Digital Down Converter

A Digital Down Converter (DDC) shifts a Discrete-Time Signal centered at some carrier frequency down to Baseband, producing a complex IQ output at a reduced Sampling Frequency. It consists of three stages:

1. Mixer: multiplies the input by a complex sinusoid $e^{-j2\pi f_{c}n}$ from a Numerically Controlled Oscillator, shifting the center frequency to zero.
2. Low-pass filter: removes out-of-band content and images.
3. Decimator: reduces the sample rate to match the signal Bandwidth.