Crystal-controlled oscillators

Chapter Crystal-controlled oscillators

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

  • material. Ferromagnetic solenoidal or toroidal cores aren’t very good for VFO coils, be-cause these materials change their permeability as the temperature varies. This changesthe inductance, in turn affecting the oscillator frequency.Engineers spend much time and effort in finding components that will minimizedrift (unwanted changes in frequency over time) in VFOs.Reliability of performanceAn oscillator should always start working as soon as power is supplied. It should keeposcillating under all normal conditions, not quitting if the load changes slightly or if thetemperature rises or falls. A “finicky” oscillator is a great annoyance. The failure of a sin-gle oscillator can cause an entire receiver, transmitter, or transceiver to stop working.An oscillator is sometimes called unstable if it has to be “coaxed” into starting, or if itquits unpredictably.Some oscillator circuits are more reliable than others. The circuits generalized inthis chapter are those that engineers have found, through trial and error over the years,to work the best.When an oscillator is built and put to use in a radio receiver, transmitter, or audiodevice, debugging is always necessary. This is a trial-and-error process of getting theflaws, or “bugs” out of the circuit. Rarely can an engineer build something straight fromthe drawing board and have it work just right the first time. In fact, if two oscillators arebuilt from the same diagram, with the same component types and values in the samegeometric arrangement, one circuit might work fine, and the other might be unstable.This usually happens because of differences in the quality of components that don’tshow up until the “acid test.”Oscillators are designed to work into a certain range of load impedances. It’s im-portant that the load impedance not be too low. (You need never be concerned that itmight be too high. In general, the higher the load impedance, the better.) If the load im-pedance is too low, the load will try to draw power from an oscillator. Then, even awell-designed oscillator might be unstable. Oscillators aren’t meant to produce power-ful signals. High power can be obtained using amplification after the oscillator.Crystal-controlled oscillatorsQuartz crystals can be used in place of tuned LC circuits in RF oscillators, if it isn’t nec-essary to change the frequency often. Crystal oscillators offer excellent frequency sta-bility— far superior to that of LC-tuned VFOs.There are several ways that crystals can be connected in bipolar or FET circuits to getoscillation. One common circuit is the Pierce oscillator. An N-channel JFET and quartz crys-tal are connected in a Pierce configuration as shown in the schematic diagram of Fig. 25-6.The crystal frequency can be varied somewhat (by about 0.1 percent) by means of aninductor or capacitor in parallel with the crystal. But the frequency is determined mainlyby the thickness of the crystal, and by the angle at which it is cut from the quartz rock.Crystals change in frequency as the temperature changes. But they are far morestable than LC circuits, most of the time. Some crystal oscillators are housed in tem-perature-controlled chambers called ovens. They maintain their frequency so well that464 Oscillators