Filter Guide; Overview; Tx Signal Path; Tx Digital Filter Blocks - Analog Devices AD9361 Reference Manual

AD9361 Reference Manual

FILTER GUIDE

OVERVIEW

This section contains a description of the analog and digital
filtering available in both the Tx and Rx signal paths of the
AD9361.

Tx SIGNAL PATH

The AD9361 Tx signal path receives 12-bit twos complement
data in I-Q format from the
channel (I and Q) passes this data through four digital
interpolating filters to a 12-bit DAC. Each of the four
interpolating filters can be bypassed. The DAC's analog output
is passed through two low-pass filters prior to the RF mixer.
The corner frequency for each low-pass filter is programmable
with the ad9361_set_tx_rf_bandwidth function. Figure 16
shows a block diagram for the
that both the I and the Q paths are schematically identical to
each other.
Overranging is detected in the Tx digital signal path at each
stage and limited to the maximum code value (preventing data
wrapping). An overrange occurrence can be monitored in the
Overflow registers or at the control output pins.

Tx DIGITAL FILTER BLOCKS

The four blocks leading up to the DAC in Figure 16 comprise
the digital filtering for the transmit path. These programmable
filters provide the bandwidth limiting required prior to
conversion from digital to analog. They also provide
interpolation to translate from the input data rate to the rate
needed for proper digital to analog conversion. In each filter,
interpolation is performed first, followed by the filter transfer
function. The digital filters are configured using the
ad9361_calculate_rf_clock_chain functions. ADC_CLK (the
ADC sample clock) is the fastest signal path clock available in
the device's digital processing section, so it is used as the master
clock reference for the digital filters in the Tx signal path. The
sample clock of each fixed-coefficient filter is always equal to
the output data clock. The following sections describe the
details of these blocks.
PROG
TX FIR
AD9361 digital interface, and each
AD9361 Tx signal path. Note
HB1 HB2
Figure 16.
Tx FIR
The first digital filter in the Tx signal path is a programmable
polyphase FIR filter. The Tx FIR filter can also interpolate by a
factor of 1, 2, or 4, or it can be bypassed if not needed. This
filter is controlled in the ad9361_set_tx_fir_config. The filter
taps are stored in 16-bit twos complement format, and the
number of taps is configurable between a minimum of 16 taps
and a maximum of 128 taps in groups of 16. The Tx FIR also
has a programmable gain setting of 0 dB or −6 dB. Each
coefficient is stored in two registers as a 16-bit number.
The Tx FIR uses DAC_CLK (Tx DAC sample clock) as its
sample clock. DAC_CLK is either set equal to ADC_CLK or is
set to ADC_CLK/2. The Tx FIR calculates 16 taps per clock
cycle. This limits the number of available taps to the ratio of
DAC_CLK to the input data rate multiplied by 16. For example,
if the input data rate is 25 MHz and DAC_CLK is 100 MHz,
then the ratio of DAC_CLK to the input data rate is 100/25 or 4.
In this scenario, the total number of taps available is 64.
Another limitation is the memory inside the filter. The total
number of operations that can be performed is limited to 64
per clock cycle. This means that the number of taps available is
limited to 64 if the interpolation factor is set to 1. If an
interpolation rate greater than 1 is used, then the memory space
can be utilized to include more taps. Table 15 lists the allowable
number of taps for each interpolation rate.
Table 15. Filter Taps Based on Interpolation Rate
Interpolation
1
2
4
Tx HB1
Tx HB1 is a fixed-coefficient half-band interpolating filter. Tx
HB1 can interpolate by a factor of 2, or it may be bypassed. Tx
HB1 has the following coefficients: [−53, 0, 313, 0, −1155, 0,
4989, 8192, 4989, 0, −1155, 0, 313, 0, −53]. Note that the full-
scale range for this filter is 8192 (2
Tx HB2
Tx HB2 is a fixed-coefficient half-band interpolating filter. Tx
HB2 can interpolate by a factor of 2, or it may be bypassed. Tx
HB2 has the following coefficients: [−9, 0, 73, 128, 73, 0, −9].
Note that the full-scale range for this filter is 128 (2
HB3/
DAC
INT3
AD9361 Tx Signal Path
Rev. A | Page 31 of 128
Maximum Number of Taps
64
128
128
13 ).
ND
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LPF LPF
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