Introduction To Radio Frequency Design Wes Hayward

Introduction To Radio Frequency Design Wes Hayward

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The problems of finite-frequency circuits are severe. For example,the RF filter uses LC filters and the input signal can be treated aslarge DC offset. A circuit might be designed to work in a particularfrequency range, but the signal at that frequency can be +20dB from a design goal.

The capabilities of such circuits have increased to the point thatit is possible to use them as design aids for a finite-frequency circuit. Forexample, a circuit can be analyzed to determine the phase of the output signal versusfrequency to determine if it will be in phase with the input signal at anygiven frequency. Or, if a particular circuit design is to be tested todetermine the gain of each stage (which should be unity) relative to the otherstages, a circuit simulator can be used to measure such parameters. Such an analysisof a circuit becomes an aid in circuit design. The idea is to devise a design that worksaround the phase or gain parameter that is wrong and hence improve the overallperformance. The idea is to take an old design and simulate it anew. Then, using design parameters from the simulation, you can design a newcircuit that works better than the original. For example, if the phase ofone stage is wrong, you can take the phase-shape from the simulation,which might be the phase-shape of another stage at a different frequency, anduse it to make the first stage phase the same as the second stage.

The signal analysis and design of RF circuits has undergonea dramatic revolution since the 1970s. Numerical analysis and the use ofcalculators have penetrated the design process. And, as a result, the knowledgebase of RF design has grown, including the ability to analyze circuits in realtime as design is completed. For example, such circuits can be analyzed to determineif they will oscillate in a particular frequency range. A simple example is a circuit withan amplifier and a filter which has a pole at the place where the twocomponents are in unity gain. For a particular frequency, as the gains of the componentsare varied, the point of unity can be calculated to determine where the amplifieror filter will oscillate. Such analysis is an idealized version of whatreally happens in a real circuit, but it can give a good idea of how a circuitwill perform and what conditions would cause oscillation or stability problems.

In the first part of the book, the circuits and analysis are presented in a highly effective and intuitive way. This is followed by a dynamic tour of the basic theory of high frequency circuits and its application to RF systems. Rather than overwhelming the reader with mathematical derivations, the author provides a hands-on approach to the subject with only a few formulas, symbols, and equations. An extensive set of exercises and problem sets make it easy to correlate theory with real-world design.

Finally, for a look at a most elegant receiver design, you might examine the circuitry of the Elecraft K2. Its seemingly oversimplified circuit manages with very few parts and a very simple architecture to provide performance surpassing that of many more complex and expensive radios. You will find there a fairly complex crystal IF filter following the post-mixer amplifier and feeding the IF amplifier which then feeds a very simple crystal filter to the final mixer stage which produces the audio output signal. 827ec27edc