This paper presents an innovative x3 sub-sampling frequency downconverter for converting signals around 77 GHz from a 26 GHz LO signal. With a LO frequency divided by 3 this topology allows drastic simplifications on the receiver's LO distribution chain. The sub-sampling mixer operation principle is described and implemented with 28-nm FD-SOI CMOS technology. Simulation results show a 1dB input-referred compression power (ICP1dB) of +3 dBm and a 14.4 dB Noise Figure (NF) with a-2.5 dB conversion gain. The mixer's LO signal shaper consumption is 36 mW on a 1.2V supply while the passive mixer core doesn't require DC power.