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Digital down converter
In digital signal processing, a digital down-converter (DDC) converts a digitized real signal centered at an intermediate frequency (IF) to a basebanded complex signal centered at zero frequency. In addition to downconversion, DDC’s typically decimate to a lower sampling rate, allowing follow-on signal processing by lower speed processors.
== Architecture ==
A DDC consists of three subcomponents: a direct digital synthesizer (DDS), a low-pass filter (LPF), and a downsampler (which may be integrated into the low-pass filter).
The DDS generates a complex sinusoid at the intermediate frequency. Multiplication of the intermediate frequency with the input signal creates images centered at the sum and difference frequency (which follows from the frequency shifting properties of the Fourier transform). The lowpass filters pass the difference (i.e. baseband) frequency while rejecting the sum frequency image, resulting in a complex baseband representation of the original signal. Assuming judicious choice of IF and LPF bandwidth, the complex baseband signal is mathematically equivalent to the original signal. In its new form, it can readily be downsampled and is more convenient to many DSP algorithms.
Any suitable low-pass filter can be used including FIR, IIR and CIC filters. The most common choice is a FIR filter for low amounts of decimation (less than ten) or a CIC filter followed by a FIR filter for larger downsampling ratios.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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