Si1000/1/2/3/4/5
D7
D6
D5
D4
D3
D2
D1
D0
POR Def.
Add R/W
Function/
Description
71 R/W Modulation Mode trclk[1] trclk[0] dtmod[1] dtmod[0] eninv fd[8] modtyp[1] modtyp[0]
Control 2
00h
20h
72 R/W Frequency Devia- fd[7]
tion
fd[6]
fd[5]
fd[4]
fd[3] fd[2]
fd[1]
fd[0]
23.3.6. Frequency Offset Adjustment
When the AFC is disabled the frequency offset can be adjusted manually by fo[9:0] in registers 73h and
74h. It is not possible to have both AFC and offset as internally they share the same register. The fre-
quency offset adjustment and the AFC both are implemented by shifting the Synthesizer Local Oscillator
frequency. This register is a signed register so in order to get a negative offset it is necessary to take the
twos complement of the positive offset number. The offset can be calculated by the following:
DesiredOffset 156.25Hz(hbsel 1) fo[9:0]
DesiredOffset
fo[9:0]
156.25Hz(hbsel 1)
The adjustment range in high band is ±160 kHz and in low band it is ±80 kHz. For example to compute an
offset of +50 kHz in high band mode fo[9:0] should be set to 0A0h. For an offset of –50 kHz in high band
mode the fo[9:0] register should be set to 360h.
D7
D6
D5
D4
D3
D2
D1
D0
POR
Def.
Add R/W
Function/
Description
73 R/W Frequency Offset
74 R/W Frequency Offset
fo[7]
fo[6]
fo[5]
fo[4]
fo[3]
fo[2]
fo[1] fo[0]
fo[9] fo[8]
00h
00h
23.3.7. Automatic Frequency Control (AFC)
All AFC settings can be easily obtained from the settings calculator. This is the recommended method to
program all AFC settings. This section is intended to describe the operation of the AFC in more detail to
help understand the trade-offs of using AFC.The receiver supports automatic frequency control (AFC) to
compensate for frequency differences between the transmitter and receiver reference frequencies. These
differences can be caused by the absolute accuracy and temperature dependencies of the reference crys-
tals. Due to frequency offset compensation in the modem, the receiver is tolerant to frequency offsets up to
0.25 times the IF bandwidth when the AFC is disabled. When the AFC is enabled, the received signal will
be centered in the pass-band of the IF filter, providing optimal sensitivity and selectivity over a wider range
of frequency offsets up to 0.35 times the IF bandwidth. The trade-off of receiver sensitivity (at 1% PER)
versus carrier offset and the impact of AFC are illustrated in Figure 23.5.
Rev. 1.0
249
SILICON [ SILICON ]
