Filter circuit

A filter circuit for communications/control, characterized in that an input signal (whose quantity of Laplace transformation is represented by Ux) of a filter circuit of a two-input/two-output system is branched into two signals, one of the two signals is supplied to an input of the filter circuit, the remainder is passed through a circuit (j circuit) which generates a signal that is 90 degrees out of phase with Ux, and supplied as an imaginary input signal (Uy) to another input of the filter circuit, the filter circuit outputs two output signals Vx, Vy in response to the input signals Ux, Uy supplied thereto, the two input signals and the two output signals are handled as respective complex variables (U=Ux+jUy, V=Vx+jVy) using an imaginary number unit j, a transfer function of a lowpass or highpass filter of real coefficients is represented by F(s), a central angular frequency to be passed or stopped is represented by ω, a complex coefficient transfer function produced by replacing the Laplace operator “s” with “s−jω” in the transfer function F(s) is represented by F(s−jω), and when the input signal U is passed through the complex coefficient transfer function F(s−jω), an output signal is expressed by V, a transfer expression is represented by:

• U·F(s−jω)=V
• characterized in that both sides of the transfer expression are multiplied by the denominator of F(s−jω), and connections are made with a transfer element of real coefficients in order to equalize real and imaginary parts of the transfer expression.