关键词: 超低通滤波器, 微计算机, 快速富里叶变换, 幅频特性

Abstract: This paper introduces how to best the frequency response of a superlow-pass filter by a microcomputer. The customary signal exciting the filter is a slowly swept sine. Due to the development of digital signal processing algorithms and especially the efficient implementation of the discrete Fourier transform (DFT) with the fast Fourier transform (FFT), it becomes possible to use more complex input signals. Instead of exciting the filter under test frequency by frequency, it is possible to generate arbitrary waveform with a broadband spectrum, using a digital-to-analog (D/A) converter, to gather all the spectral information in one measurement. In many applications, the prescribed spectral shape is a flab spectrum. One waveform that has such a flat spectrum is a sharp impulse, but in this case all the power is concentrated at one point in time, leading to a maximum peak power. This results in an important reduction of the measurement time, bub also in an undesired accuracy loss of the measurement results if no spacial precautions were taken. According to Schroeder's method a waveform that is the sum of a number of harmonically related cosine waves with equal amplitudes and with modulated phases for reducing the peak-factor has been generated. The data file, which is discretely calculated by the microcomputer according to one period of the waveform, has been created. While the filter is tested, the data file will be read into the memory of the microcomputer. By interrupting the CPU of the microcomputer at regular intervals, the waveform is sent to the filter through the digital-to-analog converter.At the same time, the output of the filter is collected by the microcomputer through an analog-to-digital converter. The frequency response of the filter can be obtained by FFT for the output. Thus the test can be performed in a shorter time with better accuracy.

Key words: Superlow-pass filter, FFT, frequency response, microcomputer