The first, second, and third pairs of DC voltages which set a modulated signal to a predetermined reference level while the levels of I and Q signals supplied to a quadrature modulator are set at \u201c0\u201d are sequentially obtained. The micro-switch may be formed of microelectromechanical system technology or microelectronics technology. The step for separating the proteins from the proteoglycans can be carried out using a sepharose column.