Complete Electronics Self-Teaching Guide with Projects

Practical Oscillator Design

26 This section briefly covers some practical problems with oscillators.

Before you proceed, review the important points of this chapter by answering the following questions.

Questions

A. What three elements must an oscillator have present to work? _____

B. What determines the frequency of an oscillator's output signal?

C. What provides the feedback?

D. How many feedback methods for oscillators have been discussed?

E. What do you need to start the oscillations once the circuit has been built?

Answers

A. An amplifier, a resonant LC circuit (or some other frequency determining components), and feedback.

B. The frequency of the output signal is the same as the resonance frequency.

C. A voltage divider on the resonant circuit.

D. Three: the Colpitts, Hartley, and the Armstrong.

E. Nothing: The oscillations should start spontaneously if the component values in the circuit are correct.

The main practical problem with building oscillators is selecting the coil. For mass production, a manufacturer can specify and purchase the exact coil required. But in a lab or workshop (where you are building only a single circuit), it is often difficult or impossible to find the exact inductor specified in a circuit design. What usually happens is that you use the most readily available coil, and design the rest of the circuit around it. This presents three possible problems:

• You may not know the exact value of the inductance.
• The inductance value may ...