Title: Qam modulation using direct digital synthesis and upconversion
Application Number: 98810942 Application Date: 1998.11.06
Publication Number: 1278976 Publication Date: 2001.01.03
Approval Pub. Date: 2003.10.29 Granted Pub. Date: 2003.10.29
International Classifi-cation: H03C3/00;H04L27/36
Applicant(s) Name: Telefonaktiebolaget Lm Ericsson (Publ) Address:
Inventor(s) Name: K.-G. Sahlman
Attorney & Agent: zou guangxin
Abstract:
    A transmitter subsystem (100 and 200) is disclosed whereby the output frequency spectrum of a DDS device (202), for use in the transmitter subsystem, is frequency multiplied to achieve greater frequency coverage at the RF bands. In one embodiment of the present invention, a DDS device is structured in such a way that the DDS output signal spectra can be multiplied directly for direct conversion to RF or IF bands. As such, the requirements placed on the DACs (108 and 116) in the DDS device are lessened, because the DACs require much lower sampling frequencies than those used in the existing technology. Also, the requirements placed on the implemented bandpass filters (114, 122 and 208) used for image rejection in the applied upconverters are lessened, because of the larger frequency difference created between the desired output and the image frequency band. In a second embodiment of the present invention, a DDS device (302) is also structured in such a way that multiplication of the baseband spectra can be accomplished at the IF. Consequently, in accordance with the present invention, less complex or costly DACs can be used for generating wideband signals at higher frequencies, but relatively high linearity signals can still be attained.
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