I've seen the acronym QDUC -- what does that stand for and how does it relate to DDS?
QDUC is an acronym for Quadrature Digital Up-Converter, which is an all-digital version of the age-old analog quadrature modulator. At present, ADI's flagship QDUC is the AD9957.
An analog quadrature modulator relies on a quadrature analog LO (local oscillator) -- an oscillator that provides two sine waves having the same frequency but phase shifted by 90-degrees relative to each other. The LO drives an analog quadrature mixer and is capable of generating a single-sideband suppressed-carrier (SSBSC) signal along with other forms of quadrature modulation.
In a QDUC, a DDS with both sine and cosine outputs replaces the analog quadrature oscillator and digital multipliers take the place of the analog mixer. The advantage of having a DDS function as the LO, is a user can program the carrier frequency anywhere in the DDS's frequency range. A DDS also allows for rapid (virtually instantaneous) switching from one carrier frequency to another. A QDUC also has a quadrature baseband upconverter. It accepts a digital quadrature (I & Q) baseband input signal sampled a rate that is less than the QDUC's output sample rate. The QDUC contains digital interpolation filters that translate the baseband I & Q input signals from the lower input sample rate to the QDUC's higher output sample rate. The digital filters take care of suppressing the Nyquist images that are a normal consequence of the upsampling process. The end result is that the QDUC generates a digital quadrature modulated signal. It can readily handle virtually any form of quadrature modulation, including QAM, QPSK, GMSK, and OFDM.
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