digital frequency-locked phase-locked loop is used for controlling the output voltage of a DAC to control the frequency of a reference VCXO, so that carrier tracking of a radio frequency-intermediate frequency-digital large loop closed loop is completed; downlink coherent carriers can be directly obtained by PLL (Phase Locked Loop), and the requirement of downlink / uplink forwarding ratio 880 / 749 is strictly met.

Description

Technical Field

[0001] The present invention relates to a technology in the field of satellite communication, in particular to a lunar exploration cubic satellite tracking and control transponder system and a method for generating a coherent carrier of a transmission signal thereof. Background Art

[0002] In the field of satellite measurement and control, the onboard measurement and control transponder cooperates with the ground station to complete the satellite's uplink and downlink data transmission, ranging, speed measurement and orbit determination. Among them, the coherent carrier forwarding function of the transponder is a technical approach to achieve high-precision satellite speed measurement. At present, the measurement and control transponder mainly adopts analog large closed loop method or intermediate frequency digital compensation method to realize carrier tracking and coherent carrier forwarding. The analog large closed loop method uses a voltage-controlled crystal oscillator (VCXO) with a frequency of f0 as the frequency reference. The multiple frequency conversion local oscillator signals of the receiving channel are all integer multiples of f0. The voltage control signal that controls the crystal frequency is generated by the error signal of the analog carrier tracking loop. The downlink coherent carrier signal is obtained by phase-locked frequency multiplication with f0 as the reference. The main disadvantage of the analog transponder is that the center frequency f0 of the crystal oscillator is usually not an integer, and the device needs to be customized, resulting in a long single-machine development cycle. The intermediate frequency digital compensation method adopts a heterodyne receiving structure. After sampling the intermediate frequency signal through the ADC, the carrier tracking and frequency difference calculation of the uplink signal are completed through digital signal processing. After the frequency difference compensation, the transmitting intermediate frequency is passed through the DAC and up-mixed to obtain the final downlink coherent carrier. The all-digital intermediate frequency method needs to go through at least one up-conversion mixing to generate the downlink carrier, which has the problem of impure transmission spectrum caused by local oscillator leakage and has high requirements for the transmission filter. Summary of the invention

[0003] Aiming at the shortcomings of existing satellite measurement and control analog or intermediate frequency digital transponders, such as special crystal oscillator frequency, complex receiving carrier tracking and transmitting coherent carrier generation circuit structure, the present invention proposes a lunar exploration cubic satellite measurement and control transponder system and a transmission signal coherent carrier generation method thereof, which achieves carrier tracking of a RF-IF-digital large closed loop by controlling the DAC output voltage through a digital frequency-locked phase-locked loop through an analog-digital combination method; the downlink coherent carrier can be directly obtained by using a PLL, the emission spectrum has low spuriousness and high purity, and strictly meets the requirement of the downlink / uplink forwarding ratio of 880 / 749.

[0004] The present invention is achieved through the following technical solutions:

[0005] The present invention relates to a lunar exploration cubic satellite measurement and control transponder system, comprising: a mixer, a receiving local oscillator phase-locked loop, an intermediate frequency sampling analog-to-digital converter, a voltage-controlled oscillator, a digital-to-analog converter, a transmitting local oscillator phase-locked loop and a carrier tracking loop, wherein: the mixer generates an intermediate frequency signal according to a radio frequency input and a local oscillator signal, and outputs it to the intermediate frequency sampling analog-to-digital converter after bandpass filtering; the intermediate frequency sampling analog-to-digital converter performs a down-conversion process of an X-band radio frequency signal on the intermediate frequency signal and outputs it to the carrier tracking loop; the carrier tracking loop outputs a digital signal to the digital-to-analog converter to generate an analog voltage for controlling a voltage-controlled oscillator; the voltage-controlled oscillator controls the output frequency through the voltage of the digital-to-analog converter; the receiving local oscillator phase-locked loop performs phase-locked frequency multiplication processing according to a reference signal output by the voltage-controlled crystal oscillator and generates a local oscillator signal; the transmitting local oscillator phase-locked loop performs phase-locked frequency multiplication processing on the reference signal.

[0006] The carrier tracking loop includes: a digitally controlled oscillator, a digital orthogonal mixer, a frequency detector and a loop filter, wherein: the digital orthogonal mixer demodulates the intermediate frequency digital signal output by the intermediate frequency sampling analog-to-digital converter and the local oscillator digital signal to generate a baseband signal and outputs it to the frequency detector; the frequency detector outputs the frequency error signal and the phase error signal to the loop filter respectively; the loop filter performs low-pass filtering on the frequency error signal and the phase error signal and then outputs them to the digital-to-analog converter to realize feedback control.

[0007] The mixer is a diode passive double-balanced mixer, and the center frequency of the output signal at its intermediate frequency output port is f IF It is 70MHz.

[0008] The nominal center frequency of the voltage-controlled crystal oscillator is 100 MHz, and the output frequency of the crystal oscillator after voltage control is f REF As the reference clock frequency of the whole machine.

[0009] The sampling frequency of the intermediate frequency sampling analog-to-digital converter is f ADC , which is equal to the output frequency f of the VCXO REF .

[0010] The receiving local oscillator phase-locked loop is a phase-locked frequency multiplication circuit, and the input is the reference frequency f REF , the output is the receiving local oscillator frequency f RXLO .

[0011] The transmitting local oscillator phase-locked loop is a phase-locked frequency multiplication circuit, and its input is the output frequency f of the voltage-controlled crystal oscillator. REF , the output frequency is the downlink carrier frequency f DOWN , f DOWN The range is approximately 8395MHz to 8495MHz.

[0012] The present invention relates to a method for generating a coherent carrier of a transmission signal of the above system, comprising:

[0013] Step 1: System initialization: Set the receiving radio frequency to a specified frequency point f within the range of 7145MHz to 7230MHz UP .

[0014] Step 2: According to the satellite's overall recommended measurement and control uplink frequency f UP , downlink frequency f DOWN , calculate the transmit frequency multiplication ratio R of the transmit local oscillator phase-locked loop PLLD =f DOWN / f REF According to the X-band measurement and control standard, the frequency point selection must meet f UP / f DOWN =749 / 880.

[0015] The frequency point is selected as the frequency value that is divisible by 749 at the precision of 1KHz. UP , then the corresponding f DOWN It can also be accurately expressed with 1KHz precision. At this time, the transmitting frequency multiplication ratio R PLLD can be expressed with an exact finite decimal number (such as f UP =7219611KHz, f DOWN =8482320KHz, at this time R PLLD =84.8232).

[0016] Step 3: Calculate the receiving frequency multiplication ratio R of the receiving local oscillator phase-locked loop PLLU =(f UP -f IF ) / f REF The received frequency multiplication ratio is also expressed as an accurate finite decimal (as described in step 2 UP =7219611KHz, then R PLLU =(7219.611-70) / 100= 71.49611).

[0017] Step 4: Set the phase error signal output by the loop filter to the DAC: According to the undersampling principle, the output frequency of the digital controlled oscillator must meet f NCO =f REF -f IF =30MHz,

[0018] The output frequency of the numerically controlled oscillator is determined by setting the NCO frequency control word W NCO =f NCO / f REF ×2 N = 30 / 100×2 N Implementation, that is, f NCO =W NCO×f REF / 2 N , where: N is the NCO bit width, usually 32bit to 48bit, and the NCO clock frequency is f REF .

[0019] Step 5: Verify that in the presence of Doppler frequency deviation, after the carrier loop is closed, the downlink / uplink frequency ratio of the transponder is 880 / 749. Specifically, assume that the actual receiving frequency f' of the satellite tracking and control transponder is UP =f UP (1+Δ), after the receiving carrier tracking loop is closed, the actual receiving intermediate frequency f' IF =f' REF -f' NCO (The phase-locked loop tracking frequency difference is zero), that is, the actual receiving frequency f' UP -R PLLU ×f' REF = f' REF -W NCO ×f' REF / 2 N , we can get f' UP =(1-W NCO / 2 N +R PLLU )×f' REF ;f UP =(1-W NCO / 2 N +R PLLU )×f REF , that is, f' REF =f REF (1+Δ), f' DOWN =f' REF ×R PLLD =f REF (1+Δ)×R PLLD =f DOWN (1+Δ), and finally get: f' DOWN / f' UP =f DOWN / f UP =880 / 749, where: f' DOWN 、f' REF 、f' IF 、f' NCO Indicates the actual downlink transmission frequency, actual crystal frequency, actual receiving intermediate frequency and actual NCO operating frequency, R PLLU , R PLLD and W NCO / 2 N The fixed setting value is not affected by Doppler. Technical Effects

[0020] The present invention adopts a large closed-loop tracking transponder structure combining analog and digital, that is, the DAC outputs analog voltage through the digital frequency-locked phase-locked loop inside the FPGA, and the frequency of the VCXO is controlled as the reference of the transmitting and receiving local oscillator PLL, thereby forming a closed-loop control of the RF-IF-digital large loop. Compared with the prior art, the large closed-loop tracking structure combining analog and digital can simultaneously complete the carrier tracking of the received signal and the generation of the coherent carrier of the transmitted signal, and the frequency forwarding ratio is strictly 880 / 749. The structure has the advantages of more flexible selection of crystal oscillator frequency than the existing analog transponder, simpler than the existing digital transponder transmitter coherent carrier generation circuit (no need for transmission up-conversion) and high spectrum purity (no frequency conversion local oscillator leakage). BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the system of the present invention;

[0022] Figure 2 is a flow chart of an embodiment;

[0023] Figure 3-Figure 6 It is a schematic diagram of the effect of the embodiment. DETAILED DESCRIPTION

[0024] like Figure 1 As shown, a lunar exploration cube satellite tracking and control transponder system involved in this embodiment includes: a mixer, a receiving local oscillator phase-locked loop, an intermediate frequency sampling analog-to-digital converter, a voltage-controlled oscillator, a digital-to-analog converter, a transmitting local oscillator phase-locked loop and a carrier tracking loop, wherein: the carrier tracking loop includes: a digitally controlled oscillator, a digital orthogonal mixer, a frequency detector and a loop filter.

[0025] The mixer is a diode passive double-balanced mixer, the ADC sampling frequency is greater than 100MHz, the quantization bit is 14bit, the FPGA uses a general device with a scale greater than 1 million gates, the DAC update frequency is greater than 10MHz, the quantization bit is 14bit, the voltage-controlled crystal oscillator center frequency is 100MHz, the controllable frequency range is greater than ±100ppm, the receiving and transmitting local oscillator phase-locked frequency multiplication adopts fractional frequency PLL, and the output frequency covers the X frequency band.

[0026] like Figure 2 As shown, this embodiment relates to a lunar exploration cube satellite tracking and control transponder system and a method for generating a coherent carrier of a transmission signal thereof, including:

[0027] Step 1: Construction Figure 1 The lunar exploration cube satellite tracking and control transponder system is shown.

[0028] Step 2: Select a set of UP / f DOWN = X-band uplink / downlink frequency of 749 / 880, such as fUP =7219.611MHz, f DOWN =8482.320MHz, then the multiplication ratio R of the transmitting local oscillator frequency PLL can be calculated PLLD =f DOWN / f REF =84.8232, according to the R PLLD The transmit PLL chip parameter configuration is completed.

[0029] Step 3: Calculate the multiplication ratio R of the receiving PLL PLLU =(f UP -f IF ) / f REF =(7219.611-70) / 100=71.49611, according to the R PLLU The value completes the receive PLL chip parameter configuration.

[0030] Step 4: Construct the digital circuit part of the carrier tracking loop in the FPGA, which consists of a numerically controlled oscillator (NCO), a digital quadrature mixer, a frequency detector, a loop filter, etc. The error control signal output by the loop filter is sent to the DAC to convert it into an analog voltage signal. NCO =f REF -f IF =30MHz, take NOC bit width as 32bit, set NCO frequency control word W NCO =f NCO / f REF ×2 N =30 / 100×2 32 , the NCO clock frequency is f REF =100MHz.

[0031] Step 5: It can be verified that in the presence of Doppler frequency deviation, after the carrier loop is closed, the downlink / uplink frequency ratio of the transponder is 880 / 749. For example: if the Doppler frequency is 100KHz at a certain moment, the actual receiving frequency is f' UP =7219.711 Calculate the carrier tracking loop after closing the loop f' REF =100.0013851161790MHz, f' IF =70.0009695813251MHz, f' DOWN =8482.4374899866500, satisfies f' DOWN / f' UP =880 / 749.

[0032] After specific experiments, the Pakistani CubeSat tracking and control transponder was used for the first time in my country's lunar exploration project "Chang'e 6", and the actual on-orbit speed measurement error of the satellite was less than 1mm / s (rms). The satellite on-orbit data measured by the Kashgar Deep Space Station of the Xi'an Satellite Tracking and Control Center of China is shown in the following table. The measured results are better than the speed measurement error of general CubeSats (greater than 1mm / s), and most CubeSats do not have speed measurement functions due to volume and weight restrictions, and only have remote control and telemetry functions.

[0033] Table 1 Random error test results of uplink and downlink bidirectional speed measurement of 35m antenna at Kashgar Deep Space Station

[0034] Table 2 Random error test results of three-way velocity measurement of 18-meter antenna uplink and 35-meter antenna downlink at Kashgar Deep Space Station (Doppler velocity measurement)

[0035] Table 3 Random error test results of three-way speed measurement of 18-meter antenna uplink and 35-meter antenna downlink at Kashgar Deep Space Station (carrier phase speed measurement)

[0036] like Figure 3 As shown, it is the digital transmission envelope signal of the downlink signal S / Φ=80dBH of the Kashgar Deep Space Station.

[0037] Table 4. Coherent forwarding ratio test record of Kashgar deep space tracking and control station. The corresponding signal spectrum is as follows Figure 4-Figure 6 shown.

[0038] Compared with the prior art, the present invention only needs one voltage-controlled crystal oscillator, and the crystal oscillator frequency can be selected as a conventional integer frequency such as 100MHz; the transmitting hardware circuit can generate a coherent carrier by only one PLL module, which greatly simplifies the transmitter structure; considering the actual situation of miniaturization and low power consumption requirements of the lunar exploration cube satellite measurement and control transponder, on the basis of traditional satellite analog and digital measurement and control transponders, a carrier tracking loop is realized by combining analog and digital methods, and the error control signal obtained by the digital frequency and phase detector is output through DAC to control the voltage control pin of VCXO to complete closed-loop control. At the same time, the downlink transmission signal carrier is directly multiplied by the same VCXO through PLL. In the case of Doppler frequency deviation in the received signal, the downlink carrier transmitted by this transponder is strictly coherent with the uplink carrier frequency, and the coherent forwarding ratio is equal to 880 / 749. The coherent forwarding function can eliminate the influence of the transponder crystal oscillator frequency accuracy on the satellite speed measurement error. The transmitting circuit of this transponder does not require up-conversion and mixing, which is conducive to the miniaturization and low-power design of the product.

[0039] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. A lunar exploration cube satellite tracking and control transponder system, characterized in that: include: A mixer, a receiving local oscillator phase-locked loop, an intermediate frequency sampling analog-to-digital converter, a voltage-controlled oscillator, a digital-to-analog converter, a transmitting local oscillator phase-locked loop and a carrier tracking loop, wherein: the mixer generates an intermediate frequency signal according to the radio frequency input and the local oscillator signal, and outputs it to the intermediate frequency sampling analog-to-digital converter after bandpass filtering; the intermediate frequency sampling analog-to-digital converter outputs the intermediate frequency signal to the carrier tracking loop after down-conversion processing of the X-band radio frequency signal; the carrier tracking loop outputs a digital signal to the digital-to-analog converter to generate an analog voltage for controlling the voltage-controlled oscillator; the voltage-controlled oscillator controls the output frequency through the voltage of the digital-to-analog converter; the receiving local oscillator phase-locked loop performs phase-locked frequency multiplication processing according to the reference signal output by the voltage-controlled crystal oscillator and generates a local oscillator signal; the transmitting local oscillator phase-locked loop performs phase-locked frequency multiplication processing on the reference signal.

2. The lunar exploration CubeSat tracking and control transponder system according to claim 1 is characterized in that: The carrier tracking loop includes: a digitally controlled oscillator, a digital orthogonal mixer, a frequency detector and a loop filter, wherein: the digital orthogonal mixer demodulates the intermediate frequency digital signal output by the intermediate frequency sampling analog-to-digital converter and the local oscillator digital signal to generate a baseband signal and outputs it to the frequency detector; the frequency detector outputs the frequency error signal and the phase error signal to the loop filter respectively; the loop filter performs low-pass filtering on the frequency error signal and the phase error signal and then outputs them to the digital-to-analog converter to realize feedback control.

3. A method for generating a coherent carrier of a transmission signal based on the system of claim 1 or 2, characterized in that: include: Step 1: System initialization: Set the receiving radio frequency to a specified frequency point f within the range of 7145MHz to 7230MHz UP ; Step 2: According to the satellite's overall recommended measurement and control uplink frequency f UP , downlink frequency f DOWN , calculate the transmit frequency multiplication ratio R of the transmit local oscillator phase-locked loop PLLD =f DOWN / f REF According to the X-band measurement and control standard, the frequency point selection must meet f UP / f DOWN =749 / 880; Step 3: Calculate the receiving frequency multiplication ratio R of the receiving local oscillator phase-locked loop PLLU =(f UP -f IF ) / f REF ; Step 4: Set the phase error signal output by the loop filter to the DAC: The output frequency of the digital controlled oscillator must meet f NCO =f REF -f IF =30MHz.

4. The method for generating a coherent carrier wave of a transmission signal according to claim 3, wherein: The frequency point is selected as the frequency value that is divisible by 749 at the precision of 1KHz. UP , the corresponding f DOWN Use 1KHz precision to accurately express, at this time the transmission frequency ratio R PLLD Can be expressed with accurate finite decimal places; The receiving frequency multiplication ratio is expressed using an accurate finite-digit decimal number.

5. The method for generating a coherent carrier wave of a transmission signal according to claim 3, wherein: The output frequency of the numerically controlled oscillator is determined by setting the NCO frequency control word W NCO =f NCO / f REF ×2 N =30 / 100×2 N Implementation, that is, f NCO =W NCO ×f REF ×2 N , where: N is the NCO bit width, and the NCO clock frequency is f REF .

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