Generate Square Wave with Fast Rise Time for Testing
The technology to produce this is called Current Mode Logic or CML formerly called ECL.
You don’t need a frequency to test an active prove or GHz DSO, just the 10~90% risetime to measure 0.35/Tr = f-3dB
Then apply FFT on the result for spurious ripple.
Here is a novel avalanche stripline picosecond pulser:
Designed by the late Jim Williams who started when I did in Analog electronics in the ‘60’s.

http://www.siliconvalleygarage.com/projects/picosecond-pulser.html