Function generator, like a sine, wave, triangle wave generator?
At 600 Mhz, you either have pure sine waves or a trapezoidal waveform.
Because of the large amount of harmonics present in a square wave, it is simply not feasible to generate a square wave. Specially if you are pretending a 5 volt swing.
Please Google LVDS, such that you the constraints you'll be facing..
You also need to think about what the lower end of the frequency range is.
There are a lot of circuits out there for "600MHz vco" (the google search I did) but many only have a limited range (say 300MHz to 600MHz)
Susan
The AWG presented in the link below, perhaps is the only device on the market that goes up to 600MHz. Usually the AWG generators sold by the big manufacturers goes only up to 120MHz.
In the link is presented the block diagram of the device: