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fpga/usrp3: Fix DC truncation bias by adding rounding to DDC chain.

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A digital DC bias caused by various truncations in the DDC chain was
previously noticable with small signal levels, especially with high
decimation rates. This patch eliminates the bias by replacing the
truncation with rounding or simply keeping more bits for longer where it
makes sense.

This is essentially a forward port of a similar prior fix to the usrp2
DDC chain: https://github.com/EttusResearch/fpga/pull/4.

Signed-off-by: Ryan Volz <ryan.volz@gmail.com>


Original-commit: a57c162a0d11ab82a83c911a94272ceb6f7ff70a
This commit is contained in:
Ryan Volz
2024-06-11 10:20:08 +02:00
committed by Joerg Hofrichter
parent 523212fa04
commit 1451ae066b
3 changed files with 88 additions and 29 deletions
Download Patch File Download Diff File Expand all files Collapse all files
lib/dsp/ddc_chain.v
+54 -17
View File
@@ -191,8 +191,8 @@ module ddc_chain
.coef_din(coef_din), // input [17 : 0] coef_din
.rfd(rfd2), // output rfd
.nd(nd2), // input nd
.din_1(i_hb1[23+HB1_SCALE:HB1_SCALE]), // input [23 : 0] din_1
.din_2(q_hb1[23+HB1_SCALE:HB1_SCALE]), // input [23 : 0] din_2
.din_1(i_hb1[WIDTH-1+HB1_SCALE:HB1_SCALE]), // input [23 : 0] din_1
.din_2(q_hb1[WIDTH-1+HB1_SCALE:HB1_SCALE]), // input [23 : 0] din_2
.rdy(rdy2), // output rdy
.data_valid(data_valid2), // output data_valid
.dout_1(i_hb2), // output [46 : 0] dout_1
@@ -200,8 +200,8 @@ module ddc_chain
reg [18:0] i_unscaled, q_unscaled;
reg strobe_unscaled;
reg [WIDTH-1:0] i_unscaled, q_unscaled;
reg strobe_unscaled;
always @(posedge clk)
case({enable_hb1,enable_hb2})
@@ -209,42 +209,61 @@ module ddc_chain
2'd0 :
begin
strobe_unscaled <= strobe_cic;
i_unscaled <= i_cic[23:5];
q_unscaled <= q_cic[23:5];
i_unscaled <= i_cic;
q_unscaled <= q_cic;
end
// ILLEGAL. Only half sample rate half band enabled.
2'd1 :
begin
strobe_unscaled <= strobe_cic;
i_unscaled <= i_cic[23:5];
q_unscaled <= q_cic[23:5];
i_unscaled <= i_cic;
q_unscaled <= q_cic;
end
// One Halfband enabled, decimate by 2.
2'd2 :
begin
strobe_unscaled <= strobe_hb1;
i_unscaled <= i_hb1[23+HB1_SCALE:5+HB1_SCALE];
q_unscaled <= q_hb1[23+HB1_SCALE:5+HB1_SCALE];
i_unscaled <= i_hb1[WIDTH-1+HB1_SCALE:HB1_SCALE];
q_unscaled <= q_hb1[WIDTH-1+HB1_SCALE:HB1_SCALE];
end
// Both Halfbands enabled, decimate by 4.
2'd3 :
begin
strobe_unscaled <= strobe_hb2;
i_unscaled <= i_hb2[23+HB2_SCALE:5+HB2_SCALE];
q_unscaled <= q_hb2[23+HB2_SCALE:5+HB2_SCALE];
i_unscaled <= i_hb2[WIDTH-1+HB2_SCALE:HB2_SCALE];
q_unscaled <= q_hb2[WIDTH-1+HB2_SCALE:HB2_SCALE];
end
endcase // case (hb_rate)
// Need to clip 1 bit here or we loose small signal performance out the truncated LSB's for worst case CIC gain cases.
// round to 19 bits for clip (->18) followed by multiplication (total gain of 6 bits)
wire [18:0] i_unscaled_rnd, q_unscaled_rnd;
round #(
.bits_in(WIDTH),
.bits_out(19)
) unscaled_rnd_i (
.in(i_unscaled),
.out(i_unscaled_rnd)
);
round #(
.bits_in(WIDTH),
.bits_out(19)
) unscaled_rnd_q (
.in(q_unscaled),
.out(q_unscaled_rnd)
);
// Need to clip 1 bit here or we lose small signal performance out the
// truncated LSB's for worst case CIC gain cases.
// NOTE: We can only clip here with CORDIC rotating, CIC in it's highest gain configurations and an input signal thats
// saturated.
wire strobe_unscaled_clip;
wire [17:0] i_unscaled_clip, q_unscaled_clip;
clip_reg #(.bits_in(19), .bits_out(18), .STROBED(1)) unscaled_clip_i
(.clk(clk), .in(i_unscaled[18:0]), .strobe_in(strobe_unscaled), .out(i_unscaled_clip[17:0]), .strobe_out(strobe_unscaled_clip));
(.clk(clk), .in(i_unscaled_rnd), .strobe_in(strobe_unscaled), .out(i_unscaled_clip[17:0]), .strobe_out(strobe_unscaled_clip));
clip_reg #(.bits_in(19), .bits_out(18), .STROBED(1)) unscaled_clip_q
(.clk(clk), .in(q_unscaled[18:0]), .strobe_in(strobe_unscaled), .out(q_unscaled_clip[17:0]), .strobe_out());
(.clk(clk), .in(q_unscaled_rnd), .strobe_in(strobe_unscaled), .out(q_unscaled_clip[17:0]), .strobe_out());
// Apply scaling gain to compensate for CORDIC and CIC gain adjustments so that signal swing over network transport has
// optimal dynamic range.
@@ -315,14 +334,32 @@ module ddc_chain
(.clk(clk),.rst(rst),.bypass(~enable_hb2),.run(run),.cpi(cpi_hb),
.stb_in(strobe_hb1),.data_in(q_hb1),.stb_out(),.data_out(q_hb2));
// round to 19 bits for clip (->18) followed by multiplication (total gain of 6 bits)
wire [18:0] i_hb_out_rnd, q_hb_out_rnd;
round #(
.bits_in(WIDTH),
.bits_out(19)
) hb_out_rnd_i (
.in(i_hb2),
.out(i_hb_out_rnd)
);
round #(
.bits_in(WIDTH),
.bits_out(19)
) hb_out_rnd_q (
.in(q_hb2),
.out(q_hb_out_rnd)
);
// Need to clip 1 bit here or we loose small signal performance out the truncated LSB's for worst case CIC gain cases.
wire strobe_unscaled_clip;
wire [17:0] i_unscaled_clip, q_unscaled_clip;
clip_reg #(.bits_in(19), .bits_out(18), .STROBED(1)) unscaled_clip_i
(.clk(clk), .in(i_hb2[WIDTH-1:WIDTH-19]), .strobe_in(strobe_hb2), .out(i_unscaled_clip[17:0]), .strobe_out(strobe_unscaled_clip));
(.clk(clk), .in(i_hb_out_rnd), .strobe_in(strobe_hb2), .out(i_unscaled_clip[17:0]), .strobe_out(strobe_unscaled_clip));
clip_reg #(.bits_in(19), .bits_out(18), .STROBED(1)) unscaled_clip_q
(.clk(clk), .in(q_hb2[WIDTH-1:WIDTH-19]), .strobe_in(strobe_hb2), .out(q_unscaled_clip[17:0]), .strobe_out());
(.clk(clk), .in(q_hb_out_rnd), .strobe_in(strobe_hb2), .out(q_unscaled_clip[17:0]), .strobe_out());
//scalar operation (gain of 6 bits)
wire [35:0] prod_i, prod_q;
lib/dsp/hb_dec.v
+20 -10
View File
@@ -26,7 +26,7 @@ module hb_dec
output reg [WIDTH-1:0] data_out);
localparam INTWIDTH = 17;
localparam ACCWIDTH = WIDTH + 3;
localparam ACCWIDTH = 36;
// Round off inputs to 17 bits because of 18 bit multipliers
wire [INTWIDTH-1:0] data_rnd;
@@ -174,20 +174,30 @@ module hb_dec
always @(posedge clk) sum_of_prod <= prod1 + prod2; // Can't overflow
wire [ACCWIDTH-1:0] acc_out;
acc #(.IWIDTH(ACCWIDTH-2),.OWIDTH(ACCWIDTH))
acc (.clk(clk),.clear(clear),.acc(do_acc),.in(sum_of_prod[35:38-ACCWIDTH]),.out(acc_out));
acc #(.IWIDTH(36),.OWIDTH(ACCWIDTH))
acc (.clk(clk),.clear(clear),.acc(do_acc),.in(sum_of_prod),.out(acc_out));
wire [ACCWIDTH-1:0] data_even_signext;
wire [WIDTH:0] acc_out_rnd;
round #(
.bits_in(ACCWIDTH),
.bits_out(WIDTH+1)
) round_acc (
.in(acc_out),
.out(acc_out_rnd)
);
localparam SHIFT_FACTOR = 6;
wire [WIDTH:0] data_even_signext;
sign_extend #(.bits_in(INTWIDTH),.bits_out(ACCWIDTH-SHIFT_FACTOR)) signext_data_even
(.in(data_even),.out(data_even_signext[ACCWIDTH-1:SHIFT_FACTOR]));
assign data_even_signext[SHIFT_FACTOR-1:0] = 0;
localparam SHIFT_FACTOR = 17 - (ACCWIDTH - (WIDTH+1));
always @(posedge clk) final_sum <= acc_out + data_even_signext;
sign_extend #(.bits_in(INTWIDTH),.bits_out(WIDTH+1-SHIFT_FACTOR)) signext_data_even
(.in(data_even),.out(data_even_signext[WIDTH:SHIFT_FACTOR]));
assign data_even_signext[SHIFT_FACTOR-1:0] = 0;
clip #(.bits_in(WIDTH+1), .bits_out(WIDTH)) clip (.in(final_sum), .out(final_sum_clip));
always @(posedge clk) final_sum <= acc_out_rnd + data_even_signext;
clip #(.bits_in(WIDTH+1),.bits_out(WIDTH)) clip_finalsum
(.in(final_sum), .out(final_sum_clip));
// Output MUX to allow for bypass
wire selected_stb = bypass ? stb_in : stb_out_pre[8];
lib/dsp/small_hb_dec.v
+14 -2
View File
@@ -110,13 +110,25 @@ module small_hb_dec
localparam ACCWIDTH = 30;
reg [ACCWIDTH-1:0] accum;
wire [ACCWIDTH-1:0] prod_acc_rnd;
round #(
.bits_in (36),
.bits_out (ACCWIDTH),
.round_to_zero (1),
.round_to_nearest(0),
.trunc (0)
) round_prod (
.in(prod),
.out(prod_acc_rnd)
);
always @(posedge clk)
if(rst)
accum <= 0;
else if(go_d2)
accum <= {middle_d1[17],middle_d1[17],middle_d1,{(16+ACCWIDTH-36){1'b0}}} + {prod[35:36-ACCWIDTH]};
accum <= {middle_d1[17],middle_d1[17],middle_d1,{(16+ACCWIDTH-36){1'b0}}} + prod_acc_rnd;
else if(go_d3)
accum <= accum + {prod[35:36-ACCWIDTH]};
accum <= accum + prod_acc_rnd;
wire [WIDTH:0] accum_rnd;
wire [WIDTH-1:0] accum_rnd_clip;