Design of the adaptive log-domain low-pass filter

This paper presents the structure and realization method of the current-controlled adaptive low-pass filter which based on the design of LC ladder network. After amplifying, amplitude limiting and shaping, the input voltage signal was converted into frequency signal. Then by the broadband frequency-current conversion circuit it was converted into current signal which has a linear relationship with input frequency. The current signal was as the bias current to adjust the cut-off frequency of the log-domain low-pass filter. So that frequency of the filter can automatically track the input signal frequency. In this paper, the design principle was introduced in detail, the design formulas were derived and the circuit of adaptive second order log-domain low-pass filter was given. The filter can realize the scope of the cut-off frequency up to 30MHz. The tracking range for the input frequency is from 1MHz to 30MHz. The simulation results show the effectiveness of design method.