0
5
10
Time Index (1/f
)
DATA
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
-
0.2
Amplitude(dB)
15 20 25 30 35 40 45 50 55
60
65
Linear Phase Filter
Minimum Phase Filter
0
0.1
0.2
Normalized Input Frequency (f /f
)
IN DATA
20
0
-
-
-
-
-
-
-
-
20
40
60
80
100
120
140
160
Magnitude(dB)
1.0
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
0.05
Normalized Input Frequency (f /f
)
IN DATA
2.0
1.5
1.0
0.5
0
0.5
1.0
1.5
2.0
-
-
-
-
Magnitude(mdB)
0.10
0.15
0.20
0.40
0.25
0.30
0.35
SBAS418I – SEPTEMBER 2007 – REVISED MARCH 2015
As shown in
, the FIR frequency response
back (or alias) into the passband and cause errors. A
provides a flat passband to 0.375 of the data rate
low-pass signal filter reduces the effect of aliasing.
(±0.003dB passband ripple).
shows the
Often, the RC low-pass filter provided by the PGA
transition from passband to stop band.
output resistors and the external capacitor connected
to
CAPP
and
CAPN
provides
sufficient
signal
attenuation.
9.15 Group Delay and Step Response
The FIR block is implemented as a multi-stage FIR
structure with selectable linear or minimum phase
response. The passband, transition band, and stop
band responses of the filters are nearly identical but
differ in the respective phase responses.
9.15.1 Linear Phase Response
Linear phase filters exhibit constant delay time versus
input frequency (that is, constant group delay). Linear
phase filters have the property that the time delay
from any instant of the input signal to the same
instant of the output data is constant and is
Figure 41. FIR Passband Magnitude Response
independent of the signal nature. This filter behavior
(f
DATA
= 500Hz)
results in essentially zero phase error when analyzing
multi-tone signals. However, the group delay and
settling time of the linear phase filter are somewhat
larger than the minimum phase filter, as shown in
Figure 42. FIR Transition Band Magnitude
Response
Although not shown in
, the passband
Figure 43. FIR Step Response
response repeats at multiples of the modulator
frequency (Nf
MOD
– f
0
and Nf
MOD
+ f
0
, where N = 1, 2,
etc. and f
0
= passband). These image frequencies, if
present in the signal and not externally filtered, fold
20
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