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for example in a communications system the the signal band may only be narrow, KHz wide, but the signal band could be centered at RF frequencies, at many MHz. If the signal is sampled according to the Nyquist criterea, i.e. twice the highest frequency, then the data rate for the RF signal will be very high. Processing the data at this high
rate is both difficult and expensive in terms of the amount of hardware required. A DDC will select the
frequency band of interest and shift it down in frequency, which allows the data rate required to retain all the information to be much lower, and consequently reduces the complexity of any further processing.
DDC is a digital downconverter. it is implemented in order to reduce the data rate for further processing. it comprises a "digital mixer" i.e. a complex multiplier followed by aN FIR lowpass filter. the LPF is followed by a decimator.