Development of a 90–600 MHz Meter-wave Solar Radio Spectrometer

Radio observation is important for understanding coronal mass ejections (CMEs), coronal shock waves, and high-energy electron acceleration. Here, we developed a new Chashan broadband solar radio spectrometer at a meter wavelength for observing the (super)fine structure of the solar radio burst spectrum. In the signal-receiving unit, we adopt an antenna system consisting of a 12 m large-aperture parabolic reflector and dual-line polarized logarithmic periodic feed source, as well as a high-precision Sun-tracking turntable system, all of which ensure the high-precision acquisition of solar radiation signals. For the digital receiver, we use a high-speed analog-to-digital converter with a sampling rate of 1.25 GSPS to directly sample the signal amplified and filtered by the analog receiver, simplifying the structure of the analog receiver, and design a 16k-point fast Fourier transform algorithm in the field programmable gate array to perform time–frequency transformation on the sampled signals. The default frequency and temporal resolution of the system are 76.294 kHz and 0.839 ms (up to 0.21 ms), respectively. The noise coefficient of the system is less than 1 dB, the dynamic range is more than 60 dB, and the sensitivity is as high as 1 sfu. We have observed a large number of radio bursts, including type I radio storms, hundreds of type III, ∼20 type II, and ∼15 type IV bursts in the past year. These high-quality data are useful in the further study of CMEs and associated particle acceleration and the origins of solar radio bursts.


Introduction
Solar radio bursts are usually used to evaluate properties such as the plasma temperature, density, and magnetic field in the source region according to their emission mechanism in different frequency ranges (Tan et al. 2019;Yan 2021;Yang et al. 2022).Therefore, these data are important for studying and understanding the dynamic processes of solar eruptions (Yan et al. 2006;Tan et al. 2021b;Hegedus et al. 2021;Judge et al. 2021).
Solar radio bursts in the meter-wave band are believed to be generated at above 0.2 solar radii in the corona (Gary & Keller 2004).Various types of bursts appear in this frequency band and are typically characterized as type I-V bursts (Suzuki & Dulk 1985).Specifically, type II solar radio bursts are excited by energetic electrons (Payne-Scott et al. 1947;Wild et al. 1963;Nelson & Melrose 1985) and it is generally believed that coronal shocks accelerate these electrons (Cairns 2004;Chen et al. 2014;Feng et al. 2015;Kong et al. 2015).In the past few years, researchers have conducted extensive investigations on the spectral morphology, radiation mechanism, radio source location, and source region diagnosis of type II radio bursts (Lin et al. 2006;Feng et al. 2012;Kong et al. 2012Kong et al. , 2015;;Feng et al. 2013;Chen et al. 2014;Ding et al. 2014;Du et al. 2014Du et al. , 2015)).Type II radio bursts are the best tracers for shock waves of coronal mass ejections (CMEs), reflecting the formation of shock waves and the transport of energetic electrons (Feng & Lv 2021).The frequency drift of type II radio bursts together with an appropriate coronal density model can be used to estimate the location and velocity of CME shocks (Reiner et al. 2003;Shanmugaraju et al. 2009;Krupar et al. 2019), thereby estimating the time when the CMEs shock waves reach Earth (Dulk et al. 1999).In addition, the frequency-band-splitting structures of Type II radio bursts are commonly used to estimate the magnetic field intensity of the corona and the interplanetary regions (Vasanth et al. 2014).Moreover, solar meter-wave type II radio bursts and interplanetary type II radio bursts are closely related (Rahman et al. 2012).
Type III and type III-like solar radio bursts in the metric wave band have also been studied in previous works (Goldman 1983;Huang 1998;Wu et al. 2002;Reid & Ratcliffe 2014;Tun Beltran et al. 2015;Kong et al. 2016).Type III radio bursts are tracers of nonthermal electrons in coronal and interplanetary space.By detecting and tracking radio type III bursts, the generation, propagation, and evolution of high-energy particle flows can be studied (Lin et al. 1981;Tan et al. 2021a), which are all highly important for studying physical processes such as energy release and particle acceleration in flare/CME processes.The values of properties such as the magnetic field, plasma density, and temperature in the radiation source region can be estimated by extracting fine structural parameter information from radio bursts and combining this information with a radiation mechanism model (Yan 2021), which can aid in obtaining a deeper understanding of the solar burst process.
Therefore, considering the use of radio-based observations in the study of solar eruptions, it is particularly necessary to develop a meter-wave solar radio spectrometer with high temporal resolution, high-frequency resolution, and high sensitivity to study flare and CME phenomena during solar eruptions.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
High-quality observation data can lead to high-quality research achievements, enabled by the use of high-performance observation instruments.Many meter-wave radio spectrum observation systems are currently available (as shown in Table 1), such as RSTN/Learmonth (Kennewell & Cornelius 1983), Phoenix-3 (Benz et al. 2009), the 70-700 MHz lowfrequency solar radio spectrometer at the Yunnan Observatory (Shi et al. 2011), and the 6 m antenna at the Rongcheng Chashan solar radio observatory (CSO; E122°.30, N36°.84;Du et al. 2017) in China.Most of these instruments were built at the end of the last century and the first decade of this century.Due to the limited performance of electronic devices at that time, they cannot simultaneously achieve high performance for all important performance characteristics, such as high temporal resolution and high-frequency resolution.Therefore, ideal structural information is often not obtained, particularly for the monitoring of the fine structures of solar eruptions, such as peak bursts and zebra patterns.
Therefore, we developed the Chashan broadband solar radio spectrometer at meter wavelengths (CBSm) with high performance in the frequency range of 90-600 MHz.It can address the performance shortcomings of existing observation equipment and fill the gaps in geographical location and observation time.The CBSm is an instrument of the Sunplanetary-interstellar monitoring chain subsystem of Phase II of the Chinese Meridian Project, a major national science and technology infrastructure in China, and is also one of the four sets of wide-band solar radio spectral monitoring systems (Wang et al. 2024).We adopt a 12 m aperture mesh parabolic reflector with prime focus and feed-forward mode and a dual linear polarization logarithmic periodic antenna feed in this observation equipment, and we use a 1.25 GSPS high-speed analog-to-digital converter (ADC) to directly sample the signals amplified and filtered by an analog receiver.The observation equipment has a frequency resolution of 76.294 kHz and a temporal resolution of 0.839 ms (up to 0.21 ms).It has some advantages, such as high frequency and temporal resolution, a low noise coefficient, high sensitivity, and a large dynamic range, and is at the state of the art compared with similar equipment domestically and internationally.
In the rest of this paper, we describe the system structure in Section 2, present the analysis of the data calibration in Section 3, and illustrate the effectiveness of the system observations and provide some observation data in Section 4. We summarize the progress achieved in this work in Section 5.

System Overview
There are four subsystems in the CBSm, namely a signalreceiving unit, an analog receiver, a digital receiver, and a host computer and data-storage management unit.It can be used to achieve the set scientific goals and space weather monitoring goals.The structural diagram of CBSm is shown in Figure 1.

Signal-receiving Unit
The signal-receiving unit includes a high-precision Suntracking turntable and an antenna system, as shown in Figure 2.
This system has a fully automatic control function and the ability to track the Sun at high precision in real time.
Table 2 shows the main parameters and technical characteristics of the signal-receiving unit.
In the antenna system, we use a forward-fed parabolic antenna with a 12 m diameter parabolic reflector and a dual-line polarized feed system to achieve high-precision acquisition of solar emission electromagnetic signals and reduce interference from other signals.The electromagnetic signal of solar emission converges to the feed system located at the focal point through a reflecting surface, thereby achieving highdirectionality collection of solar emission signals.In general, the incident signals will experience defocusing when reflected onto the feed source through the antenna reflector.In order to ensure balanced longitudinal focusing throughout the frequency range of 90-600 MHz, the aperture size of the antenna reflector needs to meet certain design requirements, that is, it should be 4 times or more the incident wavelength.The selected antenna reflector aperture size is 12 m, which not only meets the design requirements but also meets the cost requirements.At a low frequency of 90 MHz, the 12 m antenna has a diameter of only 3.6 wavelengths, making it unsuitable for use as a Cassegrain reflector antenna.Therefore, a dual linear polarized logarithmic periodic antenna was selected as the feed to achieve broadband characteristics at the antenna frequencies ranging from 90 to 600 MHz considering the special requirements of small size, low weight, and high reliability of the feed of the mesh parabolic antenna.Considering the antenna aperture, the shielding effect (Lu et al. 2022) of the surrounding hills on the system, and the obstruction of the system to other equipment at the CSO, the base of the high-precision Sun-tracking turntable is fixed on a 3 m high foundation pier, and the azimuth turntable is 9 m away from the top of the foundation pier.
Figure 3 shows the simulations of the antenna pattern of the 0°cross section (i.e., E plane) and 90°cross section (i.e., H plane) of the 12 m aperture parabolic antenna when the signal frequencies are 90 and 600 MHz.The red line is the simulation of the antenna pattern of the 0°cross section and the black line is that of the 90°cross section.
Figure 3(a) shows that the actual antenna gain is greater than 18 dBi at 90 MHz, the 3 dB beamwidth is better than 19°, and the side-lobe level is better than 18 dB.Figure 3(b) shows that the actual antenna gain can reach 34 dBi at 600 MHz, the 3 dB beamwidth is better than 3°, and the side-lobe level is better than 37 dB.This antenna has excellent emission characteristics of high gain and high directionality in the frequency range of 90-600 MHz, making it suitable for tracking and observing small-angle targets such as the Sun.
The test results in a microwave anechoic chamber show that the vertical polarization return loss of the 90-600 MMz logarithmic periodic feed is no greater than −10.286 dB, and the standingwave ratio is no greater than 1.88:1.Moreover, the horizontal polarization return loss is no greater than −10.054 dB, and the standing-wave ratio is no greater than 1.92:1.

Analog Receiver
For the 12 m aperture antenna, the solar radio signal strength of the horizontal and vertical channels formed by the signalreceiving unit is generally approximately 2.826 × 10 −12 mW (i.e., −115.5 dBm); this value is obtained according to the radio signal power spectrum formula (Ji et al. 2006) given in Equation (1) for the solar emission-flux density S equal to 10 sfu, the receiving antenna efficiency η a equal to 0.5, and the Such a weak analog signal cannot be directly digitized by the ADC board in the following digital receiver (the input signal requirement is approximately −60 ∼ 10 dBm); thus, it is necessary to amplify the signal, and the amplification factor should not be less than 50 dB.Considering the actual electromagnetic environment at CSO, the corresponding filtering process must be carried out according to seasonal changes.In addition to signal amplification and filtering functions, analog receivers also must achieve calibration of the receiver system noise and gain.Due to the subsequent digital receiver directly sampling the collected 90-600 MHz signals, there is no need for mixers or related devices.In summary, the electronic devices included in analog receivers mainly include low noise amplifiers (LNAs), radio frequency (RF) switches, standard noise sources, and filters.Figure 4 shows a schematic diagram of the analog receiver, including bandstop filters (BSFs).
The signals of horizontal and vertical orthogonal polarization received by the antenna enter the horizontal and vertical signal channels of the analog receiver, respectively.In each signal channel, the signal first passes through a single-pole three-throw RF switch, and then it is amplified by the primary LNA with a noise coefficient of only 0.7 dB and a gain of 22 dB.The signal is then filtered by a 90-600 MHz bandpass filter and two notch filters with center frequencies of 147 and 361 MHz, respectively.Finally, the amplifier is amplified by the secondary LNA with a gain of 40 dB and passed through a limiter to ensure that the output signal is in the linear working area of the subsequent digital signal receiver.The amplified and filtered signal is transmitted to the digital receiver for further processing through a 25 m coaxial cable.Because the noise coefficient of the first LNA is only 0.7 dB, the noise coefficient of the entire analog receiver can be controlled within 1 dB.
The analog receiver contains two noise sources with excess noise ratios of 35 and 15 dB.Each of these is connected to a power divider, and the four output terminals of the two power dividers are connected to the corresponding interfaces of the two RF switches.Solar radio signals are received, and the system temperature and gain are calibrated by using microcontroller units to control the RF switches to connect the antenna and two noise sources.In addition, the system temperature is calibrated five times a day at Universal Times (UTs) of 00:00, 02:00, 04:00, 06:00 and 08:00 using a program written to automatically control the RF switches connected to noise sources.
Specifically, in our analog receiver, there are only two signal channels, and no mixer or related devices are used, which can result in a relatively simple structure.The reason is that we use an ADC acquisition card with a high sampling rate to directly sample the received signals in the digital receiver.

Digital Receiver
The digital receiver is responsible for converting the signals from the horizontal and vertical signal channels of the analog receiver into time-domain digital signals through analog-todigital (AD) conversion.After a series of digital signal-  processing steps, spectrum data are obtained and then uploaded to the host computer and data-storage management unit through the PCIe interface.A schematic diagram of the digital receiver is shown in Figure 5.
The digital receiver consists of a high-speed acquisition card and a computer, and the acquisition card is composed of a highspeed ADC and a field programmable gate array (FPGA).Here, the FPGA mezzanine card (FMC) subcard is used in combination with the FMC mother card.Because the frequency range of the processed signals is 90-600 MHz, the signalsampling rate should not be lower than 1.2 GHz according to the Nyquist sampling theorem.Therefore, in the receiver, we adopt an ADC with a dual channel 14 bit quantization and a sampling rate of 1.25 GSPS (Yan et al. 2020).The main specifications of the ADC include a theoretical dynamic range of 84 dB, a spurious-free dynamic range of 64.7-78.4dB, a signal-to-noise and distortion of 49.1-57.2dB, and an effective number of bits of >7.86, all of which can ensure that the linear dynamic range of the system is not less than 60 dB.
We use a high-speed computing device FPGA chip KU115 board to ensure operational speed in the digital receiver.After AD conversion, the signal is sent to the FPGA through the JES204B interface.The FPGA performs cumulative integration operations within one temporal resolution unit or some other equivalent but potentially shorter time slot, which can be set according to certain requirements.To facilitate spectrum analysis, we window the solar radio digital signal after AD conversion in the FPGA.Here, we use a Hanning window and then convert the time-domain signal into frequency-domain data via the fast Fourier transform (FFT).We design a 16k-point FFT algorithm after considering the requirements of the system frequency resolution and the quantity of the transmitted data.The output-spectrum data are subsequently truncated (Yan et  The frequency resolution of the digital receiver is approximately 76.294 kHz due to the ADC sampling rate of 1.25 GSPS and the FPGA performing 16k-point FFT on the AD-converted data.It is necessary to perform cumulative integration operations on the obtained frequency-domain data within one temporal resolution based on the actual data quantity and subsequent PCIe transmission capacity, which can be set according to consumer requirements.Under normal circumstances, the frequencydomain data after the FFT are accumulated 64 times, so that the temporal resolution is approximately 0.839 ms.At the same time, the system can support up to 16 accumulations, so that the maximum temporal resolution can reach 0.21 ms.

Host Computer and Data-storage Management
After the spectrum data processed by the digital receiver are uploaded to the host computer through the PCIe interface, the data are sequentially displayed on it and stored on the disk.The host software first analyzes the received data, and then displays the spectral data through a spectrogram and dynamic spectrogram, which can support zooming in and out of the displayed values.The host software also supports real-time modification and display of the vertical coordinate scale to facilitate the observation of the fine spectral structure of solar radio bursts.To improve the observation quality and highlight solar radio bursts, the display system performs background subtraction processing on the spectral data.The initial background noise is subtracted from the actual observation data when pointing and aligning to the Sun, and new observation values are obtained when the antenna points toward the sky.This approach can suppress or eliminate the influence of background and observation system noise, which helps to obtain clearer observations of new burst data.This system uses a disk array with a capacity of 100 TB to store the original spectrum data received by the host according to the temporal resolution of the system.

Data Calibration
The CBSm can perform effective observations more than 8 hr per day and more than 350 days per year.Three types of data products can be generated, namely the solar radio spectrum data product fits file, the solar radio spectrum fast-view image jpg file and the original solar radio spectrum data product dat file.
In the signal-receiving unit, we adopt a mature fully automatically operated high-precision turntable.The antennapointing accuracy and tracking accuracy are better than 0°.2, and the horizontal and vertical polarization isolation of the antenna feed is greater than 31 dB.There may be differences in sensitivity, frequency response, and noise performance among different frequencies and observation equipments due to the effect of the geographical location and working environment, which will result in raw data that cannot be compared to other data.Therefore, it is necessary to calibrate the obtained raw solar radio data to ensure data comparability.
We adopt the widely used relative calibration method (Benz et al. 2009;Lu et al. 2015;Du et al. 2017;Shang et al. 2022;Yan et al. 2023) in CBSm.This is described by where F obs ( f ) is the solar radio flux value corresponding to the actual observation value of frequency f in solar flux units, and f is in MHz.R obs ( f ), R sky ( f ), and R qs ( f ), respectively, correspond to the actual observation value, the cold-sky observation value, and the quiet-Sun observation value at frequency point f, all in mW.F qs ( f ) is the radio flux value corresponding to the quiet Sun at frequency point f in solar flux units.
where z is the number of sunspots, which can be obtained from WDC-SILSO, Royal Observatory of Brussels. 3ecause F qs ( f ) in Equation (3) is actually a solar minimum spectrum, it does not apply to the nonflaring Sun during higher activity levels.Therefore, we use the quiet solar flux measurements provided by RSTN/Learmonth as F qs ( f ).
In Figures 6(a  Using the flux measurements of RSTN/Learmonth as the quiet solar flux F qs ( f ) can avoid some shortcomings of Equation (3), and they are closely related to the current solar surface activities, which can improve the accuracy of calibration.However, there may also be certain errors.The main reasons for errors include the accuracy of the selection of quiet solar regions, the selection of solar-quiet flux measurements and flux measurements of RSTN/Learmonth, the external disturbance of nonsolar signals, and the impact of atmospheres in different regions on the absorption of solar radio signals, etc.In the next stage, we will make improvements in the aspects mentioned above, striving to achieve more accurate calibration data.

Effectiveness of Solar Pointing
The CBSm is designed to track the Sun and acquire radio information about the solar activity in the meter-wave band.Therefore, it is important to effectively point and aim at the Sun. Figure 7(a) shows the process of the antenna gradually pointing toward and aligning with the Sun during the absence of obvious solar activity from 05:04:30 UT to 05:08:57 UT on 2023 February 20.
Figure 7(a) shows that in the 90-600 MHz frequency band, particularly in the 150-600 MHz frequency band, a significant difference in signal strength is observed when the antenna is aimed at the Sun compared to pointing to the cold sky, which indicates that the system has good solar directionality.

Signal-to-noise Ratio
The S/N is an important indicator of system performance.S/N represents the ratio between the relative strength or power of a signal and noise.A higher S/N corresponds to a higher signal quality relative to noise.
The signal strengths observed when the antenna is pointing toward the Sun and toward the cold sky are assumed to be P obs (in mW) and P sky (in mW), respectively.(P obs -P sky ) can be considered the strength of the solar radio signal, and the S/N of the receiver is given by  7(a)).The data obtained when the antenna was pointed toward the cold sky at approximately 05:05:00 UT are specifically considered as noise (at the white vertical line in Figure 7(a)).
As shown in Figures 7(b) and (c), due to the influence of FM radio stations and the local environment, the S/N (in dB) of the system in the 90-150 MHz frequency band is basically negative, indicating that the solar radio signals within this frequency range are submerged by the radio interference signals, and the noise difference between the signal aimed at the Sun and the signal aimed at the cold sky is essentially 0. In the 150-600 MHz frequency band, the S/N is 3-12 dB, and the majority of the frequency bands have an S/N between 5 and 12 dB, indicating good discrimination and demonstrating the effectiveness of the antenna from another perspective.

Sensitivity
The sensitivity reflects the ability of radio spectrometer equipment to measure the minimum signal and is an important parameter of solar radio receivers.A higher sensitivity of the solar radio receiver indicates that it can receive a weaker signal, providing better data support for solar radio researchers studying the fine structure of solar radio storms.By using two noise sources of the analog receiver, the Y-factor method can be used to measure the system gain and system noise temperature.By combining the system and cold-sky temperature as well as the effective area of the antenna, the sensitivity of the system can be analyzed.Based on the experimental test data, it is derived that under the conditions of an integration time of 1 ms and a bandwidth of 100 kHz, the sensitivity of the left-handed channel of CBSm is 0.942 sfu @300 MHz, and the sensitivity of the right-handed channel is 0.992 sfu @300 MHz.

Examples of Solar Radio Burst Events
We started the trial operation on 2022 November 10, and to date, we have observed a large number of radio bursts, including type I radio storms and hundreds of type III, ∼20 type II, and ∼15 type IV bursts.Figures 8-10 show the detailed structure of some solar radio burst events observed by CBSm.
Figure 8 shows the detailed structure of type I noise storms in the order of increasing frequency resolution and temporal resolutions, step by step.Figure 8 Figure 8 shows that CBSm with high performance provides people with a clearer and more detailed structure of type I noise storms; these fine structures indicate that type I noise storms are actually composed of a series of radio-spike   bursts with an average lifespan of only a few milliseconds to tens of milliseconds, as well as some dot bursts and smallscale burst clusters.These small bursts have a short average lifespan and narrow average frequency bandwidth among all bursts in the same band, and are likely to become structural units that make up other bursts, representing a special burst process.
Figure 9 shows the events of a type II radio burst following type III radio bursts observed by our system on 2023 May 8, from 08:22:23 UT to 08:28:58 UT, with a duration of approximately 6 minutes and 36 s.  structure, from 08:25:20 UT to 08:25:55 UT in the frequency range of 210-270 MHz, with a temporal resolution of 15 ms and frequency resolution of 76.294 kHz, respectively.At the same time, a large range of green shaded areas in Figure 9(a) can be observed around the main bodies of type II and III bursts, due to the high sensitivity of the system, which may be of great significance for the study of solar activity.
Figure 10 shows type III radio bursts and their fine structures.Figure 10(a) shows the dynamic spectrum observed by Yamagawa from 06:19:00 UT to 06:23:00 UT on 2023 February 13, with a temporal resolution of 1 s and frequency resolution of 1 MHz, respectively.Comparing Figures 10(a) and (b), it is easy to find that the data quality of CBSm is significantly better than Yamagawa, due to its superior performance.Moreover, the vertical bar structures in Figures 10(a) and (b) at low temporal resolution (such as 500 ms or even lower) exhibit very impressive fine structures at high resolution (such as 50 ms or even higher).Meanwhile, Figure 10(c) shows that the type III radio bursts are also composed of many small bursts, that is, beside the main structure, there are many dotlike and cluster-like structures with an average lifespan of only a few hundred milliseconds to a few seconds in the dynamic spectrum.All of these will provide useful assistance for further research on solar activity.
We have stored a large amount of observation data of CBSm, and some of the data have been used for scientific research by many institutions and individuals.For example, Hou et al. (2023) used solar radio observation data obtained by the system from 02:18 UT to 02:33 UT on 2022 November 12 to investigate coronal jet-driven type II radio bursts.For more observation data, please visit our online data website at http:// 47.104.87.104/MWRS/.

Conclusion
The main observation target of the high-performance 90-600 MHz Chashan broadband solar radio spectrograph at meter wavelengths (CBSm) is the (super)fine structure of the meter-wave band solar radio burst spectrum.The system can record solar radio dynamic spectrum data at a frequency range of 90-600 MHz, a temporal resolution of 0.839 ms (up to 0.21 ms), a frequency resolution of 76.294 kHz, a dynamic range of no less than 60 dB, and a system sensitivity of 1 sfu (integration time of 1 ms, bandwidth of 100 kHz @300 MHz).High-precision bicircular polarization observation is achieved through digital polarization synthesis.The obtained data products include solar radio spectrum data, solar radio spectrum fast-view images (all preprocessed L1-level data) and raw solar-spectrum data.The completion and successful operation of CBSm can provide high-quality scientific data for the study of the fine structure of the meter-wave solar radio spectrum and lay the foundation for future participation in space weather applications and support services.

Figure 3 .
Figure 3. Simulations of antenna pattern of the 0°cross section (i.e., E plane) (the red line) and the 90°cross section (i.e., H plane) (the black line) of the 12 m aperture parabolic antenna.The horizontal axis is theta (in degrees), and the vertical axis is gain (in dBi).(a) For the signal with the frequency of 90 MHz.(b) For the signal with the frequency of 600 MHz.

Figure 4 .
Figure 4. Design block diagram of the analog receiver.
al. 2021) to ensure the effective number of operation bits.Digital polarization synthesis is performed in the frequency domain to digitally synthesize the horizontal and vertical linear polarization solar radio signals into left-handed and right-handed circular polarization signals, which does not change the isolation of linear polarization signals.Because the isolation of the linear polarization in the signalreceiving unit exceeds 31 dB, it still exceeds 31 dB after circular polarization.Then averaging operations are performed within one temporal resolution unit to reduce noise for improving the signalto-noise ratio (S/N) and reduce the amount of data in the highspeed data stream.Finally, the frequency-domain data of the lefthanded and right-handed signals are packaged and uploaded to the host computer and data-storage management unit through the PCIe interface for further processing.

Figure 5 .
Figure 5.A schematic diagram of the digital receiver.
) and (b), we compare the relative calibration results of CBSm and the results of the RSTN/Learmonth observatory in Australia at 245 and 410 MHz from 06:41:00 UT to 06:44:00 UT on 2023 May 8.
Figures 6(a) and (b) show that the overall trend is consistent at 245 and 410 MHz during the selected time period.Although there are erroneous fluxes between CBSm and RSTN/Learmonth, the errors are generally within ± 5% (except for some points affected by sudden changes of local geomagnetic interference signals).Therefore, the data can meet the needs of scientific research and application, and they are effective.Using the flux measurements of RSTN/Learmonth as the quiet solar flux F qs ( f ) can avoid some shortcomings of Equation (3), and they are closely related to the current solar surface activities, which can improve the accuracy of calibration.However, there may also be certain errors.The main reasons for errors include the accuracy of the selection of quiet solar regions, the selection of solar-quiet flux measurements and flux measurements of RSTN/Learmonth, the external disturbance of nonsolar signals, and the impact of atmospheres in different regions on the absorption of solar radio signals, etc.In the next stage, we will make improvements in the aspects mentioned above, striving to achieve more accurate calibration data.
Figures 7(b) and (c) show the S/N of the left-hand channel of CBSm for the observation data presented in Figure 7(a) and (a) shows the dynamic spectrum observed from 04:42:30 UT to 04:47:30 UT on 2023 May 7, with a temporal resolution of 500 ms and frequency resolution of 850 kHz, respectively.The duration of the signal for this figure is 5 minutes, and type I noise storms are mainly concentrated in the range of 90-200 MHz. Figure 8(b) shows the fine structure shown in Figure 8(a) (within the white-dashed box) from 04:43:30 UT to 04:44:30 UT between 100 MHz and 200 MHz, with a temporal resolution of 100 ms and frequency resolution of 170 kHz, respectively.And Figure 8(c) shows the fine structure shown in Figure 8(b) (within the white-dashed box) from 04:44:16 UT to 04:44:20 UT between 120 and 140 MHz, with a temporal resolution of 6 ms and frequency resolution of 76.294 kHz, respectively.

Figure 7 .
Figure 7. System observation effectiveness.(a) The dynamic spectrum of the left-handed channel of the antenna from pointing to cold sky to tracking the Sun from 05:04:30 UT to 05:08:57 UT on 2023 February 20, for a frequency range of 90-600 MHz, frequency resolution of 76.294 kHz and temporal resolution of 3.356 ms, respectively.(b) S/N test results.(c) Test results of the difference between signal power and noise power.

Figure 8 .
Figure 8. Type I radio noise storms observed by CBSm.(a) The dynamic spectrum observed from 04:42:30 UT to 04:47:30 UT on 2023 May 7, with a temporal resolution of 500 ms and frequency resolution of 850 kHz, respectively.(b) The dynamic spectrum observed from 04:43:30 UT to 04:44:30 UT on 2023 May 7, with a temporal resolution of 100 ms and frequency resolution of 170 kHz, respectively.(c) The dynamic spectrum observed from 04:44:16 UT to 04:44:20 UT on 2023 May 7, with a temporal resolution of 6 ms and frequency resolution of 76.294 kHz, respectively.

Figure 9 .
Figure 9.The fine structures of type III and type II solar radio bursts.(a) Type III and type II radio bursts observed by CBSm from 08:22:23 UT to 08:28:58 UT on 2023 May 8, with a temporal resolution of 21 ms and frequency resolution of 76.294 kHz, respectively.(b) The zebra patterns observed by CBSm, with the frequency range of 210-270 MHz, from 08:25:20 UT to 08:25:55 UT on 2023 May 8, with a temporal resolution of 15 ms, and a frequency resolution of 76.294 kHz, respectively.

Figure 9
Figure9(a) shows that the type II radio burst was accompanied by a strong type III radio burst, with intervals of approximately 2 minutes and 30 s.It also presented a harmonic structure, with drift from high frequency to low frequency for both bursts.One of the harmonic frequencies ranged from approximately 150-240 MHz, lasting for approximately 1 minute and 30 s, with a frequency drift speed of approximately 0.4 MHz s −1 .The other harmonic frequency range is approximately 100-120 MHz, lasting for approximately 1 minute, with a frequency drift rate of approximately 0.3 MHz s −1 .Specifically, Figure9(b) shows a zebra pattern

Figure 10 .
Figure 10.Type III radio bursts observed by Yamagawa and CBSm.(a) The dynamic spectrum of Yamagawa with a temporal resolution of 1 s and frequency resolution of 1 MHz, respectively.(b) The dynamic spectrum of CBSm with a temporal resolution of 500 ms and frequency resolution of 610 kHz, respectively.(c) The fine structure of the part between two white-dashed lines in (b), with a temporal resolution of 50 ms and frequency resolution of 305 kHz, respectively.
Figure 10(b) shows the dynamic spectrum of CBSm during the same time period, with a temporal resolution of 500 ms and frequency resolution of 610 kHz, respectively.And Figure 10(c) shows the fine structure from 06:20:50 UT to 06:21:50 UT shown in Figure 10(b), i.e., the part between the two white-dashed lines in Figure 10(b), with a temporal resolution of 50 ms and frequency resolution of 305 kHz, respectively.

Table 1
Comparison of the Main Meter-wave Solar Radio Observation Instruments a

Table 2
Main Parameters and Technical Characteristics of the Signal-receiving Unit frequency resolution Δf equal to 100 kHz: