From ad533eb6416c76d2a986c97fff846785ee56567e Mon Sep 17 00:00:00 2001 From: michael-west Date: Mon, 20 Jul 2020 10:15:12 -0700 Subject: 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 --- host/docs/res/twinrx_alias.png | Bin 0 -> 53359 bytes host/docs/twinrx.dox | 14 ++++++++++++++ 2 files changed, 14 insertions(+) create mode 100644 host/docs/res/twinrx_alias.png (limited to 'host/docs') diff --git a/host/docs/res/twinrx_alias.png b/host/docs/res/twinrx_alias.png new file mode 100644 index 000000000..1be1ca985 Binary files /dev/null and b/host/docs/res/twinrx_alias.png differ 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" -- cgit v1.2.3