the problem about pll spur

I designed a PLL circuit at 5GHz and 10MHZ pfd frequency as ref. In PLL output phase noise spectrum,a spur at 5MHz offset is found，higher than 10MHz offset，where is it come from？

Please provide more detail on your circuit pdf, cp

well, either u are dividing the 10 MHz reference down by 2 somewhere in the chip, or u have an oscillating op amp somewhere.

Have you tested your reference spectrum purity? Many oscillators specified for 10 MHz generate also 5 MHz.

Your reference oscillator might work at realy 5MHz to obtain 2nd overtone.
It may be a reson...