this post is getting way out there! filtering harmonics? your signal is going to be about 40dB down, about as high as the noise floor in a digital chip - i would be surprised if someone could come up with a schematic that actually works. 1.25Ghz? lol!
my vote is for a pll. if you aren't familiar, a pll is a little closed-loop system that drives a VCO to match the input clock, both in frequency and phase. If you want a multiplication, you simply divide the VCO output by 10, now the system drives the VCO until Fout/10 matches the reference frequency, or Fout=10*Fin.
I think you should be searching google for both the basics of the pll, and datasheets of pll's that can generate GHz signals - these are not trivial.