Design of automatic test system for antenna pattern

0 Introduction

Antenna is an important part of radar, and the test of antenna pattern plays an extremely important role in radar performance test. In the early days, people used manual method to measure the pattern, and the data acquisition, pattern drawing and parameter calculation were all manual, which was complicated, labor-intensive, time-consuming and low-precision. With the rapid development of microelectronics technology and computer technology, automatic antenna pattern test has gradually replaced manual measurement, realizing the automation of signal acquisition, data processing and pattern drawing, greatly improving the measurement speed and accuracy. This paper introduces an automatic measurement system for radar antenna pattern, and analyzes the hardware and software structure and principle.

1 Directional pattern automatic test principle and experimental configuration

According to the antenna reciprocity principle, the antenna under test is used as the receiving antenna and the fixed radiating antenna is used as the transmitting antenna. The transmitting antenna emits electromagnetic waves, and the antenna under test is rotated to receive the waves. The signal levels at different angles within the test range are measured, and the radiation pattern of the antenna under test can be obtained [1].

The principle of automatic measurement of radiation pattern is the same as that of manual measurement. The difference is that the automation of data acquisition, processing and radiation pattern drawing is realized by using electronic and computer technology. Figure 1 shows the experimental configuration of automatic test of radiation pattern of a radar antenna.

Automatic measurement of the radiation pattern belongs to dynamic measurement. During the measurement, the antenna under test rotates continuously and receives the microwave signal emitted by the signal source through the horn antenna. The received signal is sent to the antenna amplitude signal acquisition circuit, and after transformation, amplification and A/D conversion, it is sent to the

When the antenna rotates, the antenna angle signal acquisition device converts the antenna position into an angle digital signal and sends it to the microcomputer. In this way, the amplitude signal level of each position within the measurement range can be obtained. Based on this set of data, the microcomputer can process the data and output the calculation results through the output device.

2 Hardware Circuit Design

The system hardware includes a microcomputer control part, an antenna amplitude signal acquisition device, an antenna angle signal acquisition device and a plotter. The block diagram is shown in Figure 2.

2.1 Microcomputer control circuit

The microcomputer control circuit uses a 51 series single-chip microcomputer, which consists of a CPU , program memory, external data memory, and address decoder [2].

2.2 Antenna amplitude signal acquisition device

The amplitude signal acquisition device consists of a measurement amplifier , a sampling/holding circuit s/H and an A/D conversion circuit.

The microwave signal received by the antenna is sent to the measuring amplifier, which performs high-frequency detection on the microwave signal and outputs a modulated square wave signal. Then, it undergoes amplification, detection, filtering , and other processing to output a DC signal with an amplitude that meets the requirements and a good waveform. The signal is sampled by S/H and sent to A/D. Under the control of the single-chip microcomputer, A/D converts the analog signal into a digital signal and stores it in an external data memory, thereby completing the acquisition of the amplitude signal.

The measuring amplifier is the signal conversion and amplification circuit of the system, which has high sensitivity, large dynamic range, stable working characteristics and fast response speed.

The principle of S/H selection is: if the change of input signal level is less than 1 LSB during A/D conversion, S/H can be omitted; otherwise, S/H must be added. The following formula is the relationship that the signal change rate should satisfy when S/H is not added:

Where: is the absolute value of the signal change rate; Vm is the full-scale voltage of the A/D; n is the number of bits of the A/D; T is the conversion time of the A/D.

The A/D chip uses AD574, and its parameters are: Vm = 10 V, n = 12, T = 25 μs. Substituting into formula (1), we get : The maximum change rate of the radar beam is not less than 10 V/(.), the antenna speed is generally not less than 2 r/min, and the signal voltage change rate is not less than: 10 × (2 × 360/60) = 120 V/s, which exceeds the limit value of 97.7 V/s, so S/H must be added.

AD574 has the advantages of small quantization error (2.44 mV), large dynamic range (72 dB), and high conversion speed (25 μs).

2.3 Antenna angle signal recording device

The antenna angle signal is recorded by the microcontroller-controlled shaft encoder. The shaft encoder directly takes the signal from the three-phase winding and excitation winding of the synchronous receiver of the antenna transmission mechanism and outputs a 12-bit angle digital signal. Compared with general angle encoders, it adopts a tracking closed loop, which has the advantages of fast tracking speed and high accuracy. Its principle is shown in Figure 3. [page]

The voltage of the three-phase winding on the synchronous machine is converted into two-phase voltages V1 and V2 through a three-phase/two-phase transformer:

Where: υm is the voltage amplitude; ω is the angular frequency of the synchronous machine winding voltage; θ is the angle through which the antenna rotates.

V1 and V2 are multiplied by cosφ and sinφ in the sine-cosine multiplier (cosφ and sinφ are generated by the sine-cosine function generator, and φ is the digital value of the counter), and then subtracted and amplified in the error amplifier to obtain:

Where: k is the gain of the error amplifier.

After phase-sensitive detection, the signal outputs an error signal θ-φ, which is then integrated by an integrator to control the voltage-controlled oscillator so that θ-φ approaches 0. At this time, the reversible counter outputs a digital value representing the angle θ.

The shaft angle encoder can output 12-bit angle digital signal with high angle resolution. When taking the signal from the coarse synchronous machine, the minimum resolvable angle is: 6 000/212=1.46 mils. To further improve the resolution, taking the signal from the fine synchronous machine, since the coarse and fine speed ratio is 1:20, the minimum resolvable angle can reach 1.46/20=0.073 mils.

The smaller the acquisition interval, the higher the measurement accuracy, but the minimum acquisition interval is limited by the antenna rotation speed, A/D conversion time and necessary data processing time. This system uses equal angle interval acquisition, and the acquisition interval is 23×0.073=0.58 mils.

3 Software Design

3.1 Software composition and execution process

The system software consists of three parts: signal acquisition, data processing, and pattern parameter calculation and drawing.

The signal acquisition program includes the amplitude signal acquisition program and the angle signal acquisition program, that is, the startup and data output program of the A/D and shaft angle encoder. The acquired antenna amplitude signal is stored in the external data memory; since only the change of angle is needed for drawing the directional pattern and calculating the parameters, the absolute value of the angle is not needed. Therefore, the angle digital signal does not need to be stored in the memory like the antenna amplitude signal. Instead, the last few bits of the memory address are used to represent the angle signal, that is, the address of the amplitude signal at a certain position contains the angle information of the position. This processing not only saves storage space, but also greatly simplifies data processing.

The program execution flow chart is shown in Figure 4.

After the measurement starts, the single-chip computer determines whether the angle of the antenna is △α (the angle interval between two adjacent data points) according to the angle signal output by the shaft angle encoder. If it has not rotated past △a, it waits; if it has rotated past △a, the CPU issues an instruction to "start" A/D and stores the A/D conversion result in the external data memory. In this way, under the control of the single-chip computer, the antenna amplitude signal is recorded once every time the antenna rotates △a. After the recording is completed, data calibration, filtering processing and parameter calculation are performed, and finally the directional diagram is drawn and the directional diagram parameters are printed.

Antenna pattern parameters include main lobe width, side lobe level and crossover point level. Since the antenna amplitude signal uses logarithmic values, multiplication and division operations become addition and subtraction operations, which simplifies programming. The key to solving the three parameters is to find the main lobe maximum point, the first side lobe maximum point and the crossover point. After finding these points, simple addition and subtraction operations can be performed to find the parameter values. The following introduces the measurement amplifier correction program and the "burr elimination" program in the data processing program. [page]

3.2 Measuring amplifier calibration procedure

The measuring amplifier is the main error source of the system. It is an approximate logarithmic amplifier. In order to obtain an output voltage proportional to the logarithm of the input signal , it needs to be calibrated.

The calibration method of the measuring amplifier of this system is: use the experimental method to measure the output voltage value of the measuring amplifier corresponding to the calibration value of the whole decibel point, make a voltage-decibel table and store it in the program memory, compile a table lookup program to obtain the output voltage value corresponding to the calibration value of the whole decibel point, and the calibration value of the non-whole decibel point is obtained by linear interpolation. The calculation formula is:

Wherein: i=1, 2, ..., Y is the integer decibel correction value; X is the output voltage value corresponding to Y; y is the non-integer decibel correction value between yi-1 and Yi; x is the output voltage value corresponding to y.

The voltage-decibel table can be obtained through the following experiment. The experimental configuration is shown in Figure 5.

The experimental steps are as follows:

a) Connect the circuit as shown in Figure 5, turn on the power, and make the instrument work normally.

b) Set the precision attenuator to 0 dB and adjust the signal source "Attenuation" knob so that the A/D output is close to full scale.

c) Increase the attenuation of the precision attenuator and read the A/D output value every 1 dB change until the output is 0.

d) Use the decibel value as the address (last few digits) and solidify the corresponding A/D output value in the EPROM.

Figure 6 is a calibration curve of the measuring amplifier drawn by a plotter. Experiments show that after calibration, the error generated by the measuring amplifier is greatly reduced, but it is still greater than 0.25 dB, and the error is close to 0.5 dB when the signal is small. The reason for the error is that the precision attenuator itself has an error of 0.1 dB during calibration, especially the calibration environment (temperature, humidity, etc.) is different from that during measurement, which causes a greater impact on the calibration error. Therefore, in practice, multiple decibel-voltmeters are generally made and applied to different experimental environments.

3.3 Elimination of "burr" procedures

As the electromagnetic environment becomes increasingly complex and harsh, there are many electromagnetic interferences at the test site. The most frequent one is pulse interference, which appears as small "burrs" in the drawn directional diagram. These burrs bring great trouble to data processing. If they are not eliminated, they may cause measurement errors to increase and parameter calculation errors to occur. For example, the "burrs" that appear on the main lobe will be misjudged as side lobes, resulting in errors in the calculation of the side lobe level.

Commonly used methods for eliminating "burrs" include limited filtering, arithmetic mean, median filtering, and first-order lag filtering. Experimental verification shows that the limited filtering method has the best filtering effect on "burrs" that appear in the measurement. The limited filtering method is to subtract two adjacent sampling values, calculate the absolute value of the increment, and then compare it with the maximum difference △Y allowed between the two samples. If it is not greater than △Y, the data is considered valid and retained; otherwise, the value of the upper point is taken as the data for this time. That is,

Where: K = 1, 2, ...; X is the collected data; Y is the filtered data.

The selection of △Y is very important. If it is too large or too small, the filtering effect is not ideal and needs to be obtained through repeated experiments.

4 Conclusion

This paper introduces the hardware and software principles of a radar antenna pattern automatic test system. Using this system for field measurement, excluding the time for setting up and power-on preparation, the entire test process from signal acquisition, data processing to drawing the pattern and printing the calculation results does not exceed 1.5 minutes. The system has the characteristics of high accuracy, high measurement speed, stable performance, low price and strong adaptability to the environment. It is suitable for both laboratory measurements and online measurements of antennas in the field.