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A 15-GHz high-isolation sub-harmonic mixer (SHM) with delay compensation is demonstrated in this letter using 0.35 mum SiGe heterojunction bipolar transistor (HBT) technology. The proposed SHM consisting of two parallel stacked-LO mixer cores with opposite LO I/Q inputs substantially improves port-to-port isolation. A mathematical analysis regarding the isolation properties is also included in this letter. Further, a conventional stacked-LO SHM (w/o compensation) is also implemented with the same device sizes and bias conditions for a fair comparison. SHMs w/ and w/o compensation have similar conversion gain of 9/10 dB and noise figure of 14/13.5 dB when fLO = 7.5 GHz. However, the delay compensation technique improves the 2LO-RF/IF isolation by 34/35 dB, the LO-RF/IF isolation by 8/9 dB, and the RF-IF isolation by 22 dB.