An Analog LO Harmonic Suppression Technique for SDR Receivers

A low-complexity analog technique to suppress the local oscillator (LO) harmonics in software-defined radios is presented. Accurate mathematical analyses show that an effective attenuation of the LO harmonics is achieved by modulating the transconductance of the low-noise transconductance amplifier (LNTA) with a raised-cosine signal. This modulation is performed through the bias network of a cascode device with a negligible increase in the LNTA noise figure. The proposed technique results in a notch at the third harmonic and at least 36 dB of attenuation at the fifth and the seventh harmonics. Experimental results in 130-nm CMOS and postlayout simulation results in 65-nm CMOS verify the proper functionality of the proposed technique and the accuracy of the proposed analyses.