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Based on the switched capacitor array, the oscillation frequency of the LC-tank VCO is defined by Eq. 9:

fosc = 1/2π[L(Cv + Carray + Cpar]0.5   (9)


Cpar = the parasitic capacitance,

Carray = the switchable capacitor bank, and

Cv = the capacitance value of the varactor.

The oscillator frequency, fosc, is coarsely controlled by Carray and finely tuned by Cv, with its value determined by the control voltage, Vctr. The VCO gain, KVCO, can be derived using Eq. 10:

KVCO = ∂fosc/∂Vctr = -[1/4π√L(Cv + Carray + Cpar1.5] × (∂Cv/∂Cvctr)   (10)

It can be shown from Eq. 10 that KVCO is a strong function of the capacitance of the switched capacitor array. The use of the switched capacitor array not only broadens the VCO frequency tuning range but also results in a more ideal value for KVCO.

Novel LC Tank Steers Low-Power VCO, Table 1

The proposed LC VCO design of Fig. 1 was fabricated with an 0.18-μm silicon CMOS semiconductor technology. Table 1 lists the parameters for the VCO’s components. Figure 6 shows the differential output voltage swing of the VCO core. The circuit draws DC current of 0.310 mA from a +0.8-VDC supply, which means the proposed VCO topology has DC power consumption of 248 μW. Figure 7 shows the measured frequency tuning curves. The 16 overlapping tuning curves cover the desired frequencies with sufficient overlaps between adjacent bands and with good tuning linearity. The VCO circuit exhibits a tuning range of 40.1% from 2.21 to 3.33 GHz with tuning voltages ranging from 0 to +0.8 VDC. Figure 8 shows that the VCO achieves phase noise of -112.7 dBc/Hz at a far offset of 1 MHz from a 3.33-GHz carrier.

Novel LC Tank Steers Low-Power VCO, Fig. 6

Novel LC Tank Steers Low-Power VCO, Fig. 7Novel LC Tank Steers Low-Power VCO, Fig. 8

A figure-of-merit (FOM) parameter was used to evaluate the three performance parameters for VCO: frequency, phase noise and power consumption, defined by Eq. 11:

FOM = Lfoffset - 20log(f0/foffset) + 10log[PDC/(1 mW)]   (11)

where P, the power dissipated by the oscillator core (in mW), L(Δω, is the total single-sideband (SSB) phase-noise spectral density at offset frequency Δω from the carrier, Ω0, with a higher value of the FOM implying better performance.

Novel LC Tank Steers Low-Power VCO, Table 2

Based on these parameters and calculations, the performance of this novel VCO design is presented in Table 2, alongside of recent oscillator developments and their performance levels. As Table 2 shows, the new VCO features the lowest power consumption of all the oscillators. As can be seen in the other designs, power consumption is traded off for phase noise performance, frequency turning range, and the output swing.