Comments on "Higher harmonic generation in CMOS/SOS ring oscillators"

Arguments presented in the referenced paper [1] imply that higher order modes of oscillation should not be expected in ring oscillators with a small number of stages. In contrast to this, third and fifth harmonic oscillations have been observed in 15-stage Si MESFET ring oscillators using BFL inverters. It is explained that the decay mechanism presented in the referenced paper applies primarily to even harmonics in ring oscillators with an even number of stages. Higher order modes of oscillation can be sustained in ring oscillators with a relatively small odd number of stages if the rise and fall times of the inverters used are short in comparison with their total delay.