Interface circuit design between radar simulator and radar

0 Introduction

Radar plays a vital role in modern warfare. With the rapid development of radar technology, higher requirements are put forward for radar equipment testing, performance testing, fault diagnosis, support and training. At present, with the rapid development and popularization of computer electronic technology, PC-based radar simulator has become the best choice to solve these problems with its advantages.

The radar simulator continuously generates simulated echo signals by communicating data with the host computer (PC) and the radar. In order for the simulated echo signals generated by the radar simulator to reflect all the information of the real target received by the radar, it is necessary to collect the state parameters of the radar according to the radar signal construction model and the design concept of the overall system. The radar simulator collects the radar state parameters required for echo signal simulation through the interface circuit, and transmits them to the simulator's control motherboard after conditioning. The main radar state parameters are the radar antenna angle signal, radar main pulse, cone scan reference signal and carrier frequency. The simulator collects the radar antenna angle signal as the current beam pointing of the radar receiving antenna to determine whether to generate simulated echo signals and determine the size of the antenna gain; collects the radar main pulse signal as the time reference for radar signal simulation to keep the simulated signal synchronized with the radar; collects the radar cone scan reference signal to provide a phase reference for the cone scan modulation of the difference signal; and collects the radar carrier frequency to adjust the local oscillator output frequency of the echo simulator radio frequency card to ensure that the generated radio frequency echo signal can be received by the radar. The following mainly introduces the hardware design of the radar simulator and the radar interface circuit.

1 Hardware Design of Interface Circuit

By analyzing the working parameters of the radar equipment antenna angle signal, main pulse signal, cone scan reference signal and radar carrier frequency, we can know that the forms, performances and parameters of these signals are different. Therefore, the radar interface circuit needs to design corresponding acquisition and conditioning circuits according to the differences of each signal.

1.1 Antenna axis angle circuit design

The function of the antenna azimuth and elevation conversion circuit is to convert the analog angle information of the radar antenna into digital angle information, and transmit the digitized angle information to the simulator control main board. The main board determines the range of the echo beam and the antenna gain of the target echo based on the radar antenna angle information.

Radar antenna angle information conversion is realized by rotary transformer-digital converter. The hardware circuit is based on single chip microcomputer, including antenna axis angle adjustment circuit, reference signal formation circuit and serial communication circuit, etc. The circuit block diagram is shown in Figure 1.

The signal output by the rotary transformer is generally an AC signal. Taking a certain type of radar as an example, the rotary transformer outputs an AC voltage signal with an amplitude of about ±10 V. Because the ADC conversion analog voltage range of the single-chip microcomputer (C8051F020) used in the circuit is 0~VRFE (VRFE=2.45 V), the analog signal must be reduced and de-negated before A/D conversion. These tasks are completed by the shaft angle signal conditioning circuit, as shown in Figure 2. After the input shaft angle signal is subjected to the action of the resistor voltage divider and the negative threshold voltage of the diode, it is output through the resistor voltage divider to meet the needs of the single-chip microcomputer.

The excitation voltage of the rotary transformer is used as the trigger signal for peak sampling. Since the excitation voltage amplitude is greater than the driving voltage of the internal comparator of the single-chip microcomputer, the excitation voltage signal is shaped into an approximate square wave signal by using the limiting effect of the diode, and then the signal is divided by a resistor to ensure that the sampling trigger signal is within the external driving voltage range (-0.25~+0.25 V) that the comparator can withstand. The circuit is shown in Figure 2. The excitation signal generates a reference signal under the action of the reference signal forming circuit and sends it to the entrance of the single-chip microcomputer comparator. The comparator generates an interrupt on the rising edge of the reference signal. In the interrupt, the A/D port of the single-chip microcomputer is enabled to convert the antenna azimuth angle signal conditioned by the shaft angle signal conditioning circuit into a digital signal. Under the control of the entire single-chip microcomputer program, the circuit completes the digitization of the radar antenna angle information and transmits it to the simulator control mainboard through the serial communication circuit.

1.2 Radar main pulse signal acquisition circuit

The timing control system in the radar system provides all the timing signals and various control signals required for the normal operation of the radar. The radar main pulse signal is formed by the system's repetition frequency control circuit and is used as an external synchronization signal during the debugging of the radar main station and various subsystems.

The program design of the simulator control mainboard uses the radar main pulse as the time reference for echo data calculation and transmission, so as to ensure synchronization with the radar working sequence. The specific implementation method is to configure a bidirectional programmable flag pin of the control board main control chip as input mode and interrupt generation mode, connect it to the output end of the main pulse acquisition circuit, and set it to rising edge interrupt mode. This pin will generate an interrupt immediately after receiving the rising edge of the radar main pulse signal. When it is determined that an interrupt has occurred, the program will calculate and send the radar echo data.

Since the radar main pulse amplitude is a negative pulse signal of about -10 V, and the I/O port of the control motherboard withstands a voltage of about 3.3 V, before the radar main pulse is connected to the motherboard setting pin as the time reference for echo generation, it must be signal conditioned to achieve amplitude reduction processing. The conditioning circuit designed for the radar main pulse signal is shown in Figure 3. When the input voltage is about 0 V, diodes D4 and D5 are cut off, and the output voltage is the voltage divided by resistors R9 and R10 on the 5 V power supply, which is about 3.3 V. When the input voltage is -10 V, diode D5 is turned on, and the output voltage is the threshold voltage divided by diode D5, which is about -0.7 V.

1.3 Radar cone scanning reference signal acquisition circuit

Driven by the cone-scan motor, the magnetic coupling ring rotates synchronously with the reference voltage generator, coupling the magnetic field in the circular waveguide to form a high-frequency modulated difference signal. When the radar automatically tracks the target, if the antenna electrical axis deviates from the target direction, an error signal will be generated. The error signal is an AC signal, and its frequency is the same as the low-frequency modulation frequency of the feeder system. The cone-scan reference signal output by the reference voltage generator is also used as the voltage reference of the phase-sensitive detector to detect the error signal. The detected angular error voltage drives the motor to drive the antenna to track the target movement.

During the actual operation of the radar, the target echo may be received at any time. Therefore, if the radar composite difference signal △ is to be simulated, the phase of each echo relative to the cone scan reference signal must be determined. The phase zero moment of the coupling loop is taken out by designing the cone scan reference signal acquisition circuit, and the phase of each echo relative to the cone scan reference signal is determined accordingly. The schematic diagram of the cone scan reference signal acquisition circuit is shown in Figure 4. The circuit uses the voltage comparator chip LM239D, 3.3 V power supply, and uses a diode to limit and shape the input cone scan reference signal. The output of the circuit is a 3.3 V square wave signal with the same period as the input signal. The rising edge of the square wave is considered to be the phase zero point of the cone scan reference signal. The square wave signal output by the cone scan reference signal acquisition circuit is connected to the control mainboard timer 0, which is set as the input pin, and its pulse width counting and capture mode are used to count the square wave signal.

1.4 Radar transmitter operating frequency acquisition circuit

The radar simulator control mainboard models the target model, motion rules and complex electromagnetic environment set by the host computer, and calculates the video band echo signal in real time. The echo signal is processed by digital up-conversion to obtain two intermediate frequency radar echo signals, which are then modulated to the radio frequency band by the radio frequency component and radiated through the antenna. Because the simulated echo signal ultimately generated by the radar simulator is in the radio frequency band, the radio frequency component needs to consider a series of processes of actual radar transmission and reception when designing it, ensuring that the generated simulated echo signal is within the radar receiver bandwidth and can change with the change of the radar frequency hopping combination frequency, and that the radar's operating frequency at each moment can be displayed on the host computer display system.

Radars usually have multiple operating frequency points to achieve anti-interference, and the control of the radar's operating frequency and change mode is achieved by a frequency hopping control system. The frequency hopping control system works under the comprehensive control of the frequency hopping control pulse and the system panel control button, outputs the code of the current specified frequency to the receiver, and adjusts the operating voltage of the voltage-controlled oscillator (VCO) component accordingly, thereby changing the output frequency of the VCO, which also changes the radar's operating frequency.

The frequency value corresponding to the radar's operating frequency point is fixed. As long as the frequency code output by the current frequency hopping control system is known, the current operating frequency of the radar can be known. A circuit can be designed to determine the current operating frequency of the radar by directly collecting the frequency point code. The acquisition circuit uses the microcontroller used in the antenna azimuth signal conversion circuit for control. Because the frequency point feature code is a digital signal, it can be directly connected to the digital I/O pin of the microcontroller. The frequency point feature code is transmitted and output to the RF component of the simulator through the microcontroller to control the size of its oscillator output frequency. The circuit block diagram is shown in Figure 5.

1.5 Interface serial communication circuit

The antenna data information and carrier frequency information collected by the interface circuit are transmitted to the simulator control mainboard through the serial communication circuit. Considering that the radar simulator may be far away from the radar when it is actually working, this circuit uses the RS 485 interface bus. The bus uses differential signal transmission, which can effectively suppress noise interference in long-distance transmission. The maximum transmission distance can reach 1.2 km, and the transmission speed is also fast, up to 10 Mb/s.

2 Interface Circuit Performance Analysis

The acquisition circuits of the above four signals are integrated on the same interface circuit board. Experimental tests were conducted on a certain type of fire control radar. After analyzing the collected radar signals, it can be concluded that the digital converter can complete the digitization of the antenna angle information. Statistics of the errors show that the angle conversion error is no more than 1 mil. The radar main pulse signal is well limited and reduced in voltage after acquisition and conditioning, providing a trigger pulse for the calculation and transmission of the simulator echo signal, and also providing a time reference for the echo delay time control and target signal simulation. Since the main pulse signal has a large amplitude, it will bring some interference to the entire interface circuit. The radar cone scan reference signal is detected by an oscilloscope and is known to be a sine wave. It is converted into a square wave signal with the same frequency through a zero comparison circuit. The rising edge of the square wave signal is used as the phase zero moment of the cone scan reference signal, and the phase difference of the echo signal relative to the cone scan reference signal can be accurately calculated. For the radar carrier frequency, because the number and frequency values ​​of the radar's operating frequencies are fixed, the operating frequency can be accurately located as long as the frequency code is sampled. Also, because the actual frequency values ​​corresponding to the same frequency point codes may be different for different radars, the specific frequency values ​​corresponding to the codes of each frequency point of the radar need to be tested and input into the main control board before the simulator is used.

3 Conclusion

Based on the above radar signal performance analysis, the interface circuit between the simulator and the radar is designed. The interface circuit has a simple structure and is easy to operate. The experimental measurement shows that the interface circuit signal conversion is correct and the isolation effect is good. It not only realizes the accurate and real-time collection of radar status information, but also does not affect the normal operation of the radar. Through the comparative study of the working system characteristics and signal characteristics of different radars, a unified universal radar interface circuit design can be realized based on the interface circuit design.