Antenna Pattern Automatic Test System Based on MSP430 Single Chip Microcomputer

    Antenna pattern is an important indicator of antenna. The purpose of antenna test is to test the radiation characteristics of antenna. Early antenna test mainly relied on manual work, which had large errors and low efficiency. With the development of computer application technology, automatic control systems for antenna test have emerged, which effectively improved the test accuracy and work efficiency. However, the systems developed in the past are mainly used in the commercial field, with high costs and long R&D cycles. In order to meet the actual needs of experimental research, teaching practice and small antenna engineering test for antenna test systems, an antenna pattern automatic test system based on MSP430 is proposed and designed. The system can complete the accurate test of antennas.

1 Theory and scheme design of the pattern automatic test system
    According to the antenna reciprocity principle, the rotating antenna method is adopted. The fixed radiating antenna is used as the transmitting antenna to transmit electromagnetic waves, and the antenna under test is rotated to receive. The signal levels at different angles within the measured range are measured to obtain the pattern of the antenna under test. In actual measurement, it is not necessary to carefully study every point in space. It is only necessary to measure the pattern of the E and H planes that are perpendicular to each other and contain the maximum radiation direction. For antennas erected on the ground, this system takes the horizontal pattern parallel to the ground as the research object and draws it on the liquid crystal in a rectangular coordinate system.
    The test system mainly includes the following parts: wireless signal transmission module, signal acquisition control module, data processing, display and storage module. The principle block diagram is shown in Figure 1.

2 Design of Directional Pattern Automatic Test System The
    design requirements of this system are as follows: 1) Use MSP430 to control the antenna turntable to rotate according to the test requirements; 2) Collect and store antenna signals; 3) Draw the directivity pattern based on the collected data. Based on the above requirements, the entire circuit is designed based on the principles of system stability, comprehensive functions, convenient control, and cost saving. The system hardware structure is shown in Figure 2.

2.1 The automatic control module of the test system realizes
    the automatic test of the antenna pattern. The automatic control in the system mainly refers to the control of the antenna turntable by MSP430. During the measurement process, the determinant non-encoded keyboard is connected to the P1 port of the MSP430 with an interrupt function. The relevant control instruction parameters (including start, stop, automatic and single-step operation) are sent to the MSP430 in the interrupt query working mode. The MSP430 converts it into an electric pulse and drives the turntable to rotate smoothly through the drive circuit. The working voltage of the antenna turntable is AC 24 V, the rated load is 18 kG, and the horizontal rotation speed is 6° per second. According to experiments, the rotation speed is accurate within the rated load. Since only the change in angle is required to draw the directional pattern, the timer TA module in the MSP430 can be used to generate a fixed angle by using the interrupt timing method to make the antenna rotate according to the test requirements. [page]

2.2 Signal acquisition and storage module implementation
    The signal acquisition unit is mainly composed of a measuring receiver and an A/D converter. The measuring receiver converts the received signal into current/voltage and amplifies it to meet the needs of A/D conversion: the A/D converter converts the signal into a digital quantity to enable corresponding software processing. The MSP430F149 has an internal integrated ADC12 module, which has high data acquisition accuracy and all functions can be independently configured by user software. The A/D module is initialized to use the internal reference voltage; the P6.0 port is configured as an external channel, which is connected to the signal that meets the needs of A/D conversion after conversion and amplification by the receiver; a single-channel single conversion mode is used to save software and test time. The conversion is turned on when the TA interrupt signal is generated, and it is turned off during the rotation process to save system energy consumption.
    As an instant data acquisition unit, the MSP430 is designed to interface with the USB device to achieve timely and safe data storage. Compared with other serial buses, USB obviously has a speed advantage; compared with ISA and PCI vertical expansion boards, USB is undoubtedly flexible and convenient. The system consists of MSP430 and CH375 interface chips to form a control circuit. CH375 is a universal interface chip for USB bus. It has built-in dedicated communication protocol firmware for processing mass storage devices. External MCU can directly read and write with sectors as basic units, simplifying USB devices into an external data storage device, thereby reducing the difficulty of protocol processing and simplifying programming. In the circuit, the TXD pin of CH375 is grounded to connect it to MSP430 in parallel mode. During the test, the system works in host mode by default. After receiving the instruction to transmit data, MSP430 initializes CH375, receives the interrupt signal to determine that the connection is valid, and writes the data in the sector (512B) to the USB device. This design is suitable for single-chip microcomputer systems that read and write large amounts of data in real time.
2.3 Signal display processing module
    After the test signal is collected and digitized, it is displayed dynamically on the LCD in real time through the drawing program. The system uses XY12864G, a graphic dot matrix LCD with Chinese font library, which is connected to MSP430 in parallel indirect control mode. This module uses ST7920 as the core, which can display characters, graphics, and mixed characters and graphics. Since XY12864G comes with a Chinese character library, the programming of Chinese character display is relatively simple. This article only introduces the relevant methods of dynamic graphic display. The principle of directional diagram drawing is similar.
    The key to graphic display is to determine the corresponding position of the point to be displayed in the graphic coordinate system on the LCD screen and the address in GDRAM. The corresponding relationship between GDRAM and LCD screen is shown in Figure 3.

    1) In order to make full use of the resources of the entire screen, the first dot column is used as the ordinate. Considering that the real-time voltage value needs to be displayed below the waveform and the ROM contains 16x16 dot Chinese fonts, the 47th row is set as the abscissa.
    2) Determine the width and height of the graphics displayed on the screen.
    The ordinate Y is converted according to the AD conversion accuracy and the Y-axis resolution of the coordinate system, representing the signal voltage. The system AD conversion accuracy is 12 bits, and ADC12MEM1 is the conversion register. It is not difficult to derive the coordinate conversion formula:
    Y=(ADC12MEM1x48)/212 (1)
    The abscissa X represents the fixed angle number. The turntable rotates through a certain angle, and X is equal to that number. When the count value is 128, it means that one screen is full and the screen needs to be changed. This cycle continues until the end mark appears.
    3) Although the abscissa and ordinate of the waveform data have been determined, they do not directly correspond to the dot address on the LCD screen. As shown in Figure 3, only 128x64 dots can be displayed on the LCD screen, corresponding to 64 rows × 16WORLD/row in GDRAM. When the horizontal coordinate of GDRAM is less than 8 and the vertical coordinate is less than 32, its content will be displayed in the upper half of the screen; when the horizontal coordinate of GDRAM is 8-15 and the vertical coordinate is less than 32, its content will be displayed in the lower half of the screen. Combined with the display characteristics of the graphic display area, the formulas for the horizontal and vertical coordinates (H, V) corresponding to the point (X, Y) in the screen coordinate system in GDRAM are derived as follows:
   
    4) Calculate the number of bits of the WORD corresponding to this point with (H, V) as the GDRAM coordinates. Let a=X%16. If a=16, the highest bit of WORD is 1, otherwise the ath bit from the highest bit of WORD is 1. If this bit is set to 1, the corresponding point (X, Y) on the screen can be lit.
    In the actual drawing process, since each GDRAM address corresponds to 16 addresses, if this drawing point unit data is directly written into the address, it will affect the original waveform data of the byte on the LCD screen, causing the waveform to be displayed intermittently. To avoid this situation, the value of the cell is read out in advance, and then written as the final value after being ORed with the value to be written this time. This will not destroy the original waveform display data and maintain the integrity and continuity of the waveform.
    When the GDRAM address of XY12864G is known, the steps for writing the drawing RAM are as follows:
    1) Write the vertical V coordinate command (0~47) first
    2) Then write the horizontal H coordinate command (0~15)
    3) Write the data
    4) Write the data

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3 Test process and experimental results
    The test takes a small horn antenna as the object, and absorbing materials are laid on the ground and around the indoor test site to reduce the impact of the environment on the measurement results; the maximum aperture D and wavelength λ of the antenna are analyzed, and the minimum test distance is determined according to formula (4).
    After turning on the power, the AD conversion is started, and the signal voltage is displayed as shown in Figure 5(a). The turntable is rotated by single-step control to adjust the receiving intensity in the main direction and the system zero point in the zero radiation direction. When the signal voltage is greater than 1 V, the button starts the initialization of each module and the test is carried out.
    According to the test function requirement analysis, the software design structure diagram shown in Figure 4 is obtained.

    The antenna turntable speed is 6° per second. The timer TA can be set as follows to fix the turntable step angle:
    TACTL=TASSEL0+TACLR;
    CCTL0=CCIE;
    CCR0=32768/12;
    TACTL|=MC0;

    Select ACLK as the counting pulse of TA, increase the counting mode, turn on the TA interrupt, set the initial value of the comparator to 32768/12 (the antenna turntable takes 0.5° per step, 720 steps for one turn, and 720 times of signal strength acquisition), turn on TA, judge the end mark in the interrupt service program, turn off TA and stop the turntable, turn on A/D to collect and store data, and display the signal in real time as shown in Figure 5(b) to judge the system operation status. Finally, when the antenna test is completed, the test result display interface Figure 5(c) is displayed, and MSP430 calls the relevant program to perform corresponding operations according to the received key value. The experimental results are shown in Figure 5(d). The entire test process is smooth, the software performance is stable, the time is short, the data acquisition is accurate, and the directional pattern drawing is vivid and intuitive.

4 Conclusion
    This paper discusses in detail the construction of an antenna directional pattern automatic test system based on MSP430 microcontroller. The system has a manual control test function, which controls the movement of the turntable and the acquisition, display, storage and drawing of antenna signals through external input signals; the configuration of the timer produces a smaller step angle, increases the test sampling points, and makes the collected data more accurate. From the experimental results, the design principle is feasible, and the automatic test control of the antenna pattern is well realized. The measurement accuracy and reliability are relatively high, which can meet the needs of laboratory research and small antenna testing.