The simplicity of the direct conversion receiver offers some important advantages over the superheterodyne architecture. The RF signal is filtered, amplified, and then converted to baseband directly. There is no IF signal, and hence the image reject filter, IF amplifier, and high-Q IF channel select filter are no longer required. Without these components, the homodyne receiver can achieve a higher level of integration than the heterodyne counterpart, and integration can translate into cost savings.
However, the direct conversion architecture also has some disadvantages that make its implementation difficult. DC offsets, 1/f noise, and second-order distortion from the mixer will all fall in the signal band. Another problem is LO self-mixing. Since the RF and LO are at the same frequency, any leakage from the LO port to the input of the mixer is going to produce a DC component at the output, adding to the DC-offset problem.
The goal of this project is to study the use of sub-harmonic mixers in the direct conversion receiver. Because the LO is now at a fraction of the RF frequency, any LO leakage to the input of the mixer will be mixed to a frequency outside the signal band. Currently, a sub-harmonic mixer in a 0.13-Ám CMOS process is being designed.