Chapter Stability

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

  • it isn’t hard to get it to oscillate. Another advantage of the Clapp circuit is that it allowsthe use of a variable capacitor for frequency control, while still accomplishing feedbackthrough a capacitive voltage divider.StabilityThe term stability is used often by engineers when they talk about oscillators. In an os-cillator, stability has two meanings: constancy of frequency and reliability of perfor-mance. Obviously, both of these considerations are important in the design of a goodoscillator circuit.Constancy of frequencyThe foregoing oscillator types allow for frequency adjustment using variable capacitorsor variable inductors. The component values are affected by temperature, and some-times by humidity. When designing a variable-frequency oscillator (VFO), it’s crucialthat the components maintain constant values, as much as possible, under all antici-pated conditions.Some types of capacitors maintain their values better than others, when the tem-perature goes up or down. Among the best are polystyrene capacitors. Silver-mica ca-pacitors also work well when polystyrene units can’t be found.Inductors are most temperature-stable when they have air cores. They should bewound, when possible, from stiff wire with strips of plastic to keep the windings in place.Some air-core coils are wound on hollow cylindrical cores, made of ceramic or phenolicStability46325-5Series-tuned Colpitts oscillators using an N-channel JFET.