Measurement of high frequency AC signals based on the true root mean square value

The root mean square (RMS) of high frequency AC signals is the key characteristic for data acquisition, but its value is hard to be measured directly by common data acquisition system. In this paper, we gave a brief introduction about the measurement for RMS value of high frequency AC signal. The sampling computation method is an intuitive way but it needs high hardware requirements. The precision rectification method is easy to realize by operational amplifiers but it can only be used in several special waveforms and isn´t a true RMS value method. The thermal conversion method is a true RMS value method and good at wide bandwidth but its accuracy is low. The true RMS value method based on logarithmic computing has high accuracy but its bandwidth is too narrow. In this paper, we adopted the true RMS value method based on logarithmic computing in engine manipulation ground experiment. With this principle, we designed a kind of signal conditioning module which can change the high frequency AC signals into DC values and be acquired by data acquisition cards. In the end, we checked the accuracy with serial experiments such as the RMS accuracy test and frequency response test. We proved that the accuracy of the signal conditioning module is better than 1% when the highest frequency of input signal is less than 500k Hz.