Here are square waves 32 MHz, 300 MHz and 2.6 GHz of a HP8133A
measured by a Tektronix TDS820, 6 GHz oscilloscope with 44 ps rise time:
Square wave 32 MHz of the HP8133A, x-axis 5ns/div, y-axis 200mV/div:
Square wave 300 MHz of the HP8133A, x-axis 500ps/div, y-axis 200mV/div:
Square wave 2.6 GHz of the HP8133A, x-axis 100ps/div, y-axis 200mV/div:
The rise time displayed in the last image is about 60 ps