M
ODEL
SL900A V
ERSION
V_1.0
P
REPARED BY
H/W
D
ATE
2006.06.30
S
UBJECT
T
ECHNICAL
M
ANUAL
P
AGE
32/51
SL900A
T
ECHNICAL
M
ANUAL
Figure 29. Timing Mask for normal VAPC.
8.4
Automatic Frequency Control Unit
Automatic Frequency Control unit provides the direct control of the oscillator for frequency offset and Doppler
shift compensation. The Block diagram is depicted in Figure 30. It utilizes a 13-bit D/A converter to achieve
high-resolution control. The AFC is always inputted to VCTCXO to generate 26Mhz. The Aanlog voltage is
about 1.5V and AFC_DAC is about 4200 decimally.
Figure 30. Block diagram of the AFC Controller
9.
Baseband Front End
Baseband Front End is a modem interface between Tx/Rx mixed-signal modules and digital signal processor.
We can divide this block into two parts. The first is the uplink(transmitting) path, which converts bit-stream
from DSP into digital in-phase and quadrature signals for TX mixed-signal module. The second part is the
downlink(receiving) path, which receives digital in-phase and quadrature signals from RX mixed-signal module,
performs FIR filtering and then sends results to DSP. The uplink path is mainly composed of GMSK Modulator
and uplink parts of Baseband Serial Ports, and the downlink path is mainly composed of RX digital FIR filter
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