Design And Implementation Of Frequency Characteristic Tester Based On Zero Intermediate Frequency Quadrature Demodulation

Frequency characteristic tester is the instrument measurement of frequency characteristic of circuits, widely used in electronic product testing and electronic science experiments. As is the most key,frequency characteristics measuring principle is still the focus of research. At present, advanced frequency characteristic tester also have the drawbacks of high-price, large bulk,especially complicated operation and service difficulty. In contrast, a high performance, low cost frequency characteristic tester is more easily accepted by the market.With the development of modern electronic measuring technology, all kinds of the measuring principle of frequency characteristics emerged as the times require. From the diode detector and the first-order linear interpolation theory of phase detection to digital system and the discrete Fourier transform (DFT), which based on digital orthogonal transform. The frequency characteristics measuring principle is developed to some extent, but always can’t achieve good tradeoff between cost and performance. It is unable to meet market needs of frequency characteristic tester in low-frequency.Zero intermediate frequency(Zero-IF) orthogonal demodulation principle directly converts the high frequency AC signal to DC signal by using orthogonal demodulation technology, designers and operators are extremely concerned for its simple structure and low cost. In this paper, through insighting into Zero-IF orthogonal demodulation and analyzing its deficiencies of the application in the field of radio-frequency (RF) receiver, we propose an improved Zero-IF orthogonal demodulation method which will apply to frequency characteristic measurement. The method improves the frequency characteristic measurement accuracy and reduces the measurement error and the cost of products, which has overcome a series of defectthe, such as system complexity, poor working stability, high price and so on. It is a highly cost-effective frequency measurement method.This paper expounds the limitations of the diode detection technique and balanced phase detection principle and DFT on measuring frequency characteristics, focuses on the problem analysis of traditional Zero-IF orthogonal demodulation principle, presents an improved Zero-IF orthogonal demodulation method which has been analyzed in theory.The paper has also given the hardware project design and circuit design details base on the Zero-IF orthogonal demodulation method, and focuses on discussing the control project based on STM32F429 and LCD driving project based on RA8875. This paper also detailedly introduces the software design details and GUI design process which is based on STemWin. Finally, this paper testes the frequency characteristic testing system, the measurement accuracy of amplitude frequency characteristic has reached 0.1 dB, the absolute error is less than 10%; the phase frequency characteristic measurement accuracy has reached lo, the absolute error is less than 5o. Compared to frequency characteristic tester in the same category, It has more advantages and stable performance, which will be able to meet the needs of users.