Stable Transistor Oscillator

Frequency stability is an essential requirement for most transistor oscillators as for many other oscillators. However, unlike vacuum tube oscillators, transistor oscillators usually suffer a great change in frequency with variation in operating characteristics. The most serious causes of frequency instability are the effect of temperature on parameters of transistors and the variation of supply voltage. By using certain circuit arrangements and a crystal as frequency-controlled element, a high degree of stability could be achieved over a wide range of temperature and supply voltages. As a practical example, a simple crystal controlled 100 kc oscillator is described, capable to deliver 3 volts rms, from supply voltage 12 volts. The frequency stability of the oscillator is better than 10 parts per million per 1 volt supply voltage, and two parts per million per degree centigrade, for a wide range of load resistance ranging from a few ohms to 100,000 ohms. At a frequency of 1 mc, a stability better than 5 parts per ten million per 1 volt supply voltage and one part per ten million per degree centigrade was obtained.