Datasheet

LM97593
www.ti.com SNWS019B –JULY 2007–REVISED APRIL 2013
Second Programmable FIR Filter (F2)
The second stage decimate by two or four filter also uses externally downloaded filter coefficients. F2 determines
the final channel filter response. The filter coefficients are 16-bit 2’s complement numbers. Unity gain will be
achieved through the filter if the sum of the 63 coefficients is equal to 2
16
. If the sum is not 2
16
, then the F2 will
introduce a gain equal to (sum of coefficients)/2
16
.
The 63 coefficients are identified as h
2
(n), n = 0, ..., 62 where h
2
(31) is the center tap. The coefficients are
symmetric, so only the first 32 are loaded into the chip. An example filter (STD F2 coefficients, see Figure 68)
with 80dB out-of-band rejection, gain of 1.00, and 0.03 dB peak to peak passband ripple is created by this set of
32 unique coefficients:
-14, -20, 19, 73, 43, -70, -82, 84, 171, -49, -269,
-34, 374, 192, -449,
-430, 460,751, -357, -1144, 81, 1581, 443, -2026,
-1337, 2437, 2886,
-2770, -6127, 2987, 20544, 29647
A second set of F2 coefficients (GSM set, see Figure 69) suitable for meeting the stringent wideband GSM
requirements with a gain of 0.999 are:
-536, -986, 42, 962, 869, 225, 141, 93, -280,
-708, -774, -579, -384,
-79, 536, 1056, 1152, 1067, 789, 32, -935, -1668,
-2104, -2137, -1444,
71, 2130, 4450, 6884, 9053, 10413, 10832
The filter coefficients of both filters can be used to tailor the spectral response to the user’s needs. For example,
the first can be loaded with the standard set to provide a flat response through to the second filter. The latter can
then be programmed as a Nyquist (typically a root-raised-cosine) filter for matched filtering of digital data.
Figure 68. F2 STD frequency response
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