This paper presents the design and implementation of a passive resonant sensor interrogator. The interrogator implements frequency modulation spectroscopy (FMS) using a digital modulation technique, such as binary frequency shift keying. The implementation of a digital modulation technique allows for accurate interrogation of high-Q resonant sensors in the microwave spectrum. FMS is capable of detecting the absorption and dispersion curves of a passive resonant sensor; consequently, the resonance frequency is obtained. The interrogator is successfully simulated using Mathworks MATLAB and implemented in a software-defined radio platform. The interrogation scheme was tested on a resonant cavity filter with a resonance frequency of 589.85 MHz. The interrogator achieved the resonance detection with an error of 0.025%.