TwinRX: Remove frontend filter

Removing the FIR filter in the frontend to reclaim resources and remove
redundancy when using a DDC block.  The default image has a DDC block,
so only users making custom RFNoC images and using TwinRX will need to
take care to properly downconvert the full bandwidth coming from the
radio block.

Signed-off-by: michael-west <michael.west@ettus.com>

2 files changed, 14 insertions, 0 deletions

diff --git a/host/docs/res/twinrx_alias.png b/host/docs/res/twinrx_alias.png
new file mode 100644
index 000000000..1be1ca985
--- /dev/null
+++ b/host/docs/res/twinrx_alias.png
diff --git a/host/docs/twinrx.dox b/host/docs/twinrx.dox
index d8dd20f6d..e0d327499 100644
--- a/host/docs/twinrx.dox
+++ b/host/docs/twinrx.dox
@@ -45,6 +45,20 @@ custom RFNoC FPGA images, but the DDC can be replaced by a custom block
 as long as it consumes the 400 Msps and produces an aggregate of less than
 200 Msps.
 
+Starting with UHD release 4.1, the X310/X300 FPGA images no longer contain a
+halfband anti-aliasing filter directly after the ADC when using TwinRX. The
+filter was left out because the radio no longer decimates the sampling rate for
+TwinRX, instead relying on the DDC RFNoC block to adjust the sampling rate, and
+the DSP utilization of the filter was permanent, even when no TwinRX daughtercard
+was used. This means the output of the radio block (which produces a 200 Msps
+output signal when using TwinRX) contains aliases of the signal. The following
+image shows the output of the X310+TwinRX when configuring it to run at 200 Msps.
+The center frequency is set to 2.1 GHz, and a signal is applied at approx. 2.13 GHz.
+
+\image html twinrx_alias.png "TwinRX Signal Aliases"
+
+Due to aliasing, a mirrored alias signal will also appear at 2.17 GHz. The
+statically connected DDC block will remove these aliases.
 
 \image html TwinRX_Block_Diagram.png "TwinRX Block Diagram"