abdou2014 send me schematic of your FG-90 and PCB pictures at both side of device. Upload at some Iranin host site and send me a link privately here at mailbox.
I will see, could I help you with your RF-sniffing mod. OK ? There are many "underwather reefs" which must be taken to the account if you want to work it properly and detect the gold.
The problem you have is magnetic cross-coupling. Could be solved. Need more info.
Regards !