Hi alistu,
1- I chose BPF to select only the sixth harmony of 3 GHz which is 18 GHz (i achieved that).
2- The harmonics of 1.5 GHz form double sides (DSB) on each harmony of 3 GHz.
3- so I chose LPF to select lower side of harmonics 1.5 GHz on the 18 GHZ.
N.B. I chose 18 GHz to perform local carrier which carry the harmonics of 1.5 GHz.
Regards,