We present a study of tail-bias-current 1/f noise upconversion into 1/f³ phase noise for both Colpitts and differential-pair LC oscillators. We focus on the Groszkowski effect, i.e. the shift in oscillation frequency induced by the higher current harmonics flowing into the LC tank of a harmonic oscillator. Quantitative results match very well numerical simulations run with spectreRF, and show that the Groszkowski effect may indeed be a dominant cause of 1/f³ phase noise generation.