Updated descriptions.

libraries/base/common/src/vhdl/common_create_strobes_from_valid.vhd

@@ -42,6 +42,8 @@
 --    out_eop  __________| |_______| |_______| |_______| |_______| |___
 --
 -- Remark:
+-- . Use VHDL coding template from:
+--   https://support.astron.nl/confluence/display/SBe/VHDL+design+patterns+for+RTL+coding
 -- . The out_sop and out_eop are created as well, for reference.
 -- . The out_sync1 for LOFAR1 style is only avaiable if g_pipeline = TRUE,
 --   because the pipeline is needed to let the out_sync1 preceed the

libraries/dsp/fft/src/vhdl/fft_r2_pipe.vhd

@@ -39,15 +39,22 @@
 --                            an alternating way: A(0), B(0), A(1), B(1).... etc
 --
 --
--- Remarks: When g_fft.nof_chan is used the spectrums at the output will be interleaved
---          per spectrum and NOT per sample. So in case g_fft.nof_chan = 1 there will be
---          two multiplexed channels at the input (c0t0 means channel 0, timestamp 0) :
+-- Remarks:
+-- . When g_fft.use_separate = TRUE, then the two real inputs are pseudo randomly
+--   multiplied by +1 or -1 every block of input samples in fft_switch. At the
+--   FFT output this is undone by fft_unswitch. In this way any crosstalk due
+--   to quantization noise between the two real inputs gets scrambled and thus
+--   averages to zero when integrated over multiple blocks.
+--
+-- . When g_fft.nof_chan is used the spectrums at the output will be interleaved
+--   per spectrum and NOT per sample. So in case g_fft.nof_chan = 1 there will be
+--   two multiplexed channels at the input (c0t0 means channel 0, timestamp 0) :
 --         
---          c0t0 c1t0s c0t1 c1t1 c0t2 c1t2 ... c0t15 c1t15 
+--     c0t0 c1t0s c0t1 c1t1 c0t2 c1t2 ... c0t15 c1t15
 --
---          At the output will find: 
+--   At the output will find:
 --
---          c0f0 c0f1 c0f2 ... c0f15 c1f0 c1f1 c1f2 ... c1f15  (c0f0 means channel 0, frequency bin 0)
+--   c0f0 c0f1 c0f2 ... c0f15 c1f0 c1f1 c1f2 ... c1f15  (c0f0 means channel 0, frequency bin 0)
 --
 --           
 

libraries/dsp/fft/src/vhdl/fft_switch.vhd

@@ -22,6 +22,16 @@
 -- Purpose: Scramble quantization noise crosstalk between two real inputs
 -- Description:
 -- . Ported from LOFAR1, see readme_lofar1.txt
+-- . The fft_switch multiplies the samples from two real inputs A and B in a
+--   block by +1 or -1. The fft_unswitch undoes this by multiplying the FFT
+--   output again by +1 and -1. The fft_unswitch takes account of that the FFT
+--   has time mutliplexed the two spectra of the two inputs.
+-- . The input switching is pseudo random base on a LFSR (linear feedback
+--   shift register) sequence. The fft_switch and fft_unswitch start at the
+--   first in_val = '1' and then continue 'forever' until a next power cycle
+--   by rst ='1'.
+-- Remark:
+-- . Copy from applications/lofar1/RSP/pft2/src/vhdl/pft_switch.vhd
 -- . Removed in_sync, because the in_val are guaranteed to arrive in blocks of
 --   c_nof_clk_per_block samples, forever after rst release.
 --   The purpose of  the in_sync is to recover from an fractional input block,
@@ -29,7 +39,9 @@
 --   processing, without need for in_sync to recover from fractional blocks.
 --   The application that uses the FFT must guarantee to only pass on complete
 --   blocks of c_nof_clk_per_block samples to the FFT.
--- Remark: Copy from applications/lofar1/RSP/pft2/src/vhdl/pft_switch.vhd
+-- . The two real inputs each use another LFSR sequence, like for LOFAR1.
+--   For the crosstalk mitigation purpose scrambling only one input would be
+--   enough, but scrambling both inputs is fine too.
 
 LIBRARY IEEE, common_lib;
 USE IEEE.std_logic_1164.ALL;

libraries/dsp/fft/src/vhdl/fft_unswitch.vhd

@@ -22,9 +22,11 @@
 -- Purpose: Scramble quantization noise crosstalk between two real inputs
 -- Description:
 -- . Ported from LOFAR1, see readme_lofar1.txt
+-- . See fft_switch.vhd
+-- Remark:
+-- . Copy from applications/lofar1/RSP/pft2/src/vhdl/pft_unswitch.vhd
 -- . Removed in_sync, because the in_val are guaranteed to arrive in blocks of
 --   c_nof_clk_per_block samples, forever after rst release.
--- Remark: Copy from applications/lofar1/RSP/pft2/src/vhdl/pft_unswitch.vhd
 
 LIBRARY IEEE, common_lib;
 USE IEEE.std_logic_1164.ALL;