Data Sheet

LJ1269HC
Rev1.0 Page 32 Web: www.ljelect.com
3.4.14. Automatic Frequency Correction
The AFC is based on the FEI block, and therefore the same input signal and receiver setting conditions apply. W hen the AFC
procedure is done, AfcValue is directly subtracted to the register that defines the frequency of operation of the module, FRF. The
AFC can be launched:
 Each time the receiver is enabled, if AfcAutoOn = 1
 Upon user request, by setting bit AfcStart in RegAfcFei, if AfcAutoOn = 0
When the AFC is automatically triggered (AfcAutoOn = 1), the user has the option to:
 Clear the former AFC correction value, if AfcAutoClearOn = 1
 Start the AFC evaluation from the previously corrected frequency. This may be useful in systems in which the LO keeps on
drifting in the “same direction”. Ageing compensation is a good example.
The LJ1269HC offers an alternate receiver bandwidth setting during the AFC phase, to accommodate large LO drifts. If the user
considers that the received signal may be out of the receiver bandwidth, a higher channel filter bandwidth can be programmed
in RegAfcBw, at the expense of the receiver noise floor, which will impact upon sensitivity.
3.4.15. Optimized Setup for Low Modulation Index Systems
 For wide band systems, where AFC is usually not required (XTAL inaccuracies do not typically impact the sensitivity), it is
recommended to offset the LO frequency of the receiver to avoid desensitization. This can be simply done by modifying Frf
in RegFrfLsb. A good rule of thumb is to offset the receiver‟s LO by 10% of the expected transmitter frequency deviation.
 For narrow band systems, it is recommended to perform AFC. The LJ1269HC has a dedicated AFC, enabled when
AfcLowBetaOn in RegAfcCtrl is set to 1. A frequency offset, programmable through LowBetaAfcOffset in RegTestAfc, is
added and is calculated as follows:
Offset = LowBetaAfcOffset x 488 Hz
The user should ensure that the programmed offset exceeds the DC canceller‟s cutoff frequency, set through DccFreqAfc in
RegAfcBw.
Figure 14. Optimized AFC (AfcLowBetaOn=1)
Sub GHz FSK/OOK Transceiver Module DATASHEET