Interferer Rejection in Cognitive Radio Receiver Using Heterodyne Conversion and Active Feedback

Limited linearity and local oscillator (LO) harmonics mixing are two major issues that ultra-wideband receivers, and a cognitive radio (CR) in particular, are dealing with. Direct-conversion schemes, based on current-driven passive mixers, have shown to improve the linearity but unable to resolve the LO harmonic mixing problem. They are usually limited to third and fifth harmonics rejection or require a very complex and power hungry circuitry for a higher number of harmonics. This paper presents a heterodyne conversion scheme in the 180-nm CMOS technology for CR applications (54-862 MHz band) that robustly mitigates the harmonic mixing issue for all the harmonics, while by employing an active feedback loop, a comparable state-of-the-art IIP3 of better than +10 dBm is achieved. Measurements show an average noise figure of 7.5 dB when the active feedback loop is OFF (i.e., in the absence of destructive interference) and of 15.5 dB when the feedback loop is active and a 0-dBm interferer is applied, respectively.