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A fully integrated active sub-harmonic mixer (SHM) located in two Marchand baluns is presented. The proposed circuit was designed at 24 GHz for industrial, scientific and medical (ISM) band applications and fabricated in a 0.13 μm complementary metal-oxide-semiconductor (CMOS) process. The proposed circuit layout is based on allocating the active SHM into square passive baluns to reduce the chip area without degrading performance. Two design approaches, one using a multi-layer and the other using compensated capacitors, were adopted in the CMOS Marchand baluns process to increase the coupling coefficient sufficiently and to minimise the required balun size. Additionally, the conversion gain was increased by using the current-bleeding scheme in the SHM. Moreover, a conversion gain of 3.8 dB was achieved in the designed circuit at a local oscillator (LO) input power of 5 dBm. Both radio frequency-intermediate frequency (IF) and LO-IF isolations were better than 28 dB. The proposed circuit not only has a higher conversion gain than other published mixers that are operated at millimetre-wave frequencies, but also has an innovative circuit layout.