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CLC5903
mum head room th roug h the CI C filter. For optimal no ise per-
form ance the SCALE value is set to increase this level until
the CIC filter is just below the point of distort ion. A value is
normally calculated and loaded for SCALE such that
. The actual gain of the CIC
filter wi ll only be uni ty for power-of-two decim ation value s. In
other cases the gain will be somewhat less than unity.
Channel Gain
The gain of each cha nnel can be boosted up to 42 dB by
shifting th e output of the CIC filte r up by 0 to 7 bits pr ior to
rounding i t to 2 1 bi t s. For channel A , the gain of this stage is:
, where GAIN_A ranges from 0 to 7. Over-
flow due to the GAIN circuit is saturated (clipped) at plus or
minus full scale. Each channel can be given its own GAIN
setting.
First Programmable FIR Filter (F1)
The CIC/GAIN outputs are followed by two stages of filtering.
The first stag e is a 21 tap decim ate- by-2 symme tri c FIR filter
with programmable coefficients. Typically, this filter compen-
sates for a sligh t d ro op in duced by the CIC filter while remo v -
ing undesired alias images above Nyquist. In addition, it
often provides stopband assistance to F2 when deep stop
bands ar e re quir ed. T he filte r coe fficien ts a re 16 -bit 2 ’s com-
plement numbe rs. Unity gain will be achieved through th e fil-
ter if the sum of the 21 coeffi cients is equal to 216. If the sum
is not 216, then F1 will introduce a gain equal to (sum of coef-
ficients)/216. The 21 coeffici ents are i dent ified as coe fficien ts
where is the cent er tap. The
coefficients are symmetric, so only the first 11 are loaded
into the c hip.
Two exampl e sets of coe fficient s are provided here. The first
set of coefficients, referred to as the standard set (STD),
compen sa tes for the droop of th e CIC filter providin g a pass-
band which is flat (0.01 dB ripple) over 95% of the final out-
put band w idt h w i th 70dB of out- of -b and rejection ( see Figure
21). The filter has a gain of 0.999 and is symmetric with the
followin g 11 u nique taps (1| 21, 2|20, ..., 10|1 2, 11):
29, -85, -308, -56, 1068, 1405, -2056, -6009,
1303, 21121, 32703
The second set of coefficients (GSM set) are intended for
applications that need deeper stop bands or need oversam-
pled outputs. These requirements are common in cellular
systems where out of band rejection requirements can
exceed 100dB (se e Fi gure 22). They are usef ul for wideband
radio architectures where the channelization is done after the
ADC. These filter coefficients introduce a gain of 0.984 and
are: -49, -340, -1008, -1617, -1269, 425, 3027, 6030,
9115, 11620, 12606
Second Programmable FIR Filter (F2)
The second stage decimate by two or four filter also uses
externally downloaded filter co efficients. F2 deter mines the
final channel filter response. The filter coefficients are 16-bit
2’s complement numbers. Unity gain will be achiev ed through
the filter if the sum of the 63 coefficients is equal to 216. If the
sum is not 216, then the F2 will introduce a gain equal to
(sum of coefficient s) /2 16.
The 63 coefficients are identified as
where is the center tap. The coefficients are sym-
metric, so only the firs t 32 are lo ade d i nt o th e chip.
An exam ple filter (STD F2 coefficients, see Figure 23) 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 24)
suitable for meeting the stri ngent wideband GSM require-
ments w ith 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
GAINSHIFTUP GAINCIC
⋅1≤
GAIN 2GAIN_A
=
h1n() n,0…20,,=h110()
Figure 21. F1 STD fr equency respons e
0 0.1 0.2 0.3 0.4 0.5
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
0
Frequency Response of F1 Using STD Set
Magnitude (dB)
Frequency Normalized To Filter Input Sample Rate
Figure 22. F1 G SM fr equency response
0 0.1 0.2 0.3 0.4 0.5
−120
−100
−80
−60
−40
−20
0
Frequency Response of F1 Using GSM Set
Magnitude (dB)
Frequency Normalized To Filter Input Sample Rate
h2n() n0…62,,=,
h231()
Detailed Description (Continued)