AWR eBooks

1.1 Making the transition from DC and AC to Radio Frequency 1 1 Fundamentals of Electrical Circuits 1.1 Making the transition from DC and AC to Radio Frequency It has now been ten years since I was first introduced to the basic concepts of RF and Microwave theory and I recall how excited I was as an undergraduate student to start learning and understanding this 'mysterious' and fascinating subject. Since then I have been to numerous courses and have tackled a number of engineering tasks in the field. I have also had the opportunity to teach the subject myself and invariably I can see that the students seem to struggle with exactly the same concepts which I myself was a bit baffled by back in the days. This myth of RF Engineering being a bit of a black-magic branch of electronics is not totally unfounded because it often takes people years to master it and an inexperienced engineer may often be faced with seemingly inexplicable phenomena.. I feel that the problem lies with the approach which is taken to teaching RF & Microwave Engineering which, despite being rigorous and formally correct, focuses too soon and too heavily on the maths when the physical concepts and phenomena should be given priority and wide breath. Also, and more importantly, it fails to bridge the gap between what happens in AC and DC circuits, which most students are familiar with, and what happens at Radio and Microwave Frequencies. This lack of clarity, created by this lack of connection, is further exacerbated by the use of similar terminology which has a very different meaning at DC/AC and RF. With this book I will attempt to bridge this gap and make the sharp seam between low and high frequency theory and techniques, which the classical teaching creates, a great deal smoother and much easier to overcome. Most textbooks and courses start with the concept of wavelength and how this quantity allows you to determine whether you may use a lumped circuit model or whether you need to apply RF techniques. I, on the other hand, will begin with revising the basic concepts of voltage, current and impedance in DC and AC circuits and how they relate to Electric and Magnetic fields within such circuits. Subsequently I will clarify some mathematical concepts which are often used in the treatment of the subject and connect them to the physical phenomena that they represent. I will then introduce the wavelength not just as a simple formula but in terms of its physical significance and explain why it is a good indication of what techniques need to be used to analyse a circuit. From there we will move on to the concept of impedance at high frequency and how this relates to voltages, currents and, more importantly at radio frequency, Electric and Magnetic fields. A good understanding of these concepts forms the perfect platform onto which RF & Microwave knowledge may be built and is key to understanding transmission lines, ubiquitous building blocks of high frequency circuits. Transmission lines, as most of you will know, are all about delivering power from a generator to a load. The way we achieve maximum power transfer and maximum efficiency is however very different for high and low frequency circuits. This is because, while at low frequencies (e.g. 50Hz), power is conveyed predominantly as voltage and current and is 'quasi' electrostatic in field form, at high frequencies the power is predominantly conveyed as 'electromagnetic fields'. This text presents a detailed analysis of transmission lines terminated with different loads. The concepts of incident and reflected voltages are illustrated both in the case of continuous wave and pulsed signals. The applications of "special" lines are also illustrated together with the practical implementation and simulation of the most common transmission line types. Conquer Radio Frequency 1 www.cadence.com/go/awr

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