System and method for measuring noise of local oscillator signal source based on superconducting tunnel junction

Abstract

The invention provides a local oscillator signal source noise measurement system and method based on a superconducting tunnel junction, and belongs to the technical field of terahertz detection. According to the invention, a local oscillator signal source module comprises a reference signal source and a to-be-detected signal source which are respectively connected with an electrically controlled attenuator, and the electrically controlled attenuator is connected with a superconducting detector module; the matching load module is composed of a black body load and is used for providing a radio frequency signal; the local oscillator signal and the radio frequency signal are transmitted to the superconducting detector module for frequency mixing, and an intermediate frequency signal generated in the frequency mixing process is transmitted to the intermediate frequency module for amplification and acquisition; the computer performs data interaction with the bias module and the intermediate frequency module, and the bias module acts on the superconducting detector module. According to the invention, high-sensitivity local oscillator noise measurement and electric power adjustment functions are integrated, local oscillator signal source noise and mixer noise can be effectively decoupled under different local oscillator power conditions, and the stability and accuracy of a measurement result can still be ensured under the condition that the test bandwidth and the system gain are unknown.

Description

Technical Field

[0001] The present invention belongs to the technical field of terahertz detection, and particularly relates to a local oscillator signal source noise measurement system and method based on a superconducting tunnel junction. Background Art

[0002] A Superconductor-Insulator-Superconductor (SIS) mixer is a high-sensitivity and low-noise superconducting detector, which is widely used in high-resolution detection, radio astronomy, space remote sensing, deep space exploration and other fields from millimeter wave to terahertz band. In most superheterodyne superconducting SIS mixer receiver systems, its sensitivity is usually characterized by the system equivalent noise temperature.

[0003] In a superconducting SIS mixer receiver system, the Local Oscillator (LO) is the core component in the mixing process. Its function is to provide a stable local oscillator signal to drive the superconducting SIS mixer to achieve frequency conversion. Generally, the system noise mainly comes from the superconducting SIS mixer noise, the radio frequency noise caused by quasi-optical transmission loss, and the intermediate frequency noise introduced by the intermediate frequency components cascaded with the mixer, etc. However, in recent years, it has been found that the solid-state local oscillator signal source based on a microwave reference source - multi-stage frequency multiplication and amplification chain, although achieving wide-band continuous tuning and power programmable control, introduces more significant local oscillator noise when coupled to the receiver system during the mixing process. This not only affects the sensitivity and dynamic range of the system, but also has an adverse impact on the long-term stability, especially in an extremely low-noise environment and high-sensitivity detection tasks, its impact is particularly prominent. Therefore, the local oscillator noise has become one of the key factors restricting the performance improvement of high-sensitivity superconducting SIS mixer receivers.

[0004] In a traditional room-temperature Schottky diode mixer receiver, due to the relatively high noise of the mixer itself, the influence of the local oscillator noise is relatively small and is usually ignored. However, with the development of high-sensitivity superconducting SIS mixer receivers, whose noise level has approached 2 to 3 times the quantum limit and the detection sensitivity has been greatly improved, the influence of the local oscillator noise has become increasingly significant and even become the core bottleneck restricting the overall performance of the system.

[0005] In most high-sensitivity receiver systems in the terahertz frequency band, due to the lack of effective measurement techniques, it is difficult to directly separate the local oscillator (LO) noise and mixer noise under different LO coupling power conditions. The influence mechanism of the change in LO coupling power on the mixer noise characteristics is complex and significant. Specifically, this influence is mainly reflected in the following two aspects: (1) The change in LO coupling power will change the quantum tunneling characteristics and frequency conversion efficiency of the superconducting SIS mixer, thereby causing dynamic changes in quantum noise, conversion loss, and additional noise; (2) The LO noise enters the mixer system through the sideband coupling mechanism, and its coupling efficiency shows a non-linear relationship with the LO power level, resulting in a strong power dependence of the system noise characteristics, further increasing the measurement difficulty.

[0006] In view of this, there is an urgent need to develop high-precision measurement techniques to achieve accurate characterization and decoupling of the LO noise of terahertz superconducting SIS mixer receivers, so as to provide reliable theoretical and technical support for the optimized design of SIS receivers. Summary of the Invention

[0007] The present invention aims at the measurement problem caused by the cross-coupling of the LO signal source noise and mixer noise under different LO coupling power conditions in a terahertz superconducting SIS mixer receiver, and provides a LO signal source noise measurement system and method based on a superconducting tunnel junction, which is applicable to the noise measurement and optimization of high-sensitivity RF receivers.

[0008] To achieve the above object, the present invention adopts the following technical solutions: In the first aspect, the present invention provides a LO signal source noise measurement system based on a superconducting tunnel junction, including: a LO signal source module, a superconducting detector module, a matching load module, a bias module, an intermediate frequency (IF) module, and a computer; the LO signal source module is used to provide a LO signal, including a reference signal source and a signal source under test respectively connected to an electrically tunable attenuator, and the electrically tunable attenuator is connected to the superconducting detector module; the matching load module is composed of a blackbody load and is used to provide a RF signal; the LO signal and the RF signal are transmitted to the superconducting detector module for mixing, and the IF signal generated during the mixing process is transmitted to the IF module for amplification and acquisition; the computer performs data interaction with the bias module and the IF module, and the bias module acts on the superconducting detector module.

[0009] Optionally, the bias module includes a bias circuit and a bias power supply. The bias circuit is used to provide a DC bias and an output path for the IF signal to the superconducting detector module, and the bias power supply is used to stabilize the bias operating point of the superconducting detector module.

[0010] Optionally, the IF module includes an amplifier and a spectrum analyzer. After the IF signal is amplified by the amplifier, it is collected by the spectrum analyzer.

[0011] In a second aspect, the present invention provides a method for measuring the noise of a local oscillator signal source based on a superconducting tunnel junction, which uses the local oscillator signal source noise measurement system described in the first aspect for measurement, and includes the following steps: Step 1: Scan the DC characteristic I-V curve of the superconducting detector module under pumping and non-pumping conditions, and calculate the normal state resistance and the local oscillator voltage amplitude accordingly; Step 2: Turn on the reference signal source, adjust the local oscillator power using an electrically tunable attenuator, pump the bias current of the superconducting detector module to different levels respectively, and combine the normal state resistance and the local oscillator voltage amplitude obtained in Step 1 to calculate the local oscillator power corresponding to each pumping state as the local oscillator coupling power of the signal source to be measured; Step 3: Collect the intermediate frequency output power under the conditions of a room temperature blackbody thermal load and a liquid nitrogen blackbody cold load respectively, and calculate the equivalent noise temperature of the reference signal source at different intermediate frequency offsets; Step 4: Turn off the reference signal source, connect the signal source to be measured, and set the frequency of the signal source to be measured to be the same as that of the reference signal source; use an electrically tunable attenuator to adjust the local oscillator power to pump the bias current of the superconducting detector module to the same level as when the reference signal source is turned on; collect the intermediate frequency output power under the condition of a room temperature blackbody thermal load; Step 5: Calculate the additional equivalent noise temperature generated by the signal source to be measured at different intermediate frequency offsets according to the intermediate frequency output power collected under the condition of a room temperature blackbody thermal load in Step 3 and Step 4; Step 6: Obtain the corresponding relationship between the coupling power of the signal source to be measured calculated in Step 2 and the additional equivalent noise temperature calculated in Step 5.

[0012] Optionally, in Step 1, the normal state resistance is calculated according to the DC characteristic I-V curve under non-pumping conditions , and the normalized local oscillator voltage amplitude is calculated according to the DC characteristic I-V curve under pumping conditions .

[0013] Optionally, in Step 2, the local oscillator power is calculated by the following formula: ; where represents the local oscillator power corresponding to the th pumping level, is the number of superconducting tunnel junctions, is the Planck constant, is the local oscillator signal frequency, is the normalized local oscillator voltage amplitude, is the elementary charge, is the normal state resistance.

[0014] Optionally, in Step 3, the room temperature blackbody thermal load The intermediate frequency output power collected under is: ; wherein, is the Boltzmann constant, is the test bandwidth, is the intermediate frequency offset, is the equivalent gain of the system at different intermediate frequency offsets, is the equivalent noise temperature of the system at different intermediate frequency offsets; Liquid nitrogen blackbody cold load The intermediate frequency output power collected under is: ; Define and The ratio of is the Y factor : ; Using the Y factor method, calculate the equivalent noise temperature of the reference signal source at different intermediate frequency offsets is: .

[0015] Optionally, in step 4, the intermediate frequency output power collected under the condition of a normal temperature blackbody heat load is: ; wherein, is the additional equivalent noise temperature generated by the signal source under test at different intermediate frequency offsets.

[0016] Optionally, in step 5, according to the intermediate frequency output powers collected in step 3 and step 4 respectively under the condition of a normal temperature blackbody heat load, first calculate the additional local oscillator noise power generated by the signal source under test at different intermediate frequency offsets: ; Furthermore, calculate the additional equivalent noise temperature generated by the signal source under test at different intermediate frequency offsets: .

[0017] Optionally, in step 6, according to the corresponding relationship between the local oscillator coupling power of the signal source under test and the additional equivalent noise temperature , present the relationship between the local oscillator signal source noise and the intermediate frequency offset under different local oscillator coupling power adjustments.

[0018] The beneficial effects of the present invention are as follows: The present invention integrates high-sensitivity local oscillator noise measurement and electronically tunable power functions, and can effectively decouple the local oscillator signal source noise and mixer noise under different local oscillator power conditions. Even under the conditions where the test bandwidth and system gain are unknown, the system can still ensure the stability and accuracy of the measurement results. The present invention provides a reliable means for characterizing the local oscillator noise for the sensitivity optimization of terahertz receivers, and is of great significance for the design and performance improvement of high-performance terahertz receiving systems. Brief Description of the Drawings

[0019] Figure 1 is a structural diagram of a local oscillator signal source noise measurement system based on a superconducting tunnel junction.

[0020] Figure 2 is a flowchart of a method for measuring the local oscillator signal source noise based on a superconducting tunnel junction.

[0021] Figure 3 is a graph showing the relationship between the bias power supply and the coupling power of a superconducting SIS mixer.

[0022] Figure 4 is a schematic diagram of the additional equivalent noise temperature under different local oscillator coupling powers. Detailed Embodiments

[0023] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application.

[0024] Embodiment 1 This embodiment proposes a local oscillator signal source noise measurement system based on a superconducting tunnel junction, as Figure 1 shown, mainly composed of a local oscillator signal source module, a superconducting detector module (i.e., a superconducting SIS mixer), a matching load module, a bias module, an intermediate frequency module, and a computer.

[0025] The local oscillator signal source module is composed of a reference signal source, a signal source to be measured, and an electronically tunable attenuator. In addition to providing a local oscillator signal with a basic signal frequency, it also calibrates the noise of the signal source to be measured at different coupling powers through the reference signal source, and limits the required local oscillator sideband noise level to ensure the accuracy of the measurement.

[0026] Assume that the required local oscillator power of the superconducting SIS mixer is , in dBm, and the self-noise temperature of the superconducting SIS mixer is . The additional noise contribution of an ideal reference signal source to the superconducting SIS mixer should be less than 5% (i.e., ), and the corresponding noise power spectral density is , with the unit of dBm / Hz. Usually, the intermediate frequency bandwidth of the amplifier is higher than 0.1 GHz. Therefore, when the frequency offset of the reference signal source is higher than 0.1 GHz, the sideband noise power spectral density should meet specific conditions relative to the carrier power, as follows: ; where represents the single-sideband phase noise. According to specific requirements, the of the selected reference signal source should be lower than dBc / Hz, and it is used as a high-precision reference signal source.

[0027] The electrically tunable attenuator achieves precise attenuation control through voltage regulation, with high resolution and excellent adjustment convenience. By changing the voltage applied to the electrically tunable attenuator, the attenuation amount can be precisely adjusted, providing a very fine adjustment step size to meet the requirement for precise control of the bias current of the superconducting SIS mixer.

[0028] The superconducting detector module is designed based on the superconducting tunnel junction and is mainly used to detect weak signals. The matching load module is composed of a room-temperature blackbody or a cryogenic blackbody load to generate radio frequency (RF) signals for measuring the equivalent noise temperature of the system. The bias module consists of a bias circuit and a bias power supply. The main function of the bias circuit is to provide the DC bias for the detector and the output path for the intermediate frequency signal. To effectively isolate the DC and intermediate frequency signals, choke inductors and DC-blocking capacitors are carefully selected for the bias circuit, and protection resistors are added at the DC bias voltage terminal and the current terminal; the bias power supply adopts the four-wire method to eliminate the error caused by the leads and improve the accuracy of the bias current through closed-loop regulation, thereby ensuring the stability of the bias operating point of the superconducting SIS mixer. The intermediate frequency module includes a low-noise amplifier, a room-temperature amplifier, and a spectrum analyzer to achieve the amplification and spectrum detection of the intermediate frequency signal. The computer interacts with the bias module and the intermediate frequency module through communication to realize the automatic scanning process of the superconducting SIS mixer and collect the bias voltage, bias current, and intermediate frequency output spectrum in real time and conveniently.

[0029] The basic working process of the system is as follows: The local oscillator signal and the RF signal are synthesized and then transmitted to the superconducting detector module. The intermediate frequency signal generated during the mixing process is first preliminarily amplified by the low-noise amplifier, and then the amplified intermediate frequency signal is transmitted to the room-temperature amplifier for further amplification, and finally collected and read by the spectrum analyzer.

[0030] Embodiment 2 Based on the system proposed in Embodiment 1, this embodiment proposes a method for measuring the noise of the local oscillator signal source based on the superconducting tunnel junction, as Figure 2 shown, including the following steps: Step 1: Calculate the normal-state resistance With the normalized local oscillator voltage amplitude .

[0031] Using the bias module, the program automatically scans the DC characteristic I-V curves of the superconducting SIS mixer under pumped and unpumped conditions ( and ), and the DC tunneling current of the tunnel junction under the action of the local oscillator signal is given by the following formula: ; where is the DC bias voltage; is the voltage amplitude of the local oscillator signal; is the Bessel function; is the normalized local oscillator voltage amplitude; takes values from to , is the Planck constant, is the local oscillator signal frequency, is the elementary charge amount.

[0032] The normal state resistance is calculated according to the I-V curve under the unpumped condition. In addition, fitting is performed according to the I-V curve under the pumped condition, and the local oscillator current at the bias point is used as the fitting reference to obtain the normalized local oscillator voltage amplitude .

[0033] Step 2: Obtain the local oscillator power at different pump currents.

[0034] Turn on the reference signal source and use the electronically tunable attenuator to precisely adjust the local oscillator power so that the current of the superconducting SIS mixer is pumped to different heights ( take 1, 2, 3...), and calculate the corresponding local oscillator power respectively, as shown in Figure 3 . The local oscillator power is calculated by the Tucker formula, that is: ; where represents the local oscillator power corresponding to the th pump height, is the number of superconducting tunnel junctions, taking 1 for a single junction and 2 for a double junction. Through this formula, the local oscillator power at the corresponding pump level can be accurately determined, that is, the local oscillator power corresponding to the bias current is obtained.

[0035] Step 3: Calculate the system equivalent noise temperature under the reference signal source condition.

[0036] At each pump state, first at the room temperature blackbody thermal load Collect the corresponding intermediate-frequency output power under the condition that (the ambient blackbody load noise temperature is subject to the actual ambient temperature). , that is: ; Among them, is the Boltzmann constant, is the test bandwidth, is the intermediate-frequency offset, is the equivalent gain of the system at different intermediate-frequency offsets, is the equivalent noise temperature of the system at different intermediate-frequency offsets.

[0037] Secondly, collect the intermediate-frequency output power under the condition of a liquid nitrogen blackbody cold load (the liquid nitrogen blackbody temperature is usually taken as 77K). , that is: ; Define the power ratio of the ambient blackbody thermal load to the liquid nitrogen blackbody cold load as the Y factor , as follows: ; Using the Y factor method, calculate the equivalent noise temperature of the reference signal source at different intermediate-frequency offsets as: .

[0038] Step 4: Decouple the local oscillator noise and the superconducting detector noise.

[0039] Turn off the reference signal source, connect the signal source to be measured, and set its frequency to be the same as that of the reference signal source. Subsequently, precisely adjust the local oscillator power through an electrically tunable attenuator, and ensure that the pump is at the same level as the reference signal source by monitoring the bias current of the superconducting SIS mixer in real time, so as to ensure that the noise of the superconducting SIS mixer remains basically unchanged. This process effectively decouples the local oscillator noise and the detector noise, making the equivalent noise other than the local oscillator noise remain constant. In addition, this method can also be used for comparative analysis between signal sources to be measured with different output powers.

[0040] Finally, collect the corresponding intermediate-frequency output power under the condition of the ambient blackbody thermal load . , that is: .

[0041] Step 5: Calculate the additional local oscillator noise power and the equivalent noise contribution of the local oscillator to the system at different frequency offsets.

[0042] First, obtain the additional local oscillator noise power generated by the signal source to be measured at different intermediate frequency offsets. : ; Furthermore, calculate the additional equivalent noise temperature generated by the signal source to be measured at different intermediate frequency offsets under corresponding pump conditions. , that is: .

[0043] Step 6: Characterize the relationship between the local oscillator coupling power and the local oscillator equivalent noise temperature at different intermediate frequency offsets.

[0044] Obtain the coupling power of the signal source to be measured and the additional equivalent noise temperature and present the relationship between the sideband noise of the local oscillator signal source and the intermediate frequency offset under different local oscillator coupling power adjustments, as Figure 4 shown.

[0045] Those of ordinary skill in the art can realize that, combining the units and algorithm steps of each example described in the embodiments disclosed in this application, can be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of this application.

[0046] The above is only the preferred implementation manner of the present invention. The protection scope of the present invention is not limited to the above embodiments. All technical solutions falling within the idea of the present invention belong to the protection scope of the present invention. It should be noted that for those of ordinary skill in the art in this technical field, several improvements and refinements made without departing from the principle of the present invention should be regarded as within the protection scope of the present invention.

Claims

1. A local oscillator signal source noise measurement system based on a superconducting tunnel junction, characterized in that: include: A local oscillator signal source module, a superconducting detector module, a matching load module, a bias module, an intermediate frequency module and a computer; The local oscillator signal source module is used to provide a local oscillator signal, including a reference signal source and a signal source to be measured, which are respectively connected to an electrically adjustable attenuator, and the electrically adjustable attenuator is connected to the superconducting detector module; the matching load module is composed of a black body load and is used to provide a radio frequency signal; The local oscillator signal and the radio frequency signal are transmitted to the superconducting detector module for mixing, and the intermediate frequency signal generated in the mixing process is transmitted to the intermediate frequency module for amplification and collection; the computer interacts with the bias module and the intermediate frequency module for data, and the bias module acts on the superconducting detector module.

2. The local oscillator signal source noise measurement system based on superconducting tunnel junction according to claim 1, characterized in that: The bias module includes a bias circuit and a bias power supply. The bias circuit is used to provide a DC bias and an output path of an intermediate frequency signal to the superconducting detector module. The bias power supply is used to stabilize the bias operating point of the superconducting detector module.

3. The local oscillator signal source noise measurement system based on superconducting tunnel junction according to claim 1, characterized in that: The intermediate frequency module includes an amplifier and a spectrum analyzer. The intermediate frequency signal is amplified by the amplifier and then collected by the spectrum analyzer.

4. A method for measuring local oscillator signal source noise based on a superconducting tunnel junction, using a local oscillator signal source noise measurement system as described in any one of claims 1 to 3 for measurement, characterized in that: The steps include: Step 1: Scan the DC characteristic IV curve of the superconducting detector module under pumping and non-pumping conditions, and calculate the normal state resistance and local oscillation voltage amplitude based on it; Step 2: Turn on the reference signal source, use the electrically adjustable attenuator to adjust the local oscillator power, so that the bias current of the superconducting detector module is pumped to different heights respectively, and combine the normal state resistance and local oscillator voltage amplitude obtained in step 1 to calculate the local oscillator power corresponding to each pumping state as the local oscillator coupling power of the signal source to be measured; Step 3: Collect the intermediate frequency output power under the conditions of room temperature blackbody heat load and liquid nitrogen blackbody cold load, and calculate the equivalent noise temperature of the reference signal source at different intermediate frequency offsets; Step 4: Turn off the reference signal source, connect the signal source to be tested, and set the frequency of the signal source to be tested to be consistent with that of the reference signal source; The local oscillator power is adjusted by an electrically adjustable attenuator so that the bias current of the superconducting detector module is pumped to the same level as when the reference signal source is turned on; the intermediate frequency output power is collected under the condition of a blackbody heat load at room temperature; Step 5: According to the intermediate frequency output power collected in steps 3 and 4 under the condition of a black body heat load at room temperature, calculate the additional equivalent noise temperature generated by the signal source to be tested at different intermediate frequency offsets; Step 6: Obtain the corresponding relationship between the coupled power of the signal source to be measured calculated in step 2 and the additional equivalent noise temperature calculated in step 5.

5. The method for measuring noise of a local oscillator signal source based on a superconducting tunnel junction according to claim 4, characterized in that: In step 1, the normal state resistance is calculated based on the DC characteristic IV curve under no pumping conditions. , the normalized local oscillator voltage amplitude is calculated based on the DC characteristic IV curve under pumping conditions .

6. The method for measuring noise of a local oscillator signal source based on a superconducting tunnel junction according to claim 4, characterized in that: In step 2, the local oscillator power is calculated by the following formula: ； in, Indicates The local oscillator power corresponding to the pump height is is the number of superconducting tunnel junctions, is Planck's constant, is the local oscillator signal frequency, is the normalized local oscillator voltage amplitude, is the basic charge, is the normal resistance.

7. The method for measuring noise of a local oscillator signal source based on a superconducting tunnel junction according to claim 4, characterized in that: In step 3, the normal temperature blackbody heat load The intermediate frequency output power collected under the conditions for: ； in, is the Boltzmann constant, To test bandwidth, is the mid-frequency deviation, is the equivalent gain of the system at different intermediate frequency offsets, is the equivalent noise temperature of the system at different intermediate frequency offsets; Liquid nitrogen blackbody cooling load The intermediate frequency output power collected under the conditions for: ； definition and The ratio is the Y factor : ； Using the Y-factor method, the equivalent noise temperature of the reference signal source at different intermediate frequency offsets is calculated. for: 。 8. The method for measuring noise of a local oscillator signal source based on a superconducting tunnel junction according to claim 7, characterized in that: In step 4, the intermediate frequency output power collected under normal temperature blackbody heat load conditions for: ； in, is the additional equivalent noise temperature generated by the signal source under test at different intermediate frequency offsets.

9. The method for measuring noise of a local oscillator signal source based on a superconducting tunnel junction according to claim 8, characterized in that: In step 5, based on the intermediate frequency output power collected in steps 3 and 4 under normal temperature blackbody heat load conditions, the additional local oscillator noise power generated by the signal source to be tested at different intermediate frequency offsets is first calculated. : ； Then calculate the additional equivalent noise temperature generated by the signal source under test at different intermediate frequency offsets : 。 10. The method for measuring noise of a local oscillator signal source based on a superconducting tunnel junction according to claim 9, characterized in that: In step 6, according to the local oscillator coupling power of the signal source to be measured With additional equivalent noise temperature The corresponding relationship between them shows the relationship between the local oscillator signal source noise and the intermediate frequency deviation under different local oscillator coupling power adjustments.

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