參數(shù)資料
型號(hào)： Z87200
廠商： ZILOG INC
元件分類： 通信及網(wǎng)絡(luò)
英文描述： CAP 0.047UF 100V 10% X7R AXIAL TR-14
中文描述： SPECIALTY TELECOM CIRCUIT, PQFP100
封裝： PLASTIC, QFP-100
文件頁數(shù)： 47/54頁
文件大小： 449K
代理商： Z87200
Z87200
Zilog
Spread-Spectrum Transceiver
DS96WRL0400
4-47
4
Differential Demodulation
As noted in the preceding text, computation of the “Dot”
and “Cross” products is fundamental to operation of the
DPSK Demodulator and Frequency Discriminator. Let I
k
and Q
k
represent the I and Q channel inputs, respectively,
for the k
th
symbol after downconversion and despreading.
The Dot and Cross products can then be defined as:
Dot(k) = I
k
I
k-1
+ Q
k
Q
k-1
; and,
Cross(k) = Q
k
I
k-1
- I
k
Q
k-1
In the complex domain, these products can be seen to
have been defined to form the complex conjugate product
between two input samples, one symbol apart. Let the k
th
input sample, sin(k), be defined as:
s
in
(k) = I(k) + j Q(k),
where I(k) and Q(k) are the 8-bit peak power PN Matched
Filter I and Q channel outputs directed to the DPSK De-
modulator. In polar form, s
in
(k) may be conveniently de-
fined as:
s
in
(k) = A(k)e
j(k)
with
A(k)
(k) = arctan
Simple substitution then shows that the complex conjugate
product between consecutive symbols (with an arbitrary
phase shift introduced to the previous symbol value) may
be expressed as:
s
out
(k)= s
in
(k) [s
in
(k–1) .
ω
fixed
]
*
= Dot(k) + j Cross(k)
where
ω
fixed
= arbitrary fixed phase rotation;
Dot(k)= Re[s
out
(k)]; and,
Cross(k)= Im[s
out
(k)]
*
The fixed phase rotation
ω
fixed
has been introduced to later
simplify the decision criteria. The ability to express real and
imaginary parts of the complex conjugate product between
consecutive symbols with the Dot and Cross products is
the key to their use in DPSK demodulation.
DBPSK Demodulation
In DPSK, the phase difference between successive sam-
ples is due to the data modulation phase differences,
mod, plus any induced phase rotation between sym-
bols,
rot, resulting from, for example, a frequency offset
between the received signal’s I.F. and that provided by the
Downconverter. For DBPSK, the data modulation differ-
ences
mod can take only the values of 0
°
or 180
°
. Ex-
pressing the complex phase difference [(k)-(k-1)] in
terms of these components, the decision can be seen to be
based on:
Sout
(k)
=A(k) A(k-1) e
j(k)*
e
-j(k-1)
=
A(k)
*
A(k1)
*
e
j[
mod(k)+
rot
(k)]
For DBPSK, only the real part of sout(k), Dot(k), is needed
to determine the modulated phase transition:
Dot(k)= A(k)
*
A(k-1)
*
cos(
mod
(k)+
rot(k))
=
±
A(k)
*
A(k-1)
*
cos(
rot
(k))
where the sign is determined by the transmitted data since
cos[
mod
(k)] =
±
1
*
As a result,
Dot(k)
≈
±
A
2
(k)
if the amplitude of the signal is constant for consecutive
symbols and if the phase rotation
rot
(k) between sym-
bols is small. The Z87200 DPSK Demodulator can thus
use the sign of the Dot product in order to make DBPSK
symbol decisions without the introduction of any fixed
phase rotation.
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