Using NHK-FM Osaka 880Hz time tone on 15-AUG-2019:
(Airspy HF+, 384kHz sampling, 384kHz IF)

40 stages: THD 0.032%, THD+N: 0.373%
128 stages: THD 0.025%, THD+N: 0.243%
288 stages: THD 0.020%, THD+N: 0.230%

A typical result on 27-JUL-2019:
No filter: THD 0.281%, THD+N: 0.630%
