Paul A Warr

Wide tuning-range planar filters using lumped-distributed coupled resonators

This paper describes a discretely tunable filter topology based on lumped-distributed coupled transmission lines, particularly suitable for microelectromechanical systems switching devices. This topology is capable of simultaneous wide-band center frequency and bandwidth tuning, limited only by the electrical size of the transmission lines and the placement density of the switching devices. Low fractional bandwidths can be achieved without the need for large coupled-line spacings due to the antiphase relationship of the lumped capacitive and distributed electromagnetic coupling coefficients. The positions of the additional poles of attenuation due to the lumped capacitive coupling can be selected either above or below band leading to the choice of a narrow bandwidth design having good high-side performance or a design with compromised upper stopband performance, but with no bandwidth tuning limitations. The interaction between a pair of lumped-distributed coupled transmission lines is analyzed and the resulting model is used to develop a filter synthesis procedure. The synthesis procedure and filter performance are validated through theoretical and experimental comparisons using a filter with low-side attenuation poles

Flexible frequency discrimination subsystems for reconfigurable radio front ends

The required flexibility of the software-defined radio front end may currently be met with better overall performance by employing tunable narrowband circuits rather than pursuing a truly wideband approach. A key component of narrowband transceivers is appropriate filtering to reduce spurious spectral content in the transmitter and limit out-of-band interference in the receiver. In this paper, recent advances in flexible, frequency-selective, circuit components applicable to reconfigurable SDR front ends are reviewed. The paper contains discussion regarding the filtering requirements in the SDR context and the use of intelligent, adaptive control to provide environment-aware frequency discrimination. Wide tuning-range frequency-selective circuit elements are surveyed including bandpass and bandstop filters and narrowband tunable antennas. The suitability of these elements to the mobile wireless SDR environment is discussed.

Enabling technologies for software defined radio transceivers

This paper discusses the design considerations related to the transceiver hardware elements within a software defined radio (SDR). Receiver architectures are reviewed and the viability of manufacturing these in the short to medium term is considered. The filtering functions required for a conventional receiver are examined, and the problems associated with implementation of these filters within a SDR receiver discussed, including that of image filtering. Receiver linearity requirements are evaluated in terms of typical user applications and it is shown that these will be onerous in the case of a SDR receiver. A novel technique for mixer linearisation is presented as a potentially enabling element within a re-configurable transceiver and some initial performance results are reported. The possibility of constructing an electronically tunable preselection filter using micro-electromechanical systems (MEMS) technology is examined. Some simulation and prototype measurement results are presented.

Software defined radio receiver test-bed

This paper identifies the issues that are important in the design of a SDR receiver. Receiver architectures are first discussed, and the conclusion drawn that the conventional superheterodyne structure is most appropriate for a SDR receiver. Issues associated with image rejection, and receiver linearity are discussed. The design of a sweepable preselect filter is discussed in detail. Design considerations for a practical SDR test-bed are presented.

Distortion Mechanisms in Varactor Diode-Tuned Microwave Filters

This paper examines the broadband distortion behavior in flexible filters employing varactor-diode tuning elements. Series- and parallel-resonant varactor-loaded transmission-lines, both commonly used in bandpass and bandstop microwave filters, are analyzed. Nonlinear Volterra-series analysis is employed to determine the second- and third-order distortion ratios dependent on the frequencies of the incident signals. It is shown that in a bandpass filter (employing parallel tuned resonators), maximum distortion occurs in the passband, while in a bandstop filter (employing series tuned resonators), minimum distortion occurs at the minimum-loss passband. The analysis is verified by practical measurement of filters employing the two modes of resonators

A 60% PAE WCDMA Handset Transmitter Amplifier

This paper reports the design of a class-E envelope elimination and restoration (EER) based amplifier for a wideband code division multiple access handset application that attains 60% power-added efficiency at peak power output. Emphasis is placed on the envelope modulator that employs a novel split-frequency approach in order to attain an efficiency of 80% for this part of the system. In contrast to standard EER systems, the carrier is not amplitude limited, but rather predistorted to maintain both linearity and power efficiency. Performance in terms of efficiency, spectral output, and error vector magnitude is reported.

A Frequency-Reconfigurable Antenna Architecture Using Dielectric Fluids

A new frequency-reconfigurable antenna architecture is presented, in which a dielectric fluid is pumped into a cavity behind the antenna to change its resonant frequency. The continuous tuning provided by the changing fluid volume allows the resonant frequency to be adjusted to any value within the tunable range. This tuning method does not affect the power handling capability of the antenna and does not consume power while the resonant frequency is kept constant. This method of tuning also stands out in its class by offering a wide tuning range, high efficiency, and very good electrical isolation between the antenna and the control circuitry. The antenna was designed and optimized using Ansys HFSS software and several prototypes were built and tested. Measured results of the input response, radiation pattern, and efficiency are presented. Castor oil (εr = 2.7) and ethyl acetate (εr = 6) were used in physical tests as the tuning fluids to verify the simulated results. Good agreement between simulated and measured results was observed which is also in line with the behavior suggested by theory and earlier investigations.

A High Input Impedance Low Noise Integrated Front-End Amplifier for Neural Monitoring

Within neural monitoring systems, the front-end amplifier forms the critical element for signal detection and preprocessing, which determines not only the fidelity of the biosignal, but also impacts power consumption and detector size. In this paper, a novel combined feedback loop-controlled approach is proposed to compensate for input leakage currents generated by low noise amplifiers when in integrated circuit form alongside signal leakage into the input bias network. This loop topology ensures the Front-End Amplifier (FEA) maintains a high input impedance across all manufacturing and operational variations. Measured results from a prototype manufactured on the AMS 0.35 μm CMOS technology is provided. This FEA consumes 3.1 μW in 0.042 mm2, √ achieves input impedance of 42 GΩ, and 18.2 nv/ Hz inputreferred noise.

Flexible linearity profile low noise feedforward amplifiers for improving channel capacity

This paper describes a highly linear low noise amplifier (LNA) exhibiting a flexible linearity profile intended as a way to meet future flexible mobile communications standards. This has the potential of inclusion in ad-hoc software defined radio (SDR) networks where a frequency band and modulation scheme can be chosen based on the characteristics of the frequency spectrum and user requirements. The linearity profile of the LNA is tailored to cancel distortion generated as a function of the received power profile. This improves the receivers tolerance to unwanted signals present in the frequency band of interest and reduces the requirement of coding and equalization to compensate for receiver limitations whilst increasing the available capacity. The flexible linearity profile is achieved by the use of a feedforward amplifier structure which has the capability of large distortion suppression determined by the gain and phase balances within its two loops. To ensure maximum distortion cancellation in the band, adaptive cancellation is used to maximize the cancellation of harmful distortion.

The design of an analogue RF front end for a multi-role radio

The drive towards a software-defined radio (SDR) in which much of the processing is controlled by reprogrammable digital hardware is placing new demands on the analogue RF front end. In order to facilitate multi-role multi-mode operation, this must impose the minimum of constraints on the parameters that can be defined in software, while still offering a performance which is at least as good as todays application-specific radios. This paper outlines some of the areas of research being carried out at the University of Bristol, funded by DERA, in an attempt to improve the flexibility and performance of the analogue RF front-end of a multi-role radio.