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https://github.com/NawfalMotii79/PLFM_RADAR.git
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Jason
e2cb7bd2d6
Stage-7 ADC chain audit. ddc_400m.v line 273-274 converted AD9484
offset-binary 8-bit codes to MIXER_WIDTH-bit signed via
`(adc << 9) - (8'hFF << 8)`. The right operand evaluates to
`0xFF / 2 = 0x7F` (integer divide drops 0.5), so the formula
subtracted 65280 instead of the correct mid-scale offset 65536:
- code 0x80 (0V analog, mid-scale) → +256 (expected 0)
- code 0x00 (-fs) → -65280 (expected -65536)
- code 0xFF (+fs - 1 LSB) → +65280 (expected +65024)
A 0.5-LSB DC bias on the AD9484 stream is functionally invisible
in production: after the 120 MHz NCO mixer it lands at ±120 MHz,
which the CIC + FIR LPF (~50 MHz rolloff) rejects. But the offbin
and twoc paths disagreed on the same analog input, and any reuse
of `adc_signed_offbin` outside the mixer chain would carry the
bias.
Replace with an MSB-flip + sign-extend + 9-zero pad that mirrors
the twoc branch: `{~msb, ~msb, low7, 9'b0}`. Mid-scale code 0x80
now produces 0 exactly, matching the twoc path bit-for-bit.
Local FPGA regression 36/1/6 unchanged (1 FAIL = pre-existing
T-6 drift, deferred PR-M.4).