  We consider the simple model of measurement of mechanical oscillator position
via Fabry-Pero cavity pumped by detuned laser (end mirror of cavity is mass of
oscillator) in resolved sideband regime when laser is detuned from cavity's
frequency by frequency of mechanical oscillator $\pm\omega_m$ and relaxation
rate $\gamma$ of cavity is small: $\gamma\ll \omega_m$. We demonstrate
fluctuation back action cancellation in reflected wave. However, it does not
allow to circumvent Standard Quantum Limit, the reason of it is the dynamic
back action.
