6.40.
330 CHAPTER SIX
Demodulator
X
Z??“T
Decision
Device
+
??“
+
r(k)
Remaining
Receiver
CQE
FIGURE 6.39 Eye opening CQE block diagram.
r(t) = A(t) . cos[?‰ct + ??(t)]
e??“j [?‰ct + 2????† f t ]
RI(t) + jRQ(t) LPF +
FIGURE 6.40 Quadrature demodulation operation.
The receiver will estimate this residual offset in frequency (f ) and correct it by performing frequency
compensation or what is commonly called Automatic Frequency Control (AFC) [34??“37]. This
frequency offset should be diminished since its presence will degrade overall system performance. With
the help of the above block diagram we can represent the offset mathematically in the following manner
(6.111) cRI(t)
RQ(t) d ccos[u(t)] sin[u(t)]
sin[u(t)] cos[u(t)]d# ccos[2pft]
sin[2pft] d
6.6.1 Typical Receiver Architecture
In order to facilitate a technical discussion around this topic we need to introduce a receiver architecture
to be used as a baseline. In Fig. 6.41 we show a simplified block diagram that emphasizes the
frequency compensation functionality.
This diagram shows the receiver performing the spectral down conversion operation through the
use of the quadrature demodulator. The baseband section will estimate the frequency offset and then
send a correction signal to the analog front end of the receiver.
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