The present invention relates to a phase shifter used in a demodulator of a digital-signal-modulated carrier wave transmission system.
In a demodulator for demodulating a digital-signal-modulated carrier wave, a modulated signal input to the demodulator is supplied to two phase detectors, and output signals having predetermined phase angles shifted through, e.g., 90.degree. from each other are supplied from the two phase detectors. Therefore, a reference carrier signal from a VCO or the like is directly supplied to one phase detector and to the other phase detector through a 90.degree.-phase shifter. A control signal is supplied to the phase shifter so that the output from the phase shifter always accurately maintains a phase difference of 90.degree. with respect to the output from the VCO.
A conventional phase shifter used in a digital demodulator is disclosed in U.S. Pat. No. 4,549,142. This phase shifter is arranged using an LC circuit and therefore has frequency dependency. For this reason, when this phase shifter is mounted on a printed circuit board or the like, a phase shift is easily adversely affected by a parts arrangement. Therefore, this phase shifter is not suitable for an LSI arrangement.
Japanese Patent Laid-Open No. 61-157019 discloses various phase shifters using not an inductance but a capacitor, a resistor, and a transistor, thereby eliminating the above drawback. However, a phase shift amount cannot vary in these phase shifters.