the problem about pll spur

[guj1987]

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？

 

[AdvaRes]

Please provide more detail on your circuit pdf, cp

 

[biff44]

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

 

[jiripolivka]

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

 

[BigBoss]

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