Direct-conversion transmitter circuit and transceiver system

Abstract

Disclosed is a direct conversion type transmitter or transceiver circuit suitable for a mobile communication device which corresponds to broad signal output level variable width to be required by W-CDMA, which does not necessitate any high-performance low noise VCO and RF filter, capable of reducing a number of components and the cost. In the input portion of an orthogonal modulator composed of a divider, mixers, and a common load, there are provided variable attenuators. If an input signal level of the orthogonal modulator within the transmitter circuit lowers, this variable attenuator circuit is operated so as to lower the bias of the orthogonal modulator to reduce the amount of occurrence of carrier leak, and to prevent the signal during low output level and carrier leak ratio from being deteriorated. The direct conversion transmitter circuit is capable of easily realizing output level variable width of 70 dB or higher and reducing a variable amount in the high frequency circuit in which it is difficult to secure the variable gain width.