Hello,
It depends many factors, however you could freely chose all 4 variable combination(n or p cross coupled, n or p current source). The main advantage of current architecture are

1. There is no DC voltage change change across Varactor as inductor has 0 dc potential, hence less 1/f bias noise up conversion (common mode to frequency or AM to FM conversion).  You can refer The impact of device type and sizing on phase noise mechanisms for further understanding.
2. Supply noise spurs also will be very less, whenever there is noise on supply same noise noise will be appeared at node vx and vy and if loop filter also refereed to same supply then Vtune also will also be impacted by same noise, hence no noise across varactor and no spurs.

Thanks,
Raj.

Hi Raj.,


Quote:


in this case, is it better to tie the loop filter capacitors from the supply to the charge-pump output instead of from charge-pump output to ground? My thinking is the supply may not be ideally referenced to ground. If you don't do that, I guess your supply cap should be significantly larger than your loop-filter cap right?


thanks,
Aaron

Hi Raj.,


you mentioned that the loop filter it referenced to the same supply as the VCO. So what you're saying is that if there is any noise on the supply then it will appear on both terminals of the varactors, so the voltage across the varactors will be zero.

I was just saying that your loop filter has some large capacitors connected between the CP output and ground. Ideally ground and supply have the same AC potential, but in practice, its not really true, and there will be some power supply noise wrt ground. So is it better to connect the loop filter capacitors between the CP output and the supply instead. That way, the CP output will track variations in the power supply better.


regards,
Aaron

Hi Raj.


yes that's what I meant.


thanks,
Aaron