From 4e2da5912b4761caf269e72d3d29771867a3e6f2 Mon Sep 17 00:00:00 2001
From: Martin Braun <martin.braun@ettus.com>
Date: Wed, 16 Nov 2022 11:02:10 +0100
Subject: [PATCH] fpga: Fix overflows in quarterrate downconverter

The 1/4-rate downconverter naively negated the input signals (y = -x)
which doesn't work when the signal is at full scale. In particular,
signals that are at full negative scale get converted to zeros (even
though 2's complement would be to return the same number, although that
would also be incorrect).

This fix changes it such that the smallest negative value gets converted
to the largest positive value.

This is not a mathematical correction inversion, but a minor amplitude
distortion. However, since we're already at full scale, and thus
probably clipping, this is preferable to the incorrect resolution of the
regular inversion. Put differently, there is no better value in this
case.


Original-commit: 8bd982745e6247cf3cf3da52925c65126287428a
---
 .../quarter_rate_downconverter.v              | 23 ++++++++++++++-----
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/lib/rfnoc/blocks/rfnoc_block_radio/quarter_rate_downconverter.v b/lib/rfnoc/blocks/rfnoc_block_radio/quarter_rate_downconverter.v
index ded9a8c..4a17570 100644
--- a/lib/rfnoc/blocks/rfnoc_block_radio/quarter_rate_downconverter.v
+++ b/lib/rfnoc/blocks/rfnoc_block_radio/quarter_rate_downconverter.v
@@ -43,6 +43,17 @@ module quarter_rate_downconverter #(
   reg [WIDTH-1:0] tmp_i = {WIDTH{1'b0}};
   reg [WIDTH-1:0] tmp_q = {WIDTH{1'b0}};
 
+  localparam [WIDTH-1:0] MAX_NEG_VAL = -2**(WIDTH-1);
+  localparam [WIDTH-1:0] MAX_POS_VAL = 2**(WIDTH-1)-1;
+
+  function [WIDTH-1:0] invert_sig(
+    input [WIDTH-1:0] x
+  );
+    begin
+      invert_sig = x == MAX_NEG_VAL ? MAX_POS_VAL : -x;
+    end
+  endfunction
+
   // State machine types and reg
   localparam S0=0, S1=1, S2=2, S3=3;
   reg[1:0] cur_state;
@@ -108,18 +119,18 @@ module quarter_rate_downconverter #(
         end
       S1: begin
           // S(t) * i = -Q(t) + iI(t):
-          tmp_i = -q_in;
-          tmp_q = i_in;
+          tmp_i = invert_sig(q_in);
+          tmp_q =            i_in;
         end
       S2: begin
           // S(t) * -1 = -I(t) - iQ(t):
-          tmp_i = -i_in;
-          tmp_q = -q_in;
+          tmp_i = invert_sig(i_in);
+          tmp_q = invert_sig(q_in);
         end
       S3: begin
           // S(t) * -i = Q(t) - iI(t):
-          tmp_i = q_in;
-          tmp_q = -i_in;
+          tmp_i =            q_in;
+          tmp_q = invert_sig(i_in);
         end
       default: begin
           tmp_i = i_in;