AFE8221RFP datasheet(21/48 Pages) TI

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GAIN=

NCOEFF

n=1

(9)

GAIN=

NCOEFF

n=1

(10)

SECOND FIR FILTERS

ddc0_fir2a_ncoeff

ddc0_cic_dec_rate

(11)

ddc0_fir2b_ncoeff

ddc0_cic_dec_rate

(12)

AFE8221

SBAS394A – APRIL 2007 – REVISED MARCH 2008

For EVEN mode the, dc gain is shown by Equation 9:

weights must add up to (215

– 1) to achieve unity

gain through the filter. With longer filters (and

therefore, smaller coefficients), frequency response

errors may be introduced as a result of coefficient

truncation. A Shift parameter has been added to the

second FIR filter to alleviate this problem. The total of

while for ARBITRARY mode the gain is shown by

all filter tap weights must add up to (215+ddc0_fir2a_shift

Equation 10:

– 1) to achieve unity gain through FIR2A (similarly for

ddc0_fir2b_shift and FIR2B). Note that shift values for

FIR2A and FIR2B can be set separately.

Extended-Length Filter Mode

If FIR2A or FIR2B cannot provide enough filter taps

to achieve the desired frequency response, setting

The first FIR filter is followed by two parallel second

control bit ddc0_interleave puts the two filters into an

FIR filters, FIR2A and FIR2B. Duplicate filters allow

interleaved mode that doubles the length of the filter

the output of two I and Q output streams with

that can be realized. However, there are several

different bandwidths. For example, the bandwidth of

limitations:

FIR2A may be set wide to accommodate reception of

1. Only odd symmetrical filters may be realized;

digital broadcasts, while FIR2B may be set narrower

to receive an analog broadcast sharing the same

2. The filter length M must be such that (M + 1)/4 is

band. Coefficients for FIR2A are stored in memory

an integer; and

bank 1 (MEM = 1) and coefficients for FIR2B are

3. Only one filter can be realized (in ddc_interleave

stored in memory bank 2 (MEM = 2).

mode the A and B outputs are identical: IB = IA

and QB = QA).

The operation of the second FIR filter is similar to the

first FIR filter with several notable exceptions. First,

In addition to setting the ddc0_interleave bit, FIR2A

the depths of the coefficient and data memories are

must be set to EVEN mode and FIR2A must be set to

doubled to 128. This size increase allows for filters up

ODD mode. ddc0_fir2a_ncoeff and ddc0_fir2b_ncoeff

to 126 taps to be realized without running out of data

are both set to (M + 1)/4. ddc0_fir2a_shift and

memory. It also allows longer sets of filter coefficients

ddc0_fir2b_shift should be identical. There are no

to be stored in coefficient memory.

restrictions

ddc0_fir2a_base_addr

ddc0_fir2b_base_addr.

Second, because of the additional decimation by two

from the first FIR filter, twice as many MCLK cycles

The M-tap filter has (M + 1)/2 unique coefficients. The

are available to process coefficients, increasing the

first, third, fifth, etc. coefficients are loaded into the

maximum allowable value of NCOEFF, as shown in

FIR2A coefficient memory; the second, fourth, sixth,

Equation 11 and Equation 12.

etc. coefficients are loaded into the FIR2B memory.

The center coefficients of the filter end up as the last

coefficient loaded into FIR2B.

Third, in the first FIR filter the total of all the filter tap

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