libm2sdr: re-apply RX QEC K_exp loop gain after RX LO retune#146
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enjoy-digital merged 1 commit intoJul 13, 2026
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The AD9361 driver default RX quadrature-tracking loop gain (K_exp=0x15) leaves the loop effectively inert: ~25 dBc image rejection, a 3.5-7% EVM floor that corrupts wideband PDSCH LDPC. 0x0a converges to ~40-43 dBc. Each RX LO retune re-runs the AD9361 quad cal and reverts K_exp to the inert default, so re-apply it after ad9361_set_rx_lo_freq. M2SDR_QEC_KEXP overrides the value.
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Thanks @simonerni! |
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Problem
The AD9361 driver default RX quadrature-tracking loop gain (
K_exp = 0x15) leavesthe loop effectively inert: ~25 dBc image rejection, a 3.5–7 % EVM floor. On
wideband receive this corrupts PDSCH/LDPC — the image is coherent, so HARQ
combining can't recover it.
0x0aconverges to ~40–43 dBc.Each RX LO retune re-runs the AD9361 quad cal, which reverts
K_expto the inertdefault, so the fix has to be re-applied after every retune, not just at init.
Change
After
ad9361_set_rx_lo_freq()inm2sdr_set_frequency(), call a small helperthat writes
CALIBRATION_CONFIG_2/3withK_EXP_PHASE/K_EXP_AMPLITUDE.M2SDR_QEC_KEXPoverrides the value (default0x0a; stable across0x02–0x0a).Testing
test-libm2sdr,analyze-libm2sdr(cppcheck clean), ASan/UBSan, Soapy build — allpass. Measured ~25 → ~40–43 dBc image rejection on hardware, removing the wideband
EVM floor.