xe1205 Semtech Corporation, xe1205 Datasheet - Page 13

• Manufacturer Part Number: xe1205
• Description: Low-power, High Link Budget Integrated Uhf Transceiver
• Manufacturer: Semtech Corporation

The time diagram of an FEI measurement is given in the next figure. When the FEI block has been woken up and is
ready, and as long as the block is kept on, the frequency error is measured and the current result of the measurement is
loaded in the register RXParam_FEI_out(15:0) each time registers 12 is read. TS_FEI is the time required for the first
evaluation to be completed after the block has been started up and its value is given in section 4.2.2. Since the contents
of the configuration register is validated at the rising edge of the enable signal NSS_CONFIG, the FEI block is actually
started up at this time.
To guarantee proper behavior of the FEI, the operation must be done when a preamble as defined in section 5.2.3.1 is
received, and the sum of the frequency offset and the signal bandwidth (single sided) must be lower than the base band
filter bandwidth (single sided). That is:
F
Where f
(single side) equal to the sum of the bit rate divided by 2 and the frequency deviation (BR/2 + DF), and FilterBW is the
channel filter bandwidth defined by RXParam_BW(1:0) parameters.
The frequency error can be calculated by the following formula:
The frequency error = 500*int(FEI_out(15:0)) in Hz
Where int(x) is the integer value of the signed binary representation of x.
5.2.3.6
XE1205 offers two possibilities to correct the RF frequency error either by using FEI block with external microcontroller
setting the corrected LO_Frequency or by using the internal Automatic Frequency error Cancellation (AFC).
When using FEI block,
without further calculation by a microcontroller since the PLL step is 500 Hz i.e.
number of step needed to compensate the frequency error .
Saout_fei is internally generated during a read sequence of register 12 in the same way as saout_rssi (refer to Figure 7).
To use AFC block, FEI block should be switched on by writing bit RXParam_FEI to
writing bit RX_Param:AFC_start to
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offset
+ SignalBW < FilterBW.
RXParam_FEI
NSS_CONFIG
fei_out_int
saout_fei
RXParam_FEI_out
offset
Frequency Error Correction
is the difference between the carrier frequency and the LO frequency, SignalBW is the signal bandwidth
RXParam
_FEI_out(15:0) can directly be subtracted to the register MCParam_Freq_lo(15:0)
TS_FEI
XXX
. The LO_frequency error cancellation is effective providing bit RXParam_disable is
Figure 7: Timing diagram of an FEI measurement
0
val1
13
val2
Read RSSI
TS_FEI
val3
val2
RXParam
val4
then AFC should be started by
_FEI_out (15:0) represents the
Read RSSI
val5
XE1205
val5
val6
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