si4330 Silicon Laboratories, si4330 Datasheet - Page 27

• Manufacturer Part Number: si4330
• Description: Si4330 Ism Receiver
• Manufacturer: Silicon Laboratories

3.6.5. Frequency Offset Adjustment
When the AFC is disabled the frequency offset can be adjusted manually by fo[9:0] in registers 73h and 74h. The
frequency 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 you will need to take the twos
complement of the positive offset number. The offset can be calculated by the following:
The adjustment range in high band is: ±160 kHz, and adjustment range in low band 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.
When AFC is enabled the same registers can be used to read the offset value as automatically obtained by the
AFC. A stable offset value can read after preamble detection using the preamble detection or sync word detection
interrupt.
3.6.6. Auto Frequency Control (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 crystals. 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 9.
Add R/W Function/Descrip
73
74
R/W
R/W
Frequency Offset
Frequency Offset
tion
Figure 9. Sensitivity at 1% PER vs. Carrier Frequency Offset
-100
-102
-104
-86
-88
-90
-92
-94
-96
-98
-100
Reserved Reserved Reserved Reserved Reserved Reserved fo[9] fo[8]
DesiredOff
fo[7]
D7
Sensitivity vs Carrier Frequency Offset
fo
(Rb = 100 kHz, Fd = 50 kHz)
-50
[
: 9
fo[6]
set
Frequency Offset (kHz)
D6
] 0

Preliminary Rev 0.2

156
156
0
fo[5]
.
D5
25
.
DesiredOff
25
Hz
Hz


(
fo[4]
50
hbsel
D4
(
hbsel
set


) 1
fo[3]
) 1
100
D3

fo
[
: 9
] 0
AFC Disable
AFC Enable
fo[2]
D2
fo[1] fo[0]
D1
D0
Si4330
POR
Def.
00h
00h
Notes
73
27