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Xilinx Blocks
Block Interface
The CIC Block has one input and one output port. The input port can be between 1
and 32 bits (inclusive).
The two basic building blocks of a CIC filter are the integrator and the comb. A single
integrator is a single-pole IIR filter with a transfer function of:
-1
-1
H(z) = (1 – z
)
The integrator's unity feedback coefficient is y[n] = y[n-1] + x[n].
A single comb filter is an odd-symmetric FIR filter described by:
y[n] = x[n] – x[n – RM]
M is the differential delay selected in the block parameterization GUI, and R is the
selected integer rate change factor. The transfer function for a single comb stage is
-RM
H(z) = 1 –z
As seen in the two figures below, the CIC filter cascades N integrator sections together
with N comb sections. To keep the integrator and comb structures independent of
rate change, a rate change block (i.e., an up-sampler or down-sampler) is inserted
between the sections. In the interpolator, the up-sampler causes a rate increase by a
factor of R by inserting R-1 zero-valued samples between consecutive samples of the
comb section output. In the decimator, the down-sampler reduces the sample rate by
a factor of R by taking subsamples of the output from the last integrator stage.
Figure 3-46: Pipelined decimator and interpolator
DSP
71
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