Experimental Comparison of Phase Noise in Cross-Coupled RC- and LC-Oscillators

Relaxation RC-oscillators are notorious for their poor phase-noise performance. However, there are reasons to expect a phase-noise reduction in quadrature oscillators obtained by cross-coupling two relaxation oscillators. We present measurements on 5 GHz oscillators, which show that in RC-oscillators the coupling reduces both the phase-noise and quadrature error, whereas in LC-oscillators the coupling reduces the quadrature error, but increases the phase-noise. A comparison using standard figures of merit indicates that quadrature RC-oscillators may be a viable alternative to LC-oscillators when area and cost are to be minimized.