Resonant devices

Chapter Resonant devices

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • In practical circuits, variable inductors and/or variable capacitors are often placedin tuned circuits, so that small errors in the frequency can be compensated for. Themost common approach is to design the circuit for a frequency slightly higher than fo,and to use a padder capacitor in parallel with the main capacitor (Fig. 17-13).Resonant devices32117-13Padding capacitors(Cp) allow adjustmentof resonant frequencyin a series LC circuit(A) or a parallel LCcircuit (B).Resonant devicesWhile resonant circuits often consist of coils and capacitors in series or parallel, thereare other kinds of hardware that exhibit resonance. Some of these are as follows.CrystalsPieces of quartz, when cut into thin wafers and subjected to voltages, will vibrate at highfrequencies. Because of the physical dimensions of such a crystal, these vibrations oc-cur at a precise frequency fo, and also at whole-number multiples of fo. These multiples,2fo, 3fo, 4fo, and so on, are called harmonics. The frequency fo is called the funda-mental frequency of the crystal.Quartz crystals can be made to act like LC circuits in electronic devices. A crystalexhibits an impedance that varies with frequency. The reactance is zero at fo and theharmonic frequencies.CavitiesLengths of metal tubing, cut to specific dimensions, exhibit resonance at very-high, ul-tra-high, and microwave frequencies. They work in much the same way as musical in-struments resonate with sound waves. But the waves are electromagnetic, rather thanacoustic.Cavities, also called cavity resonators, have reasonable lengths at frequencies aboveabout 150 MHz. Below this frequency, a cavity can be made to work, but it will be long and