Conferences

18 GHz In, 6 GHz Out Satellite Transponders Without Local Oscillators or Mixers

RF, satellite-transceiver technology has evolved from the traditional superhetrodyne architecture used on the Inmarsat 4 and Alphasat telecommunication payloads, to a software-defined radio approach using under-sampling, wideband ADCs to directly digitise S-band carriers and return-to-zero DACs to reconstruct the digital baseband up to almost 6 GHz. The next generation of transponder will exploit advances in analogue circuit design to directly digitise 18 GHz, communications traffic and output C-band IF/RF without the use of local oscillators or mixers.

Current, state-of-the-art, RF, transponder research is able to deliver higher performance than software-defined radio, mixed-signal transceivers by exploiting novel advances in wideband circuit design and SAW-based oscillators. The result is higher SNR and better SFDR due to improved linearity.

Future, RF transponders will be smaller, lighter, lower power consuming, higher performing, more reliable and significantly less expensive to manufacture, test and sell. Production testing and measurement will remain RF in nature, without the need to characterise traditional, individual IF stages.

All of the above benefits will allow OEMs to deliver the next generation of transceivers which will enable spacecraft operators to offer competitive Ku-band services overcoming the congested bandwidths at traditional, lower, satellite-communication frequencies.

A Study of the Mechanism of the Nonlinear Input Capacitance on the RF Performance in GaN HEMT Devices

The nonlinear input capacitance characteristic in GaN HEMTs affects the linearity of the output signal and limits the device performance by generating intermodulation distortion at the input of the circuit. This effect has been discussed extensively and design approach such as pre-linearization technique has also been suggested to compensate the impact. In this paper, the mechanism behind this behaviour is examined as a function of input power and frequency using a lumped element based model. An RLC circuit formed by the gate resistance, inductance and the input capacitance is considered at the input. Its response and impact on the input signal before it reaches the intrinsic current generator are observed for both linear and nonlinear input capacitance. Simulation results show that the distortion due to the harmonics created by the nonlinear capacitance is higher at large-signal and at operating frequency lower than the self-resonance frequency of the RLC circuit. At frequency much higher than the self-resonance frequency, there are no harmonics; however the signal is strongly attenuated. The results also show that the operation at self-resonance frequency must be avoided due to potential detrimental effect on the device as the output signal can become higher than the device breakdown voltage.

GaN Microwave Transistors: How the Reliability Challenges Have Been Solved

Microwave transistors based on GaN can deliver outstanding performance because of basic material advantages of high breakdown voltage, high carrier density and good mobility. They now deliver unmatched lifetime and robustness, are space qualified, and are displacing traditional solid-state technologies in numerous applications.  However, this performance did not come without a struggle. The primary problem areas which have impacted the performance and reliability of GaN microwave devices will be identified, and the techniques discussed which have been used to control or mitigate their impact. The talk will highlight the control of current-collapse from surface and bulk traps, the suppression of high-temperature reverse-bias gate edge breakdown, and the impact of the epitaxial growth approach on the crucial parameter for reliability - thermal resistance.

Investigation of GaN FET Optimal Harmonic Terminations Using a Simplified Compact Large Signal Model

There remains ongoing debate in the high frequency PA community as to which class of operation eg Class J Class F, Class Inverse-F etc is most suitable for optimizing the efficiency of the GaN FET PA. While active load pull measurements can be useful in observing actual termination sensitivities they do not necessarily explain the class of operation at the intrinsic current generator plane, without significant de-embedding, or reveal more subtle effects of device non linearity.  A simplified compact large signal model was developed with emphasis on low number of parameters and robust, fast convergence under high harmonic reflection coefficient. The model has been constructed in a piecewise fashion to enable the effect of each of the intrinsic nonlinearities for example gate-source charge, to be easily disabled or examined. The model includes a thermal sub-circuit, while trapping effects have been neglected in the first instance.  The model has been used to examine harmonic terminations and waveforms at the intrinsic current generator plane.

Microwave Power Amplifiers, Resonant Structures and the Rapid Diagnosis of C-difficile and other Antibiotic Resistant Bacteria

The spread of antibiotic resistance is a growing threat to our ability to treat infectious disease, due largely to the “blind” prescribing of antibiotics, and is in part a consequence of our inability to determine the antibiotic susceptibility of the infecting bacteria in real time.  Current bacterial detection methods take typically several hours and sometimes days to produce accurate results and as a result, tend to be used to confirm rather than to prevent infection. This talk describes how high-efficiency microwave amplifiers and microwave resonant cavities are enabling a prototype bacterial DNA detection system that is able to identify specific pathogens within minutes; a change that promises to reduce patient suffering and morbidity, and help extend the utility of the remaining effective antibiotics.

Microwave-to-Terahertz Properties and Applications of Graphene

The unique electronic properties of graphene are attractive for a variety of device applications in microwave-to-terahertz technology. For frequencies up to several 100 GHz, graphene behaves like a dispersion-free resistive sheet, the value of its sheet resistance can be controlled by an external gate voltage. This property enables the realization of electrically controllable attenuators, fast switches and modulators for frequencies up to the terahertz range. Due to hot electrons in graphene, highly sensitive and ultrafast graphene THz detectors have been demonstrated, which have a strong potential to outperform other detector types operated at room temperature. In the terahertz-to-mid-infrared range, the excitation of plasmon type surface waves enables promising application as label free electromagnetic biosensor devices.

In this presentation, a realistic overview about microwave-to-terahertz applications of graphene will be given, and the application potential in context with the state-of-the art of graphene technology will be discussed.

Re-Assessment of the 450 Heat-spreading Approximation used in the Calculation of the Thermal Resistance of RF Power Transistors

The original paper in which the 45⁰ heat-spreading approximation was first proposed showed that in the worst case the error was around 10% for a device mounted on a semi-infinite heatsink. It will be shown in this paper that when applied to the calculation of the thermal resistance of an RF power transistor that it can under-estimate the thermal resistance by up to 30%. The effect of using smaller heat-spreading angles in an attempt to improve the accuracy is also investigated.

RF in Healthcare-Friend or Foe

The application of radiation in healthcare of whatever frequency is governed by the interaction of the radiation with tissue and the balance of risk and benefit in the delivery of patient benefit. For ionizing radiation this is well understood, however for non-ionizing radiation across the RF spectrum the balance of risk and benefit is less well understood. This is primarily due to the fact that the interaction with tissue is poorly understood except in higher power applications. The continuing concern that mobile phone use has the potential to mediate brain tumours is an exemplar of the lack of clear evidence and understanding. The application of RF in healthcare environments is further complicated by the potential for adverse interactions between electrical and electronic systems. It is not often possible to test all potential interactions of devices in the healthcare context and thus regulation tends to restrict technology adoption especially where there is documented potential for patient harm.

Within this context the application of RF in healthcare can be divided into a number of discrete classes. Firstly, communications and identification technologies where applications such as the management of medical devices and the asset base using RFID and radio communications are slowly being adopted. Healthcare will not be immune from the technology supporting ‘the internet of things’. This application has the potential to greatly increase the efficiency and safety of use of medical devices. Secondly, the generation of heating within tissues to deliver the ability to kill tumour cells whilst preserving normal tissues – hyperthermia or, at higher power, direct tissue ablation and destruction. This is used in surgery through diathermy to cauterize tissues during surgery and prevent excessive bleeding. One of the most important applications of RF in healthcare is in the development of MRI imaging. This has become one of the highest resolution’s, in-vivo imaging technologies with the growing potential to provide quantitative, functional discrimination of tissues, however the increase in magnetic field strength has led to safety concerns due to induced currents and heating within the body.  The application of NMR in-vitro is an established technique. More recently there have been developments in the application of multi-spectral RF technologies to characterize tissues. This is being commercialized to provide a method to monitor cerebral blood volume and changes through intra-cerebral bleeds or stroke. This has the potential to significantly change clinical practice in the management of traumatic brain injury.

This paper will provide a review of these key applications, the physical interactions that are exploited and often limit the application and the risks and benefits in the healthcare environment. The regulatory framework that needs to be applied will be highlighted.