A low phase noise LC oscillator using inductively coupled inductors has been proposed. Previous oscillators have been suffered from the low Q-factor of on-chip inductor. This low Q-factor is an important factor affecting large phase noise. The proposed circuit increases effective Q-factor of inductors using electromagnetic coupling. The improved Q-factor using quadruple-inductive coupling can improve phase noise of oscillators. Moreover, noise-filtering effect of the coupled inductors reduces phase noise further more.
To prove the validity of the concepts 2-coupled and 4-coupled inductors have been fabricated and characterized. Phase noise simulation results using the fabricated coupled inductor are -116.6dBe/Hz, -115.1dBc/Hz and -118.6dBc/Hz of inductor scheme, 2-coupled inductor scheme and 4-coupled inductor scheme, respectively. The improvement in phase noise is not enormous because the fabricated inductors had a low coupling coefficient about 0.35~0.55, which is much lower than expected value of 0.7 . It is expected to improve phase noise more than 9dBc/Hz when the coupling coefficient of the coupled inductors are increased to 0.7
