The Amplitude Frequency Characteristics Measurements of Oscilloscope Based on CPLD Logarithmic Circuit

摘要

In this paper we introduced the principle and implementation of the amplitude frequency characteristics measurements of oscilloscope based on CPLD, we used DDS Signal Source and common oscilloscope to take amplitude frequency characteristics measuring of amplifier come true. At first we display the logarithmic voltage as x-output of oscilloscope which is converted by output frequency of DDS Signal Source by logarithmic circuit, then we display a signal which the frequency response output of amplifier is demodulated and filtered as y-output of oscilloscope, after this we suitable adjust x-output and y-output sensitivity, the amplitude-frequency characteristic can be display on oscilloscope. In this measurement we use DDS AD9850 to get sine wave source, and the frequency-sweep is done with single chip computer. We use pipeline AD converter chip and ispMACH4128V to get the output voltage of DDS, and calculate its effective value by single chip computer. The ispMACH4128V is used in logarithmic circuit, and the logarithmic algorithm uses table look-up technique in order to take accurate and simple measurement. This paper introduces the principle of this measurement unit, the detailed schematic block diagram of each block and main circuit principle The experiments prove that this system is practical and successful, it can reach the purpose which use common oscilloscope to measure amplitude frequency characteristics.