What sets the minimum detectable signal level for a direct-sampling software defined receiver in the absence of atmospheric or thermal noise?
Why In a direct-sampling receiver the ADC digitizes the antenna signal directly, so the smallest signal it can represent is one least significant bit, and that step size equals the full-scale reference voltage divided by 2^n, where n is the sample width in bits. That quantization step sets the quantization noise floor, so a larger reference voltage spread over fewer bits raises the floor and hides weak signals, while more bits (about 6 dB of dynamic range per bit) lowers it. With no atmospheric or thermal noise present, that quantization floor is what limits the minimum detectable signal.
Watch out Sample clock phase noise and jitter are real problems, but they mainly smear strong nearby signals and degrade close-in dynamic range, much like reciprocal mixing in an analog receiver; they do not define the basic noise floor. Data storage transfer rate is a computing bottleneck, not a sensitivity limit.
Weakest signal = one LSB: Vref divided by 2^bits. About 6 dB more dynamic range per added bit.