Eliminating the influence of temporally varying channel components on the radio frequency fingerprint (RFF) extraction has been an enduring and challenging issue. To overcome this problem, we propose a channel-independent RFF extraction method inspired by the idea of 'fighting fire with fire'. Specifically, we derive the linear differential spectrum and the logarithmic differential spectrum of the channel frequency responses (CFRs) from the received signals at different times, and then calculate the ratio of the two spectrums. It is found that the division operation effectively counteracts the channel effects, while simultaneously preserving the integrity of the RFFs. Our experiments on LTE-V2X, LoRa and Wi-Fi devices show that the proposed method achieves an average identification accuracy exceeding 95% across various environments.