D/A Second Order Digital Comb Filter - Motorola DSP56156 Manual

ON-CHIP CODEC FREQUENCY RESPONSE AND GAIN ANALYSIS
6.5.2.1

D/A Second Order Digital Comb Filter

This second order comb filter interpolates the 16-bit signal coming from the DSP core. It
consists of a digital differentiator which runs at the slow clock rate followed by an integra-
tor at the fast clock rate.
Eqn. 6-2 gives the transfer function and frequency response function:
Figure 6-8 shows an example of the D/A digital comb filter log magnitude response using
a 2.048 MHz master clock and a decimation ratio of D=128. The figure shows the comb
filter frequency response in the bands 0-1024 KHz and 0-16 KHz.
MOTOROLA
Eqn. 6-2 D/A Comb Filter Transfer Function
------------------ - 1 z
H z ( )
=
128 D
1
F f ( )
------------------ -
=
×
128 D
D: interpolation ratio
F: comb filter integrator clock
DSP56156 ON-CHIP SIGMA/DELTA CODEC
2
1
D
–
–
-----------------
×
1 –
1 z
–
2
 
 2πD 
×
sin f
---------- -
 
 
2F
 
-------------------------------- -
 
 
2π
×
sin ------ - f
 
 
2F
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