Si1000/1/2/3/4/5
Figure 23.5. Sensitivity at 1% PER vs. Carrier Frequency Offset
When AFC is enabled, the preamble length needs to be long enough to settle the AFC. In general, one
byte of preamble is sufficient to settle the AFC. Disabling the AFC allows the preamble to be shortened
from 40 bits to 32 bits. Note that with the AFC disabled, the preamble length must still be long enough to
settle the receiver and to detect the preamble (see “Preamble Length” on page 266). The AFC corrects the
detected frequency offset by changing the frequency of the Fractional-N PLL. When the preamble is
detected, the AFC will freeze for the remainder of the packet. In multi-packet mode the AFC is reset at the
end of every packet and will re-acquire the frequency offset for the next packet. The AFC loop includes a
bandwidth limiting mechanism improving the rejection of out of band signals. When the AFC loop is
enabled, its pull-in-range is determined by the bandwidth limiter value (AFCLimiter) which is located in reg-
ister 2Ah.
AFC_pull_in_range = ±AFCLimiter[7:0] x (hbsel+1) x 625 Hz
The AFC Limiter register is an unsigned register and its value can be obtained from the EZRadioPRO Reg-
ister Calculator spreadsheet.
The amount of error correction feedback to the Fractional-N PLL before the preamble is detected is con-
trolled from afcgearh[2:0]. The default value 000 relates to a feedback of 100% from the measured fre-
quency error and is advised for most applications. Every bit added will half the feedback but will require a
longer preamble to settle.
The AFC operates as follows. The frequency error of the incoming signal is measured over a period of two
bit times, after which it corrects the local oscillator via the Fractional-N PLL. After this correction, some time
is allowed to settle the Fractional-N PLL to the new frequency before the next frequency error is measured.
The duration of the AFC cycle before the preamble is detected can be programmed with shwait[2:0]. It is
advised to use the default value 001, which sets the AFC cycle to 4 bit times (2 for measurement and 2 for
settling). If shwait[2:0] is programmed to 3'b000, there is no AFC correction output. It is advised to use the
default value 001, which sets the AFC cycle to 4 bit times (2 for measurement and 2 for settling).
The AFC correction value may be read from register 2Bh. The value read can be converted to kHz with the
following formula:
AFC Correction = 156.25Hz x (hbsel +1) x afc_corr[7: 0]
250
Rev. 1.0
SILICON [ SILICON ]
