Mark S. Oude Alink

A 50Mhz-To-1.5Ghz Cross-Correlation CMOS Spectrum Analyzer for Cognitive Radio with 89dB SFDR in 1Mhz RBW

Spectrum sensing for cognitive radio requires a high linearity to handle strong signals, and at the same time a low noise figure (NF) to enable detection of much weaker signals. Often there is a trade-off between linearity and noise: improving one of them degrades performance of the other. Cross-correlation can break this trade-off by reducing noise at the cost of measurement time. An existing RF front-end in CMOS-technology with IIP3=+11dBm and NF<6.5dB is duplicated and attenuators are put in front to increase linearity (IIP3=+24dBm). The attenuation degrades NF, but by using cross-correlation of the outputs of the two front-ends, the NF is reduced to below 4dB. In total this results in a spurious-free dynamic range (SFDR) of 89dB in 1MHz resolution bandwidth (RBW).

A CMOS-Compatible Spectrum Analyzer for Cognitive Radio Exploiting Crosscorrelation to Improve Linearity and Noise Performance

A spectrum analyzer requires a high linearity to handle strong signals, and at the same time a low NF to enable detection of much weaker signals. This is not only important for lab equipment, but also for the spectrum sensing part of cognitive radio, where low cost and integration is at a premium. Often there is a trade-off between linearity and noise: improving one degrades the other. Crosscorrelation can break this trade-off by reducing noise at the expense of measurement time. An existing RF frontend in CMOS-technology with IIP3 = +11 dBm and NF = 5.5 dB is duplicated and attenuators are put in front to increase linearity to IIP3 = +24 dBm. The attenuation degrades NF, but by using crosscorrelation of the outputs of the two frontends, the effective NF is reduced to around 5 dB. In total, this results in a spurious-free dynamic range of 88 dB in 1 MHz resolution bandwidth.

Cancellation of OpAmp Virtual Ground Imperfections by a Negative Conductance Applied to Improve RF Receiver Linearity

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 9 dBm to >20 dBm, and IIP2 from 51 to 61 dBm, at the cost of increasing the noise figure from 6 to 7.5 dB and <;10% power penalty. In 1 MHz bandwidth, a Spurious-Free Dynamic Range of 85 dB is achieved at <;27 mA up to 2 GHz for 1.2 V supply voltage.

RF Circuit Linearity Optimization Using a General Weak Nonlinearity Model

This paper focuses on optimizing the linearity in known RF circuits, by exploring the circuit design space that is usually available in todays deep submicron CMOS technologies. Instead of using brute force numerical optimizers we apply a generalized weak nonlinearity model that only involves AC transfer functions to derive simple equations for obtaining design insights. The generalized weak nonlinearity model is applied to three known RF circuits: a cascode common source amplifier, a common gate LNA and a CMOS attenuator. It is shown that in deep submicron CMOS technologies the cascode transistor in both the common source amplifier and in the common gate amplifier significantly contributes IM3 distortion. Some design insights are presented for reducing the cascode transistor related distortion, among which moderate inversion biasing that improves IIP3 by 10 dB up to 5 GHz in a 90 nm CMOS process. For the attenuator, a wideband IM3 cancellation technique is introduced and demonstrated using simulations.

Spectrum Sensing With High Sensitivity and Interferer Robustness Using Cross-Correlation Energy Detection

Dynamic spectrum access relying on spectrum sensing requires reliable detection of signals in negative signal-to-noise ratio (SNR) conditions to prevent harmful interference to licensed users. Energy detection (ED) is a quite general solution, which does not require any knowledge of the signals to be detected. Unfortunately, it suffers from noise uncertainty in the receiver, which results in an SNR-wall below which signals cannot be reliably detected. Furthermore, distortion components originating from nonlinearity in the sensing receiver cannot be distinguished from true input signals, and is thus another effect that may obscure weak signals and cause false alarms or missed detections. Cross-correlation was recently proposed to reduce the SNR-wall and, at the same time, allow the receiver to be designed for high linearity. This allows for high-fidelity spectrum sensing, both in the presence of strong interference as well as for signals with a negative SNR. In this work, an integrated complementary metal-oxide-semiconductor prototype exploiting cross correlation is presented and tested in practice. The prototype achieves a high linearity of +25 dBm IIP3 at a sensitivity of -184 dBm/Hz, 10 dB below the kT noise floor. The measured results agree well with theory, and, compared to the traditional ED-approach, show both a significant improvement in sensing time, as well as a reduction of 12 dB in the SNR-wall itself. Overall, cross-correlation makes ED faster, more sensitive, more resilient to strong interferers, and more energy-efficient.

A Wideband IM3 Cancellation Technique for CMOS

A wideband IM3 cancellation technique for CMOS attenuators is presented. With proper transistor width ratios, the dominant distortion currents of transistor switches cancel each other. As a result, a high IIP3 robust to PVT variations can be achieved without using large transistors. Two prototypes in a 0.16 μm standard bulk CMOS process are presented: a Π-attenuator with four discrete settings obtains +26 dBm IIP3 and +3 dBm 1 dB-compression point (CP) for 50 MHz to 5 GHz with only 0.0054 mm2 active area, and a similar T-attenuator system which obtains +27 dBm IIP3 and +13 dBm CP for 50 MHz to 5.6 GHz with only 0.0067 mm2 active area.

{\bf \Pi}

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 IIP3 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.

- and T-Attenuators

The product, convolution, correlation, Wigner distribution function (WDF) and ambiguity function (AF) of two Hermite functions of arbitrary order n and m are derived and expressed as a bounded, weighted sum of n+m Hermite functions. It was already known that these mathematical operations performed on Gaussians (Hermite functions of the zeroth-order) lead to a result which can be expressed as a Gaussian function again. We generalize this reciprocity to Hermite functions of arbitrary order. The product, convolution, correlation, WDF, and AF operations performed on two Hermite functions of arbitrary order lead to remarkably similar closed-form expressions, where the difference between the operations is primarily determined by distinct phase changes of the weights of the Hermite functions in the result. The closed-form expressions are generalized to the class of square-integrable functions. A key insight from the closed-form expressions is applied to the design of orthogonal, time-frequency localized communication signals which are characterized by an AF with rotational symmetry. In addition to this application, the theoretical expressions may prove useful for signal analysis in fields ranging from communications, radar and image processing to quantum mechanics.