Altera CIC MegaCore Function - Manual (Page 22)

Hogenauer Pruning
Hogenauer pruning uses truncation in intermediate stages with the retained number of bits decreasing
monotonically from stage to stage. The total error introduced is still no greater than the quantization
error introduced by rounding the full precision output. This technique helps to reduce the number of
logic cells used by the filter and gives better performance.
The existing algorithms for computing the Hogenauer bit width growth for large N and R values are
computationally expensive.
For more information about these algorithms, refer to U. Meyer-Baese, Digital Signal Processing with Field
Programmable Gate Arrays, 2nd Edition, Spinger, 2004.
The CIC IP core has precalculated Hogenauer pruning bit widths. The CIC does not have to calculate
Hogenauer pruning bit widths if you enable Hogenauer pruning for a decimation filter.
Note: Hogenauer pruning is only available to decimation filters when the selected output data width is
smaller than the full output resolution data width.
FIR Filter Compensation Coefficients
CIC filters have a low-pass filter characteristic. Three parameters (the rate change factor R, the number of
stages N, and the differential delay M) allow you to change the passband characteristics and aliasing or
imaging rejection.
Typically, decimation or interpolation filtering applications require flat passband and narrow transition
region filter performance. However, the CIC filter has drooping passband gains and wide transition
regions. To overcome these problems connect the decimation or interpolation CIC filter to a compensa‐
tion FIR filter, which narrows the output bandwidth and flattens the passband gain.
Figure 3-5: Using a CIC Compensation FIR Filter
Decimation CIC
Filter
Compensation FIR
Filter
Compensation FIR
Filter
Interpolation CIC
Filter
Decimation
Interpolation
You can use a frequency sampling method to determine the coefficients of a FIR filter that equalizes the
undesirable passband droop of the CIC and construct an ideal frequency response.
Determine the ideal frequency response by sampling the normalized magnitude response of the CIC filter
before inverting the response.
3-6 Hogenauer Pruning
UG-CIC
2014.12.15
Altera Corporation CIC IP Core Functional Description
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