Notch Filter Eliminates UWB System Interference

FOR ULTRAWIDEBAND (UWB) SYSTEMS, interference comes in many forms: Bluetooth, IEEE 802.11b/g systems, cellular phones, and a variety of in-band and out-of-band WiMAX services. These sources of interference can cause receiver-gain compression. In addition, second- and third-order modulation products can fall in-band. Because another portion of spectrum is available for UWB at a higher frequency, a lowpass filter cannot be used in a full-band UWB system. Recently, a 0.13-m CMOS fourth-order notch filter for the rejection of 5-to-6-GHz interference in UWB front ends was reported by Allesio Vallese of Canova Tech Srl; Andrea Bevilacqua, Andrea Gerosa, and Andrea Neviani from Universit di Padova; and Christoph Sandner and Marc Tiebout from Infineon Technologies Austria AG.

This filter is integrated into an analog front end for UWB Mode 1. It provides more than 35 dB of rejection. The filter has a tuning range of 900 MHz and increases the power consumption of the low-noise amplifier (LNA) by less than 30 percent. With the filter off, the LNA offers an out-of-band input-third-order intercept point above 13.2 dBm. The out-of-band blocker is reduced by at least 6 dB in the front end. The integrated assembly boasts conversion gain of 25 dB per channel. The front end features an average noise figure below 6.2 dB. Its in-band 1-dB compression point is higher than 30 dBm while consuming 32 mW.

The researchers proposed an interferencerejection scheme based on a tunable narrowband notch filter that can achieve high attenuation with a reduced number of passive components. A strategy for the automatic tuning and calibration of the filter also is proposed and demonstrated. See "Analysis and Design of an Integrated Notch Filter for the Rejection of Interference in UWB Systems," IEEE Journal Of Solid-State Circuits, February 2009, p. 331.

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