DISCRETE-ANALOGUE FILTER OF THE SECOND ORDER ON SWITCHED CAPACITORS WITH TUNING OF POLE FREQUENCY BY DIGITAL POTENTIOMETER
Abstract
A discrete-analogue filter of the second order on two frequency-switching capacitors is developed and investigated. The proposed circuit contains two inputs (In_LPF_HPF, In_BPF_NPF) and four outputs (Out_LPF, Out_BPF, Out_HPF, Out_NPF). The filter type (numerator of the transfer function) is determined by connecting a signal source to the corresponding input of the circuit and taking a signal from the corresponding output. The pole attenuation depends on the resistance of a single resistor R5, which does not affect the other parameters. Therefore, the pole attenuation can be tuned using this resistor. To set the passband gain at a given level, it is appropriate to use resistor R1 in the LPF and HPF, and resistor R2 for the BPF and NPF. Changing these resistors will not cause changes in other parameters of the filter circuit. It is established that the pole frequency depends on the resistance of the resistor R8 or digital potentiometer Kdp (Kf), the transmission coefficient of which can be changed by changing the binary digital code Kf, fed to its control inputs, and the other parameters of the filter link do not depend on them, so by changing the resistance of this resistor or the transmission coefficient of the digital potentiometer the pole frequency can be tuned in a wide range while preserving other parameters. Computer modelling of the investigated discrete-analogue filter is performed in Micro-Cap environment. The sequences of pulses controlling electronic keys are given. Graphs of output voltages at the circuit outputs (Out_LPF, Out_BPF, Out_HPF, Out_NPF) are shown. The application of a digital potentiometer in the filter circuit is extremely promising in the construction of adaptive signal processing systems.
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