Which scope below $300 has best FFT function? How important is bit memory/record length to the quality of this function compared to other specs?
The budget scopes like Rigol or Siglent are lousy.
Most important is resolution, because that limits your dynamic range.
What I do is sample the tracks into a PC via SCPI and do the FFT there. MUCH better and flexible.
Expensive scopes with a 16bit resolution mode (oversampling plus averaging) create a good FFT, normally (Keysight Infiniium, e.g.).