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This paper reports a 40 GHz sub-harmonic Gilbert upconverter with a leveled-LO configuration using 0.15-μm AlGaAs/InGaAs pseudomorphic high electron mobility transistor (PHEMT) technology. The sub-harmonic structure is employed because the LO frequency is only half of the RF frequency. A two-stage RC-CR polyphase filter forms a 20-GHz quadrature generation for the LO port of the upconverter while an LC current combiner at the RF output port performs the balance-to-unbalance transformation and conversion gain enhancement. A self-biases current source is used to achieve single voltage supply. The demonstrated pHEMT sub-harmonic Gilbert upconverter has a conversion gain of -1 dB, an output return loss of -16.5 dB at 40 GHz, LO-to-RF isolation of 47 dB, and 2LO-to-RF isolation of 41 dB. OP1dB and OIP3 are -20 dBm and -5 dBm, respectively. The current consumption is 12.9 mA at a 4 V supply voltage.