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Implementing low-power voltage-controlled-oscillator (VCO) circuitry with high-frequency silicon CMOs semiconductor technology can provide effective signal generation for a wide range of wireless applications. By adopting a VCO design with an inductive-capacitive (LC) tank circuit, the oscillator can achieve outstanding performance at RF/microwave frequencies while minimizing the supply voltage.

Typically, tradeoffs are required when designing a VCO for low power consumption, a wide frequency tuning range, and low phase noise, and a number of low-power CMOS oscillators have been reported for applications operating at less than 1 V.1-5 These designs typically sacrifice in output power or phase noise at their low operating voltages. Fortunately, a VCO with tuning range of better than 40% has been achieved in silicon CMOS, tuning 2.21 to 3.33 GHz with 0 to +0.8 VDC and consuming only 0.248 mW power for a supply voltage of less than +0.8 VDC while maintaining low phase noise.

Many of the earlier CMOS oscillators suffer from reduced output swing and degraded phase noise because of the limited supply voltage. Improvements in output swing and phase noise have been achieved by incorporating capacitive feedback and forward-body-bias (FBB) techniques in a cross-coupled CMOS VCO approach.6 Unfortunately, the frequency tuning range is still relatively narrow due to the limited supply voltage. Two techniques have typically been used to increase the tuning range: capacitive and inductive coarse tuning techniques. The capacitive coarse tuning approach is based on the variation of the capacitive part of the LC tank circuit,7-11 while the inductive coarse tuning methods is based on varying the inductive part of the LC tank circuit.12-16

Compared to the switched capacitor approach, the main drawbacks of the inductive coarse tuning techniques are related to a higher degradation of the overall LC tank quality. Moreover, the additional parasitic capacitance introduced by a variable inductor must be minimized to preserve the tuning range. The advantages of switched-capacitor coarse tuning techniques include a small VCO gain parameter, KVCO, and a wide frequency tuning range. A wide variation between the minimum gain parameter, KVCO min, and maximum gain parameter, KVCO max, can impact VCO performance in several ways.17

A wide variation in the gain parameter can affect phase noise in the lower or middle parts of the oscillator frequency range, assuming the varactor stage does not constantly amplify the different noise sources (1/f and thermal noise from sustaining amplifier transistors and bias stage). In phase-locked-loop (PLL) applications, large variations in KVCO could lead to PLL instability due to variations in its open-loop gain, as well as degradation in closed-loop integrated phase noise and settling time.

The current solution involves achieving a wide frequency tuning range with a small value of KVCO. This wideband, low-power VCO exploits an integrated LC tank based on the use of a FBB technique and switched capacitor array. The novel topology and use of FBB result in changes in threshold voltage with changes in body-bias voltage, making it possible to achieve a lower threshold voltage for given operating conditions. By using a switched capacitor bank, it is possible to achieve a wide frequency tuning range.

Novel LC Tank Steers Low-Power VCO, Fig. 1

Figure 1 shows a schematic diagram of the proposed VCO, with its cross-coupled complementary topology. This approach yields robust start-up operation and lower 1/f noise compared to NMOS-only VCOs. To reduce the supply voltage required, the tail current transistor in a conventional cross-coupled VCO has been replaced in this design by on-chip inductors.18 As a result, the DC voltage drop over the inductors becomes negligible, allowing the source node to swing below ground and leave maximum voltage headroom for the oscillating signal.

The inductors are tuned to provide the source nodes of devices M3 and M4 with high impedances at twice the oscillation frequency, to reduce phase noise. These inductors contribute very little noise. The supply voltage for the cross-coupled complementary device pair is +0.8 VDC; the resonator consists of a spiral inductor, a pair of AC-coupled accumulation mode varactor diodes, and a switched capacitor array.

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