Tx Analog Filter Blocks - Analog Devices AD9361 Reference Manual

UG-570
Tx HB3/INT3
Tx HB3/INT3 provides the choice between two different fixed-
coefficient interpolating filters. Tx HB3/INT3 can interpolate
by a factor of 2 or 3, or it may be bypassed. Tx HB3 has the
following coefficients: [1, 2, 1], and Tx INT3 has the following
coefficients: [36, −19, 0, −156, −12, 0, 479, 223, 0, −1215, −993,
0, 3569, 6277, 8192, 6277, 3569, 0, −993, −1215, 0, 223, 479, 0,
−12, −156, 0, −19, 36]. Note that the full-scale range for the Tx
HB3 filter is 2, and the full-scale range for the Tx INT3 filter is
8192 (2 13 ). It is important to note that if the interpolation factor
for this filter is set to 3, then the decimation factor for the Rx
HB3/DEC3 filter must also be set to 3 so the clocks are properly
aligned.
Digital Tx Block Delay
The digital Tx filter blocks are designed to minimize delay
caused by mathematical operations so that the total delay is
dominated by the inherent filter delays. Each block's
contribution to the total data latency is approximated using
the following relationship:
N 1
∆ = ×
t
data
2 f
S
where:
N is the filter order (number of taps)
f is the output sampling clock frequency (after any
S
interpolation).

Tx ANALOG FILTER BLOCKS

Analog filtering after the DAC reduces spurious outputs by
removing sampling artifacts and providing general low-pass
filtering prior to upconversion. The corner frequency for these
filters is programmed with the ad9361_set_tx_rf_bandwidth
function call.
Tx BB LPF
The Tx BB LPF is a third-order Butterworth low-pass filter with
a programmable 3 dB corner frequency. The Tx BB LPF corner
frequency is programmable over the range of 625 kHz to
32 MHz. The Tx BB LPF is typically calibrated to 1.6 times the
baseband channel bandwidth.
Tx Secondary LPF
The Tx secondary LPF is a single-pole low-pass filter with a
programmable 3 dB corner frequency. The Tx secondary LPF
corner frequency is programmable over the range of 2.7 MHz
to 100 MHz. The Tx Secondary LPF is typically calibrated to
5 times the baseband channel bandwidth.
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Rev. A | Page 32 of 128
AD9361 Reference Manual