about the harmonic coupling of the varactor in vco

hi!
how to reduce the harmonic coupling of the varactor to the control voltage.the control voltage in my pll oscillates because the 2nd harmonic coupling of the varactor.and the peak-to-peak value is related to the loop filter capacitance,and it can even achieves to 20mv.the larger the capacitance of the loop filter ,the smaller the peak-to-peak value.i guess the reason is that the impedance from the control node to gnd is small if the capacitance is large. many papers talk about how to reduce the noise,but few papers refer to the harmonic coupling.is there a range of peak-to-peak value that we can accept.please give me some explanations.thanks a lot
best regards

Do you mean the 2nd harmonic at the Vtune of the varactor? Put a huge cap at Vtune.

You want the impedance at this node to be small so as to attenuate the 2nd harmonic.

It has nothing to do with phase noise.

thanks a lot
yes.i mean the 2nd harmonic at the Vtune of the varactor.does putting a huge cap at Vtune mean putting a huge cap between Vtune and gnd?however,if the cap is large,it will take so large area in the chip.furthermore,some paper says the quality factor will decrease if the impedance at the Vtune node is small,so the phase noise will be affected.maybe it is very small.

Hi psmon,

You should pay attention to the fact that adding big capacitor on Vtune will have an impact on the loop filter and stability...
Regards.

Mehdi

The question really bothers me these days.in my opinion,the coupling of the varactor is totally different from the ripple.vco is a autonomous system,and it produces coupling while it builds the oscillation.maybe the coupling and the oscillating signals together determine the large signal capacitance of the varactor and the oscillation frequency.however,ripple is external factor which destroys the oscillation. maybe the coupling is acceptable.but i can not make sure the above is right,because transient reponses in papers are all perfect.i am a freshman in pll.give me some advice please.
best regards

There's a 2nd order LPF on Vtune. The size of the LPF is big anyway, should short any ripple on the control.

thank psmon for your reply.you mean just make use of the capacitor of the lpf,right?and several hundred pF capacitors in chip is acceptable?they take so large areaes.best regards