Baseband Rx Analog Filter Calibration - Analog Devices AD9361 Reference Manual

AD9361 Reference Manual

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.
����������
��������
��������������,������
Table 5. Typical Rx Baseband Filter Calibration Times
Desired
Standard BBBW (MHz)
LTE 5 MHz 2.5
LTE 10MHz 5
LTE 15 MHz 7.5
LTE 20 MHz 10
�
× ���� (2)
������
× 2.8 × ��
BBPLL
Frequency (MHz) RxBBF Divider (decimal)
983.04 31
983.04 16
737.28 8
983.04 8
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.
������������,������
���������������������� ��������
(3)
Actual BBBW (MHz)
2.499
4.841
7.262
9.683
Rev. A | Page 9 of 128
=
���������� × ln (2)
������
2.8 × π × ��������������
= 610 ×
ln (2)
����
��������
������������,������
Maximum Calibration
Time (µs)
19.236
9.928
6.619
4.964
UG-570
(4)
× 2.8 × π
(5)