Hi, folks:

I've got a question on DCO's sigma-delta modulation. The modulator itself is out of the question. However, since the DAC is accomplished inside DCO through discrete small capacitors' on/off, the high intrinsic non-linearity there will destroy the spectrum.

It is impossible to make those capacitor tuning gain linear in my opinion.

I do not know how people solve that problem. Please suggest, thanks.

Reference:  JSSCC’2005 Nov. Robert Bogdan "a digitally controlled oscillator in a 90nm digital cmos process for mobile phones"