A 500-600 MHz VFO is no pushover,either!
You're not going to make a stable free running one,you are looking at a PLL or similar for that sort of frequency.
Oh,& its output should be within a dB or so over that range!
Maybe you can crystal lock the 500MHz fixed oscillator,but you will probably have to either multiply it up from a lower frequency,or use another PLL.
You are then stuck with designing a mixer & output stage which is flat from 0--100MHz!
Complicated?----No,but incredibly difficult to actually do!
This is why test equipment manufacturers charge big bucks!