Abstract: Virtual instrument (VI) is a product of the combination of modern computer technology, communication technology, and measurement technology. This paper introduces the characteristics and applications of virtual instruments, and describes the design of a virtual digital filter based on the LabVIEW virtual instrument development platform. Keywords : FIR filter; LabVIEW; programming 1 Introduction Traditional instruments are closed mechanisms with predefined functions designed by manufacturers, each instrument performing a specific function. With the combination of computer technology and traditional instrument technology, a new type of instrument—virtual instrument—has emerged, which, with the help of necessary data acquisition hardware and computers, implements all the functions of a physical instrument through software. As a new type of instrument construction technology, virtual instrument technology can partially or even fully realize the functions of physical instruments. Compared to traditional instruments, digital filters offer numerous advantages: they can handle more complex and faster processing of test quantities, provide richer and more diverse ways to express test results, facilitate convenient storage and exchange of test data, are inexpensive, and offer rapid technological updates. Their most significant feature is that they transform the way instrument manufacturers define instrument functions into a user-defined system, meeting a wide range of application needs. Digital filters are a crucial component of digital signal analysis, enabling the filtering, extraction, and enhancement of useful components while reducing useless ones. Classical filters are characterized by using different frequency bands for the useful and unwanted frequency components of the input signal, achieving filtering through a suitable frequency-selective filter. However, if the frequencies of the signal and interference overlap, classical filters are ineffective, necessitating the use of modern filters such as Wiener filters, Kalman filters, and adaptive filters. Based on the implemented network structure or unit impulse response, digital filters can be classified into Infinite Impulse Response (IIR) filters and Finite Impulse Response (FIR) filters. Compared to IIR filters, FIR digital filters can be designed to have linear phase characteristics. Therefore, it has wide applications in applications requiring linear phase. There are many design methods for digital filters, among which the window function design method and frequency sampling design method are more commonly used. This article introduces a virtual filter designed using a window function. 2. Main Functions and Features of LabVIEW Software LabVIEW (Laboratory Virtual Instrument Engineering Workbench) is the world's first 32-bit compiled program development system for instruments, launched by National Instruments (NI). Its goal is to simplify program development and improve programming efficiency. It is currently the most widely used and most powerful virtual instrument platform. LabVIEW's advanced software library has powerful data processing capabilities, including signal generation, data signal processing, measurement, data filtering, probability and statistics, linear algebra, curve fitting, numerical analysis, and other software analysis functions. LabVIEW uses visualization technology to create a human-machine interface, providing many control objects in the instrument panel, such as meters, knobs, switches, and coordinate plane diagrams. Because the testing functions and panel controls of the virtual instrument are all software-based, any user can change its functions and scale by modifying the virtual instrument's software, which fully embodies the design concept that software is the instrument. From a macroscopic perspective, LabVIEW's operating mechanism is no longer the traditional execution mode of the von Neumann computer architecture. The sequential execution structure in traditional computer languages ​​(such as C) is replaced by a parallel mechanism in LabVIEW. Essentially, it is a data flow mode with a graphical control flow structure, which ensures that a function node in the program can only be executed after it has obtained all its data. That is to say, in this concept of data flow program, the execution of the program is data-driven and is not affected by factors such as the operating system or computer. LabVIEW supports multiple operating system platforms, and LabVIEW applications developed on any platform can be directly ported to other platforms. 3 Design steps of digital filter Therefore, if M is chosen to be large, it can be guaranteed that the effective value is approximated within the window. In actual calculation (3), the M-point sampled values ​​can be used to perform M-point IDFT (IFFT) to obtain the filter. Second step: According to the requirements of transition band and stopband attenuation, select the form of the window function and estimate the window length N. Let the transition band of the filter to be determined be represented by , which is approximately equal to the main lobe width of the window function. Since the transition band is approximately inversely proportional to the window length N, A depends on the window form, for example, A=4π for a rectangular window, A=8π for a Hamming window, etc. Select the window function form according to the attenuation of the transition band and the stopband. The principle is to select a window function with a narrow main lobe as much as possible while ensuring that the stopband attenuation meets the requirements. Step 3: Calculate the unit sample response h(n) of the filter, (5) where (5) is the window function selected above. If linear phase is required, it is required that both and are symmetrical about (N-1)/2. Step 4: Use (1) to verify whether the technical indicators meet the requirements. If not, repeat steps 2, 3, and 4 according to the specific situation until the requirements are met. 4 Software Implementation 4.1 Front Panel Design Figure 1 shows the front panel of the designed digital filter. The front panel is used to set the input values ​​and observe the output quantities, and is used to simulate the front panel of the real filter. Since the virtual panel is directly facing the user, it is the core of the virtual filter control software. When designing this part, the main considerations are that the interface is beautiful and the operation is simple, and the user can control the operation of the virtual filter through various buttons, switches and other control keys on the panel. In practice, the signal to be tested can be acquired and filtered in real time by a data acquisition card, or it can be acquired by a data acquisition card and saved in a file format that LabVIEW can recognize, and then analyzed and filtered by LabVIEW. Here, basic signals (sine wave, cosine wave, square wave, sawtooth wave) are used to simulate the original signal. The program adopts the window function method calculation flow, convolving the window function with the signal to be filtered to achieve signal filtering. Users can set the original signal, noise signal, and filter parameters. The waveform of the original signal and the filtering result can be displayed in real time. In this way, after the program runs successfully, the result can be obtained from the display area, making the result more intuitive. 4.2 Flowchart Design The back panel of this digital filter, i.e., the program code block diagram, is shown in Figure 2. The block diagram program consists of four elements: nodes, endpoints, frames, and connections. Nodes are similar to statements, functions, or subroutines in text language programs. Each object endpoint in the block diagram corresponds one-to-one with an object (control or display) on the front panel. Different line types represent different data types, and each data type is also emphasized with different colors on the color display. [align=center] Figure 1 Front panel of digital filter Figure 2 Rear panel of filter[/align] 5 Conclusions Digital filters can be programmed to implement various systems to meet different needs, and the coefficients can be changed at any time to adjust the filter parameters and select the best solution. Using the LabVIEW software platform to develop virtual instruments such as electrical parameter measuring instruments. It achieves higher efficiency, saves more hardware expenses, facilitates system maintenance and reduces the burden of instrument updates. The use of virtual instruments to gradually replace traditional instruments has become a trend in the development of the testing field. However, in practical applications, it is still necessary to optimize the program and combine software and hardware according to the specific situation to make the virtual instrument perform better. The innovation of the author of this paper: A virtual digital filter was designed using LabVIEW. The virtual digital filter can filter sine wave, triangular wave, square wave and sawtooth wave signals with interference signals according to the requirements. It has the characteristics of good human-computer interaction and easy operation, and can be widely used in teaching. References: [1] Ding Yumei, Gao Xiquan. Digital Signal Processing [M]. Xi'an: Xi'an University of Electronic Science and Technology Press, 2003. [2] Robert H. Bishop [US]. Practical Tutorial for LabVIEW 7 [M]. Beijing: Electronic Industry Press, 2005. [3] Instrumnts Catalogue. US National Instruments, 2003. Zhou Weilin, Yang Huayong, Li Qingfeng. Design of Digital Filters Based on LabVIEW [J]. Microcomputer Information, 2006, 5-1: 163-164.