Radar systems suffer severe performance degradation under the combined influence of complex electromagnetic signals, including radar signals, radar jamming signals, multipath signals, and clutter signals. To clarify the mechanisms and patterns of influence between input signals and output phenomena in radar systems, research was conducted on radar system receiver front-end technology with radio frequency monitoring capabilities. This receiver front-end, based on a CPCI bus architecture, employs multiple signal coupling and sampling modes to extract signals from key nodes in the radar system receiver front-end channel. This provides a fundamental test platform and measurement data for studying the mechanisms of multipath coupling effects and the superposition effects of multiple factors in complex electromagnetic environments in radar signal receiving channels.
introduction
Complex electromagnetic environments are one of the most prominent features of modern and future battlefields. As electronic information equipment, radar systems are severely constrained by electromagnetic environments in terms of detection capabilities [1]. Current research on the performance of radar systems under complex electromagnetic environment effects mainly relies on radar countermeasures effect evaluation systems. These systems are not only expensive, but their hardware conditions are also fixed and packaged. There are no intermediate quantity test ports reserved or brought out specifically for the study of complex electromagnetic environment effects. The interference phenomenon can only be analyzed based on the final display results of the actual radar. This analysis is feasible for the impact of a single interference source on the radar. However, when there are multiple interference sources or an increase in complex electromagnetic environment elements, it is very difficult to analyze the impact of complex electromagnetic environments on radar systems based solely on the final results [2].
Therefore, it is crucial to research and establish an evaluation platform and experimental methods for radar detection capabilities under complex electromagnetic environments. This paper aims to design and study a radar receiver front-end with a radio frequency monitoring port. This front-end device enables signal monitoring at key signal flow points in the receiving channel, providing a hardware foundation and necessary data support for the study of the multi-factor superposition effect mechanism of the electromagnetic environment.
1. Working principle
The receiver employs the monopulse angle measurement method commonly used in high-precision measurement radar systems. Monopulse angle measurement radar needs to simultaneously support both amplitude and phase comparison modes. In amplitude comparison mode, the radar receiver's RF front-end has two receiving branches: a sum channel and a difference channel. In phase comparison mode, there are two channels in the coordinate plane. Therefore, the receiving front-end is designed as a dual-channel link structure based on a CPCI bus architecture, forming two receiving branches for sum and difference signals, respectively.
The receiving front-end employs a superheterodyne frequency conversion link, mainly composed of a limiter, low-noise amplifier, mixer, intermediate frequency amplifier, filter, and local oscillator. Couplers are used before and after each key component module to bring out the radio frequency monitoring points. The radar receiving front-end converts the target echo signal with a frequency range of 3.1GHz to 3.5GHz and a power range of -100dBm to -30dBm to a 70MHz intermediate frequency via the superheterodyne frequency conversion link. The maximum output power of the intermediate frequency is 0dBm. The block diagram of the receiving front-end is shown in Figures 1 and 2.
2 System Design
2.1 System performance requirements
Key technical indicators for the receiving front end:
Receiving frequency range: 3.1GHz～3.5GHz;
Radar signal bandwidth: 1MHz, 2.5MHz, 5MHz selectable;
Noise figure: ≤3dB;
Receiver sensitivity: -100dBm;
Receive dynamic range: -100dBm to -30dBm;
Receive channel gain: better than 70dB;
Intermediate frequency: 70MHz, bandwidth: 5MHz;
Output intermediate frequency level: ≥0dBm;
In-band ripple: ≤3dB;
Image suppression: ≥60dBc;
Harmonic suppression: ≥30dBc;
Stray suppression: ≥50dBc.
2.2 Circuit Design
The receiving front end mainly includes a sum and difference channel, both using the same design link. Due to the characteristics of superheterodyne receivers, such as low spurious emissions and high image rejection, a double-conversion scheme is adopted, and an intermediate frequency (IF) is selected to reduce the impact of mixing interference on the target signal. The design mixes the target echo signal (input frequency range 3.1GHz～3.5GHz, power range -100dBm～-30dBm) with a variable local oscillator (LO) signal (frequency range 7GHz～7.4GHz), outputting a first IF signal at 3.9GHz. This first IF signal is then mixed with a fixed LO signal at 3.97GHz, outputting a 70MHz IF signal, which is sent to the IF acquisition module for subsequent sampling and demodulation. Based on sensitivity, output signal-to-noise ratio (SNR), and output IF level, the receiver gain is calculated to be 80dB. The specific gain is allocated to the RF low-noise amplifier and the first and second IF amplifiers. Detailed circuit design is shown in Figure 3.
3 Key Technical Indicator Design Analysis
3.1 Analysis of the Influence of Directional Couplers
The radio frequency (RF) monitoring interface is a non-functional port of the radar receiver front end. Therefore, the design of this interface will not affect the RF channel functionally, while still achieving the purpose of bringing out monitoring points and monitoring signals at key nodes of the RF channel. By analyzing the signals brought out from the monitoring interface, key parameters such as the frequency and power of the main path signal in the RF channel at that node can be obtained.
In this design, directional couplers are used to extend the monitoring points outward from the RF channel. Because the insertion loss of the directional coupler affects the receiver's sensitivity, placing a directional coupler with an insertion loss of 0.3dB before the low-noise amplifier reduces the receiver sensitivity by approximately 0.3dB. Since the signal has already been amplified, any subsequent directional couplers added after this node will no longer affect the receiver sensitivity. To avoid the simultaneous impact of multiple couplers on system sensitivity, a power compensation method is used to adjust the effects of the couplers. There are two power compensation methods: one is stepwise power compensation, where the power at this point is adjusted before the coupler is added during processing, first performing gain amplification; the other is adding a low-noise amplifier at the final output of all couplers for a final, one-time power compensation of the module.
The step-by-step power compensation method, due to its repeated amplification and attenuation, causes system power instability, which varies significantly with changes in ambient temperature. From a performance perspective, repeated amplification and attenuation will increase stray power, affecting system performance.
Finally, the one-time power compensation method can effectively regulate the system power attenuation caused by the insertion loss of the coupler, minimizing the spurious effects on the system. Furthermore, the receiver module is gain-controlled, with the output power maintained above -60dBm, so there will be no small signal output, and the impact of a single power adjustment on the system is negligible.
After considering the above factors, a single-stage power amplification method was ultimately adopted to compensate for the impact of coupling on power. The total insertion loss of the multi-stage directional coupler is no more than 6dB, and the receiving front-end can compensate for the insertion loss of the multi-stage directional coupler through amplifiers. Therefore, the multi-stage directional coupler has a relatively small impact on the output power of the subsequent stages of the receiving front-end.
To investigate the impact of the coupler on the system, power and phase tests were conducted on the directional coupler. The coupler was input with a continuous signal at a frequency of 3.5 GHz and a power of -20 dBm. The measured signal power at the coupling end was -9.98 dBm, while the signal power measured at the coupler output end was -20.57 dBm. The test results show that adding a coupler attenuates the main signal by less than 0.6 dB. The test results are shown in Figures 4-6.
The power and phase tests above show that, within the 3.1GHz to 3.5GHz frequency band, the addition of a coupler will not affect the phase of the main signal. The addition of a single coupler will result in an insertion loss of no more than 0.5dB in the main signal. Power compensation will not affect the change in main signal power.
3.2 Gain Allocation Design
Because the receiver has a large dynamic range and requires high sensitivity, the gain needs to be allocated in the receiving channel to the RF low noise amplifier, the first intermediate frequency of 3.9 GHz, and the second intermediate frequency of 70 MHz.
The limiter used in the receiver front-end channel design has an insertion loss of 1dB, the low-noise amplifier has a gain of 30dB, the mixer has an insertion loss of 8dB, and the intermediate frequency amplifier group uses an automatic gain control amplifier. As shown in the detailed circuit design diagram in Figure 3, the total channel gain is approximately 85dB, which meets the design specifications.
Meanwhile, to ensure the local oscillator power of the mixer remains within a reasonable range, a detection circuit was designed during the frequency conversion process at the receiving front end. During the frequency conversion process, the detector samples and detects the power of the input signal. The detected level reflects the power at that sampling point. Simultaneously, the analog-to-digital converter in the detection circuit samples this level value and controls the attenuator in real time, thereby achieving automatic control of the output power.
3.3 Mixer Spurious Design
The radar receiver front-end employs a superheterodyne receiver frequency conversion method, down-converting the radar target signal to an intermediate frequency (IF) signal through two frequency conversions. The first mixing outputs a fixed-frequency IF signal of 3.7 GHz. This signal is then filtered by a narrowband filter and mixed a second time with a fixed-frequency 3.97 GHz local oscillator signal, resulting in a 70 MHz IF signal. This IF signal is then output after narrowband filtering. Therefore, mixing spurious emissions are mainly caused by the first mixing. Software simulation calculations are used to calculate the first mixing spurious emissions of the receiver front-end. The input signal frequency range is 3.1 GHz to 3.5 GHz, and the harmonics of the input signal are calculated to the third order. The local oscillator signal frequency range is 7 GHz to 7.4 GHz, and the harmonics of the local oscillator signal are calculated to the third order. The calculated output mixing spurious emissions are then used.
As shown in Figure 7, the spurious emission calculation results of the mixing frequency show that the 5th order spurious emission suppression is greater than 65dB in the design, which meets the design requirements.
4. Performance Test Results and Analysis
The technical specifications achieved by the designed and developed radar receiver front-end are shown in Table 1. The input test signal frequency range is 3.1GHz to 3.5GHz, and the power range is -100dBm to 10dBm. The first local oscillator signal frequency range is 7GHz to 7.4GHz, and the power range is -10dBm to 10dBm. The second local oscillator signal frequency is 3.97GHz, and the power range is -10dBm to 10dBm.
As shown in Table 1, the overall test results of the radar receiver front-end meet the design requirements. Furthermore, the receiver front-end incorporates directional couplers before and after the low-noise amplifier and mixer in the frequency conversion link for radio frequency (RF) monitoring, providing a total of six RF monitoring ports for external equipment to perform measurement and analysis.
5. Conclusion
This paper proposes a radar receiver front-end technology with radio frequency monitoring capabilities, which solves the problem that existing radar and radar test systems cannot effectively analyze and measure the effects of complex electromagnetic environments during the receiving process. This technology is used to test the single-channel characteristics and inter-channel characteristics of the receiver under the action of multiple interference sources and multiple superimposed electromagnetic environments. The test results can be used as a reference for radar system designers and debuggers, providing necessary testing methods to improve the overall performance of radar systems [3]. At the same time, this receiver front-end is based on the CPCI bus architecture, which is small in size, easy to operate and control, and easy to improve and secondary develop.
References
[1] Hu Jin. Quantitative description of radar detection capability under complex electromagnetic environment [J]. Aerospace Electronic Countermeasures, 2017, 33(1):32-35.
[2] Wang Peizhang, Shao Wei, Yu Tongbin, et al. Research on broadband integrated receiver front-end technology [J]. Journal of Microwave, 2012(8):319-321.
[3] Meng Yan, Zhang Linrang, Peng Weijie. Design and implementation of radar multi-channel receiver test system [J]. Electronic Science and Technology, 2017, 30(10): 5-7, 11.
