For contrast and completeness' sake below results I got from an Analog Discovery 2 with the BNC adapter module:
(output shunted with built-in 50Ohm terminator, set 1V symmetric square wave)
input was not terminated (uses ~1MOhm "high impedance" input, DC coupled)
top to bottom:
- 8' BNC cable, 5MHz (the signal generator offers 10MHz, but then not much of a square is left)
- 8' BNC cable, 1MHz
- 100MHz 10x probe, 1MHz (system is in a bit of a noisy environment, I think the probe picks up some RFI)
- 100MHz 10x probe, 5MHz
- 8' BNC cable, input terminated via T-piece and 50Ohm terminator, 5MHz
- FFT of the last one