A Superheterodyne Receiver Front-End With On-Chip Automatically Q-Tuned Notch Filters

Abstract

A superheterodyne receiver front-end with on-chip automatically Q-tuned notch filters is proposed. The front-end includes a differential LNA and a Gilbert down-converter, each block is coupled with an on-chip image-rejection notch filter to get high image-rejection ratio. The notch filter is formed by one LC network and one negative-resistance cross-coupled pair to compensate the loss of the LC network. The current through the cross-coupled pair is automatically adjusted by a master Q tuning circuit so that the loss of the notch filter is compensated to achieve a deep notch. The master Q tuning circuit is an analog-digital mixed signal circuit, and successive approximation register algorithm is used to search for the optimum current value. The circuit has been implemented in 0.18 um CMOS, and the experimental results show that the circuit achieves an image rejection ratio of 80 dB while the two image-rejection filters only draw 222 uA current from a 1.8 V power supply.