about the 2nd harmonic coupling of varactor in lc tank vco

[calven303]

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

 

[neoflash]

Kick back noise on VCO input will modulate Capacitance in 2Wo. However, this is not a problem since the Capacitance is already modulated by swinging output VCO voltage, even if Vctrl stays constant.

I guess that is why people do not talk about it on literature. That's just OK.

 

[calven303]

Re: about the 2nd harmonic coupling of varactor in lc tank v

thanks a lot.it is just the reply that i expect.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. but i can not make sure whether the above is right,because transient reponse waveforms of vco control voltage in papers are all perfect.thanks again.
best regards

Kick back noise on VCO input will modulate Capacitance in 2Wo. However, this is not a problem since the Capacitance is already modulated by swinging output VCO voltage, even if Vctrl stays constant.

I guess that is why people do not talk about it on literature. That's just OK.