1. Field of the Invention
The present invention relates to demodulators specifically, to a demodulator capable of avoiding phase and amplitude shifts between a first modulated signal and a second modulated signal by making the input impedance of a first matching section employed in a splitting/matching section for receiving the first modulated signal equal to the input impedance of a second matching section which is also employed in the splitting/matching section for receiving the second modulated signal.
2. Description of the Related Art
In recent years, high-speed transmission technologies making use of a high-frequency band such as a millimeter-wave frequency band are being intensively and extensively researched with an aim to transmit a signal at smaller power consumption and a lower transmission cost due to a smaller circuit scale through the use of CMOS (complementary metal oxide semiconductor) technology. A signal transmission apparatus making use of a high-frequency band is configured to employ a modulator for transmitting a modulated signal of a millimeter-wave frequency band and a demodulator for receiving the modulated signal from the modulator and generating the modulated signal.
A demodulator 600 employed in the existing signal transmission apparatus is explained. As shown in FIG. 10, a modulated signal received by an antenna 510 is amplified by an amplifier 520. The amplified signal is split into a first modulated signal and a second modulated signal at a branch point Bo which is provided between the amplifier 520 and a squaring circuit 530. The squaring circuit 530 is a section for multiplying the first modulated signal by the second modulated signal in order to demodulate the modulated signal. That is to say, the squaring circuit 530 generates a demodulated signal as a result of the multiplication. The squaring circuit 530 supplies the demodulated signal to an amplifier 540 which then amplifies the demodulated signal and outputs the amplified signal.
In addition, the demodulator also referred to as a signal detection circuit can have another proposed typical configuration described as follows. In this proposed configuration, for example, a signal output by the signal detection circuit is compared with a reference voltage and a direct-current component of the result of the comparison is supplied to an IF (intermediate frequency) amplifier and fed back to the signal detection circuit. See, Japanese Patent Laid-open No. Sho 57-37905 (hereinafter, as Patent Document 1). In accordance with this signal detection circuit, a direct-current voltage of the detection output can be made stable.