A new technique to measure the frequency response of a fiber-optic tunable filter (TF) is proposed and demonstrated. This technique is based on the linearity of the sinusoidal function in the vicinity of $m\pi$ , where m is an integer. The free spectral range (FSR) voltage, $V_{\mathrm{FSR}}$ , is evaluated fast and accurately by measuring the time interval corresponding to the bandwidth of TF. The time interval is obtained by adjusting the dc offset of the driving signal to locate the transmission peak of TF at the maximum slope of the driving signal of $<1V_{p\text{-}p}$ . The measured FSR voltages range from $500\mathrm{mV}\text{to}100\mathrm{V}$ over the frequency up to $200\mathrm{kHz}$ .