Low-noise RF integrated oscillator design

In this work the different trade-offs in the design of low phase noise integrated oscillators are being evaluated. Although the focus is on bipolar implementations with tank circuits that include spiral inductors, the discussion and the results apply in general to any LC oscillator. A typical cross-coupled oscillator is swept from soft to hard switching operation and its phase noise performance is evaluated. The major conclusion is that hard switching, class-C, operation is desired because in this regime the signal is larger. For a specific tank circuit the magnitude of the voltage swing across the tank circuit is therefore the most critical parameter for phase noise. At the same time it is the most neglected parameter in commonly found implementations that make use of a degenerated tail current source for biasing and limit the available headroom. The limited swing is the main reason why CMOS oscillators seem more attractive than bipolar circuits even though the bipolar transistor has superior noise properties. This limitation becomes more evident when the circuit operates from a lower supply voltage as the available headroom decreases. A new architecture that enables the magnitude of the voltage swing to develop to the maximum level available is being proposed and is implemented as a 1.8-GHz VCO in a SiGe process that supports Cu spiral inductors. Both simulation and experimental results indicate that a phase noise improvement of 8 dB can be expected. Even though the specific fabricated circuit does not fully-exploit the hard switching operation regime due to unexpectedly higher parasitic capacitance of the bipolar transistors, a phase noise of −119 dBc/Hz was still obtained at a 600 KHz offset which is 3 dB below the DCS-1800 receiver specification for the mobile station. From a phase noise point of view the improved-swing bipolar oscillator is superior to CMOS implementations, especially at low frequency offsets where the high 1/f noise of CMOS devices is the dominant noise contributor.