Altera Corporation 7–25
September 2004 Stratix Device Handbook, Volume 2
Implementing High Performance DSP Functions in Stratix & Stratix GX Devices
Figure 7–14. Block Diagram Representation of Decimation
Decimation filters reverse the effect of the interpolation filters. Before the
decimation process, a low pass filter is applied to the signal to attenuate
noise and aliases present beyond the Nyquist frequency. The filtered
signal is then applied to the decimation filter, which processes every D-th
input. Therefore the values between samples D, D-1, D-2 etc. are ignored.
This allows the filter to run M times slower than the input data rate.
In a typical system, after the analog to digital conversion is complete, the
data needs to be filtered to remove aliases inherent in the sampled data.
Further, at this point there is no need to continue to process this data at
the higher sample (oversampled) rate. Therefore, a decimation FIR filter
at the output of the ADC lowers the data rate to a value that can be
processed digitally.
Figure 7–15 shows a specific example where a signal spread over 8 MHz
is decimated by a factor of 4 to 2 MHz. The Nyquist frequency of the
downsampled signal must be greater than 2 MHz, and is chosen to be
2.25 MHz in this example.
D
LPF
Input Output
sample rate f
s
sample rate f
s
/D
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