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

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_i = invert_sig(q_in);
           tmp_q =            i_in;
         end
       S2: begin
           // S(t) * -1 = -I(t) - iQ(t):
-          tmp_i = -i_in;
+          tmp_i = invert_sig(i_in);
-          tmp_q = -q_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_q = invert_sig(i_in);
         end
       default: begin
           tmp_i = i_in;