Linear distortion correction using a feedforward system

The application of the feedforward amplifier linearization technique to the removal of both harmonic and intermodulation distortions is well understood and has been discussed in the literature. An additional benefit of the feedforward technique, which has received relatively little attention, is that of so-called linear distortion removal. Linear distortion is a term used to describe the nonideal gain and phase response (e.g., frequency response ripple) present in all practical amplifiers, and it is possible to significantly improve these parameters, in a practical application, using the feedforward technique. This paper discusses the improvements in this area, which may be achieved by utilizing the feedforward technique, demonstrates mathematically how they arise, and presents both simulation and practical results to demonstrate the effect for various types of gain and phase response nonlinearity     

Gain-weighted technique for directly measuring loop bandwidth of split-loop phase-locked loop

A technique is described, for directly measuring the loop bandwidth of the recently proposed split-loop phase-locked receiver. This technique takes advantage of recent advances in laboratory test equipment, and is both rapid and straightforward. The characteristics produced permit the actual loop parameters to be determined, thus providing verification of the calculated performance.<>     

Around the world by balloon: an electrical perspective

A number of teams are currently attempting to become the first to circumnavigate the globe by balloon. In each case, their equipment must be capable of operating continuously, without malfunction, for over 500 hours. This is a significant engineering challenge, as the many failures to date have highlighted. The purpose of this paper is to outline some of these challenges, particularly in the electrical and communication engineering fields, and to examine the various solutions adopted by some of the main contenders in this the `last great aviation challenge'     

Power amplifiers and transmitters for RF and microwave

The generation of RF/microwave power is required not only in wireless communications, but also in applications such as jamming, imaging, RF heating, and miniature dc/dc converters. Each application has its own unique requirements for frequency, bandwidth, load, power, efficiency, linearity, and cost. RF power is generated by a wide variety of techniques, implementations, and active devices. Power amplifiers are incorporated into transmitters in a similarly wide variety of architectures, including linear, Kalm, envelope tracking, outphasing, and Doherty. Linearity can be improved through techniques such as feedback, feedforward, and predistortion     

Transposer systems for digital terrestrial television

The UK digital terrestrial television (DTT) network has completed its first phase of roll-out, with over 70% of the population now able to receive all six digital multiplexes. Whilst this is an impressive achievement, the current analogue TV network can cover 99.7% of the UK population and digital satellite systems can boast virtually 100% coverage, so DTT still has some way to go. DTT coverage must improve in order to make analogue switch-off a politically acceptable reality and to compete directly with satellite in value-added services, such as shopping or banking. Analogue switch-off is a particularly desirable goal, as it would release a significant amount of valuable spectrum for use by other commercial services (e.g. mobile telephony). This paper addresses the use of transposers, in a manner similar to that of the current analogue TV network, in order to extend coverage cost-effectively to those viewers currently unable to receive all of the digital multiplexes. A number of transposer designs are considered, employing both DSP and high-temperature superconducting (HTS) techniques, and results presented on some of the technologies employed     

Analysis of instability in feedforward loop

Although the feedforward system is theoretically unconditionally stable, there are circumstances under which oscillation can occur. These oscillations are possible in both the error-determining loop and the error-cancellation loop and may occur in-band or out-of-band depending upon the parameters of the design. The authors analyse the circumstances under which oscillation is possible, and highlight the corrective action which may be taken in order to eliminate the possibility of its occurrence     

Emerging technologies for software radio

Multimode, multiband transceivers were once seen as a desirable but unachievable goal, particularly when considering small, battery-powered terminals. There are, however, significant market-pulls requiring just such a device and dual-mode systems (albeit with largely separate hardware) are now a reality. The next revolution will be the production of so-called flexible architecture terminals, in which the same hardware is used for all modulation formats and bands, with the internal software determining the mode to be utilised. Considerable research is being directed toward this goal worldwide, and practical results are beginning to appear which indicate that the unachievable may soon become a reality     

Living in harmony-industrial microwave heating and cellular radio

The 900 MHz band has been used for industrial microwave heating for many years, for such diverse applications as thawing or tempering of large blocks of frozen food, continuous baking and continuous vulcanisation of rubber. The UK, TACS (Total Access Communication System) cellular radio network is also assigned to this band and the two operations have happily co-existed without serious problems. Recently, however, the large growth of microwave heating applications and the proliferation of cellular radio has led to concern over the continuance of this harmonious co-existence. This has led to a unique partnership between the cellular operators and the microwave heating equipment manufacturers in order to ensure that both services can continue without detriment to either of their respective user communities     

Electronic tracking systems for space communications

Since the first television pictures were received from Telstar by aerial 1 at Goonhilly Down in Cornwall, commercial operation of satellite systems has dictated that the earth-space link must be maintained as consistently as possible. Any break in transmission or lowering of quality results in huge losses for the operator and as a result the performance of the antenna tracking system has received a great deal of attention. Recently a new technique, termed `electronic beam-squinting', has been developed to solve this problem both elegantly and economically. It is capable of a performance approaching that of the most expensive systems currently available for fixed earth stations and is also applicable to mobile terminals