Instantaneous frequency measurements (IFM) are of great importance in modern electronic warfare. However, traditional methods based on electronics are facing considerable challenges because of their shortcomings. Particular attention has been paid to photon-assisted technologies owing to their large bandwidth, low loss, small size, lightweight, and electromagnetic interference immunity characteristics. In this study, the operational fundamentals of IFM are presented and analyzed based on a dual-path-imbalanced optical link. A demo system is also constructed with ordinary devices to measure the signal (0.5-18.5 GHz). It is demonstrated that the experimental and theoretical results are consistent, and we achieve excellent measurement accuracy (within ±60 MHz). This method has great potential for military applications owing to the simplicity of its front-end structure, which benefits from the use of the ordinary, phase-modulated optical link.