problems on integer N PLL harmonic locking

wfcawy

Hi,
It's a Integer PLL with N=8,

During PLL test, we find the 8th-order harmonic of the reference signal can make the oscillator itself locking at 8*fref , and the pll loop seems to have no help on the tuning of the PLL.

It's called something like "Injection and Locking or Pulling" effect, and does anyone have similar experience or How can I reduce the coupling and make the PLL work normal ?
Look forwards,