power spur when FPGA work

[foxgan]

I've got a trouble. When the FPGA SPARTAN 3A3400 work (about 80% resource used), a 7KHz spur and its harmonics occure on the ground. The spur‘s level is about 30mVp-p. when I reduce the FPGA resource usage, the spur become little.
I try to add 220uF tan capcitor, but it does not work.
plz help!