17.2 A CMOS Differential Noise-Shifting Colpitts VCO

Demand for higher numbers of communication channels impos-es tighter phase noise performance for the local oscillators. Cross-coupled oscillators are widely used due to ease of imple-mentation and differential operation (Figure 17.2.1a) [1,2,3]. Unfortunately, in cross-coupled VCOs the maximum noise gen-eration instant coincides with maximum phase noise sensitivity and hence the cross-coupled VCOs do not use the full potential of the resonator [3]. This oscillator topology achieves improved phase noise for a given quality factor and bias current by alter-nating the bias current and aligning the maximum noise with the least sensitive time in the cycle. The phase noise of an oscillator is given by [4]: (1) where foff is the offset frequency from the carrier, qmax is the max-imum signal charge swing, i2n /∆f is the power spectral density o