I've noticed that the majority of "amateur" SDRs have ADCs with a sampling rate not higher than 20 MSps, but still the HackRF One for example can scan it's whole frequency range (1MHz to 6GHz) in the same time.
How does that happen?
If a SDR were to have a ADC with a 20MSps sampling rate, it could safely sample a signal with a maximum frequency of 20MHz (if quadrature, 10MHz if else). So basically it could work with signals from 0Hz to 20MHz and that range is shifted with the help of a down converter, but it still remains the same width. Do they work with the Nyquist zones or something, somehow reconstructing the spectrum for frequencies higher than the Nyquist frequency?