A magnetically tuned quadrature oscillator

Continuous frequency tuning by control of the magnetic field of a transformer-capacitor tank, in a selective oscillator, is explored in this work. A quadrature generator is built connecting two identical transformer-capacitor oscillator cells in a feedback loop. The topology itself assures the currents in the transformer windings are aligned in phase, while their relative amplitude determines, via magnetic coupling, oscillators' tank reactance, i.e., oscillation frequency. This paper introduces the idea, analyzes oscillation amplitude, frequency tuning band, phase noise, and phase accuracy, and discusses design and experiments. Prototypes, realized in 65 nm CMOS, employing MOS varactors to further extend operation bandwidth, show the following performances: 3.2 GHz and 7.3 GHz minimum and maximum oscillation frequency, respectively. Phase noise figure of merit of 176.5 dB at 3.2 GHz and 170.5 dB at 6.4 GHz is observed, with 24 mW maximum power consumption and 1.5 degrees maximum deviation from quadrature.