Introduction In RF/microwave systems, it is necessary to properly separate the signal spectrum. The component that performs this function is the filter. Numerically controlled frequency hopping filters (NCFM filters) are increasingly favored by users due to their multi-frequency switching, flexible switching, high speed, and strong adaptability. With the continuous improvement of simulation software and the advancement of production materials and processes, mass production of NCFM filters has become possible. Faced with hundreds or even thousands of NCFM filter testing tasks and test data processing work, how to efficiently conduct product testing and reduce the workload of testing personnel has become a challenge for manufacturers. Therefore, after extensive practical experience in NCFM filter testing, a user-friendly, easy-to-operate, and stable computer-aided testing (CAT) system for NCFM filters was designed using computer software and hardware technology, combined with a vector network analyzer. The system has proven to be highly effective. 1 Hardware Composition and Design 1.1 Working Principle of CNC Frequency Hopping Filter The CNC frequency hopping filter is a product of the combination of digital control technology and filter technology. By inputting different combinations of high and low levels to the control pins of the CNC frequency hopping filter, the capacitor array in the filter is selected, thereby achieving the purpose of selecting different operating frequency points of the filter. Its principle is shown in Figure 1. If a high level is represented by 1 and a low level by 0, then different combinations of high and low levels are equivalent to binary encoding. For example, if a certain CNC frequency hopping filter has 8 control pins, then its binary encoding range is 00000000～11111111, which corresponds to 2⁸ = 256 frequency points. While understanding the working principle of the CNC frequency hopping filter, a large amount of testing practice was carried out, solving two key technologies of the hardware composition of the CNC frequency hopping filter CAT system: automatic frequency switching and automatic acquisition of test data, thus realizing the hardware architecture of the CNC frequency hopping filter computer-aided testing system. 1.2 Automatic Frequency Switching The computer's parallel port (also known as the printer port) is a 25-pin D-shaped connector with an 8-bit parallel data port for writing and reading data. The pins are PIN2 to PIN9, defined as shown in Table 1. If these eight pins are considered as general digital output pins, they are essentially eight digital I/O outputs. When data is transmitted through the data port, the voltage levels of these eight pins are changed; the receiver interprets this according to the same encoding principle to obtain the transmitted data. Connecting the 8-bit data port pins of the computer's parallel port to the control pins of the digitally controlled frequency hopping filter forms the physical link shown in Figure 2. Computer software programming controls the automatic output and updating of data at the data port to select different capacitor arrays in the digitally controlled frequency hopping filter, thereby achieving automatic frequency switching of the filter. 1.3 Automatic Acquisition of Test Data The vector network analyzer is the main test instrument for numerically controlled frequency hopping filters. To meet the needs of users for automatic testing, manufacturers typically provide interfaces such as USB, LAN, and GPIB for communication with a computer, as well as dynamic link libraries and COM components associated with programming these interfaces. Since the vector network analyzer used in this design is an 8735D, GPIB communication will be used as an example for explanation. First, a PCI-GPIB card needs to be installed on the computer, along with the corresponding driver. Then, a GPIB cable is used to connect the computer and the vector network analyzer through the GPIB interface, and the GPIB communication address of the vector network analyzer is configured, forming the physical link for obtaining test data as shown in Figure 3. Finally, using the dynamic library provided by the manufacturer, commands are automatically sent to extract data such as insertion loss, BW3dB, VSWR, rectangular coefficient, far-end rejection, and Q value, obtaining the corresponding test data. 2. Software Architecture and Design The CNC frequency hopping filter computer-aided testing system is based on the Windows operating system platform and designed using Microsoft Visual C# 2005 development tools and object-oriented programming methods. It achieves high digital detection accuracy, a complete user interface, rich operational functions, scalability, reusability, and maintainability, and provides valuable utility call functions. 2.1 Application of Object-Oriented Technology and Methods Modern object-oriented technology is based on an intuitive way of thinking, describing the objective world in a natural way. It views the things in the objective world as different objects, each with its own attributes and methods, and objects communicate with each other through mechanisms such as messages. Its basic characteristics include data abstraction, encapsulation, inheritance, polymorphism, and message communication, which have significant advantages over procedural methods in terms of development efficiency, code reusability, software scalability, and maintainability. The objects in the diagram include the system's testing, processing, control, result expression, and storage processes, which are encapsulated using dynamic link libraries, COM components, and C# classes and templates, respectively. 2.2 Software Architecture The Visual C# development environment provides an excellent software architecture model, making the designed program structure clearer and development efficiency higher. The communication and connection relationships between specific objects are shown in Figure 5. 2.3 Product Qualification Judgment Algorithm There are generally two methods for judging whether a CNC frequency hopping filter product is qualified: one is to judge the test data of all frequency points; the other is to judge the test data of key frequency points. Regardless of which judgment method is used, the process shown in Figure 6 must be followed. 2.4 Software Interface In Visual C#, forms are mainly implemented in two ways: SDI (single document interface) and MDI (multiple document interface). This system adopts the MDI multi-document structure, which facilitates users to compare test data and is also conducive to the expansion of test objects. The system software interface is shown in Figure 7. 3 Conclusion After CNC frequency hopping filter products can be mass-produced, automated testing replacing manual testing is an inevitable trend. The computer-aided testing (CAT) system for numerically controlled frequency hopping filters designed in this paper was developed under such circumstances. It features low design cost, simple operation, and high testing efficiency. It is now widely used in actual testing work, greatly reducing the workload of testers, who are very satisfied with its use.