Xilinx V2.1 Manuel d'utilisateur télécharger pdf (Page 71)

DSP 71

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 twobasic building blocks of aCIC ﬁlter arethe integrator and the comb. A single

integrator is a single-pole IIR ﬁlter with a transfer function of:

H(z) = (1 – z

-1

)

-1

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

H(z) = 1 –z

-RM

As seenin thetwo ﬁgures below, theCIC ﬁltercascades Nintegrator 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

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Xilinx V2.1 Manuel d'utilisateur Page 71