XE1205 Datasheet, PDF (13/48 Pages) Semtech Corporation – 180 MHz - 1GHz Low-Power, High Link Budget Integrated UHF Transceiver

XE1205
5.2.3.5 Frequency Error Indicator in continuous mode – FEI
The block is switched ON by writing bit RXParam_FEI to ‘1’.This function provides information about the frequency error
of the local oscillator compared with the input carrier frequency and can be used to implement an external AFC. The
condition on the modulation index for proper behavior of the FEI function is:
β = ^2⋅Δf ≥ 2,
BR
Where Δf is the frequency deviation and BR is the bit rate.
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.
RXParam_FEI
NSS_CONFIG
fei_out_int
TS_FEI
XXX
val1
Read RSSI
TS_FEI
Read RSSI
val2
val3
val4
val5 val6 0
saout_fei
RXParam_FEI_out
0
val2
val5
Figure 7: Timing diagram of an FEI measurement
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_offset + SignalBW < FilterBW.
Where f_offset is the difference between the carrier frequency and the LO frequency, SignalBW is the signal bandwidth
(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.
© Semtech 2008
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