This particular architecture performs
the spectral down conversion in the analog domain; if the application pushes this operation closer
toward the antenna, then an analog correction may not be needed. In any event this frequency control
loop is continually running since the offset can vary over temperature and time as discussed above.
6.6.2 ARCTAN Based
In this section, we will provide a frequency offset estimation technique using differential detection
and /4-DQPSK modulation scheme. Recall the differential detector??™s output constellation consists
of four states since there are only four possible transitions allowed. Also the presence of any constant
frequency offsets prior to the differential detector will show up as constant phase offsets at the output
of the detector. The amount of phase offset is directly proportional to the amount of frequency offset
present on the received signal. Moreover, the direction of the phase offset depends on if the offset is
a positive value or a negative value. Hence the differential detector output can be used as an indication
of the amount of frequency offset present on the received signal (see Fig. 6.42).
The AFC loop will generally consist of three components: the phase detector, Voltage Control
Oscillator (VCO), and loop filter.
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