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The impact of digital noise coupling through the substrate on RF MOSFETs was investigated in terms of the noise figure (NF) of the device up to 26.5 GHz. Previous works on the substrate digital noise coupling have treated the effect mostly in terms of the electrical isolation between ports, rather than actual devices, which does not provide direct information on the degradation of actual device performance parameters from such coupling. In this work, an actual NMOSFET was employed for test and the effect was described in terms of NF, a practical device performance parameter. The results show that NF is significantly degraded as the device enters the weak inversion state and/or V ds becomes smaller, suggesting a trade-off between low power operation and immunity against the substrate noise coupling. Also, it is experimentally verified that devices with a dual guard ring showed much smaller NF than those with a single guard ring.