This article presents a concept of a long pulse envelope detector (ED) with a high sensitivity value based on Microwave Photonics (MWP). The process of envelope detecting of RF signals is achieved by the translation of the RF spectrum to the optical spectrum range followed by optical filtering to cut the lower sideband and the carrier, finally beating the remaining upper sideband into a low-speed photodetector (LSPD). The LSPD output signal is amplified by transimpedance gain. This approach depends only on the RF passing band of the phase modulator (PM) and the optical filter transfer function. This concept is experimentally demonstrated reaching envelope detector tangential signal sensitivity (TSS) values around -40 dBm for RF input frequency range from 10 GHz to 20 GHz. The proposed architecture does not use any optical or video signal amplifier. The implementation of this approach is simple, employs few components, and Commercial off-the-Shelf (COTS).