AD9361 Reference Manual
UG-570
| Page 33 of 128
Rx SIGNAL PATH
The
AD9361
Rx signal path passes downconverted signals (I
and Q) to the baseband receiver section. The baseband Rx
signal path is composed of two programmable analog low-pass
filters, a 12-bit ADC, and four stages of digital decimating
filters. Each of the four decimating filters can be bypassed.
Figure 17 shows a block diagram for the
AD9361
Rx signal
path. Note that both the I and Q paths are schematically
identical to each other.
The digital Rx HB filters are sized to eliminate over-ranging.
The Rx FIR filter can over-range based on the filter coefficients.
The Rx FIR output is limited to the maximum code value when
overranging occurs (preventing data wrapping). An overrange
occurrence in the Rx FIR filter is indicated in the Overflow
register or at the CTRL_OUT pins.
Rx ANALOG FILTER BLOCKS
Analog filtering before the ADC reduces spurious signal levels
by removing mixer products and providing general low pass
filtering prior to upconversion. The corner frequency for these
filters is programmed with the ad9361_set_rx_rf_bandwidth
function call.
Rx TIA LPF
The Rx TIA LPF is a single-pole low-pass filter with a
programmable 3 dB corner frequency. The corner frequency is
programmable over the range of 1 MHz to 70 MHz. The Rx TIA
LPF is typically calibrated to 2.5 times the baseband channel
bandwidth.
Rx BB LPF
The Rx BB LPF is a third-order Butterworth low-pass filter with
a programmable 3 dB corner frequency. The corner frequency
is programmable over the range of 200 kHz to 39.2 MHz. The
Rx BB LPF is typically calibrated to 1.4 times the baseband
channel bandwidth.
Rx DIGITAL FILTER BLOCKS
The four blocks following the ADC in Figure 17 comprise the
digital filtering for the receive path. These programmable filters
provide the bandwidth limiting and out of band noise and
spurious signal reduction after digitization. They also provide
decimation needed to generate the correct data rates. In each
filter, decimation is performed after the filtering has taken
place. ADC_CLK serves as the master clock reference for all
digital filters in the Rx signal path. The sample clock of each
fixed-coefficient filter is always equal to the input data clock.
These digital filters are configured using the ad9361_calculate_
rf_clock_chain function.
Rx HB3/DEC3
Rx HB3/DEC3 provides the choice between two different fixed-
coefficient decimating filters. Rx HB3/DEC3 can decimate by a
factor of 2 or 3, or it may be bypassed. Rx HB3 has the following
coefficients: [1, 4, 6, 4, 1], and Rx DEC3 has the following
coefficients: [55, 83, 0, −393, −580, 0, 1914, 4041, 5120, 4041,
1914, 0, −580, −393, 0, 83, 55]. Note that the full-scale range for
the Rx HB3 filter is 16 (2
4
), and the full-scale range for the Rx
DEC3 filter is 16384 (2
14
). It is important to note that the if the
decimation factor for this filter is set to 3, then the interpolation
factor for the Tx HB3/INT3 filter must also be set to 3 so the
clocks are properly aligned.
Rx HB2
Rx HB2 is a fixed-coefficient half-band decimating filter. Rx
HB2 can decimate by a factor of 2, or it may be bypassed. Rx
HB2 has the following coefficients: [−9, 0, 73, 128, 73, 0, −9].
Note that the full-scale range for this filter is 256 (2
8
).
Rx HB1
Rx HB1 is a fixed-coefficient half-band decimating filter. Rx
HB1 can decimate by a factor of 2, or it may be bypassed. Rx
HB1 has the following coefficients: [−8, 0, 42, 0, −147, 0, 619,
1013, 619, 0, −147, 0, 42, 0, −8]. Note that the full-scale range
for this filter is 2048 (2
11
).
Figure 17.
AD9361
Rx Signal Path
ADC
HB1
HB2
PROG
RX FIR
BB
LPF
TIA
LPF
HB3/
DEC3
1
1668-
018
Rev. A
