Digital control oscillator using varactor and dac

52		Hogge Detector				Continuous-Time	M.H. Perrott
52		Hogge Detector					M.H. Perrott
				Ipd	2	First Order Σ Δ ADC
				Ipd		- Isd
		D Q	D Q	retimed data(t)
						D Q
						Cint
	clk(t)					clk/2(t)
	clk(t)	Reg	Latch			clk/2(t)

data(t)

clk(t) phase error(t)

LC oscillator can be achieved through a switched-capacitor network by altering the

resonant frequency of the tank according to the amount of capacitance switched

data(t)	Phase-to- Digital			clk(t)

Fig. 7 Digital control of an LC oscillator using a switched-capacitor network

Figure 8 shows an alternative means of achieving digital control of an LC oscilla-tor, which is to simply control the input of a varactor within a hybrid VCO [7] with the output of a digital-to-analog converter (DAC). In order to limit the frequency range required of the varactor (which lowers its influence on the phase noise of the oscillator), a switched-capacitor network can be used to perform coarse calibration of the oscillator in order to remove the impact of process variations [7]. Since the unit capacitor size in the array can be much larger than the “all-digital” design shown in Fig. 7, its control network is much less complex and a simpler design effort can be applied to achieve high performance. In practice, the coarse calibration is often performed off-line with a frequency acquisition circuit, and the analog varactor is controlled by the feedback action of the phase-locked loop.

Analog Digital

data(t)	Phase-to- Digital		DAC	Hybrid VCO
data(t)	Phase-to- Digital		DAC