Details

The subject of the graduate qualification work is “Design of an LC Low-Pass Filter with Voltage-Controlled Cutoff Frequency”. The purpose of the work is to develop a third-order Butterworth LC low-pass filter with an electrically tunable cutoff frequency, including analytical circuit calculation, simulation of its characteristics, and investigation of the dependence of the frequency and time-domain characteristics of the filter on the tuning coefficient of the controlled capacitors. The work presents an analytical synthesis of a passive third-order Butterworth LC low-pass filter implemented as a pi-type circuit with a nominal cutoff frequency of 10 GHz, a source impedance of 50 Ohm, and an open-circuit load. The component values of the circuit are obtained, together with the transfer function, amplitude-frequency response, phase-frequency response, step response, and impulse response. Computer simulation of the filter is carried out in Multisim, and the simulation results are compared with the analytical calculations. The simulation results agree well with the analytical data, with the deviation of most key parameters not exceeding 1 %, while the deviation in the impulse-response peak time is about 1.8 %. As a result, an analytical law for cutoff frequency tuning is obtained, and the changes in the amplitude-frequency and time-domain characteristics of the filter at different tuning coefficients are investigated. Based on the criterion of a maximum passband deviation of 2 dB, the practical tuning range is established as k in, which corresponds to a cutoff frequency range of approximately 8.0 to 12.7 GHz.