High linearity CMOS radio receivers often exploit linear V-I conversion at RF, followed by passive down-mixing and an OpAmp-based Transimpedance Amplifier at baseband. Due to nonlinearity and finite gain in the OpAmp, virtual ground is imperfect, inducing distortion currents. This paper proposes a negative conductance concept to cancel such distortion currents. Through a simple intuitive analysis, the basic operation of the technique is explained. By mathematical analysis the optimum negative conductance value is derived and related to feedback theory. In-and out-of-band linearity, stability and Noise Figure are also analyzed. The technique is applied to linearize an RF receiver, and a prototype is implemented in 65 nm technology. Measurement results show an increase of in-band IIP 3 from 9dBm to >20dBm, and IIP2 from 51 to 61dBm, at the cost of increasing the noise figure from 6 to 7.5dB and <10% power penalty. In 1MHz bandwidth, a Spurious-Free Dynamic Range of 85dB is achieved at <27mA up to 2GHz for 1.2V supply voltage.

Abstract-Since 2008, The Federal Communications Commission (FCC) allows the operation of Cognitive Radio (CR)in unused parts (i.e. white spots) of the DTV spectrum. Due to the nonlinearity of the radio receiver and the existence of strong DTV signals, different types of distortion products will be generated in the CR-receiver. This paper analyzes the spectral location of distortion products across the white spots depending on the location of the DTV signals in the RF spectrum, focusing on 3 rd order distortion products. Based on this analysis, we show that a receiver is always limited by cross-modulation (XM3) and self-interference products. Thus true distortion free white spots do not exist if DTV signals are present after the RF-band filter. However, XM3 and self-interference distortion products are typically much weaker than 3 rd order intermodulation (IM3) products. Thus it makes sense to monitor the level and spectral location of interferes and classify the "white spots" into two types, namely IM3-spots and IM3-free spots. This paper derives equations to quantify how much the 3 rd order linearity requirements are relaxed when the CR operates at an IM3-free spot. The analysis not only takes into account narrowband interferers but also wideband interferers. The analysis is verified by measurements.Index Terms-Cognitive radio (CR), radio frequency (RF), dynamic spectrum, orthogonal frequency division multiplexing (OFDM), radio receiver, linearity requirement, input third intercept point (IIP3), third order intermodulation product (IM3), cross-modulation product (XM3), Self-interference. I. INTRODUCTIONOGNITIVE RADIO is a new emerging radio communication paradigm [1] aiming at improving the utilization efficiency of the scarce spectral resources. It senses unused "white spots" in the radio spectrum that is licensed to a primary user and adapts its communication strategy to use these parts while minimizing interference to the primary service. This work studies requirements on the 3 rd order linearity of a CR receiver for wireless applications operating in the DTV bands [2]. Cognitive Radio requires high programmability for radio transmission and reception because the position of the white spots changes dynamically with time (i.e. dynamic spectrum). Traditional radio receivers are narrowband and are typically highly dedicated to a specific RF-band, especially due to the fixed RF-filter (see Fig.1). The high out-of-band rejection of the RF-filter, usually a highlinearity highly selective Surface Acoustic Wave (SAW) filter, suppresses the interference of out-of-band interferers. Traditional radio standards define in detail how the radio-band is used, e.g. by specifying a multiple access method (TDMA, FDMA, CDMA) and blocker profiles. Consequently in-band signals are "under control", while RF-filtering reduces the outof-band interference. In such narrowband systems, typically third order intermodulation products (IM3) produced by nonlinearities in the receiver dominate as is shown in Fig. 2. Both the signal and the ...

Highly linear CMOS radio receivers increasingly exploit linear RF V-I conversion and passive down-mixing, followed by an OpAmp based Transimpedance Amplifier at baseband. Due to the finite OpAmp gain in wideband receivers operating with large signals, virtual ground is imperfect, inducing distortion currents. We propose to apply a negative conductance to cancel this distortion. In an RF receiver, this increases In-Band IIP 3 from 9dBm to >20dBm, at the cost of 1.5dB extra NF and <10% power penalty. In 1MHz bandwidth, a Spurious-Free Dynamic Range of 85dB is achieved at <27mA up to 2GHz for 1.2V supply voltage. Index Terms-Receiver linearity, in-band and out-band IIP 3 , mixer-first receiver architecture, operational amplifier.

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The exponential increase of wireless communication increasingly leads to spectrum congestion. Attempts are being made to increase RF spectrum utilization efficiency by introducing Cognitive Radio (CR) concept. A CR tries to intelligently solve the congestion problem via Dynamic Spectrum Access (DSA), i.e. determine which frequencies are temporarily and locally free, and exploit this free spectrum. Especially in the TV broadcasting bands below 1 GHz, such CR possibilities are being explored. Ideally, a CR receiver should be able to operate directly adjacent to the primary service users, e.g. Digital TV channels, which use high power levels and often leave the adjacent channels unused. Under such conditions no or very low up-front filtering of the interferer is possible. Consequently, a CR receiver must tolerate the existence of strong interferers, i.e. have a very high linearity front-end. This thesis examines CR receiver linearity requirements and explores techniques that mitigate distortion.