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A digital down converter (DDC) is a component of a digital signal processing system where it is used to convert a digitized real signal centered at an intermediate frequency to a basebanded complex signal centered at a zero frequency. It is also used to reduce the input signal to a lower sampling rate, thus allowing lower-speed processors to process incoming fast signals.
The DDC performs a fundamental function of many communication systems and is used in radio receivers. It is implemented with the help of field-programmable gate arrays or application-specific integrated circuits.
Digital down converters typically perform the down conversion of incoming signals in a digital signal processing system. They are widely used in communication systems and radio receivers where the fast analog-to-digital converters (ADCs) deliver huge amounts of data that need to be down converted. A DDC takes the output signals from the ADC and allows for intensive processing of the specific data by discarding the rest of the data.
A DDC makes use of the following mathematical relation to shift the bandwidth of interest to the baseband:
F(A)*F(B) = F(A-B) + F(A+B)
Where F() stands for the frequency of the band. The received signal is multiplied by an approximation of the original carrier to perform the down conversion.
The major components of a DDC are a direct digital synthesizer, a low-pass filter and a down sampler.
The hardware required to implement a DDC may include a local oscillator and a multiplier (mixer). The unwanted signals are removed with the help of a linear phase filter. The resulting signals are decimated to obtain a lower sampling rate.
The filter used can be any type of suitable low-pass filters like FIR, IIR and CIC filters. FIR filters are mostly used for low amounts of decimation, whereas CIC filters are used along with FIR filters to provide larger down-sampling ratios. IIR filters are a lower order than FIR and provide more efficient implementation when specifications in terms of passband, ripple, stopband and/or roll-off must be met.
Using DDC in place of analogue techniques has several advantages such as: