Radio frequency (RF) receivers are used in a wide variety of applications such as television, cellular telephones, pagers, global positioning system (GPS) receivers, cable modems, cordless phones, satellite radio receivers, and the like. One common type of RF receiver is the so-called superheterodyne receiver. A superheterodyne receiver mixes the desired data-carrying signal with the output of tunable oscillator to produce an output at a fixed intermediate frequency (IF). The fixed IF signal can then be conveniently filtered and converted back down to baseband for further processing. Thus a superheterodyne receiver requires one or more mixing steps.
A superheterodyne receiver is a receiver that mixes the desired data-carrying signal with the output of tunable oscillator to produce an output at a fixed intermediate frequency (IF). The fixed IF signal can then be conveniently filtered and converted back down to baseband for further processing. Superheterodyne receivers are useful in a wide variety of applications in which the desired channel can occur within a wide band of frequencies, such as AM and FM radio, satellite radio, etc.
To reduce the cost of a superheterodyne radio receiver, it is useful to combine as many circuit elements as possible into a single integrated circuit (IC) built using low-cost complementary metal-oxide-semiconductor (CMOS) manufacturing processes. However integration creates its own set of problems. For example a conventional CMOS mixer is formed by a local oscillator (LO) and a multiplier circuit. The multiplier circuit converts an input voltage conveying the signal to be mixed into a current signal. A portion of the multiplier known as a chopper circuit selectively diverts the current signal based on clock signals output by the LO. However the LO is usually laid out as a block on an adjacent part of the IC from the multiplier. The clock signals are then provided to the chopper switches using conductors such as metal lines.
However the chopping process distorts the output signal because the transistors cannot be perfectly matched. Furthermore the signals are usually mixed with both an in-phase LO signal and a quadrature LO signal to form an in-phase mixed signal and a quadrature mixed signal. The transistors in the in-phase mixer and the quadrature mixer will also typically not track each other so that when they are recombined at baseband the image component will not perfectly cancel out, resulting in distortion of the output signal. What is needed is a mixer that has lower distortion than such known mixers.