  We study asymptotic normality of the randomized periodogram estimator of
quadratic variation in the mixed Brownian--fractional Brownian model. In the
semimartingale case, that is, where the Hurst parameter $H$ of the fractional
part satisfies $H\in(3/4,1)$, the central limit theorem holds. In the
nonsemimartingale case, that is, where $H\in(1/2,3/4]$, the convergence toward
the normal distribution with a nonzero mean still holds if $H=3/4$, whereas for
the other values, that is, $H\in(1/2,3/4)$, the central convergence does not
take place. We also provide Berry--Esseen estimates for the estimator.
