Motorola DSP56156 Manual page 163

6.6.3.1 A/D Decimation DSP Filter
In addition to antialiasing, the decimation filter also needs to compensate the A/D comb
filter response. For a decimation ratio D=105 and a master clock F=1.68 MHz, the fre-
quency response of the A/D comb filter is:
The frequency response of the decimation filter will have to be shaped by 1/F(f) in order to
flatten the response of the A/D section. An example of such a filter is given in Figure 6-29.
Transmit Decimation and Compensation Filter
5
0
L
-5
o
-10
g
-15
M
-20
a
-25
g
-30
n
-35
i
t
-40
u
-45
d
-50
e
-55
( -60
d
-65
B
-70
)
-75
-80
0
Figure 6-29 Example of GSM Transmit Antialiasing-decimation IIR Filter
The IIR filter shown in Figure 6-29 is implemented using 4 biquadratic sections. The gain
and coefficients of the four section are listed in Table 6-18.
6 - 48
APPLICATION EXAMPLES
2
F f ( )
--------------- -
=
( )
3
105
1 1.5 2 2.5 3
DSP56156 ON-CHIP SIGMA/DELTA CODEC
×
3
 
2π 105
 
×
sin f
---------------------
 
 × 
2 1680
 
------------------------------------------- -
 
 
2π
×
sin -------------------- - f
 
 
×
2 1680
3.5 4 4.5 5 5.5
Frequency (kHz)
compad105.IIR.Filter
6 6.5 7 8
MOTOROLA