Electron paramagnetic resonance (EPR) is a powerful technique for probing the spin states of unpaired electrons and for elucidating the structure and energy-transfer pathways of matter. By retrofitting the teaching-grade Dahua EPR spectrometer with a modulation field and a lock-in amplifier, we obtain the first-derivative absorption signal and employ the NI LabVIEW platform to automate signal control and data acquisition. Using the standard DPPH sample, the relaxation time and Lander factor are determined. Through this experiment, students gain an intuitive appreciation of how modulation fields and lock-in amplifiers are used to handle weak signals in magnetic-resonance investigations, thereby enhancing their overall experimental proficiency.