P.B. Kenington

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.

The efficiency of a feedforward amplifier with delay loss

In most applications using the feedforward linearization technique, the overall amplifier efficiency is of significant concern. This paper theoretically analyzes the effect that the insertion loss of the time delay elements has on the overall efficiency of a feedforward amplifier. Practical linearity and efficiency results are given for a feedforward system, using VMOS transistor class C main and error amplifiers, to illustrate the effectiveness of the analysis. >

Power amplification techniques for linear TDMA base stations

Two techniques that facilitate the transmission of linear time-division multiaccess (TDMA) signals from a mobile radio base station are proposed. The Cartesian loop technique is applied in a first-order loop configuration and shown to exhibit an excellent transient response when driven with a rapidly switched TDMA waveform. The practical implementation of a feedforward technique is also discussed and shown to be applicable to multicarrier systems employing dynamic channel allocation. Practical results are presented for both systems.<<ETX>>

Noise performance of a Cartesian loop transmitter

The Cartesian loop transmitter is now a well-known linear transmitter architecture and is finding application in a number of mobile radio systems employing linear modulation technologies. In particular, systems utilizing /spl pi//4 DQPSK require a linear transmitter, and many emerging standards [e.g., trans-European trunked radio (TETRA)] provide applications for the Cartesian feedback linearization (CFL) technique. One problem with the CFL transmitter is that its output-noise performance is no longer dominated by that of the RF power module employed within it (as is the case in more conventional transmitter architectures). The use of significant degrees of attenuation, followed by a high level of gain, within the loop, means that the noise performance of the loop is significantly poorer than that of a conventional transmitter. There are a number of tradeoffs that are available to the designer of a CFL transmitter to aid in the optimization of the output-noise performance. This paper presents a derivation of the noise performance of a Cartesian loop transmitter and highlights the design methods that may be employed in order to optimize its noise performance. It also provides a comparison of the theoretically derived behavior with that of a practical transmitter operating in the TETRA (380-400 MHz) band.

A broadband linear power amplifier for software radio applications

This paper discusses a new type of polynomial-based RF predistorter with excellent broadband performance and very low levels of complexity. The technique employed may be designed to operate at any desired centre frequency and therefore has application in the PMR, cellular and satellite areas of communications systems. The technique is compact, very simple and of very low cost, so it will find application in handsets as well as within the base station environment.

A highly-efficient linear amplifier for satellite and cellular applications

This paper presents a scheme to linearise highly-efficient amplifiers over a broad bandwidth, whilst retaining most of their efficiency. Present wideband linearisation techniques are normally restricted to either feedforward or analogue predistortion. Feedforward has been shown to provide excellent linearity, but only modest efficiency; whilst analogue predistortion has little effect on efficiency, but limited linearity improvement. However, the combined use of both techniques has enormous potential, since the linearity improvement is greater than that attainable with feedforward alone, whilst the efficiency approaches that of the main amplifier. Practical results are presented which confirm the above predictions in terms of overall efficiency and linearity, and illustrate the effectiveness of the technique.

Tracking receiver design for the electronic beam squint tracking system in the mobile environment

The authors investigate the beam squint technique from the viewpoint of the tracking receiver in a mobile satellite environment, together with the more general problems experienced by tracking receivers in this situation. This technique is detailed with particular reference to the design and characterization of an optimum tracking receiver, and the new problems which the system poses in this area. The system is described with an identification of the factors in existing receivers which limit the performance of the electronic tracking technique. Using a computer simulation, a fundamental understanding of the physical processes involved is presented and some comprehensive experimental results are included to validate the conclusions and proposed solutions. The techniques described may also be applied to conventional tracking systems, with a useful improvement in performance.<<ETX>>

The Effect of Modulation on the Power Rating of Multi-Channel Linear Amplifiers

Recent research has shown that a single broadband, highly linear amplifier can provide an alternative to high-level combining of multiple amplifiers. The multi-carrier amplifier allows much more flexible frequency planning than a cavity combiner, and much greater power efficiency than a hybrid combiner. However, since the multi-carrier amplifier carries a complex signal, which is the sum of all the individual modulated carriers, its peak power rating must be carefully considered. For example, the peak rating of a 16-channel system may be as high as sixteen times the average power (i.e. 256 times the power in each carrier). A statistical analysis of the peak power of a multi-carrier signal, using modulated carriers, is presented, which shows that the amplifier power rating may be significantly reduced for a large number of channels, resulting in a decrease in the hardware and operating costs of multi-carrier transmitters.

Predicting the power rating of a multi-channel linear amplifier

The demand for much greater flexibility in the frequency planning of radio networks has led to the development of a new base station transmitter architecture. A single highly linear multi-channel amplifier could replace the conventional high-level combination of individual amplifiers using selective cavities. This new architecture allows much greater flexibility in frequency planning than a cavity combiner, together with an increase in power efficiency over that of a hybrid combiner. However, the power rating of this single amplifier, which carries a complex signal consisting of the sum of all the individual modulated carriers, must be carefully considered. For example, the peak envelope power of a transmitter carrying sixteen channels at 10 Watts each could be as high as 2.56 kW. The work presented in this paper complements an earlier analysis into the statistical behaviour of the envelope of a multi-carrier signal, by investigating the degree of distortion introduced by limiting the multi-carrier envelope, with a view to reducing the peak envelope power.