provided is a sub - harmonic frequency mixer having a structure in which two terminals of transistors receiving a radio frequency signal and a local oscillation signal are coupled . the frequency mixer is composed of a single - level structure of transistors and thus can be driven at a lower supply voltage compared to a common frequency mixer . a direct - conversion receiver employing such a frequency mixer needs an rf signal and an lo signal sources of single - phase . therefore , the direct - conversion receiver has an architecture that simplifies a whole rf transceiver circuit and thus can be advantageously applied in implementing soc for a low power , high integration , low price and subminiature wireless transceiver circuit .