A.2.10 nyquist filters – Comtech EF Data SDM-9000 User Manual
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SDM-9000 Satellite Modem
A–28
Rev. 4
A.2.10 Nyquist Filters
Prior to modulation, the I and Q baseband signals are passed through Nyquist filters
exhibiting a square-root, raised-cosine transfer function with a 35% rolloff factor. The
theoretical transfer function is defined by the following expression:
H f
f
f
N
( )
(
)
=
−
1
1
for
| |<
α
(
)
(
)
H f
f
f
f
f
f
f
N
N
N
N
N
( )
sin
| |
|
|
=
+
−
−
≤
≤
+
1
2
1
2
2
1
1
π
α
α
α
for
H f
f
f
N
( )
(
)
=
+
0
1
for
| |>
α
where:
α
=
.35
f
T
R
N
S
S
=
=
1
2
2
is the Nyquist frequency
The reception process uses Nyquist filters exhibiting the same transfer function as the
transmission process (matched filters). The receiving Nyquist filters are incorporated
directly after the QPSK demodulator. The I and Q outputs of these filters are digitized
and mapped to 3-bit, soft-decision symbols for subsequent de-puncturing (if required)
and Viterbi decoding.