AD9361 Reference Manual UG-570
| Page 9 of 128
BASEBAND Rx ANALOG FILTER CALIBRATION
The baseband Rx analog filter calibration tunes the cutoff
frequency of the third-order Butterworth Rx anti-aliasing filter.
The Rx filter is located just before the ADC in the Rx signal
path and is normally calibrated to 1.4× the baseband channel
bandwidth (BBBW). This calibration is important for Rx
interferer rejection. Note that the BBBW is half the complex
bandwidth and coerced between 28 MHz to 0.20 MHz for the
equations used in this filter tuning. To calibrate this filter, the
BBPLL is divided down using a divide by 1 to 511 divider
dedicated to the Rx tuner block.
=
× ()
,
× . ×
(3)
The Rx baseband analog filter calibration runs during the
ad9361_set_rx_rf_bandwidth function. Calibration completion
can be monitored on a control output pin or by reading the
calibration control register until the Rx baseband filter
calibration bit self clears.
Because the filter calculation uses a ceiling function to generate
the divider there will be some quantization of the corner
frequency. If the quantization is too large it may be necessary to
adjust the desired BBBW to compensate.
,
=
× ()
. × ×
(4)
= 610 ×
()
,
× . ×
(5)
Ta
ble 5. Typical Rx Baseband Filter Calibration Times
Standard
Desired
BBBW (MHz)
BBPLL
Frequency (MHz) RxBBF Divider (decimal) Actual BBBW (MHz)
Maximum Calibration
Time (µs)
LTE 5 MHz 2.5 983.04 31 2.499 19.236
LTE 10MHz 5 983.04 16 4.841 9.928
LTE 15 MHz 7.5 737.28 8 7.262 6.619
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